Pm-728vt conversion

[Toolmaker335xi]

Hello everyone, I’m starting to do some research for converting my mill over to cnc. I did not want to tear the mill down multiple times so I am waiting on PM’s conversion kit.

My question right now is about powering the steppers. I already have a G540 with a 48v power supply. 465oz nema 34 steppers 3.5a 4.2mH. Which would require a 65v power supply to my understanding. If I decide to make adapters to mount to the kit( which I think is being designed for nema 23’s) what are the draw backs from using the 34’s I already have? Am I only going to have 339 oz holding torque? Also will they get too hot because they won’t go into current set standby? Would I be better off losing the 34’s all together and purchase 23’s?

 

[macardoso]

Nema 34s are physically larger so that could be one issue. The motors with lower holding torque should be able to achieve higher speeds due to their lower inductance. Stepper drives can be driven with voltages lower than their max, but the peak speed the motor can reach will also be reduced. You will retain the full holding torque because that is analogous to current not voltage.

I'd say why not try it. Don't "frankenstein" the kit to shoehorn the Nema 34s in, but if you can make them fit I bet they'd do what you are looking for.

 

[Toolmaker335xi]

Nema 34s are physically larger so that could be one issue. The motors with lower holding torque should be able to achieve higher speeds due to their lower inductance. Stepper drives can be driven with voltages lower than their max, but the peak speed the motor can reach will also be reduced. You will retain the full holding torque because that is analogous to current not voltage.

I'd say why not try it. Don't "frankenstein" the kit to shoehorn the Nema 34s in, but if you can make them fit I bet they'd do what you are looking for.

Thanks for the input, speed isn’t really a concern for me right now. This kind of just a learning project for me. And if I can put a little bit of my own design on it by using the 34’s, that’s a plus to me. Don’t worry, I won’t be butchering the kit to make them fit.

 

[rwm]

Did you get anywhere with this?
I want to convert my Z axis only at this time. I just measured the torque required to move the head and I am getting around 400 oz-in. That is the upper range of the NEMA 24's. Not sure why PM would not use NEMA 34 at least on the Z.
Robert

 

[Toolmaker335xi]

Did you get anywhere with this?
I want to convert my Z axis only at this time. I just measured the torque required to move the head and I am getting around 400 oz-in. That is the upper range of the NEMA 24's. Not sure why PM would not use NEMA 34 at least on the Z.
Robert

I think they designed the kit to use a Nema 34 for the Z axis. But I can’t confirm this since I have not received my cnc kit yet from PM. I ordered it back in April and was supposed to receive it by the end of July. In the middle of July they said it was going to be another 6-8 weeks. Hopefully I receive it sometime soon.
Sorry I can’t help.

 

[kered]

Bigger is not always better, A 465oz nema 23/24 will out perform a 465oz nema 34 by a long way. Bigger motors have much higher inductance and to overcome it you need to use a much bigger PSU just to get the same torque/speed. If Im correct the 728 is a bigger heavier machine than the pm25mv and I doubt very much that a 465oz stepper will be enough, especially for the Z axis, and just so so for the other axis, lots use 566oz on the X & Y and at least 650oz or bigger for Z.
Having said that the driver and the power it has available to pass to the motor will make the most difference.

You can easily make an adapter plate to screw into the nema 23 holes and another set of holes to fix the nemer 34 to, you will need to get a slightly longer connector too but you need a bigger one anyway as the nema 34 usually have 14mm shaft compared to 8mm on the 23's and hence are bigger.

 

[Toolmaker335xi]

Bigger is not always better, A 465oz nema 23/24 will out perform a 465oz nema 34 by a long way. Bigger motors have much higher inductance and to overcome it you need to use a much bigger PSU just to get the same torque/speed. If Im correct the 728 is a bigger heavier machine than the pm25mv and I doubt very much that a 465oz stepper will be enough, especially for the Z axis, and just so so for the other axis, lots use 566oz on the X & Y and at least 650oz or bigger for Z.
Having said that the driver and the power it has available to pass to the motor will make the most difference.

You can easily make an adapter plate to screw into the nema 23 holes and another set of holes to fix the nemer 34 to, you will need to get a slightly longer connector too but you need a bigger one anyway as the nema 34 usually have 14mm shaft compared to 8mm on the 23's and hence are bigger.

Thanks for your Input
I’m using parts I already have from other projects and my initial question was if I would lose toque by not being able to supply the motors with the full 65v the 34’s required since the max voltage for the G540 is 50v. (Speed is not a concern for me.)

I didn’t calculate the required torque for the z axis, rwm commented that the 728 would require around 400oz of torque and questioned why PM didn’t design it for 34’s. And I only replied that I think they did design it for a 34. ( I am not even doing a 3 axis conversion on my mill at this time)

I realized adapters would need to be made as well as new couplers to account for the different shaft diameters and bolt patterns. I’ll post some updates whenever I receive the kit. It was supposed to be shipped in July, but still waiting.

Thanks you again for the input, after giving it some thought I might as well purchase all new motors just so I can just bolt it together and be done with it. I lost interest months ago waiting for the kit to be shipped.

 

[kered]

The Pm25mv is a much smaller machine, check out what happened to shooter123456 in this thread https://www.hobby-machinist.com/threads/new-pm-25mv-mill.65939/ first page, he started of with the 400oz motors on Z, also Arizona video has some mention in his videos about Z axis sizes and for the smaller pm25mv he recomends 656oz, I think in 1 video he mentions the next size up, I'll check and if its relevant will post a link.

Its nice to have plenty of cash to just go buy new motors but before you do look at the driver, by changing drivers only it can make a big difference, although I doubt the 400oz for your Z will cut it, it will probably need arround 1280oz to perform reasonably well.

 

[kered]

is one of the videos I mentioned, mostly refering to pm25mv size mills also the much bigger pm940 I think yours is in between.

 

[Toolmaker335xi]

Thanks for your concern, but for now I am only doing a 2axis conversion for multiple reasons. I only mentioned that the z axis mounts in PM’s CNC kit were designed for nema 34’s in regard to someone’s question.

I checked out the shooter123456 thread you referred to above. Seems like his 400oz worked ok but the driver was what gave out eventually. If I do decide to drive a z axis while still using the G540 I’m not looking to get rapids over 120ipm.

This is just the first phase I’m working on for the conversion, which has stalled out since waiting so long for the kit. Eventually once I’m ready to upgrade the controller and a bt30 spindle is in production for this mill, I’ll worry about the Z axis.

My funds and time are limited but since I haven’t spent a penny on this mill in months and I’m reusing a controller/driver I already had, I can justify new motors. Thanks again for the info and getting me interested in the mill project again

The Pm25mv is a much smaller machine, check out what happened to shooter123456 in this thread https://www.hobby-machinist.com/threads/new-pm-25mv-mill.65939/ first page, he started of with the 400oz motors on Z, also Arizona video has some mention in his videos about Z axis sizes and for the smaller pm25mv he recomends 656oz, I think in 1 video he mentions the next size up, I'll check and if its relevant will post a link.

Its nice to have plenty of cash to just go buy new motors but before you do look at the driver, by changing drivers only it can make a big difference, although I doubt the 400oz for your Z will cut it, it will probably need arround 1280oz to perform reasonably well.