[Beezle]

Of course you can. Ball screws are typically at least 90% efficient and as a result they can be easily back-driven. There are only two things besides the ball screw efficiency that fight against back driving. At low RPMs the residual magnetic field caused by the permanent magnet rotor through the air gaps and the stator exhibits a small detent torque. You can easily feel this by manually rotating an unpowered stepper shaft by hand. At higher rotation rates the back EMF will try to prevent any rotation if the winding is electrically connected to an external circuit (including just shorting the wires together).

Ah, thanks. Makes sense to me anyway.

[luv2ride]

I finally threw down and got a new X-axis ball screw. It was a process of elimination, but the fact that I was able to 'push' the table in the X direction about a thou and a half, pointed to the ball screw. All other possibilities had been eliminated: bearing preload, saddle movement.
Results were better than expected:

Old backlash/lost motion: .002
With the new screw in and the six 4mm screws torqued to about 6Nm ( not much ), the gib set loose, backlash was .0001-.0002! Yes that's right, that's tenths! I relaxed the pre-load just a hair, and tightened up the gib; backlash is now about .0003 - .0004. I can live with that!

Adam

That is quite impressive. Did you just replace the screw or the ballnut as well? I'm not sure how Tormach selsl them.

[adamvs]

That is quite impressive. Did you just replace the screw or the ballnut as well? I'm not sure how Tormach selsl them.

It's sold as a unit, screw and ball nut. About $650. The old screw/ball but actually had a rattle to it! It was done. All in all the iron on this machine is solid. Mine's about four years old no, and gets used a lot . To be able to return it to better than factory specs in a few hours is pretty amazing.

[DLawrence]

I finally threw down and got a new X-axis ball screw. It was a process of elimination, but the fact that I was able to 'push' the table in the X direction about a thou and a half, pointed to the ball screw. All other possibilities had been eliminated: bearing preload, saddle movement.
Results were better than expected:

I relaxed the pre-load just a hair, and tightened up the gib; backlash is now about .0003 - .0004. I can live with that!

Adam

Adamvs,
How is the backlash holding up with the new ballscrew and all ?

Thanks,
Dave

[jake hoback]

Sorry I haven't been posting on here, we had a baby and are getting ready to take off to Atlanta for the big knife show. So I have been insanely busy in the shop. I honestly just gave up on the stupid Tormach and just put it to work on very none critical tasks that require very loose tolerances. I may replace the y axis ball screw but I still think its BS that I would need to do that on a machine that is only 2 years old. I'm just fed up with the problems Ive had with the stupid thing in general so I'm gonna get rid of both of them in the near future and get a real machine. These things are supposed to save time, not take up a bunch of my time.

[jake hoback]

So Since Im selling this machine I wanted to follow up on this very old thread and tell y'all how I corrected the issue.

you can still move the table a bit but I was able to adjust the ball screw bearing nut preload and crank down the very hard to reach screws for the ballnut. having done that and replace a bad bearing at the motor end of the screw i got the movment down to a reasonable level that allows factory accuracy.

I have to give it to the Tormach, even if its not the most robust machine in the world, it def is easy enough to work on.