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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: https://www.teknic.com/model-info/CPM-SDHP-3421P-ELN/

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.
IMG_20210215_153701_01.jpg
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.
IMG_20210303_095411_01.jpg
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.
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and anodized black (a pretty neat process, but certainly took some fiddling to get it down to where I am happy):
IMG_20210221_160429_01.jpg

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,
JPM

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.

Re: PM728VT - Success in Progress

Posted: Wed Nov 24, 2021 10:36 am
by AlanTerry2
jpmsteadi wrote: Sat Jul 03, 2021 7:25 pm 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.

What motor and drive did you switch to? I am in the middle of the same conversion and would like to upgrade the motor as it is already shutting down while manually feeding? Thank you for the help in advance.

Alan Terry

Re: PM728VT - Success in Progress

Posted: Mon Dec 20, 2021 1:59 am
by suntravel
spineman wrote: Fri Jul 16, 2021 4:51 pm 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.
The conversion of these type of benchtop mills ist pretty easy. Done it for my hobby shop in the basement, because i can not get big machines downstairs. For a pro shop i would not do it. Way better to get an old Mori, Fadal or Haas VMC and retrofit it with Centroid CNC

Regards

Uwe

Re: PM728VT - Success in Progress

Posted: Thu Jan 20, 2022 8:19 pm
by jpmsteadi
I have been pretty busy in my small amount of freetime making parts on the 728 conversion. I am really feeling ready to graduate to a "real" VMC, but have zero space to do it so far. We are shopping for a house at the moment, but that is a crazy other story.

I have a bit off work right now and decided that, if I am going to be stuck on this mill for awhile longer might as well do a couple upgrades. First on the list was a solenoid valve so CNC12 can control the mist coolant. One less switch for me to have to flip. That was easy.

The second thing on the list was a power drawbar. Changing tools is the most cumbersome part of making parts on this mill. I have the TTS tool holders and have been using a battery powered impact driver, but that get tiring reaching up over my head every time.

As I had already changed to a bigger motor I couldn't fit a traditional double or triple pneumatic cylinder directly over the drawbar. I opted for a lever arm design that offsets a single 20mm stroke 100mm bore cylinder off the front of the spindle head and uses a 3.5:1 ratio lever to actuate the drawbar. 2 days of design and 3 days of making parts (including two runs to mcmastercarr) and here I am:
IMG_0359.jpg
I did a couple of test cuts and didn't have any pull out so far. It will remain to be seen if this is final or not.

Check out a video of how easy it is here:

Re: PM728VT - Success in Progress

Posted: Thu Jan 20, 2022 8:42 pm
by cnckeith
cool! nice work!

Re: PM728VT - Success in Progress

Posted: Thu Feb 03, 2022 9:58 am
by DesertRider
jpmsteadi wrote: Thu Jan 20, 2022 8:19 pm [...]

As I had already changed to a bigger motor [...]
Can you link to what motor did you use? Thanks.