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PM728VT - Success in Progress

Posted: Mon Mar 29, 2021 2:51 pm
by jpmsteadi
Hi all! I have been posting over in the other forum some and figured I would start a thread here to document what I consider to be a success. Still a ways to go and still learning a ton, but have been making parts for three months and been able to basically cover the cost of the conversion through selling those parts, so not too shabby!

The base machine is a Precision Matthews PM-728VT. I am using their ballscrew kit, which fit very well minus one screw size mismatch that was quickly fixed. I did have to make some Nema 24-34 adaptors on the mill manually before starting the conversion as I am using three nema34 motors.

I am sure these motors are way overkill, but I got an ebay deal on them for 250 each (normally about 650). So far I have had zero complaints with them. I am doing 3000mm/min rapids on z and 5000 mm/min rapids on x and y. A link to the motors:

The electronics enclosure is an old pelican case I had laying around. not perfect, but it works well. It used to be used to ship exposed film for NFL films. Pretty cool third life for it!
IMG_20210120_195445_01 (1).jpg
I made a very very bad looking enclosure to try and contain the chips. As I have been avoiding going to the store as much as possible I built it with scraps of wood I had laying around the house. If you look closely you can see parts of ikea chairs and a chest of drawers in there. It looks awful, but works decent. The lexan on the front needs to be turned into a sliding door badly. The magnetic catch on it right now works, but is a real pain.
I am still trying to dial in some lost motion, but based on some reasearch I have done recently I think my gibs are too tight. I am going to adjust them when I get back home and see if that solves the issue. Thankfully most of the parts I am making aren't high tolerance.

I took the dro off of my lathe and put it on the axis to do ballscrew comp on them. That helped accuracies quite a bit. I may have to do it again once I adjust the gibs, but I hope not.
Some of the parts I made in the first month or so. I work in the film industry and these are specific brackets for some gear I use.
and anodized black (a pretty neat process, but certainly took some fiddling to get it down to where I am happy):

Overall I have so enjoyed this experience of learning about CNC and converting my mill to CNC. I am by no means finished with the project either. My current list of things to do:

-New spindle motor - the BLDC motor that PM ships with the mill is just underpowered for doing the cuts I want to do. The nature of BLDC is that it bogs down and the torque tapers off as they spin faster. I would love to have a faster spindle than 4250rpm as well. It worked fine for manual milling, but for CNC....

-Maybe new spindle bearings - I found some high precision NSK bearings on ebay for pretty cheap (new old stock) and have ordered them. As I plan on spinning this spindle higher than it was initially intended I feel like putting in nicer bearing can't hurt. I am also tempted to just send it and see what happens - replace things once they wear out

-get some more TTS tooling. I have some and it works a charm, but having to swap in the drill chuck to do drilling ops is a pain. This is simple; just order more!

Please let me know if you have any questions or helpful hints. I am still new into the CNC world, but have been diving pretty quick!

All the best,

Re: PM728VT - Success in Progress

Posted: Wed Apr 07, 2021 3:49 pm
by cnckeith
fantastic! thanks for posting the photos, great work.

Re: PM728VT - Success in Progress

Posted: Sat Jul 03, 2021 7:25 pm
by jpmsteadi
Another little update, put on the bigger motor, changed the spindle bearings, and it's working beautifully. Been pushing the rapids and have gotten very happy with it all.

Re: PM728VT - Success in Progress

Posted: Wed Jul 07, 2021 11:27 am
by cnckeith
cool! nice looking. thanks for posting the video!

Re: PM728VT - Success in Progress

Posted: Fri Jul 16, 2021 4:51 pm
by spineman
Very nice work! I am considering a conversion of our PM728VT but it looks like a research project that a small commercial shop will not have the time for. The PM works well as a manual mill, but not sure its worth the effort and frustration. Have you found anyone that has a kit? The fact you had to upgrade the motor is a big deal and makes sense. Probably should have bought the larger PM to start? PM was supposed to be offering a completed CNC unit but doesn't mention it on their website any longer. Hats off to you. Awesome work. Maybe I will buy the PM from my company and use it for my home shop.