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Check Backlash
- Thread starter Blackhawk
- Start date
- Joined
- Jul 2, 2014
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- 7,594
There will always be backlash in a lead screw. 6 thou is not excessive. You have to learn to deal with the backlash. It appears that the backlash is between the lead screw and its nut. Is there an adjustment for that? Sometimes there are double nuts, sometimes an adjustable split nut. I am not familiar with your machine, maybe others are...
- Joined
- Jul 2, 2014
- Messages
- 7,594
OK, definitely should not have backlash with the ballscrews, would have to be at one of the mountings.
Edit: I checked the video again, and you did say ballscrew near the end. My bad...
Edit: I checked the video again, and you did say ballscrew near the end. My bad...
Last edited:
- Joined
- Apr 30, 2012
- Messages
- 2,401
I have made a couple videos on how I adjust the gibbs and backlash on my machine that may help.
Steve
http://www.hobby-machinist.com/threads/cnc-from-sketch-to-part-the-way-i-do-it.19633/page-5
Steve
http://www.hobby-machinist.com/threads/cnc-from-sketch-to-part-the-way-i-do-it.19633/page-5
- Joined
- Feb 1, 2015
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- 9,994
I have a Tormach CNC. According to the manual, you can expect about .0004" of backlash in the ball screw nut. Additional sources for backlash are the bearing preload and lost motion due to flexing of the machine. An example of lost motion. If you tighten the gibs up to remove play, at some point the friction will increase fairly rapidly. When ever you apply force to a system, there will be some flexing taking place. This can be in the frame itself twisting of the leadscrew, taking up play in the other axis ways, etc. It is more prevalent for smaller machines than large industrial machines. with massive castings.
You can check lash in the leadscrew system be backing the gib for that axis off. This will minimize the force required to slide the table. Use a dial indicator to measure the actual displacement as compared to dial reading or DRO reading. Run the table in both directions. The lag in the indicator reading as you change directions or the difference in DRO reading for a table position as defined by the indicator should fairly well be due to the leadscrew. You should then adjust the bearing preload to minimize this lash. You do need to be careful not to overly tighten to preload as it will increase friction and wear in your bearings. I had a previous post regarding preload adjustment by measuring torque. http://www.hobby-machinist.com/threads/measuring-ball-and-lead-screw-torque.37492/#post-320384
As you tighten the gibs, you will see the the apparent lash start to increase. You are starting to see lost motion due to stretching, compressing, and twisting metal. A good adjustment for the gibs is the point where you start to see the rise in backlash. Again, on a smaller machine, they are not rigid enough to completely eliminate backlash. I was able to get lash down to about.001 on the Tormach.
You can check lash in the leadscrew system be backing the gib for that axis off. This will minimize the force required to slide the table. Use a dial indicator to measure the actual displacement as compared to dial reading or DRO reading. Run the table in both directions. The lag in the indicator reading as you change directions or the difference in DRO reading for a table position as defined by the indicator should fairly well be due to the leadscrew. You should then adjust the bearing preload to minimize this lash. You do need to be careful not to overly tighten to preload as it will increase friction and wear in your bearings. I had a previous post regarding preload adjustment by measuring torque. http://www.hobby-machinist.com/threads/measuring-ball-and-lead-screw-torque.37492/#post-320384
As you tighten the gibs, you will see the the apparent lash start to increase. You are starting to see lost motion due to stretching, compressing, and twisting metal. A good adjustment for the gibs is the point where you start to see the rise in backlash. Again, on a smaller machine, they are not rigid enough to completely eliminate backlash. I was able to get lash down to about.001 on the Tormach.
Thanks for the replys, I backed off the gibs till sloppy, backed off angular contact bearing and tightened till play was removed, still getting same results of .002 play in ball nut.
Steve I got the same setup as you, wonder if I can repack with larger balls?
I've tried backlash compensation but it does weird things, slows way down then takes off
I'm using regular motor oil and temp is about 63 in the shop, wonder if this creates sticktion problem.
I'm using heavy duty red grease in ball nut, maybe something thicker?
Lanham
Steve I got the same setup as you, wonder if I can repack with larger balls?
I've tried backlash compensation but it does weird things, slows way down then takes off
I'm using regular motor oil and temp is about 63 in the shop, wonder if this creates sticktion problem.
I'm using heavy duty red grease in ball nut, maybe something thicker?
Lanham
- Joined
- Jun 15, 2013
- Messages
- 1,199
download a prototrak retrofit manual, it tells you how to properly set the gibs on a cnc machine. I will explain
table all the way to the right, place an indicator on the knee way, mag base, put the tip on the table, on the left, make sure the holder is rigid, now push on the right side of the table, and adjust the gib until about .001 is all that is recored, do the same with the saddle gib, placing the mag base on the saddle this time and the tip on the knee way, I know you don't have a knee but i guess its the base on yours. that's how i set my mill up. if your machine is worn then you must set it up with the table in the middle of the travel.
another source of lost motion is the ball nut mounts, on knee mills this would be the yoke. mine flexes
table all the way to the right, place an indicator on the knee way, mag base, put the tip on the table, on the left, make sure the holder is rigid, now push on the right side of the table, and adjust the gib until about .001 is all that is recored, do the same with the saddle gib, placing the mag base on the saddle this time and the tip on the knee way, I know you don't have a knee but i guess its the base on yours. that's how i set my mill up. if your machine is worn then you must set it up with the table in the middle of the travel.
another source of lost motion is the ball nut mounts, on knee mills this would be the yoke. mine flexes
Thanks for all the help
Replaced motor oil for ISO 68 on jibs
Re packed ballscrew with lithium grease
Readjusted jibs according to jumps (steve) video
X axis play .0015
Y axis play .0045( still looking at ways to make it better)
Followed hoss video on how to properly set backlash compensation, I now have no noticeable play in x or y
Lanham
Replaced motor oil for ISO 68 on jibs
Re packed ballscrew with lithium grease
Readjusted jibs according to jumps (steve) video
X axis play .0015
Y axis play .0045( still looking at ways to make it better)
Followed hoss video on how to properly set backlash compensation, I now have no noticeable play in x or y
Lanham