Linear Motion Double Ball Nuts

[JimDawson]

That seems like a lot of deflection, but it may be OK anyway. I would think that the biggest problem might be tool chatter.

With those motors, I assume that they max out at about 7 amps or so, I would set the running current at max, and cut back the idle current. Just as a side note, those motors will get hot after running for awhile, 150° F + is not uncommon.

I expect that the non-cutting torque requirement will drop as things seat in a bit with use. Try the test hole, it might be fine as it is. There is no point in chasing your tail until you know where you're at.

 

[TomS]

That seems like a lot of deflection, but it may be OK anyway. I would think that the biggest problem might be tool chatter.

With those motors, I assume that they max out at about 7 amps or so, I would set the running current at max, and cut back the idle current. Just as a side note, those motors will get hot after running for awhile, 150° F + is not uncommon.

I expect that the non-cutting torque requirement will drop as things seat in a bit with use. Try the test hole, it might be fine as it is. There is no point in chasing your tail until you know where you're at.

Thanks Jim. My X and Y axis drivers are rated at 7.8A max and the Z axis is rated at 7.0A. The motors are rated 3.5A/phase for the X and Y and the Z is rated 8.0A/phase. X and Y driver settings are 4.9A peak and 3.5A RMS. Idle current is set at half current. The Z axis is set at 7.0A. I don't see a switch for setting the idle current.

I ran a break in routine for about 1-1/2 hour and the motors didn't skip a beat. We're on the same page about not wanting to waste time trying to set the gibs to the absolute minimum clearance. I'll cut a test hole and see how round it is then adjust from there, if necessary.

Tom S.

 

[TomS]

I was able to spend a few hours in the shop today. Set up and pocketed a couple of 1-1/2" diameter holes in a piece of scrap 3/8" aluminum. I am pleased to say that both holes are round within .0004". The double ballnuts did the trick!

Tom S.

 

[JimDawson]

Sounds good!

 

[jbolt]

What was your final pre-load on the AC bearings?

 

[brav65]

Using the Bridgeport method for adjusting gibs I was able to reduce the X axis table deflection to .0035" (Bridgeport states .001" is the target). Any tighter and the 1600 oz. in. stepper motor stalls. Having never measured my table deflection before I'm not sure if .0035" is as good as it gets but it does seems a bit too much.

I've played with the velocity and acceleration settings and the motor seems happy at velocity of 150 and acceleration of 50. Base setting was 100 velocity and 18 acceleration. Could the motor stalling be a current setting issue? Or I could leave it as is and test cut a hole to see how round it is.

Tom S.

You could adjust the voltage going to the motor if your board allows for that.

 

[TomS]

You could adjust the voltage going to the motor if your board allows for that.

My drivers have current adjustment capability but not voltage adjustment. To my knowledge the breakout board has no voltage adjustment feature.

What was your final pre-load on the AC bearings?

For the X and Y I ended up with .003" - .004" preload. My method is a bit different in that I assembled the bearings and housings on the machine then measured backlash using a dial indicator on the table and saddle. Comparing the dial indicator reading to the Mach3 DRO I determined my backlash. I then added .003" to .004" to the backlash reading to come up with my shim pack thickness. Worst case is I trash the Chinese bearings.

For info I measured the bearing housing depth, the cover spigot length, and the bearing widths but wasn't able to get consistent measurements. It wasn't my measurement technique it was the quality of the machining on my Chinese bearing housings.

Edit: still have about .003" of Z backlash that I need to address. Using the method I described above I'll add .006" to .007" to the shim pack. I'm traveling for the next 6 weeks so will attack it when I get back home.

Tom S.