Precision Matthews PM-728VT CNC conversion

4th axis
Considering adding a 4th axis. The Acorn has room for the 4th, and I will just buy another Clearpath, but where do I even begin with the hardware?

Would I just get a dividing head and mount the servo unto it? Please point me in the right direction.
 
Depends on what your needs are :) I used a rotary table and stepper on my mini mill. Have not added one to my PM-728.
 
koenbro said:
4th axis
Considering adding a 4th axis. The Acorn has room for the 4th, and I will just buy another Clearpath, but where do I even begin with the hardware?

Would I just get a dividing head and mount the servo unto it? Please point me in the right direction.
Click to expand...
Here’s the issue with 4th axis units. Unlike getting ballscrews to replace lead screws for CNC, most rotary tables (worm gear) have pretty poor backlash that you can’t really get rid of. Also they have poor rigidity when unlocked since that really isn’t how they are designed to be used. That limits you to using them in an indexing fashion rather than being really great for full interpolated rotary motion. Or interpolated rotary with lots of backlash comp, and easy to machine materials


Alternate solutions include low backlash planetary gear heads (face mount) which is $$$ or harmonic drives $$$$. Both of which need to be fitted into custom enclosures. These have low enough backlash to be used for full rotary milling.

I have a big servo planetary gear head and a Yuasa super spacer body that I hope to put together to make a beefy rotary.

1638423546619.jpeg


Just my personal opinions. I made a 4th axis from a Grizzly 4” rotary and it worked but was never accurate enough to do any real precision work.
 
On my Mini-Mill CNC conversion (Mach3), I used a rotary table with a stepper motor and is turned out "decently accurate" and reliable for my purposes, very little backlash. Now that I have the PM-935, I've built a stand-alone position driver (Arduino) for it and that works very nicely manually operated. It is small I admit, but it serves my purposes and see no reason why a larger rotary table cannot be adapted likewise.
Here it is on my Grizzly CNC Mini-Mill (Mach3 driven):


...and here it is setup for use, stand-alone on the PM935 (Arduino driven):
Rotary table drive.jpg
 
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Cletus said:
On my Mini-Mill CNC conversion (Mach3), I used a rotary table [...]
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Wow looks great in the video. The precision of those slots in the tube looks adequate for my skillset and expectations.

@Cletus and everyone else: What rotary table are you using or recommending? Will an dividing head like the PM BS-0 work in this setting or are they incompatible?

I looked at the Grizzly 4" rotary that @macardoso mentioned and looks pretty good but if it is reasonable, would rather not get both a dividing head and a rotary. One advantage of the rotary though seems to be its lower height. Can't even figure out how much if any room is left in the z-axis once you mount a dividing head and turn it to point up.
 
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Yes, those units from LMS lend themselves to CNC conversion nicely.
 
Cletus said:
Yes, those units from LMS lend themselves to CNC conversion nicely.
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So those rotary tables have low enough backlash for CNC use? Or do they still need a lock function to prevent motion when the stepper motor is holding still?
 
As always, you guys are great -- thanks for the practical discussion.

I went ahead and ordered the LMS 4" precision rotary table that @jimrk linked to above. The reason I went with the LMS rotary over the PM BS-0 dividing head, as I initially intended, is the z height. The PM solution has the clear advantage of angular adjustment but am not convinced that with my z-axis limitations that's worth it. Also ordered the tailstock matching this table, and the 3-Jaw 3" lathe chuck. Have never used a lathe chuck (or a lathe :) ) so I am very excited to try it/them out. Wanted to also get the compatible dividing plate set but they are out until late January.


Am very tempted by the elegant, self-contained and stand-alone solution @Cletus has shown above, because I think I can use it as a welding positioner as well. What motor and controller are you using, @Cletus ?
 
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