Replacing a CNC Ball Screw

      When a CNC's ball screw fails, you can get an aftermarket part custom-built but is it worth the hassle? June 30, 2007

Question
We recently found out that the ball screw for the table on our late 90's Komo router is shot and needs to be replaced. A new one from Komo can run $10,000 and I was wondering if anyone else has encountered this situation and if there are any other options, i.e. rebuilding the screw or purchasing it from an outside manufacturer?

Forum Responses
(CNC Forum)
From contributor J:
You need to find out what mm diameter it is, such as 40mm, 63mm, etc., and the pitch. Measure the overall length; most machine shops can machine the ends and you should be able to get one. Then you will have to have it checked with a laser for accuracy once installed and tensioned. Probably a Star, NSK or THK. Replace the ballnut at the same time. Some leadscrews have an extra set of leads you can use when the original wears out. That was what we used on the DMS routers.



From contributor C:
Seems pretty soon to have worn out a ball screw! We've got some 20+ year old machines that have run 2 and 3 shifts without significant wear on the screws. Obviously if it's bad, you're going to have to fix it, but I'd sure want to know what the cause of all that wear is and correct it before installing a new one.

There are companies that rebuild ball screws and can bring them back to original spec, assuming that they aren't too far gone. It's been many years since I've looked for that, so I can't give you any names, but I'm sure if you Google around a bit you'll find some. The question to ask with a rebuild is, can you stand the downtime required to remove it, ship it off, have it be in their shop for days or weeks, ship it back and then re-install it?

To buy a new one from a third party may save you some money but could be tricky. If it were me, I'd want to be able to go to the original manufacturer of the screw to be sure that I'm getting the exact same spec (pitch, diameter, etc.) as the original. Then you will need to supply them with specs for the additional machining such as bearing fits, supports, servo couplings and so on. If you don't get it all right, you may end up with a screw you can't use.

The advantage to a rebuilt unit or a new one from a third party is obviously the potential cost savings. The disadvantages are the time you will invest in the project, lost production time, and the risk that it won't go well.

The advantage to a new unit from Komo is that you know it will be the right part and will work as new or you don't pay for it until it does. The disadvantage is higher cost. In our case, we'd probably swallow hard and buy the new one from the machine tool manufacturer, in this case Komo. We'd lose more in lost engineering and production time than would be saved trying to source it ourselves.

In the end you'll have to make the call based on your level of in-house expertise, production needs and risk tolerance.



From contributor R:
How did you determine that the ball screw is bad? Did you do a backlash test? Have you tried adjusting the backlash in the ball nut? If it really is the ball screw, they can be remanufactured.


The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor B:
We recently had a 20 ft. long Ball Screw failure due to a crash on our Hein CNC Router. We were told two months lead time for a replacement unit. In a bind, we sent the old Ball Screw to Wedin International in Cadillac, Michigan and they repaired the entire unit in three working days! We have been running trouble free since re-installing it in a few months ago and we are very satisfied with our results.



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