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PM-940 DIY CNC or Turn Key PM-940M-CNC-VS

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RADustin

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Hi all,

Great forum. Trying to get caught up on these smaller CNC machines.

I have some basic and broad questions to those that have some experience: is the turn key PM-940M-CNC-VS worth the initial purchase knowing that the nmotion controller and other aspects need upgrading fairly quickly? The laggy e-stop really has me scared to run the CNC-VS machine at all out the box.

This would be in comparison to a DIY PM-940M using a hardened way machine as a starting point. The cost of the CNC-VS is currently $8k. The hardened way manual machine is $3.5k which leaves $4.5K to complete the conversion.

Obviously the castings are what they are, so do many start with the CNC-VS version to gain the belt drive 2 speed head to get away from the low RPM gear head?

Are there other major differences the turn key machine has that can't be easily or affordably achieved with the converted manual machine?

Thanks!
 

jbolt

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On my PM-932 I converted 3 years ago I probably now have $7K into it including the mill with cast iron base. That also includes VFD/belt drive conversion, 6K rpm spindle. pneumatic draw bar, enclosure, flood/mist coolant systems plus some of the trial and error expenses. I'm using mach3 with a smooth stepper controller and PMDX -26 BOB and PMDX spindle controller.

Looking at the 940-cnc specs the stepper motors look to be at the minimum in size. It does not say if the ball screws are rolled or ground. I know in some of the previous pm-945-cnc's they had ground ball screws which can be nice depending on where they were made (China or Taiwan). The spindle is limited to 3200 rpm. I looks like this is due to the use of less expensive taper roller bearings used on the spindle. To get to 6k + rpm on the spindle it will need to be converted to AC bearings.

I'm using mach3 with a smooth stepper motion controller and PMDX-126 BOB and PMDX-104 spindle controller.

If you want to just get to making parts and not deal with a conversion buying a ready to go machine may be the way to go with the caveat that you may be doing additional upgrades over time. If you like the idea of doing a conversion where you can tailor the machine to your needs but at the cost of time and maybe save a few pennies.
 

RADustin

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On my PM-932 I converted 3 years ago I probably now have $7K into it including the mill with cast iron base. That also includes VFD/belt drive conversion, 6K rpm spindle. pneumatic draw bar, enclosure, flood/mist coolant systems plus some of the trial and error expenses. I'm using mach3 with a smooth stepper controller and PMDX -26 BOB and PMDX spindle controller.

Looking at the 940-cnc specs the stepper motors look to be at the minimum in size. It does not say if the ball screws are rolled or ground. I know in some of the previous pm-945-cnc's they had ground ball screws which can be nice depending on where they were made (China or Taiwan). The spindle is limited to 3200 rpm. I looks like this is due to the use of less expensive taper roller bearings used on the spindle. To get to 6k + rpm on the spindle it will need to be converted to AC bearings.

I'm using mach3 with a smooth stepper motion controller and PMDX-126 BOB and PMDX-104 spindle controller.

If you want to just get to making parts and not deal with a conversion buying a ready to go machine may be the way to go with the caveat that you may be doing additional upgrades over time. If you like the idea of doing a conversion where you can tailor the machine to your needs but at the cost of time and maybe save a few pennies.
awesome! Do you have a build thread?

How did you handle the oiling? I checked with PM and they said the hardened ways are in fact still machinable. So I could chuck the saddle in my bridgeport and ball mill some grooves in the ways to get the oil in. This is probably my largest concern LOL.

Currently I'm leaning towards converting my own because I like the acorn motion board/software and I'd like to go servo. Looking at the DMM 1kW servos for the xy and 1.8kW for the z.

I also don't mind sourcing a nice motor and VFD that I can eventually convert everything to belt drive and up the RPMs to 6-7k. At first I think I'd run under the 3k limit just to get some chips under my belt.

Arizonavideo is about to release a ball screw kit. So that would help alot.

Basically- I think I'd rather just build the machine the way I want from the beginning rather than buy something that I know I will replace basically everything except the iron.

With that in mind, what kind of life span can be expected from the hardened ways? CNC has to be tough on dovetails and gibbs.
 

jbolt

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My build thread. https://www.hobby-machinist.com/threads/pm932-cnc-build.21442/

I'm pretty sure my ways are not hardened and they are holding up fine for a hobby machine. It gets used on average about 10 hours a month. Just keep it oiled. I currently have a manual oil pump for the X & Y and a separate one for the Z. At some point I will move to an automated oiler.
 

RADustin

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I'll check out your build. Thanks!
 

RADustin

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anyone have information on running servos on a PM-940 or similar? really curious what size/brand used and if holding brakes are required.
 

jbolt

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There is an older thread on the CNC Zone of a PM-945 conversion using servos. Lots of good info in that thread.

If I were going to use servos I would call DMM and talk to them about sizing. The servo on the Z requires braking or the head will fall when powered off or disabled.

You might consider posting this in the CNC forum to get better exposure to the CNC crowd.
 

RADustin

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I'll do that. Thanks!
 

phazertwo

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I like my 940 CNC, though it took a little work to get it there. I think the base machine and most of the electronics are pretty good, however the CNC controller they use is junk. There are a bunch of 940CNC guys on here, myself included, that have changed over to an Acorn controller and are much happier with the machines.

Hind sight being 20-20... I think I would have got an 833-T and done a CNC conversion. I've been upgrading my 940, and it seems like I probably should have just started from scratch. The 833-T is a much higher quality machine (or so I have heard), so I think it would make a very nice CNC conversion. At the end of the day I think you really have to ask yourself: do I want to build a machine? or do I want to make parts? If you want to make parts, get a 940CNC and a new controller and run with it.

PZ
 

RADustin

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my end goal is definitely to make parts. With arizonavideo coming out with a full ball screw kit for the 940, it has opened the door to possibly building a machine- well rather assembling and wiring it. The only 'machine' work would be to ball mill the ways for the oiling. For me, I'd never want to sit and design and build the ballscrew parts.

Its going to come down to the spindle motor/vfd and if I want to go servo or not. If I do want to go servo I think I'll build one. If I am OK staying stepper I will probably buy a turn key and change the board. That's what things look like right now.

I'm going to make another thread about the servos in the CNC forum- see what I can learn.
 
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