# Discussion on Small CNC Mill Spindle and Axis Motor Performance



## macardoso (Feb 4, 2021)

Hi All,

I have owned a CNC converted G0704 for about 9 years now and really have a ton of hours on it. One of the things I really enjoy thinking about and have done some experimenting with is understanding how to overcome the significant limitations of these smaller machines by selecting toolpaths and strategies to take advantage of what the machine is good at and avoid what it is not.

Currently my G0704 has a 5000 rpm (11.1Nm of torque), 1.8kW servo spindle and 750W direct drive axis motors for a max rapid of 1000ipm. The tool holding system is the Tormach Tooling System (TTS). I have monitoring of spindle load and axis thrust forces in real time by pulling data from my servo drives which I will use in analysis below. My spindle belt drive consumes 8% of the spindle load in heat, for a total spindle efficiency of 92% (actually pretty good) and 144W of heat generated in the motor, belts, and bearings.

*Analysis: *The biggest limitation of these machines in rigidity. Toolpaths need to be chosen to minimize horizontal thrust forces while still maximizing material removal. The first approach is to use smaller tools. For my size machine, I find optimum performance with 3/8" endmills in aluminum and 1/4" in steel. I can use larger, but the chipload needs to be less so there is actually a loss of efficiency. Second approach is to use very sharp tools. High polish aluminum specific endmills like the YG-1 Alu-Power are great for pulling more productivity from your machine without needing to upgrade mechanics.

Now the use of smaller tools drives us towards the conclusion that the 2000-2500rpm spindles that come stock on these machines are not suited for CNC operation. As the tool gets smaller, the RPM required to maintain an acceptable surface footage at the tool goes up. I run my spindle maxed out at 5000rpm most of the time and have considered many times over to modify it to reach 10k RPM. For general machining in aluminum (usually pretty light), my spindle load sits below 15%. At 1/4" diameter endmills (aluminum) and 3/16" (steel) and below, I lack the spindle speed to make efficient use of the tool. The heaviest cut I have ever seen this machine take was 1400W in aluminum with a 3/8" endmill and I doubt I would want to do that often without a serious flood coolant setup and enclosure.

There are a few cases where I do start to utilize the torque of the spindle. These are usually in low speed (<800 RPM) roughing operations in cast iron using 1/2" tools. I can maintain about 75-80% torque utilization without having the machine flex all over the place. I also see these higher torque values when drilling with larger drills. Interestingly drilling is great for low rigidity machines (think of drill presses) since all the force is in line with the spindle. 

If I were to re-address the spindle situation, I would want a 2 speed belt driven solution with a lower gear to 2500 rpm and a high gear to 10000 rpm with the same 1.8kW power capacity. I think this is achievable with the kinds of machines we work with and actually comes pretty close to what Tormach has evolved their 770 and 1100 spindles to. If your machining needs are typically light machining in aluminum and steel, then 500W of spindle power up to 5000 rpm would likely cover your needs. 

The TTS system has been extremely reliable for me except for long extension endmilling (4"+). The straight shank fails to adequately support the moment loading and extreme chatter and tool pullout is almost always the result.

The 18k RPM water cooled spindles seen on eBay are another option. They have great speed, but lack torque and are limited to a bottom speed of 8000rpm. For this reason, they are an excellent choice for milling and routing in aluminum with 3/16", 1/8", or smaller tools. It is actually difficult to mill steel with these as the speed is usually too high and the torque too low. 

What about axis feed motors? Truth be told is I have never used 1000ipm beyond initial testing. I limit it to 300ipm and that is still more than needed. The biggest time saver in a CNC for anything we will use it for is the axis accelerations. This means a high torque motor (and low inductance in stepper motors). Milling feed rates up to 80-120ipm will cover pretty much anything you will be doing on a hobby machine. 

Going back to how to make the most of the machine you have is utilizing trochoidal "high efficiency" toolpaths. These minimize radial engagement of the cutter and peel away at the work. The axis motors will be under constant acceleration and deceleration so maximizing what your machine can do in that regard will allow you to spend more time at the programmed cutting feedrate. Since our machines are limited in rigidity, we cannot take high chipload cuts, often less than the minimum recommended chip load of the tool. For this reason, it is often advantageous to use endmills with higher flute counts, even 6 or 8 flutes in steels to give higher material removal at a given chip load. 

Finally, one strategy I would like to investigate is feedmilling. In this case a special tool is rotated relatively slowly while being fed at extreme feedrates. The resultant forces are pointed into the spindle much like drilling, while remaining extremely efficient in terms of spindle usage and material removal. Unfortunately these tools are uncommon and expensive.

Would love to hear the thoughts from other CNCers

-Mike


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## spumco (Feb 4, 2021)

macardoso said:


> a special tool is rotated relatively slowly


Not slowly.  RPM is still set by the appropriate surface speed for that diameter tool.  The big difference is a very shallow DOC and high IPM.  The change in force direction is due to the geometry of the insert/cutting edge.

So if you're using a 1/2" altin feed mill in 4140 you still need to spin it up to 3-3500rpm - same as with a regular end mill.

Commenting on your general thoughts...  I've had the same quandry ever since I started fiddling with my mill.  Lack of rigidity seems to be the first limiting factor, second is the price to performance formula for spindles.

Rigidity can be usually improved but it's a hassle.  Think filling castings with concrete, adding external bracing, etc.  Franken-mill.

My experience with TTS has been less satisfactory than yours, but we don't have the same machine so that's not a broad condemnation of TTS. Which is why I'm switching over to BT30 right now.

I think 10k, maybe 12k RPM belt-drive spindles are about the limit for home-gamers.  Higher than that and the bearings get silly expensive.

I agree with your position: the best spindle solution is a two-speed poly-v belt system, with an encoder on the spindle itself.  Add in a Fadal-type auto speed changer and you can use a cheaper motor and relatively cheap cartridge spindle.  1.5:1 and 1:1.5 ratios perhaps.  Taper of your choice, of course.

10k may be the sweet spot, now that I'm thinking about it.  Induction motor or decent servo can hit 5k, so a 2:1 and 1:2 belt system will cover everything from 500RPM high-torque big drills up to 10kRPM little guys.  Spindle encoder means you don't have to do computer gymnastics for synchronized tapping or ATC orientation.

-Ralph


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## macardoso (Feb 4, 2021)

spumco said:


> Not slowly.  RPM is still set by the appropriate surface speed for that diameter tool.  The big difference is a very shallow DOC and high IPM.  The change in force direction is due to the geometry of the insert/cutting edge.
> 
> So if you're using a 1/2" altin feed mill in 4140 you still need to spin it up to 3-3500rpm - same as with a regular end mill.
> 
> ...



Ah, my bad about the feedmill. Still want to try one. Seems like a good roughing strategy for a machine with low rigidity.

I feel like a 2HP, 3600 rpm inverter duty motor run at 5krpm with a 2:1 and 1:2 belting options would be a killer setup. Beyond that, spindle bearings, dynamic balancing, and active cooling probably become necessary.


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## spumco (Feb 5, 2021)

I think an 1800rpm 4-pole motor would be better than a 3600.  Keep in mind that most inverter duty induction motors can be spun way above base frequency.  Like 5-6krpm.

A 3600rpm motor will get hotter and not have the same torque when run at 1krpm as an 1800.

The reason an ac servo of similar power/speed specs would be better is weight and inertia.  The same power motor would be half the weight (or less), and can speed up/slow down/revers much quicker.  Downside, as usual, is cost.

I thought I posted this a while ago:





That, to me, is the high-water mark for hobby spindle setups.


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## ChrisAttebery (Feb 6, 2021)

I've had this discussion a few times on the Benchtop Mills forum on CNCzone. I had my G0704 for about 8 years now. Here's my current configuration.

Grizzly G0704 mill
Centroid Acorn controller
Centroid Wireless MPG

From Automation Technology:
3 x KL-5056D Digital Bipolar Stepper Motor Driver-32 bit DSP Based
3 x NEMA23 570oz/in 5A 3/8” Dual Shaft Stepper Motor (KL23H2100-50-4B)
KL 48VDC/12.5A Switching CNC Power Supply
Mean Well LRS-50-24 24V Switching Power Supply for inputs, fans, relays

From Automation Direct:
MTRP-002-3BD36 2hp, 56C frame, 3600rpm motor
WEG Electric CFW300 series vector drive
Metrol CS067B-L Precision Touch 5 Micron Accuracy Limit Switches

CNC Fusion's Premium Conversion Kit (stepper mounts and ball screws)
Home Brewed Belt Drive Conversion
Home Brewed Fogless Coolant Misting System
Home Brewed 1010 T Slot Enclosure
Manual Oiling System
Chinese Tool Setter from eBay

Kurt DX-4 Vise

*Controllers:*
I used Mach3 with an Ethernet Smooth Stepper for 8 years. It worked OK, but it wasn't super reliable and it would act weird often enough that I usually machining about 20% more parts than I needed just in case. I recently upgraded to the Centroid Acorn and so far it's been very reliable.

*Axis motors:*
I run 240IPM rapids on all three axis. IMHO on a machine this size you can't really use much more. They are reliable at these speeds which is more important that saving a couple seconds here and there. I've NEVER had a cut stall the axis motors without breaking the tool or stalling the spindle motor first.  

*Spindle:*
I'm running the stock spindle with Timken tapered bearings. I ran AC bearings for years but the 2HP motor and high loads ate those up in a couple weeks. I would love a 10-15k balanced spindle, but this little machine isn't worth the trouble and cost of installing one.

*Spindle motor(s):*
I started out with the stock "750W " motor, went through a couple different treadmill motors, before settling on a 1100W DC motor for about 6 years. Over the summer that motor finally melted down so I moved to a 2hp 3600rpm 56 frame inverter duty motor with a VFD. I run the motor at 4500 rpm because that's what I designed my poly V belt drive around. My belt drive has two ratios: 3:2 on the high end (6750 rpm) and 1:2 on the low end (2250 rpm). 

Now that that's all of that out of the way, you're right that the lack of rigidity is the biggest limiting factor on machines in this weight class. Moving to higher speed spindle does allow you to use smaller tools that put less load on the machine but that only helps so much. 

99% of the time I'm machining aluminum. I've settled on the Best Carbide 601-33750-1 3 Flute HSC Aluminum End Mill as my workhorse. My recipe is 6750 RPM, 81 IPM (.004" IPT), ~.5" DOC and ~.050" WOC to get 2cu/in minute MRR. I've also looked at the YG1 AluPower but haven't tried one yet. I had really good luck with the YG1 HSS end mills for aluminum though. 

I use HSS drills up to 3/8". Anything larger than that I will predrill and then use my 3/8" end mill if I can.

I rarely use the low drive ratio. When I do it's usually to use a 3" HSS slitting saw.

To be honest the biggest time savers I've found from my latest upgrades have been from the software and hardware controllers being super reliable, and not having to constantly re-home and re-locate my WCS. When I do home the machine the homing switches are so accurate (.0002") that they basically repeat to one step on the motors. According to the probing software I'm using the cheap tool setter is also repeatable to .0002". So I can hit the E-stop, leave the controller running, come back the next day, re-home the machine and not have to worry that the WCS has moved. I can also run Fusion360 on the same PC that runs the machine so I can make minor changes to the code in between cycles without having to use another PC (usually INSIDE the house). It's been refreshing.

My next upgrade is adding a Renishaw MP3 Probe. Then I can ditch the edge finder and touch plate. I should be able to probe a corner or bore in a lot less time and a lot more accurately with the probe.


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## matthewsx (Feb 7, 2021)

Watching....


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## spumco (Feb 7, 2021)

Since improving rigidity is very machine-specific and spindle motors/drives are a bit more universal, here's a teaser from that German chap.

According to his build thread he's running 1:1.5 and 3:1 ratios.

Now I'm trying to figure out where to put an encoder on the spindle.


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## macardoso (Feb 8, 2021)

That's such a slick setup. If you could extend the spindle tube up a bit, you could add a hollow bore encoder in line with the spindle. Could also belt drive it 1:1 off to the side, but that looks dumb. Maybe inside the head where the change gears used to be?

I spent a ton of time modifying my spindle for low noise. Basically I have an air tight chamber within which the belt drive (curvilinear) runs. It is about 62dBA measured 1' away from the head of the machine. This is a very comfortable level for me. I have a L series timing belt on it before, open to the air, and it was LOUD!. I would love to readdress the spindle belting, but I would not trade the air tight chamber.







My pneumatic drawbar sits on top of those posts.


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## ChrisAttebery (Feb 8, 2021)

Yes, the L belts are super loud. I moved to a 6 groove Poly V belt and it's much quieter. The dominant noise at the spindle's top RPM is from the motor's fan.

I'm basically done spending money on upgrading this machine. I'd really like to upgrade to a Tormach or small used VMC but my sales haven't been high enough to warrant that type of investment lately.


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## spumco (Feb 8, 2021)

macardoso said:


> Maybe inside the head where the change gears used to be?



Uh...  Not so much.  Mine never had change gears and the spindle cartridge is tall enough that there's no exposed spindle rotating bits in the head.

And as of two days ago there's about 20lbs of basalt fiber-reinforced CSA concrete in there.

Remember when I mentioned that improving rigidity is a hassle?  That wasn't conjecture.


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## M.T. Pockets (Feb 19, 2021)

My home shop experience has more or less come to the same conclusion as yours. I went from using Bridgeports, Haas toolroom mills at uni to a Taig mini mill at home after I graduated. Quite the step down... but what a learning experience! These smaller mills are far less forgiving than something thats 1,000+ lbs with lots of HP's which really taught me a lot about feeds and speeds and best practices to get the most our of a machine. When I first got my Taig I didn't think it was capable of very much and after spending about 3-4 years with it I've learned that you can do a lot with with a 10k spindle and some reasonable feedrates.

When I cut aluminum on that machine I'm usually doing MRR's of around 0.5-0.8 cu in/min, which isn't a lot, but its good enough for a hobbyist with an 80lb machine. I'm usually using a 3/8 or 1/4 roughing end mill, around 5-9krpm, and feed rates of 20-30ipm. Yeah, it doesn't cut very quickly but it will hold <0.003in all day long and give a nice surface finish with a descent high flute end mill at 10krpm for a finishing pass. 

I've even been able to cut 303 stainless which isn't quite as easy. That involves lower RPMs obviously and bumping up the chip load to prevent work hardening. That's when rigidity really starts to be a problem.


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## COMachinist (Apr 14, 2021)

I have just started the rebuild of my 12 yr old G0704 CNC conversion spindle for the umteenth time. I’m almost ready to pull the trigger on one of these spindles. https://www.cncdepot.net/product-page/fm-series-spindles. They really nice and about 2/3rds the weight of the 3ph/belt drive G0704 head conversion, with a PDB. These use air ISO BT30 tool holders, 12k rpms @ 3hp. they just fly through metal. Check outClough42 youtube channel  on his G0704 CNC conversion. 



I have probably spend this much over the life on my G0704 project.
Just  thinking.
CH


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## macardoso (Apr 14, 2021)

@tkalxx, when you get to assembling it, you have to share your design here!


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## tkalxx (Apr 14, 2021)

I will give some background info into my spindle motor design and post more pictures/videos as I assemble it on the mill. I can't take credit for the idea as I copied the German fellow referred to earlier in this thread who copied the fadal belt actuator design. My goals were to increase the maximum rpm to at least 7500rpm and still have enough torque in the low end (500-2000rpm) for machining steels. Rigid tapping was never a design requirement for me but was a "could be nice to have" feature. I have 2 belt ratios utilizing J6 poly V belts; low gear is achieved through a 1:2 belt ratio (150-3000rpm) and high gear is achieved through a 4:3 belt ratio (3000-8000rpm)

The selected motor is an Adlee 2.2kW BLDC motor controlled by 0-10V analog. It's a standard 130 mm frame motor which is critical for packaging the entire system. Here are some key motor specs:
*Motor Weight: *23.6 lb
*Drive Input: *220V 1ph (18A max)
*Motor Input: *220V 3ph (9A max)
*Motor RPM Range:* 300-6000 rpm
*Continuous Torque @6000rpm:* 3.7 Nm
*Peak Torque @6000rpm: *7.2 Nm

Here's what this means in terms of my belt ratios.

_Low Gear (150-3000rpm):_
*Continuous Torque: *7.2 Nm
*Peak Torque: *14.4 Nm

_High Gear (3000-8000rpm):_
*Continuous Torque: *2.8 Nm
*Peak Torque: *5.4 Nm

I had originally planned on using a 1.8kW servo motor from DMM-tech. The servo definitely outperforms the BLDC motor in terms of torque in the lower 60% of the motors rpm range, however, the BLDC motor slightly outperforms the servo in the upper 40% of the motors rpm range (most likely due to their different power rating). The BLDC was also cheaper, and proved to be much easier to integrate with my motion controller. In order to achieve 8000rpm with a typical 3000rpm servo, the belt ratio needs to be 8:3 and there simply isn't enough room on the head of the PM25/G0704 to fit this kind of ratio along with a pneumatic belt tensioner and power draw bar cylinder. Some people (*cough @macardoso cough*) have been able to source 5000rpm servos, but there is a very limited selection and they demand quite a price.












-Adam.


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## spumco (Apr 14, 2021)

Adam,

You're my new hero.  Well done.

If it brightens your day at all, I think you'll be happier with the BLDC than with the DMM servo.  I've seen a number of people reporting (youtube and the linuxcnc forum) that the DMM's have very noisy encoder outputs, and aren't doing very well with 0-10v input.  Step & direction - fine, no issues.  But not as great with analog input, especially in torque mode.

Nothing against DMM - I just installed four of them on a plasma.  But I'm not convinced they're the best thing around for a spindle motor.

You didn't mention what control you're using, but in either case (servo vs. BLDC) you can fit an encoder direct to the spindle and send that signal to the control.  No need for a servo, and no fiddling with interpolating the encoder count to belt ratio.

Down the road you can always drive that BLDC with a servo drive.  Think of that BLDC as a servo with halls but no incremental encoder.  If you replace the Halls with a commutation encoder (i.e. CUI Devices) you can drive it with any of the agnostic servo drives (Copley Xenus, Allen Bradley, etc.).

The nice thing about using a higher-quality (or perhaps more feature-rich) servo drive than the DMM is that the encoder outputs are clean, and they have internal scaling.  Plus user-programmable inputs.  Think about having the same signal that changes your belt drive triggering a different encoder output ratio being sent from the drive to your control.  No scaling needed in the control.

The only fly in the ointment is that you still need a spindle index trigger, and if you've got any belt slip you're back to needing a direct spindle encoder.

And some of the servo drives have analog outputs - imagine using the actual spindle load to manage the cutting feedrate...

In fact, I've got a 130mm 2.2kw BLDC sitting on my bench about to get reinstalled on my mill.  It's probably the same thing you have, but about 10 years older.  And a Copley Xenus XSL-230-40 drives it just fine, both in velocity mode and as a positioning servo.  That's about the only single-phase drive I've seen capable of 20+ amps continuous (besides some of the ancient AMC 'dumb' PWM drives).

Have fun with your awesome belt system, and if your drive blows up or you want spindle positioning, let me know and we'll see about finding you a '_Macardoso-worthy_' spindle drive.

-Ralph


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## tkalxx (Apr 15, 2021)

Don't praise me yet, I haven't even proven that everything works!

I'm running Mach4 and Pokeys57CNC. I've heard some of the horrors of running DMM servos with analog output. I've also heard of issues where people are constantly faulting the servo in step/dir mode when it gets overdriven which seems incredibly annoying for a spindle motor.

Fitting a commutation encoder and running the BLDC with a servo drive is an interesting idea that I never considered. I know down the road I'll probably implement some sort of feedback. During the design phase, I thought ahead and made a recess in the bottom of the spindle pulley so that I could fit an encoder wheel and an old posic encoder that I have sitting around from work; this is not shown in any of the pictures I posted above. I was hoping to experiment with that sometime in the future - I would need to buy a code wheel though https://www.posic.com/EN/codewheel-tpcd07-180.html. Pokeys has an ultra-fast encoder input and should handle the posic just fine. A servo drive with programmable inputs/outputs seems pretty appealing but I would be worried about belt slip (since I can't run timing belts) and not having any feedback from the spindle itself.

It's going to be fun trying to automate everything in Mach. Currently, I have 2 speed ranges configured, and the range needs to be manually selected (better than physically changing the belts but not good enough in my books). I'm working on figuring out a solution where I can input a commanded spindle RPM and the correct belt engages/RPM achieved. From my initial digging, it looks like this will require changes to the core code that calculates analog output voltage.

What are the master plans for that BLDC on your bench? I assume you'll be running a timing belt?

-Adam.


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## macardoso (Apr 15, 2021)

COMachinist said:


> I have just started the rebuild of my 12 yr old G0704 CNC conversion spindle for the umteenth time. I’m almost ready to pull the trigger on one of these spindles. https://www.cncdepot.net/product-page/fm-series-spindles. They really nice and about 2/3rds the weight of the 3ph/belt drive G0704 head conversion, with a PDB. These use air ISO BT30 tool holders, 12k rpms @ 3hp. they just fly through metal. Check outClough42 youtube channel  on his G0704 CNC conversion.
> 
> 
> 
> ...



I've eyed those up as one of the only small, ATC ready spindles out there. I wonder if it has any low end torque for steel or larger tools. Drilling for example uses quite a bit of torque at lower speeds. I see they have the 12k rpm 4 pole motor, but do not have torque/speed curves for any of the motors. Just curious because this thing looks sweet!


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## macardoso (Apr 15, 2021)

spumco said:


> Adam,
> 
> You're my new hero.  Well done.
> 
> ...


Tkalxx shared a picture of this with me and I knew you guys would love it. If it works well, I could see copying this design at some point down the road!.

I haven't heard of problems with DMM before. Not surprising, but unfortunate. I'd honestly like to keep seeing them do well. Handling noise in an industrial environment is a tough problem. Not to mention that servos have the worst of both worlds: low voltage, highly sensitive encoder signals, right next to high power, high voltage, high dV/dt PWM outputs to the motor. It is a recipe for trouble. I would question the installation practices of those who are having problems. Even with the nice AB stuff I get to play with, if you have a bad ground or shield/case/subpanel/enclosure bonding, there will be noise problems. A ton of well trained people mess this up so I bet the hobby guys do as well. Curious it is only with analog input though... makes it sound like an analog wiring shield issue. Anyways, I'm rambling.

I'd be curious to see the BLDC performance on a servo drive. BLDC and AC servos are wound differently and generate different back EMF signatures. The drives either need to sinusoidally (AC servo) or trapezoidally (BLDC) commutate the motor. The only drive I've seen give you this option is the Ultra 3000, but maybe others do too. Even then I'd suspect there would be unwanted torque ripple. Probably does not matter for us hobby guys. 

Fitting a basic optical spindle index sensor into the design wouldn't be too hard but having a full spindle encoder for rigid tapping would be nice. Perhaps a hollow bore? Only problem is $$$.

Anyways, @spumco and I are servo nerds, so if you ever get the bug, hit us up!


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## JimDawson (Apr 15, 2021)

Time for me to jump in with some thoughts and personal experiences.

First DMM servos.  I have installed a total of 6 of their 1.8KW units (4 on my lathe, and 2 on another machine) and have operated them in both step & direction and analog torque mode.  Under no condition have I noted any encoder noise using differential quadrature inputs, rock solid.  None of these installations were in a spindle application.  I have heard of others having problems when attempting to use these for a spindle application, most notably, not reaching rated speed in step & direction mode, but seemed to work OK in analog torque mode.

IMHO a spindle is not a good application for an inexpensive servo or BLDC motor.  There are purpose built servos deigned for spindle use, but $$$$.

I have found that a standard 3 phase induction motor works fine for spindles, and even better is a 3 phase induction motor purpose designed for spindle use.  Most quality, standard NEMA frame induction motors <5hp are rated for at least 4500 RPM max, and some are rated as high as 6000 RPM (Baldor).  The Fanuc 10HP spindle motor on my lathe is rated at 6000 RPM, but is a very expensive 3 phase induction motor.  Any of these can be operated with a standard sensorless vector VFD and are very stable throughout the operating range with near 100% torque down to about 10 RPM.

In the case of my lathe, I use a +/- 10V analog signal to control the spindle speed via the motion controller and VFD, the speed is monitored by the spindle encoder and fed back to the motion controller.  The encoder is driven by a small timing belt from the spindle.  Rigid tapping can be easily accomplished by hanging an encoder on the spindle, and use electronic gearing.  I have rigid tapped thousands of parts this way on my lathe.  The spindle is an axis in the motion controller.

My mill is not set up for rigid tapping because I don't have a spare axis available in that motion controller, I do however have an encoder attached to the motor to give me a RPM readout on my computer screen, the speed is manually controlled via a pot on the spindle control panel.

In conclusion, I see no advantage to using a servo or BLDC motor for a spindle drive when a standard induction motor is less expensive and easier to control with inexpensive hardware.


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## tkalxx (Apr 15, 2021)

JimDawson said:


> In conclusion, I see no advantage to using a servo or BLDC motor for a spindle drive when a standard induction motor is less expensive and easier to control with inexpensive hardware.



I completely agree that an induction motor would be most suitable in terms of cost/complexity, but in my eyes I do see an advantage to a servo/BLDC. With the smaller benchtop mills, it gets difficult to find a suitable induction motor that is small enough to fit on the head with a powerdraw bar cylinder. Not only are they more challenging to package elegantly, they are also heavier.

There are work arounds to fit an induction motor on a PM25/G0704, like making a larger head spacer which (in my specific scenario) would push the spindle center line outside the optimal Y travels of the mill and increase the moment induced on the Z-axis dovetails. There is also the option of offsetting the motor in the X direction, however, this introduces new loads to the Z-axis and in my opinion is not an elegant solution. There are also other options for a powerdraw bar that would give you additional space - like ones that act on a lever. I think this is a big reason people who own benchtop mills and want to replace the spindle motor default to a servo or BLDC.

The very first motor I considered was a baldor induction motor, but I couldn't design a way to mount it efficiently with the additional features that I wanted. I can fit a NEMA 42/42C frame motor with the current design, but now you're limited to 1/4hp MAX? - that's no good.  In addition, the BLDC +drive cost me the exact same price as a new baldor +VFD and at this point in time - straight out of the box - is no more complicated to control than a standard VFD.


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## spumco (Apr 16, 2021)

All the "real" mills ive seen/used have an induction motor just like Jim has suggested. Not the 15kRPM+ spindles, but everything under that.  Modern VFDs are really blurring the line between servos and traditional dymb spindle drives.

The problem i have with induction motor/vfd setups is simply space and weight. Unless you can beg/borrow/steal a really high performance spindle-specific induction motor you're stuck with fairly standard motors. And standard induction motors, up to about 10hp or so, arent as power-dense as PM servos or BLDCs.

Thats the reason Ive selected a PM motor for a spindle. Weight and size advantage means my Z axis isnt trying to lift 100lbs of motor  and I can fit a standard PDB indtead of  some kludge lever thing.

If i had more room, or if the motor was a smaller percentage of spindle/head assembly weight I'd probably use an inductio motor, vfd, and spindle-mounted encoder like Jim described


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## tkalxx (Apr 27, 2021)

I managed to get the spindle motor and belt system temporarily mounted on the mill. There is definitely some fine tuning to be done. Due to the orientation of the pulleys, the high gear belt works perfectly, but the low gear belt doesn't get tight enough for my liking. This is due to the larger pulley being on the motor for high gear and on the spindle for low gear; hence the geometry of the belts are different. This was an oversight on my part and i'll have to re machine a new lever for the low gear. One thing that needs to be considered in a design like this, is that each tensioner needs some over travel to compensate for belt stretch. The powerdraw bar will come later - I need to solidify the belt system before I move onto other things. 

I've mounted all the spindle motor electronics in a separate enclosure to mitigate any noise issues and so far everything is running smoothly. 








I haven't been able to run any programs to put the new motor through its paces, but I couldn't resist doing a few test cuts at 8000 rpm. I have to say, what a crazy difference it makes over the stock 2500 rpm motor. 3/8" 2 flute, 0.5" DOC, 0.100" WOC at 70ipm puts a massive grin on my face.


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## macardoso (Apr 27, 2021)

tkalxx said:


> I managed to get the spindle motor and belt system temporarily mounted on the mill. There is definitely some fine tuning to be done. Due to the orientation of the pulleys, the high gear belt works perfectly, but the low gear belt doesn't get tight enough for my liking. This is due to the larger pulley being on the motor for high gear and on the spindle for low gear; hence the geometry of the belts are different. This was an oversight on my part and i'll have to re machine a new lever for the low gear. One thing that needs to be considered in a design like this, is that each tensioner needs some over travel to compensate for belt stretch. The powerdraw bar will come later - I need to solidify the belt system before I move onto other things.
> 
> I've mounted all the spindle motor electronics in a separate enclosure to mitigate any noise issues and so far everything is running smoothly.
> 
> ...


Looks amazing! You have to share some video of the belt change and maybe some cutting performance. Awesome work!


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## tkalxx (Apr 27, 2021)

macardoso said:


> Looks amazing! You have to share some video of the belt change and maybe some cutting performance. Awesome work!



Absolutely. I'll be sure to get some video.


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## spumco (Apr 27, 2021)

Speaking of motors, I might have stumbled across a couple of candidates...  $100/ea, $15 for the cables.

It appears the Coply can accept sin/cosine encoder signals, so no butching the Siemens stuff.

Check out the specs on the dataplate.


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## macardoso (Apr 27, 2021)

spumco said:


> Speaking of motors, I might have stumbled across a couple of candidates...  $100/ea, $15 for the cables.
> 
> It appears the Coply can accept sin/cosine encoder signals, so no butching the Siemens stuff.
> 
> ...


Very nice. I had a few Siemens motors but they all had resolvers. Sold them for a good chunk of change


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## spumco (Apr 27, 2021)

I'm pretty excited to be able to get 8kRPM with a 1:1 spindle now.  And no extra spindle encoder/index signal needed - just using the drive output.


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## mattthemuppet2 (Apr 27, 2021)

tkalxx said:


> I haven't been able to run any programs to put the new motor through its paces, but I couldn't resist doing a few test cuts at 8000 rpm. I have to say, what a crazy difference it makes over the stock 2500 rpm motor. 3/8" 2 flute, 0.5" DOC, 0.100" WOC at 70ipm puts a massive grin on my face.


that is really slick and very impressive. You could even do something like that manually for a non-CNC machine


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## spumco (Apr 27, 2021)

mattthemuppet2 said:


> that is really slick and very impressive. You could even do something like that manually for a non-CNC machine



Good thinking.  A couple of over-center levers (I'm thinking toggle clamp-type things) to select each ratio.  And pull the second one to lock the spindle during a drawbar tool change.


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## tkalxx (May 2, 2021)

I replaced my spindle bearings this weekend with SKF 32005X/Q and 32007X/Q tapered roller bearings. The larger of the two bearings has a reference speed of 8500 and a maximum of 10,000. Using Kluber isoflex nbu-15 grease, I'm unable to run the spindle at 8000rpm comfortably. During the break in period, temps shot up very quickly within the first 5 minutes running at 8000rpm (upwards of 95C). 7000rpm seemed OK - I ran it for 45mins and temps leveled out around 80-85C. I tried playing around with the pre-load and did not see any different results.

Throughout this thread it's been mentioned that 10,000rpm would be the desired top end for our hobby cnc's. Unfortunately, I can confidently say you wouldn't be able to run at those speeds with the top of the line SKF tapered bearings. This might push me down the road of investigating alternative bearings or lubrication methods. On the plus side, I'm measuring between .0001"-.00015" of runout on the R8 taper with these new bearings. 

Does anyone know how tormach pulls off 10,000rpm? As far as I'm aware, they don't even use an oil drip feed to the spindle bearings. Maybe angular contacts would improve top speed figures, but you risk increasing total runout. Another option is a new spindle cartridge rated to higher rpms as Spumco has mentioned, but I'm not really interested in heading down that path.


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## macardoso (May 2, 2021)

tkalxx said:


> I replaced my spindle bearings this weekend with SKF 32005X/Q and 32007X/Q tapered roller bearings. The larger of the two bearings has a reference speed of 8500 and a maximum of 10,000. Using Kluber isoflex nbu-15 grease, I'm unable to run the spindle at 8000rpm comfortably. During the break in period, temps shot up very quickly within the first 5 minutes running at 8000rpm (upwards of 95C). 7000rpm seemed OK - I ran it for 45mins and temps leveled out around 80-85C. I tried playing around with the pre-load and did not see any different results.
> 
> Throughout this thread it's been mentioned that 10,000rpm would be the desired top end for our hobby cnc's. Unfortunately, I can confidently say you wouldn't be able to run at those speeds with the top of the line SKF tapered bearings. This might push me down the road of investigating alternative bearings or lubrication methods. On the plus side, I'm measuring between .0001"-.00015" of runout on the R8 taper with these new bearings.
> 
> Does anyone know how tormach pulls off 10,000rpm? As far as I'm aware, they don't even use an oil drip feed to the spindle bearings. Maybe angular contacts would improve top speed figures, but you risk increasing total runout. Another option is a new spindle cartridge rated to higher rpms as Spumco has mentioned, but I'm not really interested in heading down that path.


Sorry to hear that - bummer.
Out of curiosity, how much grease did you add to each bearing? How did you measure it out and pack it?


----------



## tkalxx (May 3, 2021)

My apologies, I may have posted the last update a little prematurely. I spent the last few hours continuing to bed the bearings. I actually ended up getting a stable temperature at 8000rpm - it seems the bearings just needed more cycles getting hot, cooling down and then hot again. I did 3 additional cycles bringing the bearings up to 100C (about as hot as I felt comfortable) and then letting them cool to ~40C. On my last run, it took about 15 minutes running at 8000rpm to get them to 90C and they were stable at 91-93C for the following 45 minutes. The upper bearing runs a little cooler at 75C. Overall, these bearings run HOT... too hot to comfortably grab the spindle nose for any extended period of time. I'll have to monitor them over the next couple of weeks and report back. SKF states the maximum operating temperature is 120C, I'm not sure what the spec is for Kluber NBU-15.



macardoso said:


> Out of curiosity, how much grease did you add to each bearing? How did you measure it out and pack it?



SKF has an online tool to calculate grease quantity and relubrication intervals.





						SKF Bearing Select
					

Please, use our online bearing selection tool, SKF Bearing Select, which has a fully updated, clean and user-friendly interface. Calculation results can be obtained fast and are presented directly near the required input data. SKF Bearing Select has been designed in alignment with the Bearing...




					www.skfbearingselect.com
				




For the lower bearing (the one I'm more concerned about) the tool told me 6g of grease and a relubrication interval of 110hr based on 8000rpm. I used a medical syringe and a food scale to measure out the grease quantity. The lower bearing was packed like I would with a wheel bearing. The upper bearing was a lot more difficult to pack "properly". My spindle requires the upper bearing to be heated in order to slip over the shaft. Because of this, there wasn't a lot of time between pulling it out of the oven and installing it on the shaft to pack it like a wheel bearing (not to mention the bearing is hot and difficult to handle). I ended up measuring out the grease with my syringe, squirting some on the race that was installed in the quill and some on the rolling elements of the bearing before installing it on the shaft. After the bearing was installed I came back with a 22 gauge needle on the syringe and was able to stick it in between the rolling elements and squirt the rest of the grease in the bearing.


----------



## spumco (May 3, 2021)

You might want to double-check the SKF bearing select.  It looks like the reference speed for the 32007x is generated using oil - but I could be wrong as I just had a quick look.

AC bearings don't increase runout.  Every high-speed spindle I've seen uses AC bearings (not talking about air bearings).  Lathes have a big cylindrical roller at the front for radial loads, but otherwise they're all AC and not tapered rollers.  If you haven't seen it yet, SKF's site has a great series of spindle/bearing configuration diagrams - and none of the higher-speed spindles they show use tapered rollers.


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## tkalxx (May 3, 2021)

spumco said:


> You might want to double-check the SKF bearing select.  It looks like the reference speed for the 32007x is generated using oil - but I could be wrong as I just had a quick look.


Although not explicitly stated, it's my understanding that the reference speed for SKF bearings are generated using grease. It is stated throughout multiple documentation that if exceeding the reference speed and approaching the limiting speed, "oil lubrication may be required".


spumco said:


> AC bearings don't increase runout.  Every high-speed spindle I've seen uses AC bearings (not talking about air bearings).  Lathes have a big cylindrical roller at the front for radial loads, but otherwise they're all AC and not tapered rollers.  If you haven't seen it yet, SKF's site has a great series of spindle/bearing configuration diagrams - and none of the higher-speed spindles they show use tapered rollers.


You're right. Almost every CNC I've seen uses AC bearings. In fact, there are a lot more options available for low runout AC bearings compared to tapered rollers. I guess what I was trying to allude to is the fact that these spindles were not designed to be used with AC bearings and thus, from what I've seen, there are very few AC bearing options that fit our spindles. All of the AC bearing options I have come across appear to be a lower tolerance grade than the X/Q tapered bearings I purchased and so I just assumed runout may increase if going with those options. 

It's probably worth doing a deeper dive into AC bearings. I couldn't find any SKF's that would fit our spindles. I think the most common choice are the Nachi bearings (I believe @macardoso has been running them for a while). Maybe they're good enough for a hobby application?


----------



## macardoso (May 3, 2021)

I don't have enough experience to comment if I am right or wrong, but I have had decent luck with my AC bearings purchased from VXB. 

They required shims to be added to take up for the difference in size from the stock TR bearings, and they are not paired truly back to back like a spindle designed for AC bearings would be. In the case of the G0704/PM25, the two AC bearings are different sizes and preloaded by the adjuster nut which gives poor control of preload, and likely thermal variability and likely bow in the spindle from the poor quality of that nut.

The first build of the spindle with AC bearings ran HOT. Too hot to comfortably touch and extended hours at 5000rpm could get a rag to gently sizzle on the lower bearing housing. The was set too high of preload and packed 100% fill with standard synthetic grease. These bearings were shot after 4 years. My second attempt was more careful during assembly, proper 30% fill with Kluber IsoFlex NBU15, and a much lighter preload. The lower bearing gets warm, but plenty comfortable to hold onto (~110*F). The AC servo running my spindle gets the hottest at about 160*F during extended operation - this is normal. Total TIR is 3.5 tenths at the spindle taper, but I cannot comment how much of that comes from the spindle taper being ground not true to the bearing seats vs. inaccuracy in the bearings. I want to say the bearings are rated <2 tenths TIR so assuming I did OK in the assembly, some of that has to be in the taper. 

In practice, with the Tormach TTS collet in the mix, I get about 4-5 tenths of runout. Not ideal but doesn't mess up the work I do. 

If I ever scrape my machine and get more accurate screws, I'd love to hard turn the spindle taper in-situ to give an absolute minimum TIR.

I did a fun measurement using the current feedback on my servo and found that the unloaded spindle and drive train consume about 11% of the spindle load which is about 200W of heat being generated within the head of the machine. This is actually pretty efficient for a belt drive + 6 bearings.

-Mike


----------



## spumco (May 3, 2021)

Found this on the SKF site.  They state that low-viscosity grease is 'also valid' but that greased bearings will run hot on run-in.

Your results may be to be expected if the preload is a bit high.  I think investigating some AC bearings might be the ticket.

And Tormach gets 10kRPM with AC bearings, just like everybody else.  I believe they have a 5kRPM motor, and the high range of the two-speed drive gets the spindle up to 10k.


----------



## spumco (May 3, 2021)

In spindle motor news...  I've run in to a bummer with those Siemens servos.

I'm unable to get them to spin faster than about 4200rpm.  Looks like I didn't do enough homework on the datasheet and my bus voltage (~340vac) just isn't enough to get them to rated speed.  That, or I'm completely lost on the drive tuning and motor parameters.

Unless someone has some suggestions on drive tuning, looks like I may be back to the BLDC plan.


----------



## tkalxx (May 4, 2021)

macardoso said:


> My second attempt was more careful during assembly, proper 30% fill with Kluber IsoFlex NBU15, and a much lighter preload. The lower bearing gets warm, but plenty comfortable to hold onto (~110*F). The AC servo running my spindle gets the hottest at about 160*F during extended operation - this is normal. Total TIR is 3.5 tenths at the spindle taper, but I cannot comment how much of that comes from the spindle taper being ground not true to the bearing seats vs. inaccuracy in the bearings. I want to say the bearings are rated <2 tenths TIR so assuming I did OK in the assembly, some of that has to be in the taper.



If bearing temps vs. speed is at all a linear correlation, it seems like your AC bearings run a lot cooler (which is to be expected, and a little encouraging). I recorded my TR bearings leveling out at 65C (150F for you american folks ) at 5000rpm. 3.5 tenths doesn't seem too bad.



spumco said:


> Found this on the SKF site.  They state that low-viscosity grease is 'also valid' but that greased bearings will run hot on run-in.
> 
> Your results may be to be expected if the preload is a bit high.  I think investigating some AC bearings might be the ticket.
> 
> View attachment 364780



I completely missed that in my research, thanks! I'm going to stick with the TR bearings for a little bit and see what happens. In the meantime, I'll dive into researching some AC bearing options. I'm definitely intrigued to try out some different bearings and report back my findings. As for preload, I'm afraid if I reduce the preload any more than I already have, I'll end up with axial play, but more than likely there could be some fine-tuning to be done (the preload nut is extremely touchy).



spumco said:


> I'm unable to get them to spin faster than about 4200rpm.  Looks like I didn't do enough homework on the datasheet and my bus voltage (~340vac) just isn't enough to get them to rated speed.  That, or I'm completely lost on the drive tuning and motor parameters.



That's super unfortunate... I was going to comment earlier but forgot. It seems like those servos are rated for 480vac?


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## spumco (May 4, 2021)

tkalxx said:


> That's super unfortunate... I was going to comment earlier but forgot. It seems like those servos are rated for 480vac?



Not according to the datasheet.  Ke is 50v/Krpm, so it _should_ go faster.  But it doesn't.

I'm not an expert by any stretch, thus I'm hoping its a drive setting I can fiddle with.

Curves from the datasheet.  "Vmot" is defined in the obscure "general data" brochure as the bus voltage:


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## tkalxx (May 4, 2021)

I just wanted to share this Timken resource that I came across. It specifically addresses machine tool spindle design and I have found pages 22-31 particularly insightful. 



			https://www.timken.com/wp-content/uploads/2016/12/5918_Machine-Tool-Catalog.pdf


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## macardoso (May 4, 2021)

spumco said:


> In spindle motor news...  I've run in to a bummer with those Siemens servos.
> 
> I'm unable to get them to spin faster than about 4200rpm.  Looks like I didn't do enough homework on the datasheet and my bus voltage (~340vac) just isn't enough to get them to rated speed.  That, or I'm completely lost on the drive tuning and motor parameters.
> 
> Unless someone has some suggestions on drive tuning, looks like I may be back to the BLDC plan.


If you want, shoot me a PM and lets chat. It might depend on the motor rated voltage and the DC bus voltage you have available. The the drive's DC bus voltage is too low, there will come a speed at which the drive output voltage minus the motor back EMF is insufficient to saturate the windings of the motor (based on winding inductance) before the rotor has passed and the next winding must start its commutation cycle. 

The motor will perform with full torque up to around this critical speed and then performance quickly falls off. I can run Allen Bradley 400V class servos on a 200V drive system and get about 60% of the top speed of the motor. Likewise I can run a 200V system on 120VAC input to the drive and again get about 60% of the top speed of the motor.

This is somewhat the same phenomena that is seen in stepper motors and why running a stepper drive at a higher DC input voltage yields higher motor top speeds.


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## macardoso (May 4, 2021)

spumco said:


> Not according to the datasheet.  Ke is 50v/Krpm, so it _should_ go faster.  But it doesn't.
> 
> I'm not an expert by any stretch, thus I'm hoping its a drive setting I can fiddle with.
> 
> ...


OK, so those motors seems to be rated for a 380V system which would be common overseas. Given that information, I'd expect you to be able to run them at 63-75% of their rated speed on the "b" curve, which would give around 5000-6000 rpm, still very respectable but sadly not the 9100 rpm you were hoping for. This was all assuming you were powering that Copley with 240VAC 1P (US residential) giving a DC bus of 325VDC.


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## spumco (May 4, 2021)

macardoso said:


> OK, so those motors seems to be rated for a 380V system



What did I miss on the torque curve sheet that indicates they're designed for 380VAC line voltage?    I didn't think I'd get 9kRPM out of them, but they simply die at about 4200rpm.  4000rpm sounds great, very responsive, etc.  Bump it up a hair and they saturate and won't go any faster.

And yes, you've got it right - I'm trying to use the Copley at 240vac.  According to the drive, bus voltage is 330-340vdc (depending on supply variations).

Datasheet for your review (these are the -6AK7).  Maybe you can help me identify what I should have been looking for.

Back to the BLDC, or keep searching for a 200v unicorn motor.  Good news is that I can return them to HGR (one has dodgy bearings anyway).


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## macardoso (May 4, 2021)

I was basing them on this info in your graph.




Reference speeds [a] through [c]. DC Link = DC Bus which is ~ 1.35*ACrms Line Voltage. So 240VACrms = 325VDC.

In the Siemens motors, 540VDC/1.35 = 400VACrms and 600VDC/1.35 = 444VACrms.

Also the motor voltages (Vmot) are given in a range of 340Vrms - 425Vrms. This is roughly centered around 380Vrms

Bit of rough math, but this puts you in the 380VAC market (Asia) or the 400VAC market (Germany????). These motors would also run fine on a 480VAC (650VDC) system common in the USA. Either way, you're a bit under that voltage and I would expect the behavior you are seeing.

I got to your speed by taking the ratio of real to rated DC bus voltages: 325VDC/540VDC = 0.6. Then multiplying that by the limiting speed from reference line [a] (7000rpm) to get 4213 rpm.

I guess redoing the math with different assumptions got me even closer to the speed you are seeing - cool.

Remember this is the max speed with zero torque available, so any applied torque will pull you back from this maximum.


EDIT: My initial analysis was based on reference curve, this analysis was based on reference curve [a]


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## macardoso (May 4, 2021)

As you commented, high speed motors in a 200V class flavor are rare. It is difficult for motor manufacturers to design motors with sufficiently high inductance to generate the magnetic field strength at the stator/rotor interface to create optimal torque while simultaneously keeping the inductance low enough that the windings can be driven to the desired current levels with minimum applied voltage. Higher motor speeds require faster changes in the winding currents.

Higher inductance resists change in current and higher driving voltage is required to change the current more quickly. This is the reason you see higher speeds in higher voltage motors and why small motors are rated for high speed while big motors must spin slowly (big motors = large windings = more inductance).

Conversely, low inductance motors exist and can run quickly, but lack the torque of slower motors as the lower inductance results in lower magnetic field strength and lower torque.

Kind of cool. You always play this tradeoff game between torque and speed. The only thing that can improve both factors is a higher driving voltage.

The best motors I've seen for the application are the Fanuc asynchronous spindle motors (seen on RoboDrills and other small high speed machines).


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## spumco (May 4, 2021)

Thanks for the analysis - looks like I misunderstood the curves.  Siemens defines Vmot (in another document) as bus voltage, so I mentally glossed over the "540VDC" and focused on the "Vmot = 340V" bit.  My bad.

And yes, I get the need for higher voltage when you want more speed - that's why I'm running a 100vdc PS and fairly low inductance steppers.

Anyone have a Fanuc A06B-0852 laying around they want to donate?


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## macardoso (May 4, 2021)

spumco said:


> Thanks for the analysis - looks like I misunderstood the curves.  Siemens defines Vmot (in another document) as bus voltage, so I mentally glossed over the "540VDC" and focused on the "Vmot = 340V" bit.  My bad.
> 
> And yes, I get the need for higher voltage when you want more speed - that's why I'm running a 100vdc PS and fairly low inductance steppers.
> 
> Anyone have a Fanuc A06B-0852 laying around they want to donate?


I take Vmot to be Volts AC/PWM (RMS). 

If they do, I'm gonna grab that sucker before you can


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## spumco (May 4, 2021)

HGR has about a hundred A06B-0235's right now.  I'm wondering if I can over-speed it a bit and get away with a 1:1.5 belt.


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## Yukonjack (May 7, 2021)

COMachinist said:


> I have just started the rebuild of my 12 yr old G0704 CNC conversion spindle for the umteenth time. I’m almost ready to pull the trigger on one of these spindles. https://www.cncdepot.net/product-page/fm-series-spindles. They really nice and about 2/3rds the weight of the 3ph/belt drive G0704 head conversion, with a PDB. These use air ISO BT30 tool holders, 12k rpms @ 3hp. they just fly through metal. Check outClough42 youtube channel  on his G0704 CNC conversion.
> 
> 
> 
> ...


Anyone else try this? Lowest practical RPM? Drilling performance in steel?
Is there any ATC spindle that is capable of real milling in steel (sub $5,000)?


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## spumco (May 7, 2021)

Yukonjack said:


> Is there any ATC spindle that is capable of real milling in steel (sub $5,000)?


Assuming you mean a unitized cartridge/motor spindle, and 'real' milling in steel = 1/2" end mill or larger - then no.  Assuming they can be safely run at 50% rpm, you aren't going to use a 1/2" drill in steel at 6kRPM.  I'm sure it'd be fine to use a 1/4" carbide end mill to do hole interpolation, but you're SOL for reamers, big drills, ect.

If you want a package deal, you can get a BT30 spindle and a 2.2kW servo spindle with an 8 or 10k RPM top end from Automation Technologies or direct from China.  With a 1:1 drive that should do just fine in steel.

The servo above _may_ be a bit too large for the G0704.  I think it's a 150mm frame, whereas the DMM and other similar sized are 130mm servos.  Wider isn't really the problem...  but the extra frame size may interfere with a PDB cylinder over the spindle.

I've been mulling over spindles & motors for 3 years now, and there's no magic answer.  I'll stand by my statement I've made a few times/places: 

_The most flexible spindle arrangement I can imagine (for a hobby-sized mill) is the following:_
- two-speed poly-v belt drive (preferably with auto belt engagement a-la Fadal pozi-drive)
- around 1.5:1 and 1:1.5 ratios
- 2 or 3hp PMSM or AC servo (to cut down on weight/size vs an inducton motor, and easy spindle homing/orientation)
- BT30 cartridge spindle, with 8k to 12k bearings
- Air or air over hydraulic PDB
- Direct spindle encoder (depends output/control system), and/or spindle index trigger signal


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## Yukonjack (May 7, 2021)

I agree with your assessment after talking with CNCDepot about that spindle.
I'm at a crossroads. I want to CNC a PM940. PM has some 940M's inbound but no 940V's until the end of the year.
I was considering buying the 940M and making a new head.
However, the "most flexible spindle arrangement" you show above, seems like a 2-yr project with no trailblazing from anyone.
Makes me come back to why bother and just start looking for a Fadal.

Hard to believe there isn't more available to address your "most flexible spindle arrangement". Maybe because the ratio of home router to home mill guys seems to be 1000:1.


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## spumco (May 7, 2021)

Trailblazing is underway right now.

Im working on the "drive a BT30 to 8krpm with a 2.2kw BLDC using surplus servo amp and cheap encoder" phase.

The "2 speed Pozi-drive via BLDC and OEM drive" also appears to be underway.

I've actually spun & servoed my BLDC with this drive, just not made chips on anger yet. It will be another month or so before I can report on actual in-cut performance as I've switched to LinuxCNC and the learning curve is very painful (for me).

Phase 3, depending on my BLDC performance, will be a guide to shopping for surplus (or cheap) servo/drive combos that will work with each other and various controls. I've already failed with those Siemens due to the voltage mismatch, but there are plenty of other candidates.


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## spumco (May 7, 2021)

Yukonjack said:


> Makes me come back to why bother and just start looking for a Fadal.


If you've got the room, power, and concrete thickness, that's not a bad idea.  But even a small Fadal 2016 is not in the same league as a G0704, PM940, or my mill.  Different animal.

Given the limitations of the dovetail ways and overall stiffness, I don't understand why folks are so keen on CNC'ing the RF45's and similar.  They're very good 'hobby' sized manual mills, but seem to me like a sub-optimal cost-to-potential performance choice once they are converted to CNC.

I guess it's difficult to find used 'real' CNC mills in some locations, whereas it's easy to get a PM940 delivered to your house.


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## Reddinr (May 7, 2021)

I know it's not optimal but I've made many parts and have earned a good living in part due to the capabilities of the IH RF-45-like mill with CNC conversion kit.  Not sure what is available that is significantly better for anywhere near the low price I paid for it.  The larger than standard envelope of this mill has been very useful.  On the other hand, if price, weight and space were no object I would like something a bit larger that I don't have to baby as much.  I may start a new thread on that subject.


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## spumco (May 7, 2021)

Reddinr said:


> Not sure what is available that is significantly better for anywhere near the low price I paid for it.



That right there is a good answer to my earlier speculation/rambling.  In some parts of the country you can't swing a dead cat without hitting old CNC machines - almost get them for the cost of hauling them away.

In other places, not so much.  And I shouldn't have discounted the somewhat sequential budget impact of a DIY conversion.


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## Reddinr (May 7, 2021)

Interestingly (maddeningly), my mill X axis started missing steps today.   I ran 24 parts with some simple profiled holes and about half of the holes on some of the boxes were off in X by about 20 mil.  Fortunately, that didn't ruin them because they were just conduit holes in plastic cabinets.  Looks like I have to troubleshoot that.  This is the first problem I've had with the electronics even though it is a Mach 3 lash-up of 3 or so breakout boards on two parallel ports.

It might be time to upgrade my controls.


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## spumco (May 17, 2021)

JimDawson said:


> In conclusion, I see no advantage to using a servo or BLDC motor for a spindle drive when a standard induction motor is less expensive and easier to control with inexpensive hardware.



Duh, I should have listened to Jim.

I'm abandoning my attempts to use a servo drive and BLDC as a spindle motor.  Long story short... I want my mill back and while fiddling with various combinations of motors and drives has been educational, I can't say it's been productive.

I'm quite sure there are AC servos and BLDC motors out there that would work fine, but everything I've got on hand (or returned) isn't doing it for me.  And everything I _know_ will work is too expensive to justify.

So... back to a VFD and an induction motor.  I'm thinking an induction motor is better for a spindle due to the inertia mismatch issue with PM servos - but I could be wrong.

Remember that Fanuc A06B-0851 I joked about earlier?

I stumbled across a unicorn on Friday.  It fits, mostly, and eliminates the need for a reduction or overdrive.

Brand new, still in the Mitsubishi crate.  It's a 2002 model (motor plate is from internet) but appears never to have been installed.  Spins up fine (open-loop) with my 1.5kw VFD on the bench.

VFD is on the way, as is a normal encoder and QD bush to fit the shaft.   Came with an oddball Tamagawa 15v magnetic encoder that simply looks too fussy to re-use.

Plenty of electrical enclosure work and rewiring to do, but I'll report back when I've got something interesting.

-Ralph


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## macardoso (May 18, 2021)

My jaw is on the floor and my heart full of jealousy!

If you don't mind sharing, what did you pay for it.


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## spumco (May 18, 2021)

There was this sale at HGR...
I think it was $112.00 after taxes.

You can work out for yourself how long I agonized over the decision.


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## macardoso (May 18, 2021)

spumco said:


> There was this sale at HGR...
> I think it was $112.00 after taxes.
> 
> You can work out for yourself how long I agonized over the decision.


AHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH jealous


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## tkalxx (May 18, 2021)

spumco said:


> There was this sale at HGR...
> I think it was $112.00 after taxes.
> 
> You can work out for yourself how long I agonized over the decision.



Wow. I wish there was industrial surplus local to me like there is in the states - I'm jealous. Looking forward to seeing your progress. 

On a side note, I'm still trying to work through some issues with my spindle. The good news: SKF TR bearings seem to be holding up so far. I've been running them pretty hard without seeing excess heat buildup (measuring 50-60C on average when running through programs). The bad news: I've seized the small bearings on the idler tensioner and as a result burned through a belt. At 8000rpm spindle speed, the idler is spinning at roughly 18krpm. The bearings I seized were rated to 45krpm but the dynamic load rating had essentially no factor of safety in my design. I'm in the process of redesigning the idlers with more capable bearings - small oversight on my part.


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## spumco (May 18, 2021)

macardoso said:


> AHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH jealous


Don't be.  Karma will probably catch up and I'll wind up sending a whole face mill through the car door.


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## spumco (May 18, 2021)

Any room for larger idlers?  At that diameter even 1/2" larger would cut the bearing speed down a meaningful amount.


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## tkalxx (May 18, 2021)

spumco said:


> Any room for larger idlers?  At that diameter even 1/2" larger would cut the bearing speed down a meaningful amount.



Yup, that's the plan. In addition I stepped the bearing sizes up to 26mm from a .75" which triples my dynamic load rating while maintaining the same reference speed limit.


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## spumco (May 18, 2021)

Sounds like yer in bidness!

Don't know if you're using sealed or shielded, but shielded tend to run cooler as there's no shield contact with the inner race.


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## vtcnc (Nov 12, 2021)

spumco said:


> There was this sale at HGR...
> I think it was $112.00 after taxes.
> 
> You can work out for yourself how long I agonized over the decision.


1.2 seconds?


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## tkalxx (Jan 24, 2022)

Wow, it's been a while since I've followed up with this thread! Been super busy, but my mill has been reliably trucking along making lots of parts with the new spindle configuration since I last posted. I have probably put over 100 hours on the machine with the new BLDC spindle motor and I haven't had many, if not any, issues.

I promised a video of my spindle motor on the PM25 mill and although I don't have a video specifically showing off the belt changer, I did manage to grab a video with my phone machining a new brake pedal when I was in the shop tonight. Thought I would post it here to show off the capabilities of these small mills with an 8000 rpm spindle. In retrospect, I probably could have pushed my 3/8" endmill harder without issue... My typical F360 settings for it is 8000 rpm at 0.5" DOC, 0.05" WOC and 100ipm. The part in the video is roughly 12"x4"x0.875".

Ignore the weird color hue of the video... the LED's in my enclosure appear purple in photos and videos for some reason. 











-Adam.


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## macardoso (Jun 13, 2022)

Ran across this guy today. Might be the lowest cost / highest HP AC servo that can do 5000 rpm in the hobby market.

$385 for motor, drive, and cables (1.5m). 5000 rpm, 3.2Nm continuous, 9.5Nm peak. 1000W or 1.34 HP.

This would be plenty sufficient for a PM25 or G0704 sized machine in a 1:1 belt configuration.






						T6 Series 1000W AC Servo Motor Kit 3000rpm 3.19Nm 17-Bit Encoder IP65 - T6-RS1000H2A3-M17S STEPPERONLINE
					

T6 Series 1000W AC Servo Motor Kit 3000rpm 3.19Nm 17-Bit Encoder IP65 - T6-RS1000H2A3-M17S - Servo Motor - Items in kit 1 x T6-1000RS: 1000W AC Servo Motor Driver 1 x T6M80-1000H2A3-M17S: 1000W AC Servo Motor 3000rpm 3.19Nm 17-Bit Encoder IP65 1 x CMST1M5




					www.omc-stepperonline.com


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## GunsOfNavarone (Jun 18, 2022)

Interesting reading here guys. I have a Tormach (just a 440) and I sure wish I have a higher torque motor. Now I am not nearly as versed as you guys at CNC, only about 2.5 years. I run into the most issue with straight drilling. If I'm not short/slow pecks, I can stall my motor. I use yg-1 luma-power and altin coated yg-1 for steels. I really only run SS, A6, (probably more than my mill was sold as being able to handle, but since I don't KNOW that, it hasn't slowed me down. I feel my tool paths are really limited now that F360 has cut us cheap users ($400+ a year) I have sat through so many of their classes just to find at the end, that is an add on at another $1600 a year. I hope you guys continue your convo here as I'm sitting back and taking it all in. 
Related note, anyone know if it possible (without extensive mods) to increase my motor setup? It does have a <5000 rpm pulley and a 5000~10K pulley. That drilling issue is really a huge problem for me, obviously on the slower/torquier pulley.


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## macardoso (Jun 20, 2022)

GunsOfNavarone said:


> Interesting reading here guys. I have a Tormach (just a 440) and I sure wish I have a higher torque motor. Now I am not nearly as versed as you guys at CNC, only about 2.5 years. I run into the most issue with straight drilling. If I'm not short/slow pecks, I can stall my motor. I use yg-1 luma-power and altin coated yg-1 for steels. I really only run SS, A6, (probably more than my mill was sold as being able to handle, but since I don't KNOW that, it hasn't slowed me down. I feel my tool paths are really limited now that F360 has cut us cheap users ($400+ a year) I have sat through so many of their classes just to find at the end, that is an add on at another $1600 a year. I hope you guys continue your convo here as I'm sitting back and taking it all in.
> Related note, anyone know if it possible (without extensive mods) to increase my motor setup? It does have a <5000 rpm pulley and a 5000~10K pulley. That drilling issue is really a huge problem for me, obviously on the slower/torquier pulley.



I do not use F360 and was thankful of that fact after they killed it off. I had a student license of Inventor that was still going strong until 2 years ago from when I was in college. I've since moved onto other CAD/CAM options.

I think you're going to be in a bit of a pickle with the 440 if only for the fact that you're working with a Tormach designed system, and I'm guessing they don't allow freely editing their software to deal with modded hardware configurations. I could be wrong about this of course.

One option, if there is room, is to swap the 0-5000 rpm pulley to a 0-2500 rpm. This would double your torque on the low end, but also create a gap in the 2500-5000 rpm speed band which would not be handled well by the upper pulley.

You could also swap in a motor with more horsepower/torque. Not sure what the 440 uses, but if it is an induction motor or a BLDC, then your only option would be a permanent magnet motor for the power density. You'd need to develop the drive interface and get it working with the software.

I'd also be concerned about the machine having the rigidity to use the extra oomph from a bigger motor.

Best of luck and do tell us what you try!


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