CNC PM-728V-T

[Toolmaker335xi]

I have completed the first phase of my CNC conversion. I had a couple questions in another thread, but didn’t receive any feedback so I started a new thread to hopefully get a few answers and some feedback.

First, a little bit about my setup. It’s a pm-728vt with PM’s CNC kit. G540 running Mach3 on an old desktop using the parallel port. Right now it’s only 2axis with 570oz dual shaft steppers with a sepraty DRO.

I’m happy with the CNC kit and can say the cast iron mounts were worth the 8 month wait. Everything bolted up as it should with no issues so far. I have about .0009 backlash in X and .0026 in Y. X is acceptable to me but Y seems like a lot. I tightened the lock nut till I had maybe .0001 axial play on the shaft end. So my question is can I or should I try to shim in between the double ball nut? Is it possible to have that much play in the shaft coupler? What else can I do to improve this?... as I’m typing, I realized I can swap the couplers to check there...

Also, in Mach 3 is there a way to use my homing switch as a limit switch or do I need to have two limits and a home switch for my x and y.

Overall I’m pleased with the machine, the only complaint I have is the Y axis ballscrew is not left hand thread. So since I tried to keep my hand wheels the table moves opposite as it normally would cranking it. And hopefully I can improve the backlash of the Y axis some.

Here’s a couple pics. Short term changes are going to be upgrading to Mach4. Installing the Z axis. I struggle to get fusion360 to post toolpaths with zero Z moves. ( I kind of wanted to have it like the 2 axis mill power setup I use at work and since I mainly have been running wire edm for the last 10 years. Z moves cause me anxiety. Lol.) Also need to clean up the wiring and start planning a partial enclosure.

Thanks for any feedback in advance.

 

[rwm]

That looks slick! I have a 728 that will eventually be CNC so I am watching with interest. Those limit switches look pretty large and beefy? Is that just overkill? I see you went with double ended steppers and kept the handles. I was advised not to do this. How do the handles turn when using it manually? Does it require a lot of force or could you use it manually? Those are funky sleeves you made to fit the handles. Integrated key? Any info on how you made those? What size stepper do you plan for the Z? Sorry but I have more questions than advice!
Robert

 

[macardoso]

I’m happy with the CNC kit and can say the cast iron mounts were worth the 8 month wait. Everything bolted up as it should with no issues so far. I have about .0009 backlash in X and .0026 in Y. X is acceptable to me but Y seems like a lot. I tightened the lock nut till I had maybe .0001 axial play on the shaft end. So my question is can I or should I try to shim in between the double ball nut? Is it possible to have that much play in the shaft coupler? What else can I do to improve this?... as I’m typing, I realized I can swap the couplers to check there...

You should be using shaft couplers made for CNC. These would be the "Lovejoy" style jaw couplers with an elastomer spider (must be the zero backlash kind). Even better would be a disc or bellows type coupling. The helical spring coupling are garbage for this application.

Eliminate the play in the screw mounting, then any remaining backlash must be in the ballnut. If you have double nuts, shimming between them is exactly how you should reduce backlash, although I'd be talking with the screw manufacturer or kit seller to explain why it is so bad. I bet you spent good money for the kit and they should make it right.

Also, in Mach 3 is there a way to use my homing switch as a limit switch or do I need to have two limits and a home switch for my x and y.

Yep, that is allowed and most people do just that. Assign them the same setting in ports and pins.

Here’s a couple pics. Short term changes are going to be upgrading to Mach4. Installing the Z axis. I struggle to get fusion360 to post toolpaths with zero Z moves. ( I kind of wanted to have it like the 2 axis mill power setup I use at work and since I mainly have been running wire edm for the last 10 years. Z moves cause me anxiety. Lol.) Also need to clean up the wiring and start planning a partial enclosure.

I highly recommend Mach 4. I'm a big user of it and happy to help you out if you need. I also highly recommend a motion controller like the Ethernet Smoothstepper (ESS) for Mach 4. You can use the parallel port with a paid plug-in, but it is like towing your new Ferrari behind some horses.

 

[rwm]

What is wrong with the helical couplers?

I assume they have zero backlash? No?

Robert

 

[macardoso]

Helical (beam) couplings have zero backlash but poor torsional compliance. When they experience load, they "wind up" which creates error between the motor shaft and the ballscrew position. This acts similar backlash when you are machining and represents error in the positioning that you can't be certain of.

In fact all couplings have torsional compliance, but they can be orders of magnitude less than the helical coupling. These designs will have better performance for your machine, but also may require the shafts to be aligned more closely from the get-go and may cost more. Bellows are probably the best for precision motion control applications (hobby CNC probably does not fall in this category )

This one below is a disc type.

 

[rwm]

Thanks for clarifying. I had no idea about that. That info will be quite useful!
Robert

 

[macardoso]

Not to go off topic, but shaft couplers are a very interesting topic. Different applications require very different couplings. In precision applications, zero backlash and ultra low compliance are key which necessitates precision couplings. These tolerate very little misalignment (although any misalignment is intolerable because it can cause the universal joint surging of motion, even at small scales, which is usually undesirable).

Other applications care not about backlash or compliance, but require large misalignment tolerance or vibration damping characteristics. Non-motion control rated jaw couplings are great for this as the "spider" durometer can be tuned to dampen the load.

Other applications (like pumps) require over torque protection and can rely on clutch or magnetic couplings that slip on an overload condition.

Finally some applications require fail safe couplings (rips apart when over torqued, separating the shafts) or fail functional (like jaw couplings which will ride metal to metal if the spider gets ripped apart).

 

[GB21]

Hello new guy here.
I thought the couplers might be contributing to the backlash too, I tried a few haha. Mine also had alot of wobble on the x&y axis. I ended up going with the disk type and it fixed the wobble and cleared up about .0005 backlash. It must be in the nuts/screws. My Z is pretty bad...

 

[currinh]

Toolmaker:

Yours does look like a nice setup. I have a PM728 in the shed, and a conversion kit sitting under the bench. The "manual" for the conversion kit is just an assembly drawing though. Since you've done the conversion I'm hoping you can shed a little more light on the process. How much did you need to disassemble to install the kit? Any tips would be greatly appreciated as I start on this journey.

I can't help much with the controller. I'll be using steppers from Automation Technology (425 oz-in X/Y, 920 oz-in Z) all using Gecko G201X drivers. The controller will be LinuxCNC with Mesa 5i25 and 7i76 cards. I mocked up one axis and was able to get stepper motion (woo hoo). So I'm now wiring the controller. I just discovered I need some wire I don't have, shopping Monday. SO, I thought I'd tackle the hardware side.

Any tips would be greatly appreciated. Thanks.

Stay safe.

Hugh

First, a little bit about my setup. It’s a pm-728vt with PM’s CNC kit. G540 running Mach3 on an old desktop using the parallel port. Right now it’s only 2axis with 570oz dual shaft steppers with a sepraty DRO.

 

[currinh]

Well, I went ahead with the PM728 CNC conversion installation. All the parts were in the package, and they fit the kit. I've assembled the X and Y axes, at least have the ball screws and end plates in place. Nice kit.

However, I found the assembly drawing (all the direction that came with the kit) shows M6-1x16 cap screws (part #911) holding the Y front plate to the machine. These screws came with the kit and the counter sinks in the Y plate fit these M6 cap screws. Now the kicker. The original screws, and thus threads in the base casting, are M8! Has anyone else run into a similar problem?

I suppose I'll re-drill/bore the kit's Y plate for the original M8 cap screws. May be a trick to align to the existing holes as the mill doesn't have a functional Y lead screw. Would have been easy if I'd known before taking the mill apart.

Thanks. Stay safe.

Hugh