Precision Matthews PM-728VT CNC conversion

[koenbro]

I have a few questions for the base I am about to fabricate:

1. How high do you prefer to have the bed? I am 5'9" and my workbenches are 36" high and I like that. My drill press table is at around 48" (obviously adjusts up and down a few) and like that also for closeup inspection. So I was thinking the bed should be at around 46-48" high. Any advice?

2. Should I add leveling elastomer/rubber feet or just plain plate steel at the bottom and screw them into the concrete slab (with shimming as needed)?

 

[cncjoe21]

How is your PM-728VT conversion going? have you been able to put it all together now? will you be posting videos on youtube maybe?

 

[koenbro]

How is your PM-728VT conversion going? have you been able to put it all together now? will you be posting videos on youtube maybe?

I have decided to reverse my initial intent, and do the conversion first, then fabricate a base and enclosure; I wanted to see how wide the x and y extensions are, and where I can mount the coolant tank and its pump.

So I took the mill completely apart, while it was still on its pallet. Because I have never used a mill (or even seen one up close), it took a moment (and some googling) to figure out that the gibbs are asymmetric, and by chance I wedged it tight as I moved the table sideways in an attempt to remove it. Luckily I didn't break or chip anything. Removing the lead screws was easy on axes x and y, but a bit more challenging on the z-axis, because of the 90 degree gears. You have to remove the set screws for the lower gear and a retaining clip on the upper gear before the screw can be removed.

 

[koenbro]

So after I took it apart, I hoisted everything on my workbench and started adding the components.

Installing the CNC screws was easy. The factory instructions are very minimal (basically one drawing), but interestingly enough, sufficient. It would have been nicer of course to have step-by-step instructions for somebody inexperienced like me, particularly on how to secure the bearing-retaining precision nuts. On the first try, I had a large backlash on axis z, so had to remove/reinstall it.

 

[cncjoe21]

I have decided to reverse my initial intent, and do the conversion first, then fabricate a base and enclosure; I wanted to see how wide the x and y extensions are, and where I can mount the coolant tank and its pump.

So I took the mill completely apart, while it was still on its pallet. Because I have never used a mill (or even seen one up close), it took a moment (and some googling) to figure out that the gibbs are asymmetric, and by chance I wedged it tight as I moved the table sideways in an attempt to remove it. Luckily I didn't break or chip anything. Removing the leadscrews was easy on axes x and y, but a bit more challenging on the z-axis, because of the 90 degree gears. You have to remove the set screws for the lower gear and a retaining clip on the upper gear before the screw can be removed.

Installing the CNC screws was easy. The factory instructions are very minimal (basically one drawing), but interestingly enough, sufficient. It would have been nicer of course to have step-by-step instructions for somebody inexperienced like me, particularly on how to secure the bearing-retaining precision nuts. On the first try, I had a large backlash on axis z, so had to remove/reinstall it.

thanks for the info. I have basically done the same. I have replaced all screws and put it back together. Also got my Acorn controller kit but haven't got the motors yet. How do you like the Clearpath servos? I would appreciate it if you provide me a complete list of your electronics items I see in your pictures when you have some time. I'm a CNC machinist but need to learn more about electronics and components.

 

[koenbro]

The shaft couplings packaged in the PM CNC kit are a weird choice. All three leadscrews have 10mm ends, so one side of each coupling is 10mm. The other side is however 8mm on two of them (for the NEMA 23 motors for axes x and y), while the third was 14 mm (presumably for the NEMA 23 motor on the z axis. I'd wager that most motors in the US have inch shafts; mine are 3/8" and 1/2", so I ordered from McMaster Carr the matching ones (2401K14 for 3/8" and 2401K15 for 1/2"). This set me back $147 after I paid ~1K for the conversion kit, so it is not a trivial amount. I think it's odd from PM to force unto the customer metric couplers that they will probably have to throw away; they could offer a pull-down menu with the right choice, for example, or give the options to not include the couplers in the order). Also I think PM should provide more info on the screws (such as end dia 10mm, pitch 5mm, length, etc) which are needed to calculate motor size etc.

FWIW these are the useless halves:

 

[DavidR8]

So after I took it apart, I hoisted everything on my workbench and started adding the components.


View attachment 356590

Installing the CNC screws was easy. The factory instructions are very minimal (basically one drawing), but interestingly enough, sufficient. It would have been nicer of course to have step-by-step instructions for somebody inexperienced like me, particularly on how to secure the bearing-retaining precision nuts. On the first try, I had a large backlash on axis z, so had to remove/reinstall it.

Not to hijack the thread, but curious what CNC router that is in the background.

 

[koenbro]

Let's talk about sensors.

My Avid CNC Router Pro has Pepperl + Fuchs induction proximity sensors that stop the motors before they can crash. That is what I want to replicate on the mill setup. I am not there yet, but making some progress.

I know nothing about sensors and made a few mistakes I want to share here. First, I ordered some nice Contrinex NO (normally open) PNP sensors off Automation Direct (by the way, I highly recommend automationdirect.com for their high quality products, prompt shipping, great literature, and superb customer service). These don't work, probably because they are PNP; not sure, have returned them.

Then I used some cheapo eBay NO NPN sensors that do work, however there are many reports on the internet about failures after a few months, so I do not want to go the cheapo route. I also wanted to use NC (normally closed) sensors so if there is a wiring issue, they trip and thus alert you of their failure.

After reading up some more, I ordered these sensors (NC NPN), which work.

The way you have to wire them for a normally closed circuit in the Acorn is that the white wire goes to the Acorn input (black wire is unused; brown and blue are as in the diagram):

The factory literature is here, and is useful.

 

[koenbro]

Not to hijack the thread, but curious what CNC router that is in the background.

It's the Avid CNC RouterParts 4824 Pro, which I use with the spindle for wood and sheet aluminum, and I also have the water table for plasma.

 

[koenbro]

There was no easy way to attach the sensors to the casting; tried to drill a hole, but I learned very quickly that that's not going to work. So I decided to reuse the tapped holes for the locking levers on the saddle. I designed a simple bracket in Fusion.

Then cut in on the router out of scrap 1/16" 6061.

Here it is mounted on the mill. The left sided sensor is already on; have added the second (+ limit ) sensor since.

I plan to add two sensors for axis Y and maybe one for axis Z (upper limit). Still haven't figured out how and where I mount the y-axis sensors, as I want to use existing holes in the casting.

More importantly, will need to figure out how to configure the Acorn to stop the motors. Right now, it shows on the screen that the sensor is activated, but the motors do not stop.