# Cnc Options For The G0704?



## MontanaAardvark

I'm considering options to CNC my Grizz G0704.  The first place I found was the Hoss Machine conversion page and I've also found Automation Technologies, CNC Conversion Kits, and a couple of others that look to be different levels of completion.  There was even a high-end kinda place at Cabin Fever I looked at (which was about 2x more expensive than the mill, so no thanks).

I have two things going for me.  I have a CNC Sherline mill and lathe which I pretty much built from the ground up, so I kinda know what it takes to get from here to there.  And second, that CNC mill could conceivably make the metal parts to do the conversion.  

Basically, I'm open to any recommendations on which way to go, and everyone's good or bad experiences.  I'd love to hear about what others have done.  


Thanks,
Bob


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## dave2176

Hoss is a good source of information. He worked with Automation Technologies or maybe they based their kits off his efforts. I think their ball screws come from Chai (linearmotionbearings2008?). Go with the new hybrid steppers with encoder feedback. Larger motors are ultimately better. Post a build log.
Dave


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## RVJimD

Speaking of build log, has anyone done a recent build log of a mill conversion in this size range?

Thanks for asking the original question.

Jim


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## jumps4

here is a link to one I did with a friend a while back
http://www.hobby-machinist.com/threads/g0704-build-i-helped-a-friend-here-do-in-my-shop.34583/
Steve


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## MontanaAardvark

jumps4 said:


> here is a link to one I did with a friend a while back
> http://www.hobby-machinist.com/threads/g0704-build-i-helped-a-friend-here-do-in-my-shop.34583/
> Steve



That's gorgeous!  Man, you thought of everything.  

I spent a few minutes watching the Tormach CNC machines demoed at Cabin Fever, and they have nothing on you. 


Bob


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## Al-Hala

dave2176 said:


> Hoss is a good source of information. He worked with Automation Technologies or maybe they based their kits off his efforts. I think their ball screws come from Chai (linearmotionbearings2008?). Go with the new hybrid steppers with encoder feedback. Larger motors are ultimately better. Post a build log.
> Dave



Dave has the right of it, Chai seems to the be generally accepted "go-to" for Chinese sourced screws from the research I've done.

While I agree with the general sizing statement about motors, you CAN go too large. It isn't too common to happen, but when I saw a post once about someone putting a ~1000 oz-in motor in place of a ~270, well...

When I CNC'd my machine (LMS39xx), I looked around and went with a popular motor size proven to work in others builds (Hoss was a major source of information: www.g0704.com , hossmachine.info, 78 page thread on his own(original,defunct), the Bigger Thread), and it worked out well. Just use some common sense in motor sizing, check what has worked for others, and it is hard to go wrong.

And since I'll put in my unsolicited advice on motor sizing, might as well be hung for a ... and all that. If the above is good enough for you, abandon reading this now  For the rest, you've been warned 


For those math and theory adverse, my apologies. I'll try to be brief, and post sources.

After the above, when I was planning a DIY 4th axis, I found out about the parameters I mention below , since I wanted my 4th axis to rotate at a higher speed.  It is completely true when someone tells you to fully understand the work envelope, and THEN pick components: Feed/Speeds, largest amount of mass to be worked on, pitch ratios, all of it matter *greatly *when picking drive components; *one of the reasons people often just purchase a kit or combination that has been proven to "just work" *(also prevents you from having to read and understand all of the below).

Four parameters often overlooked when people build from the components up:
- Motor Driver Selection - kinda, sorta important to motor size
- Rapid Speeds vs. Motor Torque Curves - important with regard to size of motor
- Rotor Moment of Inertia - important with regard to size of motor, related to the above
- Inertia Ratio Mismatch - important with regard to size of motor and load

*Motor Driver Selection:*
In a summary, the electro-mechanical characteristics of the stepper motor change with speed. ergo, the driving waveform should change as well. Cheap drives do not, and that means issues for larger motors (ultimately, RPM limiting at top end, starting issues at rest).

A brief statement on the importance of a good drive can be found at this thread: The founder of Gecko Drives on Stepper Drives (CNCZone). The discussion started as micro-stepping, not motor sizing. but has the following quotable section:

_"5) More than any other type of motor, step motor performance is tied to the kind of drive connected to it. More than any other type motor, a stepper can be driven from very simple drives (full-step unipolar L/R) to very complex ones (microsteppingfull-bridge bipolar synchronous PWM mid-band compensated).

Motor performance will range from "Miserable, give me a servo, I'll never use another stepper again" to "What is the big deal about servos anyway, this is just as good." It's ALL in the drive."_

Push an oversize/undersized motor (for the task) with a sophisticated drive, and it will perform better than a low-end drive.

*Rapid Speed vs. Torque Curve*
Stepper motors lose a lot of torque the moment they start turning, and continue to lose it with speed increases. The larger the motor, the worse this becomes. A larger motor has much more torque AT REST, but how often is a stepper motor actually just holding torque? A side note: larger motors than needed are wasting energy in space heating.

Examples:
Daycounter Stepper Motor RPM and Power Calculator (can calculate maximum RPM for a given motor)
Selection of Motors Showing Torque Curves
Gecko On Selecting a Proper Motor

Sample motor sizing criteria: fastest feed/speed envisioned, plus the pitch of the ballscrew/leadscrew, plus how much mass is on the table, plus the table system itself, plus friction losses = motor torque requirement, motor speed requirement. Here is a math intensive example of sizing a motor, complete will all system parameters.

Minebea; brief summary of some of stepper motor parameters

Gecko has a very comprehensive motor selection guide which goes far beyond motor size vs speed:
Gecko Drives / PDF

As for the rest of my list, generally the following items are most applicable to servo closed loop systems (hybrid steppers with encoders / pure DC/AC motors with encoders) since a stepper system is somewhat "self" regulating (the pole forces will "push back" against unintended movement unless the force is too large, acting like a spring), but taken to extremes will effect open loop steppers as well.

*Rotor Moment of Inertia*
As taught in school, inertia is the property of matter to remain at rest, or in movement, unless acted on by an outside force.  The larger the motor rotor, the more mass in the rotor, generally. This leads to increased power needed to start, stop and change acceleration in the motor; along with decrease in ultimate usable RPM. Remember, a LOT of motor reversals and acceleration changes happen in CNC.

Oversizing a motor can lead to limited ability to make quick direction or speed changes, and the moment increases drastically with size.  Make it big enough, and suddenly steps are being misplaced, or the motor will not even move if the command rate is too high (think of pushing a car with a quick shove; generally does not move. Same with stopping said car once moving).

The same applies to the load it is driving, as they are linked systems.

From a MIT PDF on applying stepper motors:
_"Consider a step motor trying to start a load. Not only must the motor contend with load inertia, it also has to get its own inertia going."_

From Oriental Motor:
_*"[...]* Since the stepping motor’s rotor and load have their own moment of inertia, lags and advances occur on the motor axis during instantaneous starting and stopping. These values change with the pulse speed, but the motor cannot follow the pulse speed beyond a certain point, so that missteps result. The pulse speed immediately before the occurrence of a misstep is called the starting frequency."_

Here is an excerpt from Schneider-Electric:
_"When determining the torque requirements of a Stepper Motor application, the effects of Inertia are often over looked.  Many stepper applications are low performance systems that accelerate, run at a low velocity, then decelerate to a stop. These systems typically do not require careful inertia calculations to size correctly since most of the torque seen by the drive is friction and/or load torque that does not vary significantly  with changes in inertia.  [...] When designing systems that are higher in performance, the torque resulting from acceleration of system inertia may actually far exceed the torque from friction and/or load.  [...] "_

*Inertia Ratio Mismatch*
Basically the same as the above, but taking into account what the motor is driving. The motor is too small/large for the load it is driving. In addition to my car analogy above, here's a reasonable analogy from PhysicsForums:
_"A good analogy is based on a pair of linked masses with a spring (and backlash) between them[...] Imagine a strong man pulling a truck with an elastic rope; the truck not moving until the rope has stretched quite a bit. The man will have a great deal of trouble trying to control the truck due to its high inertia, and the elasticity and backlash in the rope.[...] " _

Too large a mismatch (some cite say greater than 10:1 ratio, some less) will not allow a maximum of power transfer from the motor to what it is driving, can stall or back-drive the motor. _*Quick Mind Numbing Unwanted Fact:*_ one reason gearing in systems is used is to adjust this ratio: it's reduced by a square of the gear ratio, and the speed is increased by a multiple of the gear ratio. e.g. a mouse on a tread-wheel can move a dump truck, if the gear ratio is right.


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## MontanaAardvark

Al-Hala,

I appreciate the details.  I'm an engineer/techno-geek (EE, not mechanical), and know some of this stuff, but not all.  More details about design trades are always welcome.  I've seen our systems at work use 200 step motors with microstepping _and_ gears, and I've seen some that use 8 step motors and gears.  The approach seems to be up to the guy who designed it, like there's a large element of personal taste.  

For the home CNC, I think it's hard to say what the biggest work envelope will be, what Feed/Speeds, largest amount of mass to be worked on, and so on.  Yeah, I'll probably stay with "smaller" projects, but still those that fit comfortably on the G0704.  For the real small stuff, I have my Sherline.  I'd be surprised if someday 10 years from now, I don't say that I've worked on things I never thought I'd fit on the machine.  Speaking of fourth axis, I have a rotary table for the Sherline, but don't have one for the Griz yet.  I think I'd like to add one.  

I think I'll go order Hoss' G0704 CNC DVD.  Since the DVD and the digital download are the same price, may as well get the backup of the "hard copy". 


Bob


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## Al-Hala

Welcome, Bob.

Fortunately or no, there is plenty more where that came from, heh.  I suspect as an EE, you will have very little issue picking up the design cues; there are some similar concepts (resonance is resonance, in either field, for example; more mass in the structure raises the resonance point...)

I would have to agree about design methods; I once knew an mechanical engineer who insisted on utilizing a fine thread on the exterior of an hydraulic cylinder to join it to a vertical plate, which then seized in place the very first time assembly was attempted (was his personal favorite join method). What was wrong with a traditional flange and bolt circle, I am still not quite sure (bangs head).

I have always heard good things about the Sherlines; I myself had the Taig lathe (there were no local Sherline dealers) and that was far more rigid than the much larger Chinese units I was looking at, and it provided great service within the work envelope. I guess I should not really discuss the time I tried to reduce some 17-4 stainless bushings in it, however... (screech).

There are some awesome resources available on the web devoted to the Sherline units.  I almost adapted the Sherline bronze worm gear 4th axis, but in the end, it had too much backlash for my intended uses (just the nature of solid worm gear units; split or loaded are a differing story). How did you find yours?

Since I am jumping from topic to topic, have you encountered the Micro-Machine Shop or 5 Bears in your travels; some interesting information on machine construction, tooling and measurement on those two (although NOT for the Grizzly unit you were asking about).

I gather Hoss was a professionally employed machinist at one point, and has a fair grasp of design mechanics as well. Definitely one of the more prolific posters out there on the subject of tinkering and original design.  He built his own 4th, and after a lot of research, it is likely I will be one of the ones to follow in his footsteps. He specifically wanted (as do I) a unit that works in lathe and index mode, in his case utilizing the headstock of a surplus lathe and separate stepper motor.  He has built such for his (now discontinued) X2 projects, and was porting the designs over to the Grizzly unit.

He was also making a 5th axis (should be on YouTube under Hossmachine), using 3D printed housings and metal gearing (although I do not know how serious he was about it). I do not currently have the room for the larger machine, but I have been thinking of picking up his DVD just to peruse his designs as well.

If you would like to see the absolute extreme end of a Do-It-Yourself 4th Axis, research the InTurn on YouTube. It is a heavy duty dual operational mode unit, much in the same vein, and is gorgeous. The fellow had a long build post on the Mach3 forums.

With regards to feeds and speeds, along with work-area, here is how I did it. There is a nice product called G-Wizard by Bob Warfield. It is not new, and has a free trial variant. It is based on a subscription model, but works for hobbyists thus: once the initial paid subscription expires, it runs in a Lite mode; any hobby machine with 1 HP or under spindle can continue to use it. Here is the link: CNCCookbook; the site has a considerable amount of additional CNC information as well.

There are other calculators, such as the web version of FS Wizard, which also has a long development period, and free. There is also a Pro (pay) variant. Each program has interesting features.

With work area determined by measurement of X,Y, and Z travels, I simply went online, found a calculator for the most common materials I expected to be working (4140 steel and aluminum) and used it to tell me what a chunk would mass in the work envelope I had on the machine (utilizing average density of the alloys and form factor).

Then I went to G-Wizard, selected various tooling, and told it to calculate what feeds and speeds would be generally acceptable (one highlight of G-Wizard is it can de-rate the machine based on perceived rigidity; handy for those of us with less-than-rigid Chinese machines).

Once I had that information (highest mass, lower feeds and lower mass, highest feeds) I could proceed to work out the required torque and RPM limits to push said block around using similar links to what I supplied earlier.

As it turns out, the motors suggested by Hoss et al. worked out just fine. I can drive the machine harder than I feel comfortable running it.

Speaking of microstepping, that is something else that should be taken into consideration when system building. It is great at smoothing motor operation, reducing motor resonance, but not so great for precision or _incremental_ holding torque (at least in the high numbers; some hold anything past 8x as being a waste, Mariss of Gecko has stated he feels anything past 10x : Link).   Here is a favored link on the subject of microstepping.

Since steppers are not exactly known for ultimate precision, even those with a 0.9 degree step instead of the standard 1.8 (and good luck finding reasonably priced AND powerful 0.9 units), but with careful selection of the lead or ballscrew and control of backlash, the positional errors can easily be kept under those generated by the drivetrain.

Speaking of drivetrain, I just remembered that CNCfusion also apparently has a ballscrew and mount kit for the Grizzly-slash-BF20; might be of interest.

For a motor controller, I use the Dynomotion KFLOP and KSTEP (16x microstep) with standard 1.8 degree motors, and they can place the motors better than the dovetail ways can hold position (I need to learn to scrape the dovetail in; the gib shows wear on a narrow band instead of the whole surface).

The Gecko drives have adjustable microstepping, unlike the fixed value in the KSTEP. The Geckos are also independent drives, so if you destroy one, just the one needs replacement. Some find that useful. There are other systems: Granite Drives, DMM Tech are two that come to mind.

Dave, again, is of the same mind as myself regarding closed loop operation, but experience has shown a properly designed and operated stepper system does not lose steps. While I have equipped my motors with CUI AMT capacitive encoders for close loop velocity and positional operation, I have not yet actually engaged them. A fair amount of people are reporting them to have considerable noise at certain operational settings, as well as some dithering at rest (here is a link to show how bad it can get). While they apparently can be made to work, the US Digital E5 or similar encoders (actual glass) should be superior. So far, I am not missing the capability.

In the future, I may become absurd enough to use two control loops; one for actual table position (glass or magnetic scales), and the other for the motor velocity (controlling the motor position alone does not account for backlash in the drive train). The perceived accuracy is likely to greatly exceed the actual repeatable positioning of the mill, with the rough castings, stiction and deflection properties under load.


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## MontanaAardvark

Thanks for all the info!  I was busy playing in the shop and didn't notice this reply yesterday.  I've been over some of this material before, although not in about 10 years.  I tend to think of microstepping as only for smoother motion, not enhanced torque.  My Sherline drives (Xylotex) are 1/8 step - on 200 step motors.  Since they're direct drive, one turn of the leadscrew is .050".  I'm not naive enough to think that when I turn the 200 steps into 1600 steps, I'm really getting 0.00003125" accuracy.  

As a guy who designs control loops for a living, I was thinking of going to servos and feedback loops in this one.  Since there are two hardware signal paths, that means the hardware has to be more expensive.  Plus, I know millions of systems are shipped with open loop steppers every year, and if you treat them well, they won't get you in trouble.  

Anyway, I'm making some shelves for the shop, so back to it.


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## Al-Hala

MontanaAardvark said:


> Thanks for all the info!  I was busy playing in the shop and didn't notice this reply yesterday.  I've been over some of this material before, although not in about 10 years.  I tend to think of microstepping as only for smoother motion, not enhanced torque.  My Sherline drives (Xylotex) are 1/8 step - on 200 step motors.  Since they're direct drive, one turn of the leadscrew is .050".  I'm not naive enough to think that when I turn the 200 steps into 1600 steps, I'm really getting 0.00003125" accuracy.
> 
> As a guy who designs control loops for a living, I was thinking of going to servos and feedback loops in this one.  Since there are two hardware signal paths, that means the hardware has to be more expensive.  Plus, I know millions of systems are shipped with open loop steppers every year, and if you treat them well, they won't get you in trouble.
> 
> Anyway, I'm making some shelves for the shop, so back to it.



Ahh, engineering trade-offs. It would be SO DULL without them *laughs*

No worries on this end; I would be doing the same, but shop time is very limited for me this month, and the next *sigh* .

Given the size of your mill, combined with the price of some servo systems (Leadshine, DMM for examples) these days and add in your control loop experience (the usual tripping hazard those without PID experience on dovetailed Chinese mills, what with the fit, slop and backlash often reported), it could be a good choice.

For my own, once I saw some of the issues people were reporting regarding tuning headaches, I decided I would back-burner that phase for a while. Having said that, I understand some of the more advanced dedicated servo drives have self tuning. Plug it in and go, all done automatically.

Here is a 2012 thread (that includes Hoss commenting) on a servo equipped Grizzly. And here is another from 2013, and his own recommendation of hardware. The hardware page is nice in that he collected several reiterations, included pricing and links.

One quick note on the Leadshine units. The drives can use Step and Direction which requires a specific setup dwell time (5 microseconds) that some have reported is too long for the KFLOP (4 microseconds) system (the one I mentioned and use). Supposedly Leadshine has clarified they can work with the KFLOP, but if you decide to use that hardware combination, best to make certain.

Edit: Interesting. DMM has a promotion on their DYN2 AC servo system, 400W, 16 bit absolute encoder, 5K RPM for $348.  They claim they can deal with a 20:1 moment ratio.


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## MontanaAardvark

The DYN2 at $348/axis looks nice; the demo video was certainly impressive.  The current version of what I'm using from Xylotex is $375 for 4 axes.  I'm sure he must have updated his hardware, since the Allegro A3977 he was using has gone obsolete.   I don't know how to compare the ratings of the servo motor at 400W to the 425 in-oz stall torque on the Xylotex steppers, or if 425 in-oz is a reasonable torque for the G0704.

I ordered Hoss' DVD last night and got the shipping notice already, so I'll probably have it by next weekend.  I've been on vacation this past week and will be getting back on my head Monday - as the old joke goes.  Less time in the shop, but maybe I can do some more reading/planning.  It's coming up on summer and around here in Florida, summer is the time to do your indoor projects.  Yeah, we're kind of backwards from the rest of the country.


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## Al-Hala

Bob, I've been to parts of Florida in the summer; I can see why it is done that way *grin*. How'd the Xylotex work out for you? Did you ever encounter mid-band resonance/are the motors fitted with dampeners?

I`m putting up another wall of text, just in case this is useful for anyone else is still following along.

Steppers have a rated holding torque that drops off with speed increase, so the torque needed has to be calculated at the operating parameters expected.  Here's a sample curve. Faster operation means less rotary torque available to be converted into linear force.

Servo motors (excluding a encoder equipped stepper, which technically is a "servo") generally have a rated torque which is run-all-day-long value and a peak torque, typically 3 times the rated for only a limited time period that can decline with increasing speed but remains fairly flat (brushed DC motors drop off faster than BLDC or AC Servo).

Motor Sizing is dependent on what the machine is expected to do: how much weight and cutting forces must be moved at what rate and what acceleration, as well as what mechanical advantage is supplied by the rotary to linear motion mechanism (pitch of the screw). The overall goal is to make certain the torque available is sufficient during acceleration and nominal operations. On a servo, the intermittent region of the curve is for acceleration and deceleration, with the average needs within the rated torque envelope; on a stepper, the holding torque drops with speed, sometimes precipitously.

Now, before I run on further and not address Bob's comments....


MontanaAardvark said:


> I don't know how to compare the ratings of the servo motor at 400W to the 425 in-oz stall torque on the Xylotex steppers, or if 425 in-oz is a reasonable torque for the G0704.



Warning: I did make attempts to check my math, but I`ve been multi-tasking this all day *scribbles*, and this prime ground (for me) for mistakes, what with all the conversions.

*Is the 425 in-oz a reasonable torque for the G0704?*
Hoss states his bare recommended minimum is 381 oz-in steppers on the X and Y, and a 465 on the Z, and then states that 570 oz-in steppers are a great match (on the X and Y axis; 906 oz-in on Z) with stock screws (which have huge efficiency losses compared to ballscrews, and will traverse slower) or ballscrews.  So, that gives us a range. Others builds he references also seem to use similar motor ranges.

Xylotex does not seem to publish a torque vs. speed graph for the motor, so I looked at a Automation Technologies 425 unit. The graph is a bit daft, apparently missing the top value (425 oz-in is 3 N.m), and is in Pulses Per Second, not Rotations Per Minute; makes it tricky to compare.

1.8 degree motor at 8x micro-stepping would be 1 600 steps per rotation. 8000 PPS = 480 000 pulses per minute, 480 000 PPM / 1600 steps = 300 RPM.

So, according to the curve, 42.5 oz-in is remaining at 300 RPM into a (picked out of the blue) 5mm pitch (~0.2 inch per rotation) ballscrew will produce ~75 pounds of force at 53 Inches Per Minute (IPM) of travel; this is just first approximation, and disregards friction from the gibs, dovetails, opposing cutting forces, acceleration losses, etc.  _Again, this is assuming the two motors are in any way similar_; I have no way of knowing, and measuring in PPS is...unusual, to put it mildly.

Looking at the 570 oz-in Hoss recommended, it has a Torque-Speed Curve claiming at 1000 RPM it will produce ~63 oz-in translating into ~111 lbs. of linear force.

So the 570 stepper provides more rapid speed and more force.  Why 1000 RPM?  Using the above screw at 1 000 RPM will produce 200 IPM travels. Why 200 IPM?

*Feeds and Speeds*
I set out to see what the maximum rapid IPM people have been achieving. Again, the ubiquitous Hoss has achieved 300 IPM rapids (with the 570 oz-in motor). 200 IPM seems to be quoted as a more common number, so I used it in the example above. He also claims in one thread he has hit 200IPM with a 0.5 endmill during High Speed Machining operations, so it seems a reasonable number to look at for actual use (moving 200 pounds at 200IPM might be asking a wee bit much, however).

To do some basic cross-checking, I decided to take what I'd seen and compare the quoted numbers to G-Wizard rendered ones. The G0704 comes stock with a 1 HP spindle motor, end mill capacity of 0.75 inch (according to Grizzly). I took that information and built a machine in G-Wizard, and started in.

One video has Hoss demonstrating with a 0.8HP spindle motor driving a 0.5 inch four flute HSS endmill at 2250RPM, He managed a full width cut at 0.375 inches depth in 6061 Aluminum at a stated 12-15 IPM with no excessive audible motor loading noted.

G-Wizard, using this information and that from the mill specifications, gives a value of 18.4 IPM for a very aggressive cut, and 9.7 for a fine finish. The IPM is limited on the aggressive setting by available HP, and tool deflection on the fine finish. So, the theoretical roughly corresponds to the actual achieved numbers.

I then cross-checked using a milling power calculator (this is an approximation only) on 6061 Aluminum and lowered the feedrate to 13 IPM (due to the calculator claim this is 1HP motor load), I did the same using 4140 steel which nets a feed rate of between 2 (calculator) and 2.6 (G-Wizard) IPM.  Hoss`s numbers look good (not that I doubted them; he knows more about practical machining than *I* do, for certain).

What I take away from the above is: Yes, _*depending on the motor's characteristics and mode*_ (low impedance, high driving voltage, bipolar parallel more usable torque at higher RPM) and its torque-speed curve, a 425 oz-in stepper would work on the G0704* X and Y axis* with high rapid speeds, as shown by theory, actual examples and cutting rates.

*Comparing the ratings of the servo motor at 400W*
Power is work over time, either in HorsePower or Watts. Torque x RPM = power.

Quick and dirty power formula for motor : Power: Watts = (heaviest object in lbs.  * Inch Per Minute) / 531  (the 531 constant is just cutting out the conversions)
Example: ~113 Watts required to move 200 pounds (assume: 75 pounds for table and attachments, plus 125 pounds for work-piece, holding) at 300 IPM.

The 570 oz-in motor in the previous example would be generating ~46.5 Watts, and the 425 oz- in generating ~10 Watts. The G0704 1HP spindle motor, for contrast, would produce 448 oz-in at 2250 RPM to rate at 1 HP.

The DMM 640-DST-A6TS1 (DYN2, 60mm fram, 400W, Medium Inertia, 16 Bit Encoder, 60V max, Straight Shaft)  has a rated torque of 1.27 N.m (180 oz-in) and a peak torque of 3.82 N.m (540 oz-in). Here are the Torque-Speed Curves.

As such, while 180 oz-in is considerably weaker than the holding torque of the aforementioned steppers, at speed (200 IPM rate * 5 rev. per inch = 1000 RPM) the motor is still generating 180 oz-in (18x and 3.8x respectively for the previously mentioned steppers) and ~318 pounds of linear force, or 133 Watts, with 3x that much in reserve for intermittent use at that speed.

Edit: Acceleration is a critical part of system design, and I will actually (gasp) refrain from getting into it at this time; suffice it to say there should be ample reserve capacity to _put the pedal to the metal_, so to speak.
*
Additional Resources:*
- here's a handy guide put together by CNC Cookbook for motor sizing. In that post is a link for a Excel spreadsheet that has assorted variables to contemplate.

(for the above Excel file)
Mill Table Weight: I also could not find the stated weight of the mill table, so off I went to a calculator with the dimensions for a guesstimate: 73 lbs.
Part and Fixture Weight or Table Capacity: The G0704 has a table capacity of 125-150 lbs (also according to Hoss).
Cutting Force: That can be determined via calculators found on the web.
Linear aka Rapid IPM: Hoss again. 300 IPM (utilizing steppers)
Cutting IPM: Hoss claims in one thread he can hit 200IPM with a 0.5 endmill.  This is in-line with comments others have made.

Leadscrew: Not much information I could quickly research, sorry.
Lead/Pitch: I looked quickly and could not seem to find what the leadscrew is that comes with the G0704, one claimed it was 5mm.
Diameter and Length: Again, no information.
Lead Efficiency: I could also not find reference to the lead efficiency, but it is somewhere between 20-40% generally. .4 is the value I run across most often.

- Kollmorgen Motioneering Online : online (register access) utility for linear motion system design
- Choosing Stepper or Servo: Gecko Drives
- MIT on Speed-Torque Curves
- Maris Freimanis from Gecko on a CNCZone thread running some numbers as an example of sizing, gearing, power, resolution for a gantry router


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## jumps4

I'm a bit lost here, are we converting a lower end milling machine, made in china, installing Chinese ball-screws,
and strapping the trajectory computer for the lunar lander to it.


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## Al-Hala

jumps4 said:


> I'm a bit lost here, are we converting a lower end milling machine, made in china, installing Chinese ball-screws,
> and strapping the trajectory computer for the lunar lander to it.



*laughs*

That's the beauty of the world; for those that want plug and play, many people have already crunched the numbers and come up with acceptable solutions; hence the links to existing proven selections. Use "x" with "y" and go hard with full confidence that others have it working.

For others, they want to optimize and understand the choices that go into such (hence the links to all the threads where this stuff is discussed in detail), or understand why their purchase of cheap and popular offshore chopper style stepper drives (hint they are popular because they are cheap) combined with serially wired huge steppers and a high speed gantry have yielded disappointing crappy results; or the eternal steppers vs. servo discussions, and the wisdom of putting $3000 of gear on a $1000 mill. 

You should see the epoxy granite retrofit discussions  

I have had similar thoughts when reading the threads about people scraping in these mills to improve performance; still, they have me (and others) thinking about it.


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## RVJimD

Does anybody counter weight the head on their CNC mill to help reduce the required motor on the Z?  One of the FIRST things I did on my NON CNC 704 was to mount a couple pulleys on the ceiling and put about 40lbs of counterweight behind the cabinet to make it easier to raise the head.

Jim


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## MontanaAardvark

Gee, Al-Hala.  I'm going to have to save this whole thread!


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## Al-Hala

You are most welcome. 

See, I CAN write short responses! *grin*


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## dave2176

RVJimD said:


> Does anybody counter weight the head on their CNC mill to help reduce the required motor on the Z?  One of the FIRST things I did on my NON CNC 704 was to mount a couple pulleys on the ceiling and put about 40lbs of counterweight behind the cabinet to make it easier to raise the head.
> 
> Jim


I've seen many mention using gas charged supports to take some of the weight off. The type like your car has on the rear hatch to hold it up. Many options for those at McMaster.
Dave


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## Al-Hala

Hoss did not bother to do his, due to a larger motor on the Z, but I have seen example threads where it is done (mostly other mills); I understand some of the VMCs have it as well. I can see the case for it, your own experiences for starters. 

As Dave so rightly puts it, gas shocks (I was proofing this as his message popped up!) seem to be more common. CNCFusion at one time sold a gas spring add-on kit as well. 

Here is a link on a BF clone equipped with a counterweight from the Hoss Massive Thread , and the linked YouTube Video of the same. 

And from a CNC Arena thread (which seems to be a copy of the CNCzone threads):

_I know this is probably going to be a ...............well DUH question, but why does the X2 need a counter weight , and the 0704 not?_
_
An x2 wouldn't need a counterweight if you use a strong enough stepper with a proper driver and power supply.
A 381 oz/in, g540, 48v PS works well on an X2 just like a 570 oz/in, 6050 or 5056D, 48v PS works well on a g0704.
Go crazy adding weight to the head and you need more oomph or add a counterweight or gas springs.
Hoss_


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## arizonavideo

I have done a lot of playing around with the G0704. 

As far as the steppers. The 575 is a great stepper because of the low 2.1MH inductance. It is just plan faster than almost all the other steppers.  With any properly sized motor you never run out of power at low speeds. A cutter requires less power at low speeds and direction change forces are less. Steppers fail at high speeds due almost entirely to making less power at higher RPM due to inductance.

I can't count the number of people throwing stepper twice the size needed at a mill. Most of the guys are running the 575 which is a 5A stepper and driving it with a 5.6A PEAK China drive so that is 5.6A x .707 for RMS or 3.9A RMS  so they are running the stepper at about 80% of full power.

Most are also running a 900oz on the Z which is silly too but the last 10 CNC kits I have sold for the 704 they all wanted the Nema 34 Z mount for the 900 oz stepper. The 575 works great on the Z even at 4A if you run it at full power then it is even better.

So most people are limiting the power they make by the driver and not the stepper. 

The 425 is OK but has higher inductance so is slower. 

I have no idea why you would want to run a servo.  A good Gecko stepper driver works really well and you will never look back. They are much better than the China drivers. Micro stepping does help the accuracy but really makes the stepper run smoother at low speeds. I think only Gecko drives do drive morphing so they make 1/3 more power at high RPM over the China drives yet still have vary high micro stepping at low RPM for smooth running.


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