PM-728 Conversion to CNC

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jlchapman

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I'm using the CNC kit from Precision Matthews. Installing the kit for the most part is simple.

But I've never used a precision Locknut for bearings before.

So I have a couple of questions:

1. Does the inner steel go towards the bearing? See picture labeled locknut. I would think so.

2. How much preload? How do I measure that? Gut feel? Preload, Like when you install the locknut on your cars front axle:)

3. Where do you find a wrench for these locknuts? Just a spanner wrench? I looked on Yinsh.com and they warned about using maybe a screwdriver with one point of contact and tightening that way would destroy it.
 

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Well, I'm not sure as I'm guessing. But... I put the shinny (machined) ring towards the bearing. Your second picture shows this shinny ring. It should be the truest face, and also the slight raised inner section should assure only riding on the inner ring (not the seal).

There is no solid stop for pushing the bearings together so it could be too tight. I think it would be hard to over tighten though. The obvious is tight enough to prevent backlash, but not tighter. It may be best to snug them up and check backlash. Then tighten till the backlash "goes away". You could likely measure tightening torque, but not easily. Torque isn't a reliable way to set pre-load though as it depends on finishes and lubrication. But I am still guessing.

I'd think a spanner wrench would work well. Hard to assembly unless you can put the wrench on, and take it off, from the side rather than put over the end of the screw. If you find a spanner wrench that works well for this size I'd like to hear of a source.

Thanks.

Hugh
 
currinh said:
Well, I'm not sure as I'm guessing. But... I put the shinny (machined) ring towards the bearing. Your second picture shows this shinny ring. It should be the truest face, and also the slight raised inner section should assure only riding on the inner ring (not the seal).

There is no solid stop for pushing the bearings together so it could be too tight. I think it would be hard to over tighten though. The obvious is tight enough to prevent backlash, but not tighter. It may be best to snug them up and check backlash. Then tighten till the backlash "goes away". You could likely measure tightening torque, but not easily. Torque isn't a reliable way to set pre-load though as it depends on finishes and lubrication. But I am still guessing.

I'd think a spanner wrench would work well. Hard to assembly unless you can put the wrench on, and take it off, from the side rather than put over the end of the screw. If you find a spanner wrench that works well for this size I'd like to hear of a source.

Thanks.

Hugh
Click to expand...
Thanks Hugh it helps to get a second opinion!

I pretty much came to the same conclusion. I didn't think about the different material, finishes and lubrication.

My only experience with setting preload is on a Ford 9-inch rearend. You use a dial type gauge torque wrench and rotate the pinion.

I bought this spanner wrench from McMaster Carr https://www.mcmaster.com/6975A12/ The spanner wrench is an excellent tool to spin the axis before you have the stepper motor installed.

I'm finishing my bench testing of the electronics today. Tomorrow I should be able to start testing the backlash on the 3 axis. I found a really good articles that tells how to test backlash. https://www.haascnc.com/service/tro...o/how-to/vmc---ballscrew---backlash-test.html

Thanks again,

Jerry
 
Thanks for the tips on spanner wrenches. I'll have to add one next time I order from McMaster, not too much $. I doubt I'll have time to build one before I need it.

Thanks.

Hugh
 
I have since found a better article on backlash in Centroids forum. Check this out: https://centroidcncforum.com/viewtopic.php?f=63&t=1801

This topic also explains how to compensate for backlash in CNC12. It also talk about the Overall Turns ratio calculations.

I did a couple of things on my setup:

1. I removed the Clearpath servo and attached one of the PM728 wheel handles to the ball screw clamp. I adjusted the gibs using the handles. Wanted to make sure it was smooth and not to tight. Then reinstalled the Clearpath servo.
2. Then I measured for backlash. I tightened the Precision lock nut to remove almost all the backlash. Measured again.
3. Then compensated in CNC12 for the backlash that I could not remove.
4. Then measured again.
 
Last edited:
jlchapman said:
I have since found a better article on backlash in Centroids forum. Check this out: https://centroidcncforum.com/viewtopic.php?f=63&t=1801

This topic also explains how to compensate for backlash in CNC12. It also talk about the Overall Turns ratio calculations.

I did a couple of things on my setup:

1. I removed the Clearpath servo and attached one of the PM728 wheel handles to the ball screw clamp. I adjusted the gibs using the handles. Wanted to make sure it was smooth and not to tight. Then reinstalled the Clearpath servo.
2. Then I measured for backlash. I tightened the Precision lock nut to remove almost all the backlash. Measured again.
3. Then compensated in CNC12 for the backlash that I could not remove.
4. Then measured again.
Click to expand...
I have just reached the same point in my 728 to CNC conversion and was stumped by the Radial Locknut. Thanks for starting this thread. It resolved all the questions I had.

I was able to find the specifications of this particular locknut (YSR-M12x1.75P) here: https://www.yinsh.com/product/more-1-7

The maximum torque is specified as 3.5 Newton meters or 31 in-lb.

How much backlash did you have after all your adjustments?
 
sakumar said:
I have just reached the same point in my 728 to CNC conversion and was stumped by the Radial Locknut. Thanks for starting this thread. It resolved all the questions I had.

I was able to find the specifications of this particular locknut (YSR-M12x1.75P) here: https://www.yinsh.com/product/more-1-7

The maximum torque is specified as 3.5 Newton meters or 31 in-lb.

How much backlash did you have after all your adjustments?
Click to expand...
Nice that the thread worked out for you.

My backlash was between .0007 to almost .0015 on the axes.

I may revisit the backlash and torque to the spec. See if the backlash gets tighter. I have a Haimer for a probe and I'll see if the backlash is as well behaved as I measured in real use.

What did you get for backlash readings?
 
jlchapman said:
Nice that the thread worked out for you.

My backlash was between .0007 to almost .0015 on the axes.

I may revisit the backlash and torque to the spec. See if the backlash gets tighter. I have a Haimer for a probe and I'll see if the backlash is as well behaved as I measured in real use.

What did you get for backlash readings?
Click to expand...
Backlash < one thou is pretty impressive. PM classify the ball screw as C5 so I was curious as what it would be.

I have not yet reached the point of being able to measure the backlash. Still waiting for the wrench for the locknuts. I'll report back when I get the numbers.

Up to now I have used a Sherline mill with a X-Y ball screw upgrade (rated C9) and Mach 3. I was never able to get a consistent backlash reading -- it varied from day to day and eventually I turned off the backlash compensation feature altogether. I got better results that way.
 
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