Metric/CNC conversion of lathes/mills

[Oscar Lazo]

Hello community! This is my first post here. I am considering to buy some PM machines to start a home workshop. They seem to have both good quality and capability machines as well as reasonable prices. However, I personally don't like the fact that PM machines are all built in inches for both the leadscrews and the dials. My question is: Do you know if there are any metric conversion kits out there that I could use to replace the leadscrews and dials? The machines I have an eye on are the PM-1228VF-LBl lathe and the PM-728VT milling machine.

Furthermore, my dream scenario is to have these two machines be able to switch between manual mode and CNC, so that I can operate with dials/DRO or use the computer to tell stepper motors how to move the axes. Does this sound doable to you? I know that there are PM CNC conversion kits, but they seem to either replace the dials or not accomodate for different dials to be used.

Any thoughts or suggestions? Thank you.

 

[Ischgl99]

Give PM a call and see if they can order a machine with metric lead screws. I believe someone posted in the past doing that, but I’m not certain about that. The machines are normally built that way at the factory for everywhere other than North America, so they are available.

 

[markba633csi]

Hi Oscar- CNC machines usually have ballscrews for the table X/Y so they are rather tedious to crank by hand

 

[RJSakowski]

If you have DRO's on the machines, it makes little difference whether the lead screws are metric or inch as the readouts can be changed at the push of a button. An issue could be how the readout is generated. My Tormach CNC mill doesn't have an independent and position is determined by that commanded by the CNC controller. This requires software control of the three axis rather than true manual control. Consequently, there are potential positional errors due to backlash in the drive system.

On the other hand, if you have an independent DRO, position will be dictated by the scales and will be true regardless of backlash. At issue is does the DRO control the movement or does the movement control the DRO. The former would seem to be the superior system. That type of system could be driven manually or by CNC. I believe that @WobblyHand has incorporated similar features in his ELS system.

 

[Oscar Lazo]

Give PM a call and see if they can order a machine with metric lead screws. I believe someone posted in the past doing that, but I’m not certain about that. The machines are normally built that way at the factory for everywhere other than North America, so they are available.

Will do!

 

[Oscar Lazo]

Hi Oscar- CNC machines usually have ballscrews for the table X/Y so they are rather tedious to crank by hand

Does that mean it's more turns per mm using ballscrews rather than leadscrews? Do they have a finer thread or are they stiffer to crank?

 

[Oscar Lazo]

If you have DRO's on the machines, it makes little difference whether the lead screws are metric or inch as the readouts can be changed at the push of a button. An issue could be how the readout is generated. My Tormach CNC mill doesn't have an independent and position is determined by that commanded by the CNC controller. This requires software control of the three axis rather than true manual control. Consequently, there are potential positional errors due to backlash in the drive system.

On the other hand, if you have an independent DRO, position will be dictated by the scales and will be true regardless of backlash. At issue is does the DRO control the movement or does the movement control the DRO. The former would seem to be the superior system. That type of system could be driven manually or by CNC. I believe that @WobblyHand has incorporated similar features in his ELS system.

I understand that the DRO makes my insistence on metric hardware a bit silly. But It's a hefty amount of money for a hobby, so I'd like it to be exactly as I like it, if at all possible. I expect a good CNC system to use the DRO measurement as opposed to step counting, or even a rotary encoder, and I'd say a DRO is a must (at least for the milling machine).

 

[RJSakowski]

I understand that the DRO makes my insistence on metric hardware a bit silly. But It's a hefty amount of money for a hobby, so I'd like it to be exactly as I like it, if at all possible. I expect a good CNC system to use the DRO measurement as opposed to step counting, or even a rotary encoder, and I'd say a DRO is a must (at least for the milling machine).

Aside from the benefit of removing backlash from the equation, another benefit of the DRO is it eliminates counting turns when traversing. Additionally, the modest computational skills like offsets, multiple frames of reference, bolt circles, etc. make the DRO. I have a DRO on all the machines that I routinely use and consider them well worth the cost.

I would like to see a CNC mill using actual position rather than counting steps. In the hobby class systems, I haven't seen one.

Does that mean it's more turns per mm using ballscrews rather than leadscrews? Do they have a finer thread or are they stiffer to crank?

Ball screws have a coarser pitch than lead screws. If I recall correctly, the pitch on my ball screws is 6mm. Because of the ball bearings, there is very little friction so they drive easily. In fact one of the issues is that they don't work as well on manual machines because they can be back driven. The z axis on my mill has a brake that engages the ball screw when the stepper is not active to prevent the head from falling.

 

[Oscar Lazo]

I would like to see a CNC mill using actual position rather than counting steps. In the hobby class systems, I haven't seen one.

Thank you! That is the sort of insight that I was sure I'd get here.

Ball screws have a coarser pitch than lead screws. If I recall correctly, the pitch on my ball screws is 6mm. Because of the ball bearings, there is very little friction so they drive easily. In fact one of the issues is that they don't work as well on manual machines because they can be back driven. The z axis on my mill has a brake that engages the ball screw when the stepper is not active to prevent the head from falling.

Then why would it be more tedious to crank than leadscrews? It sounds like the pitch is too coarse for fine manual positioning. Have you heard of the sort of simultaneously manual and CNC mill that I am talking about?

 

[RJSakowski]

Thank you! That is the sort of insight that I was sure I'd get here.

Then why would it be more tedious to crank than leadscrews? It sounds like the pitch is too coarse for fine manual positioning. Have you heard of the sort of simultaneously manual and CNC mill that I am talking about?

I would think that the decreased friction would offset the increased torque required because of the lower mechanical advantage. Generally, manual operation of a CNC mill isn't recommended although there are those who have. A search on this forum will turn up a few.

I run my mill manually,more often than with CNC by using a jog/shuttle. It's not ideal as there is no biofeedback and motion is in steps but it works. The CNC conversion kit for the mill that you mentioned uses a 5mm pitch ball screw so one rotation of a hand crank is 5mm of travel as compared to a more typical 2.5mm for a lead screw on a small mill.