Gene Weber's PM-728VT CNC Conversion

[gene_weber]

I'm using time off over the holidays to work on my PM-728VT CNC conversion. Koenbro’s conversion thread provides a wealth of information that I’m relying on. So, I thought I’d document my conversion here as well to perhaps help someone else. This conversion uses PM’s Ultra Precision CNC Ballscrew Kit.

I tried to do this conversion as inexpensively as possible, while maintaining a high level of precision and quality. Some of that's subject, and obviously there are many trade offs. I'm maintaining a spreadsheet of almost everything I purchased for the conversion, and will publish that when the project is complete.

I'm a Linux person, and any card that requires Windows is a non-starter / non-negotiable. So this conversion will use Linux-CNC and a Mesa FPGA card with a Raspberry Pi 4B. To be honest I have a lot to learn about Linux-CNC, but I'm a computer scientist, so not too concerned.

I justified this purchase for the resto-mod of a Triumph Spitfire that I’ve started. The goal is to install a non-stock engine. Who knows how many parts I might need to make. Ha ha, I'm already $8K into this CNC mill. Obviously I could pay someone to make more custom parts than the Spitfire will ever need for that much money. The reality of this purchase is it's a hobby unto itself. I plan to retire in about a year and a half. I hope to spend many hours with this mill making things. I really enjoy pretty much any creative project. My wood shop, and wood working skills are decent, so time to setup for metal and learn some metal working skills. Actually I may use this mill on wood as well. Right now I know zero about running a mill. The adventure begins.

First question I have. What lubricant should be used on the ball screws? I asked PM and they didn’t have any recommendation.

 

[gene_weber]

The mill was purchased this summer and temporarily set up in the garage. Here is is torn down and moved into my basement. The manual provides enough information for the disassembly. I could carry everything down the stairway except the main casting. Brought that down using a dolly with the help of my college age son.

 

[gene_weber]

Teknic Clearpath servo motors are being used. A CPM-SDSK-3421P-RLS for Z axis and CPM-SDSK-2321S-RLS for X and for Y. These have the additional shaft sealing, but not the enhanced machine resolution of 0.00003" vs. 0.00025". Personally I don't think the mill has that sort of precision, so paying for that enhancement seemed a waste to me. If I get this to 0.001 accuracy I'll be thrilled. I also purchased Teknic's IPC-5 power supply. I spent a lot of time (too much) considering purchasing a cheap 1000W 75V switching supply, and making a circuit board to protect the supply and deal with counter EMF generated by the servo motors. I'm pretty sure it would have worked just fine, but the cost of the additional circuitry would have narrowed the price difference. Then there's the time to build and debug the circuit board. And if it didn't work as expected, more time, more cost. In the end it was becoming a matter of (I can do it) pride instead of practicality. I am adding a 25 CFM fan to the IPC-5 supply to get the full 500W.

 

[gene_weber]

Here are some photos detailing the removal of the Z axis threaded shaft. This was the only part of the disassembly that could use a little more info. Koenbro did point out that the clip ring on the horizontal bevel gear needs to be removed. I still struggled with it a bit, because I didn't realize that the gears have two set screws, not one, Doh!

Assembly removed from the main casting for visibility. Snap ring on the horizontal gear must be taken out of its groove. The loosen BOTH set screws, and the horizontal shaft will slide out. Loosed the nut on the vertical shaft as shown. Loosen BOTH set screws on the vertical gear and TAP on the nut with a hammer with a block of wood on top of the nut and the vertical shaft will drop out.

Access to loosen the two set screws on the vertical shaft.

 

[gene_weber]

The directions for installing the Ultra Precision CNC Ballscrew Kit are just a single exploded diagram. So the first thing I did was count and label parts.

All of the screws are grade 12.9, and you can look up the correct torque amount at numerous sites on line.

 

[gene_weber]

A YINSH YSR-M12x1.75P precision bearing lock nut is used on each of the ballscrew shafts. This thread explains installation direction and torque (31 in/lbs). NO, See post #12. I'm not big on buying a tool that I'll likely never use again, so I wasn't keen on buying a wrench for these lock nuts. Instead I make one out of a cheap old 14mm socket I had from days when I couldn't afford quality tools.

I put some masking tape over the socket and marked the location of the locknut channels. Took of some material with an angle grinder. Did most of the work with a Dremel tool with a abrasive cutoff wheel. Touched it up with a file.

Ideally one would use a deep wall socket, but this is what I had lying around. Fortunately it doesn't go too far on the shaft. But I did have to drill out the socket so the 10mm shaft could go deeply into it. There was just enough of the socket "square" left for the drive to turn it.

 

[gene_weber]

Again, I'd be interested to hear what people are using for ballscrew lubricant?

BTW: Once the bearing locknut to torqued to 31 in/lbs, it takes some force to rotate the ballscrew shaft. I guess this is OK, but it's tighter than I expected it to be. NO. See post #12

 

[gene_weber]

Note that the Z axis motor base is not symmetric. I assumed it was until I realized that the Z axis coupling was not flush with the casting finished face. Then I took a closer look and spun it around the correct way.

 

[gene_weber]

After much searching for a metal box to house the power supplies, electronics, and wiring, I stumbled upon a steel box of the appropriate dimensions on Craigslist for $15. Score!!

Even though the painted over flower decals are pretty sweet, the door opens the wrong way, and I want a plexiglass door so I can see inside. So I took an angle grinder to the welds and removed the door.

 

[gene_weber]

I built a 6' long stud wall for the mill area. It provides a place to hang a cabinet behind the mill to store mill parts and accessories, a place to hang the electronics box, and a place to put a laptop shelf. The laptop is my son's old HS/College laptop that I wiped and setup to dual boot Linux Mint and Windows 10. Yes, sometimes one is forced to use Windows.