# PM940M cnc conversion



## N2XD (May 27, 2018)

Could anyone who has made a cnc conversion on the pm-940m tell me what would be the correct size motor (either step or servo) to use on the z-axis. Thanks.


John


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## TomS (May 27, 2018)

N2XD said:


> Could anyone who has made a cnc conversion on the pm-940m tell me what would be the correct size motor (either step or servo) to use on the z-axis. Thanks.
> 
> 
> John



I converted a PM-932 a couple of years ago using a Nema 42 Stepper Motor With 4200 Oz-In Holding Torque for the Z axis.  Probably way oversized but it moves the head like it's not there and have never missed steps.  Not saying this is the "correct" size.  It's what I used.


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## Eddyde (May 27, 2018)

I am planning to CNC my mill, I bought these https://www.automationtechnologiesi...motor/nema34-1125ozin-dual-shaft-servo-motor/ 
for all axises, I plan on 2:1 reduction on the X&Y and 4:1 on the Z. From the research I did, these motors should give plenty of power and my mill is heavier than the PM 940.


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## phazertwo (Jun 3, 2018)

My 940 was CNC from the factory, with 640oz-in in the X and Y, and 1200 in the Z.  I would STRONGLY suggest 1200oz-in for the X and Y, and at least a 1700oz-in for the Z... However if I were starting from scratch like you I would go with a Nema 42 for the Z with 2800+ oz-in.  You're not going to wake up one morning and say "damn I wish I would have used smaller motors" but I wake up all the time thinking about upgrading.

Budgets are budgets though.

Also, I would suggest direct drive.  It works great, is simple, and less room for error.

PZ


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## N2XD (Jun 5, 2018)

Thanks for all the replies. I am going for 1700 oz on the x and y and a nema 42 on the z ( not sure of the oz in. yet.


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## phazertwo (Jun 5, 2018)

You will not be disappointed!


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## N2XD (Jun 13, 2018)

I went the full limit and went with a nema 42 at 4200 oz. I'm also going to use a ball screw on the z axis. Any comments on this? I also ordered the centroid acorn system. This will be a long project but I think it will be fun and useful.


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## phazertwo (Jun 13, 2018)

I would use a ball screw on each axis... Only way to go IMO.

I'm going to be jealous of your machine... I am constantly looking at upgrading motors.

PZ


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## JimDawson (Jun 13, 2018)

N2XD said:


> I went the full limit and went with a nema 42 at 4200 oz. I'm also going to use a ball screw on the z axis. Any comments on this? I also ordered the centroid acorn system. This will be a long project but I think it will be fun and useful.



A lot of holding torque, but the useful RPM is limited on those motors.  I have some experience with that particular motor, and found that below about 450 RPM it worked fine, but above that, the torque drops off rapidly and the motor can decouple running unloaded.  If your RPM and acceleration can be limited it should work fine.


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## N2XD (Jun 13, 2018)

PZ, Don't get jealous yet, it isn't even started.

Jim, Lower rpm was considered as I'm not going to do production work it is really not an issue for me. Thanks you for the input every bit is considered helpful to me. I have no plans at present to change over to a high speed spindle so going slower will be just fine. Retired here and just looking to expand my knowledge. 


Thanks Guys,

John


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## jbolt (Jun 13, 2018)

N2XD said:


> I have no plans at present to change over to a high speed spindle so going slower will be just fine.



Good luck with that. 

I'm also using the Nema 42 4200 oz stepper on my PM-932 conversion. I can set the Z rapids faster than I am comfortable with. IMHO rapid Z moves with a dovetail column mill while machining are hard on the machine if the head is not counter balanced. I tried running some 3D profiles at high speed and was afraid I was going to damage something with all the pounding and jerking the head was doing.


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## N2XD (Jun 13, 2018)

Jay,
    I'm a little confused. If I run at slower feed rates and slower rapids I would think that the pounding would be at a minimum. If I'm wrong please send me in the right direction. Thanks.


John


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## jbolt (Jun 13, 2018)

John, sorry for the confusion. 

Running what I would consider normal feed rates for that type of mill are fine. I was responding to Jim's comments about limited speed with that servo. My point being in real practice you can run the big stepper motor faster than the machine can handle so no worries on using it. 

Jay


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## N2XD (Jun 13, 2018)

No Problem Jay. This is going to be a quite a learning curve for me but it is time to bring at least one of my machines into the 21 century. Be prepared for some real dumb questions. Thanks.

John


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