[CNC] Rf 45 Backlash

[Rex Smith]

I'm going to wade in with my first post. I have converted my RF45 to CNC. I've been creating designs with Visual CAD/CAM and cutting them out successfully with Mach3. I'm now working on eliminating backlash in the mechanism. The x-axis has 25 mils of backlash. I've implemented ball screws and there's no backlash I can detect in the nut. I see all the backlash in the thrust bearing assembly. I can reduce this backlash by tightening down the cap that compresses the bearing assembly, but if I get it too tight, the x axis won't move at all. That doesn't seem right. Note that I've attached a pic that shows the assembly I'm dealing with. I'd like to hear from anyone who has experience with this. rex

 

[JimDawson]

Is it possible that the bore for the outside bearing race is bored too deep, thus not allowing the bearing to come up tight in the bore? Or maybe there is other interference in the assembly that won't allow the race to pull up?

 

[Rex Smith]

Is it possible that the bore for the outside bearing race is bored too deep, thus not allowing the bearing to come up tight in the bore? Or maybe there is other interference in the assembly that won't allow the race to pull up?

If the bore was too deep, I wouldn't think that tightening it clear down would seize the axis.
I could analyze it a little better if I could get that reduction gear off. Tried a puller and it is TIGHT. I can put more pull pressure on the gear with the puller, but I'm afraid I'm going to damage the gear.

 

[JimDawson]

There are two types of systems commonly used in lead screws: One puts the entire lead screw in tension by tightening against the bearings on both ends of the table. The other is done with only the thrust system on he motor end of the lead screw and allows the bearing on the other end to float in it's housing. Not sure which you have.

I would go get a bearing puller, it looks like you have room in there to use one. That way you can pull on the center hub and not the rim of the gear.

This is the type I'm talking about. http://www.harborfreight.com/14-piece-gear-puller-set-30305.html

 

[Rex Smith]

There are two types of systems commonly used in lead screws: One puts the entire lead screw in tension by tightening against the bearings on both ends of the table. The other is done with only the thrust system on he motor end of the lead screw and allows the bearing on the other end to float in it's housing. Not sure which you have.

I would go get a bearing puller, it looks like you have room in there to use one. That way you can pull on the center hub and not the rim of the gear.

This is the type I'm talking about. http://www.harborfreight.com/14-piece-gear-puller-set-30305.html

Thanks Jim. It looks like there's only a thrust bearing and a ball nut. I can see that the bearing is sliding back and forth on the shaft, so probably needs to be shimmed a little more against the spacer. Have to get that gear off though, so off to Harbor Freight tomorrow.

 

[JimDawson]

Fastenal sells arbor shims, also Winks or Parkrose hardware might be a good source they have pretty much everything there.

 

[Rex Smith]

Thanks Jim; you've been a great help.

 

[Blackhawk]

Don't know if you got it figured out yet but here is how jobolt helped me out.

Go to post number 100 on my build thread, jbolt really helped me understand what I was happening with his explanation and pics. I know it's not correct but I cut up a beer can for shims lol http://www.hobby-machinist.com/threads/copycat-mill-build.10024/page-5

Here is a video by jumps that really helped also

 

[Rex Smith]

Hey blackhawk, that really helped. I finally understand how that bearing/sleeve stack works and managed to reduce my backlash from 10.5 mils to under 1 mil. I also got some real way oil and the table now moves as smooth as butter.
I do have a question...
- At present, I have too many shims between the bearings, and if I don't tighten the housing down too tight, things are fine. However, these are high quality thrust bearings. I didn't think it would be so easy to tighten the housing to a point to exceed their axial load capacity. If I tighten the housing too tight, I can no longer rotate the shaft. Apparently there's more pressure being applied by the housing than I imagined? If I were an ME rather than a EE, I would probably know how to calculate the force on those bearings due to the ft/lb torque of the housing bolts.
thanks much, rex

 

[JimDawson]

do have a question...
- At present, I have too many shims between the bearings, and if I don't tighten the housing down too tight, things are fine. However, these are high quality thrust bearings. I didn't think it would be so easy to tighten the housing to a point to exceed their axial load capacity. If I tighten the housing too tight, I can no longer rotate the shaft. Apparently there's more pressure being applied by the housing than I imagined? If I were an ME rather than a EE, I would probably know how to calculate the force on those bearings due to the ft/lb torque of the housing bolts.
thanks much, rex

Taking a shim(s) out is probably the way to fix it. Normally trial & error is the best way to set up a shim pack. One thing you could do is to take all of the shims out, reassemble the assembly, lock the table, and put a dial indicator on the end of the lead screw and see what the end play actually is. Then use that number to create the shim pack, plus about 0.002 or so to get the pre-load.