Fiddling with the Lead Screw Nut on a Ball-Screw CNC

      Long description of troubleshooting and improvised repairs for a worn CNC part. September 15, 2011

Question
I have a Sabre 404. Y axis is going into error due to lead nut coming apart in tiny little chunks. It gets tight. We took it off of the shaft, cleaned it up, and put it back on. Worked a bit and no go for long at all. If I have to, I will buy a new screw and nut set. I would prefer not to. Can I buy just a new Teflon nut?

Forum Responses
(CNC Forum)
From contributor J:
My first preference is to replace all faulty parts. You should be able to just replace the nut. Call the fellows at SS.



From contributor D:
I've got a Sabre 408.

If you put any lubricant on the lead screw, it will attract dust and cause a Y axis error when moving to the Y0 position. The design of the nut causes it to tighten up while traveling to the Y0 position.

An easy way to solve this is to run the Y axis back and forth while spraying the lead screw with an aerosol brake cleaner. You can buy this from an auto parts store. Pay special attention to the nut assembly.

With the gantry in the Y48 position, you should be able to reach underneath to the nut. It's spring loaded so you should be able to rotate it against the spring. It should spring back easily. If not, it needs more cleaning.

Then get an aerosol chain lube. Chain lubes are dry, meaning they don't attract dust.
I chased the same problem for a year until I figured out the above solution. You cannot buy the nut assembly from Gerber. I have tried.



From the original questioner:
I too would like to replace faulty parts. I actually bought new bearings for the rails 4 years ago. Just put them in last month. I am busy right now and need the machine to run, run, run. What I can't afford is $1,600 for a new screw and nut set. Machine is about 10 years old.

What we found (friend helping used to repair industrial machinery)... Disconnected the gantry from the nut and moved gantry out of the way. Cleaned the nut with denatured alcohol and moved it back and forth (rinse, repeat...). It got better, but not better enough. After "what if" and "they say" conversation, we determined that broke is broke, so we can't break it more than it is now.

Gerber says you cannot remove the nut or it will come apart. Well we did take it off of the front side. Careful measure and mark. Took it apart and cleaned out grime and gunk. Found some deterioration on the brown Teflon nut (this is an anti-backlash 2 nut system). Cleaned and cleaned, made a special grind tool (bent screwdriver) to get in there and chase the little chunkies out of the damaged thread area. Reassembled and test, test, test (lot of back and forth). Feels great. Put together and run. Woo-hoo! Not.

Take apart, put finer grind on makeshift tool and treat it like a torn tendon. Remove deteriorated chunks of the threads inside the Teflon nut. Basically get rid of any parts that may continue to irritate the clearance between the moving parts. Found a teeny tiny bur on the lead screw. About 1 hour under the machine with fine stone and alcohol rinse, a lot of light and mag.

Things moving real good now (give thanks and say prayer here). Ran machine for about 2 hours after reassembly. Stopped and tested and cleaned a bunch of times. Not the perfect fix, but it is the fix I need in order to track down a new nut assembly.

Who is this SS you speak of?



From contributor B:
There are many ball screw vs rack-pinion threads here. This is what happens to ball screws overtime. If it was equipped with rack-pinion, you would never have this problem.


From contributor J:
Sorry, just keyed in on "Sabre," as in Shop Sabre - but I see it's Gerber. Gerber is known to be a bit proprietary but are they now owned by AXYZ. Parts for the nut cannot be that hard to get; those machines are a workhorse in the sign industry.


From contributor D:
As far as I know, Gerber is not owned by AXYZ. They do have lots of proprietary stuff though. For instance, they don't run G-code.

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