# Bench Top Mill CNC Conversion (basic question)



## bretthl (Mar 28, 2019)

Should I purchase a gear head bench top mill with a spindle speed of `~1500 rpm or a DC variable drive with max speed of 3000 rpm.  I like the PM833T gear head but am concerned about the low spindle speed.  The PM30MV is variable DC at 3000 rpm.  Am I making too much of this?  Can the gear head be driven faster with a 3 phase motor and VFD?


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## coherent (Mar 28, 2019)

In my opinion having the ability to set RPMs to any variable is the better option, especially considering the increased RPM.  A gear head mill may offer more torque depending on the ratings of each. The spindle type and design itself (bearing types and specs) may be a factor if you want to change the motor and run at faster speeds, but I doubt the difference between 1500 and 3000 would be a factor.  You can always use a different motor/vfd combo to meet your needs if the spindle can handle the new speeds. If not bearings are usually the limiting factor and can normally be replaced fairly easily. The least expensive option would be to simply get what you want to start with and not go through the cost and hassles of upgrading. Motors and VFDs can get a bit pricey. Another option to speed up a DC variable speed mill may be to install or modify a belt drive setup. Converting to a belt drive and choosing pulley sizes to run in the RPM ranges you want is a pretty common mod. Dual pulley diameters would even allow simple high or low RPM range changes.  I would examine to your planned specific requirements. Depending on your mill/cutter size, number of cutting edges, type etc, most materials (like aluminum) should be ok under 3000 rpm for manual milling but for CNC rpms for best performance and quality could easily exceed that.  Also remember with a CNC mill optimal RPM is directly influenced by feed/travel speeds which may depend on the machines capabilities.


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## bretthl (Mar 28, 2019)

I like the PM833T because it appears to be a heavier (more rigid) mill than the PM30MV.  I don't want to put time and effort converting a light weight mill.  I want a mill capable of performing CNC operations in steel.  I think I would be willing to trade faster feed rate for rigidity.


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## cs900 (Mar 28, 2019)

do you mean the PM30MV? I can't seem to find any specs on a 40MV model. 

I would stick with your gut on this one. If you can afford the heaver machine I'd go with it, and just plan on upgrading the spindle motor (and maybe bearings) as part of your conversion. This is a pretty common practice on these mills and is fairly well documented.


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## bretthl (Mar 28, 2019)

Yeah, PM30MV (if fixed it), thanks.  From what I have seen on belt driven machines there is no transmission between the motor and the spindle.  If you smoke the spindle bearings then change them out.  But on a gear head machine would you be speed limited by the transmission?


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## macardoso (Mar 28, 2019)

You will find the low speeds very limiting for tools under 3/8". I would suggest getting the biggest machine you can afford and fir, but if you will CNC it, you will probably want to look into modifying the spindle to reach 5000+ rpm

I would recommend a belt transmission for noise reduction.


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## cs900 (Mar 28, 2019)

Yeah, I'm not sure what the top limit of the geartrain is, but i'm very certain is under 5k. I stripped the entire geartrain out when I did my belt conversion.


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## cjtoombs (Mar 28, 2019)

1500 RPM is going to be very limmiting on a CNC mill (or a manual one, for that matter).  I have a Tormach PCNC 1100 and it's 5140 RPM max is fairly good, but for small carbide end mills in aluminum it's way too slow.  I have the speeder for that, but it does eat up some vertical real estate.  On the flip side, variable speed is great at the upper RPM, but at the lower RPM they loose power quickly, which makes using things like slitting saws a huge pain, especialy in steel.  They require running the saws faster than they really should be and taking very light cuts.  The ideal would be several speed ranges between 100 to about 5000 rpm that give good power across the RPM range.  Not something you generaly find on resonably priced manual machinery.


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## bretthl (Mar 28, 2019)

So based on spindle speed the PM30MV is the better choice because of the existing belt configuration and bearings can be upgraded and motor swapped out?  Why do people do CNC conversions on gear heads?  I think even QMT offered a CNC gear head bench mill.


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## coherent (Mar 28, 2019)

It appears the PM30MV already has the variable speed and belt drive. You may be able to increase RPM by simply changing the pulley configuration/sizes. You shouldn't need to change the motor unless you just decide you want more HP. The existing bearings may be ok to 5000 rpm or more. You can prob get the specs on the stock bearings. If not it should be a fairly simple task to buy/install higher rated bearings that fit without any mods. If your spindle/bearings run really hot, you'll know. Your first project will likely be the CNC conversion... start there and once you know the machines capabilities, travel feeds and speeds after the conversion, then you can determine the RPM required to optimize performance and go from there.


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## jbolt (Mar 29, 2019)

At the end of the day it really depends on what you will be making that will determine how much mill you need. This can be an almost impossible question to answer if you have no prior experience with a CNC mill. Often it comes down to budget and space.

The next question is do you want to build a CNC mill or just have a CNC mill to use? Building a mill takes some planning and time to complete.  You will also need a mill and possible a lathe to make the parts for the conversion. I stripped, measured and reassembled my mill to use to make parts for the conversion. 

The advantage to building a mill is being able to configure it to your own specifications / needs. You can control some of the over all costs by which components you use. If you would rather just get to making parts I would look at a commercial offering.

I converted a PM-932 to CNC about 5 years ago. My recommendation is to get the largest mill you can afford with the largest travels. More mass = more rigidity, larger cuts, better finishes and a larger work envelope. You can never have enough work space. The down side to a larger mill is everything else that goes along with the conversion is larger / costlier i.e. stepper/servos, drives, power supplies, ball screws etc.

A gear head is limited to about 3K rpms. This is due to the tapered roller bearings and the amount of heat a gear head will create. At the stock 2k rpms, running for several hours the head and spindle would get too hot to comfortably touch all the while shedding grease from the spindle bearings

As most everyone who has done a conversion on a gear head mill will tell you, spindle speed becomes a limiting factor pretty quickly. I did the 3 phase/VFD belt drive conversion and have up to 7K spindle speed with AC bearings on the spindle. I normally do not run over 6K due to the lower quality import 3ph motor/VFD I am using. With a quality motor/VFD/AC bearings some have run the spindles up to 10k. 

With the belt drive I have two drive ranges depending on the belt/pulley location, 0-1750 rpm & 1750-7000 rpm. Steels are typically done at the lower speed range and aluminum at the higher. HP, spindle speed and machine rigidity will dictate how large or small a cutter you can use. I routinely use end mills from 1/8" to 3/4".


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## bretthl (Mar 30, 2019)

Jbolt that was a really solid response and I just read your 932 build thread.  The only milling experience I have is with my PM935 and mostly in steel.  I want to explore CNC from the ground up, hence do a conversion.  It looks as though the the heaviest off the shelf belt drive bench mill option is the PM30MV.  Heavy enough to justify the time and cost?  How will it perform in steel?

At the other end Arizona99 is cutting CNC conversion parts from aluminum with the 940, no belt conversion.

I am sensing that spindle speed is critical in CNC if you want fast cutting speeds (feed rate).  True or false?


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## coherent (Mar 30, 2019)

bretthl said:


> I am sensing that spindle speed is critical in CNC if you want fast cutting speeds (feed rate).  True or false?



No doubt about it. Tuning your cutter type, spindle speed and cut speed to that "happy point" is the key. Every machine is different and has it's own limitations so you have to adjust one of the other factors if your machine is limited in one. Speed and feed calculators are handy, but I find an additional bit of fine tuning is usually required to get the best quality or best cut speed depending on your specific requirements.


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## jbolt (Mar 30, 2019)

I ran my mill with the gear head for a while before converting and it is perfectly capable of doing the job just slower and with some compromises in tool life and finish quality especially with aluminum. The noise will drive you batty after a while. 

The other benefit of a belt drive/VFD is spindle speed control through the control software. With the single phase motor you are limited to ON/OFF control. With the VFD you add speed control. I have run parts with 20+ tool changes. Having a large range of available speeds allows for tuning the mill for each tool which is easier on the machine and tools.

I am not familiar with the PM30V but I have seen a number of smaller mill conversions that have been successful. I would certainly seek them out and get some feed back. If I recall the PM30V has a brushless DC motor so I'm not sure if speed control through control software is possible. Something to check out.

When I was researching for my mill I was able to go look at a few including a Seig X2 conversion. It seemed quite capable but I was put off by the limited work envelope. 

If I were to do another conversion I would start with the PM940 with the hardened ways.


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## bretthl (Mar 31, 2019)

jbolt said:


> If I were to do another conversion I would start with the PM940 with the hardened ways.



... what about the PM833T?  I know the max spindle rpm is 1500 less than the 940.


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## jbolt (Mar 31, 2019)

bretthl said:


> ... what about the PM833T?  I know the max spindle rpm is 1500 less than the 940.


Please don't think I am trying to talk you out of anything. I am just presenting my personal opinion. If I could afford it I would have a VMC.

The 940 with hardened ways is $200 less. The 833 does not say anywhere in the specifications that the ways are hardened. The 940 is larger & heavier, has a larger table and has more travel in all axis. In my case all I am looking to keep is the base, cross slide, table, column, head case and spindle. The lead screws, head gears, quill gears, motor and electronics all get removed.

I'm sure the 833 has a better fit and finish and may have a higher degree of precision but at the end of the day there is only so much accuracy you can squeeze out of a dovetail machine of these sizes.


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## cs900 (Apr 1, 2019)

jbolt said:


> Please don't think I am trying to talk you out of anything. I am just presenting my personal opinion. If I could afford it I would have a VMC.
> 
> The 940 with hardened ways is $200 less. The 833 does not say anywhere in the specifications that the ways are hardened. The 940 is larger & heavier, has a larger table and has more travel in all axis. In my case all I am looking to keep is the base, cross slide, table, column, head case and spindle. The lead screws, head gears, quill gears, motor and electronics all get removed.
> 
> I'm sure the 833 has a better fit and finish and may have a higher degree of precision but at the end of the day there is only so much accuracy you can squeeze out of a dovetail machine of these sizes.


Just something I noticed the other day....the weights listed for all the mills do not include a stand....except the 940. I'm sure it's still a larger machine, but it is a little misleading as first glance. 

I'm with jbolt....i'd get a VMC! second choice is the 940 with hardways.


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## bretthl (Apr 2, 2019)

I totally agree with what your saying Jbolt.  If you you wind up discarding 1/3 of the machine doing a CNC conversion then the 833 does not make sense.

I too noticed the 940 included stand weight.  Not really a big deal but it is inconsistent in their comparison sheet.

I can afford a vertical CNC, I just don't have a place to put it right now besides the garage and that's not happening.  Plenty of these out there for ~10K:









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## cjtoombs (Apr 2, 2019)

bretthl said:


> I totally agree with what your saying Jbolt.  If you you wind up discarding 1/3 of the machine doing a CNC conversion then the 833 does not make sense.
> 
> I too noticed the 940 included stand weight.  Not really a big deal but it is inconsistent in their comparison sheet.
> 
> ...




Better make sure you have the power to turn that 15hp 3 phase spindle before you go to procuring one of those.  Most home shops don't have that kind of service and in many residential areas it's not even available.


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## bretthl (Apr 2, 2019)

cjtoombs said:


> Better make sure you have the power to turn that 15hp 3 phase spindle before you go to procuring one of those. Most home shops don't have that kind of service and in many residential areas it's not even available.



Yeah that's 50A minimum at 220V.  Still exploring.


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## MarkM (Apr 3, 2019)

For the money and time it will take to convert the pm 833 t I think you would be much better off going with a true cnc either of the used flavour or looking at some of the smaller cnc type machines from Novokon, Syil, Skyfire and Tormach.  I think the Pm 833 t is a fantastic machine for what it is.  It should be left as a manual machine for it s intended purpose. The gear head is a limiting factor in my opinion.  Belt drive will give you years of service and much easier to maintain.  Too much money wasted to bring the pm 833 t up to cnc standards.  Unless you want a project and enjoy that I would skip it and spend the money on a ready made cnc.  Will you really save money in the end?


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## Cadillac STS (Apr 3, 2019)

MarkM said:


> For the money and time it will take to convert the pm 833 t I think you would be much better off going with a true cnc either of the used flavour or looking at some of the smaller cnc type machines from Novokon, Syil, Skyfire and Tormach.  I think the Pm 833 t is a fantastic machine for what it is.  It should be left as a manual machine for it s intended purpose. The gear head is a limiting factor in my opinion.  Belt drive will give you years of service and much easier to maintain.  Too much money wasted to bring the pm 833 t up to cnc standards.  Unless you want a project and enjoy that I would skip it and spend the money on a ready made cnc.  Will you really save money in the end?



I would agree the PM 833t is too expensive for the conversion.  You would need the PM 833t, ballscrew conversion then the servo motors and electronics, etc..  Probably over $9000.  Would suggest sitting down and working out the costs prior to making move.  Then compare cost to a CNC mill already completed.

There is a website I found with one click that is going to offer a CNC conversion ballscrew kit for it though...


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## jbolt (Apr 3, 2019)

[QUOTE="bretthl, post: 654070, member: 48029"

I too noticed the 940 included stand weight.  Not really a big deal but it is inconsistent in their comparison sheet. 

I can afford a vertical CNC, I just don't have a place to put it right now besides the garage and that's not happening.  Plenty of these out there for ~10K:

[/QUOTE]

My best guess based on the stand for the 940 being similar to the 932 is it is somewhere around 250 lbs.

I would love to referb an older VMC but as cjtoombs points out, getting 3-phase power makes it cost prohibitive, at least at my current location.


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## jbolt (Apr 3, 2019)

Cadillac STS said:


> I would agree the PM 833t is too expensive for the conversion.  You would need the PM 833t, ballscrew conversion then the servo motors and electronics, etc..  Probably over $9000.  Would suggest sitting down and working out the costs prior to making move.  Then compare cost to a CNC mill already completed.
> 
> There is a website I found with one click that is going to offer a CNC conversion ballscrew kit for it though...



On my PM-932 conversion I have about $8,500 total into is including the mill, base, pneumatic drawbar, one-shot oiling system, coolant system, enclosure and 4th axis. About $700-$800 of that is for things I changed or upgraded over time. 

I wouldn't pigeon hole myself into someone elses ballscrew kit. Design and specify the screws and mounts to fit your needs and constraints. 

I agree on costing out the major components before deciding on what direction to go. Compare rolled screws vs ground screws, steppers vs servos, motion controllers etc. 

Personally I design and build everything in CAD before ever ordering parts or cutting material. 

I didn't build my CNC to save money, I built it for the experience and joy of building a machine. If I were trying to make a living off my CNC then I would have considered a commercial offering.


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## hanermo2 (Apr 3, 2019)

Get the PM.

Cheap, good:
Put a bigger 3-phase motor on it, direct, forget belts and gears.
VFDs don´t need any power, you can limit their draw via sw.
3-phase motors are almost free in power, as in 2-3-5 HP cost about the same in surplus center etc.

Since you mentioned You could buy a VMC, a servo motor on the spindle would be the right choice.
I have one on my 12x lathe, 2.5 kW cont.
1500€, +/-.
It´s about == to 10 HP industrial turning centers (Haas 11 kW 102 Nm max 1200 rpm, mine 90 Nm 0-1000 rpm).

A servo motor gets you 5-10x the torque and *surface finish* of anything else.
Yes it does - to my great surprise 3 years ago.
And rigid tapping.
And failsafe.
The servo stalls before breaking anything important, at 0.1 ms in time.

The acceleration of a servo is fantastic but useless, the top speed is mostly not important unless geared down like mine at 1:3.

There are zero realistic cnc mills comparable to a good servo refit.
A tormach new-model servo is probably the best, I think not yet available, around 20k$.


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## bretthl (Apr 5, 2019)

Well this discussion has been most helpful and a good reality check for me.  I can see that a mill conversion would not meet my long term goals.  I would not be able to attain precision from a low speed dovetail machine sufficient to justify the expenditure (time/money).  It would be a fun project and enhance my understanding in general but down the road I would regret not going with a smaller industrial VMC.  I will have to wait until the end of the summer when the stand alone shop is complete to have the space.  Going with a used VMC scares the you know what out of me.  I can inspect a manual mill or lathe with a high degree of confidence but I would be lost with a VMC.  I have a hunch that most used VMC are for sale for a reason and that reason is not in my best interest.


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## MarkM (Apr 5, 2019)

Not necessarily!  The smaller vmc s may have been a stepping stone and with a well documented maintenance program would give a fair bit of confidence knowing they went with something to improve the capability of the shop and just want the money from the machine they are selling.


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## bretthl (Apr 5, 2019)

In my mind it's just not worth the risk.  Most likely the used machine will come from out of state.  So unless I can find the time to travel and "inspect" (or hire an inspector) it will be sight unseen.  There is a youtube channel called Full Throttle CNC and this guy lays out the process of buying a used machine.  Rigging, transportation and setup alone can run 3K - 5K USD depending on the location.  That coupled with no warranty or returns and the fact that used machines 20 years old command a pretty high price does not make sense in the long run.  I would also need to consider availability and cost of replacement parts.  He also speaks to the advantages of financing vs. paying cash outright (hard to finance a used machine) with respect to ease of future purchases.  I'm not totally ruling out used but unless a local absolute deal appears it won't happen.


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## JimDawson (Apr 5, 2019)

Just to confuse the issue a bit more, there is an intermediate step between a benchtop and VMC.  The BP style CNC knee mills, available on the used market in several sizes from about 2500 to ~5000 LBS.  Both in V and box way configurations, from 3 to 5 HP.  I have 2 of these in my shop, one a 3V, and the other a 5V size.  Both very capable machines and the 5V has a 24 tool changer.  The downside is that they do not play well with flood coolant, in that it is difficult to contain.


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## MarkM (Apr 5, 2019)

I ll add more confuision along the same lines.  Mitutoyo /Milstar retro kits are being sold with new Knee Mills.  For the right candidate and the right timing for them.  I think long term this would give the highest dollar for dollar value for three axis.  It s for real right from the get go at an attainable price.  Probably 1.5x what you might spend on a benchtop retro but then then you would not a want or need to upgrade to a Tormach or something. Thats s where you d see the value I think especially if you may work with it.  Serviceability and support as well I would think.


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## spumco (Apr 5, 2019)

Food for thought...

If you buy a smaller, used VMC with rails (HAAS, etc.) there's not as much to worry about.  Budget replacing all the electronics except for the spindle motor and drive.  Even the linear rails can be replaced if necessary.

We're saturated with used VMC's here in the rust belt, but I'd imagine you could find an old HAAS or Brother (or similar) for a few thousand that has an obsolete control that some shop doesn't have time to fiddle with it.  Rigging for one of the smaller VMC's isn't going to be that bad, so the only thing to worry about financially is the spindle.

New ballscrews are going to be about the same price whether for a PM940 or for a HAAS VF-0.  Same thing for the control hardware, and if you can re-use the axis servos & drives - that's a savings.  Just don't count on it and budget for it.

Having done it on a small mill, I don't see much difference between taking a new manual machine and converting it and retrofitting an old VMC.  The big difference is that you don't have to fabricate ballscrew mounts or modify any of the major hardware to get it working.  The challenge will be getting the control to work with the spindle motor.

If the tool changer is too complicated to figure out at first - don't use it.  Just leave yourself some extra inputs/outputs to get it working at some point after you've gotten your CNC Controls Apprenticeship over with.

So instead of spending time researching how to manufacture ballscrew mounts, spend time looking at old VMC user manuals and find one with spindle drive inputs you can connect to with off-the-shelf CNC control hardware.

And look for one that originally ran on single phase - I think there were a few 7.5hp little ones made a while ago that could work.


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