Ballnuts

[Robin Hewitt]

If you spring the nuts it should be perfectly smooth, it can ride over imperfections without cogging. PITA to do but you get your reward when you put the DTI on it :tup:

[Web Goblin]

Robin,
there is only one spacer in between the two parts of the nut. They look to be the same type as the ones in your photo apart from the spacer being in two halves.
I do like the idea of sprung washers though. This would allow the two halves of the nut to go together tightly enough to work correctly and allow for a little bit of adjustment to align them correctly. I think I will modify them before I go any further with my build.
I am still trying to think up a good idea for replacing the seals/screw cleaners on the nuts. If anyone has any ideas they would like to share :whistling:

Ian

[Jonathan]

I do like the idea of sprung washers though. This would allow the two halves of the nut to go together tightly enough to work correctly and allow for a little bit of adjustment to align them correctly. I think I will modify them before I go any further with my build.

Springs is the standard way to do it. Just make sure that the force the spring applies is greater than the cutting force on your machine, otherwise the backlash will come back to get you...

[Web Goblin]

Jonathan,
thanks for that. What angle of washers did you use? I was thinking of 60 degree ones to give a bit of adjustment space.

Ian

[Jonathan]

thanks for that. What angle of washers did you use? I was thinking of 60 degree ones to give a bit of adjustment space.

I've not done it yet :lol: I'm not sure why you would need adjustment space.

http://en.wikipedia.org/wiki/Belleville_washer might be helpful...

[Web Goblin]

The adjustment space would allow me to align the keyways on both havles of the nut. Say they were screwed together and the keyways did not line up you could rotate the nuts till they did line up and still have enough tension between them to keep the backlash down.

[coredump]

Springs is the standard way to do it. Just make sure that the force the spring applies is greater than the cutting force on your machine, otherwise the backlash will come back to get you...

This is a common misconception. Having a spring force above cutting force is not enough to minimise backslash.
You should use a spring that have a spring constant high enough to give a displacement which is less than backslash.
displacement = Force / spring constant
Let say you have a fixed nut coupled with a spring loaded nut. The spring is preloaded 10mm to give 100N force (k = 10N/mm). If the cutting force is 10N, the spring will retract 1mm, above typical backslash, so in fact you end up like having fixed nuts with clearance between them. It's not backslash defined as having dead gap displacement when changing direction, but circle cuts will gives you ovals in these conditions as spring is compressed during direction change.

Belleville washer are the one to be used, and strong enough to minimise backslash. The best is to use the nut body itself to be the spring, this way you can achieve very high rigidity. Same thing with oversized balls.

[Robin Hewitt]

Surely the preload has to be overcome before the spring can move any further? :whistling:

[coredump]

Yes my deflection example is wrong when the total force applied to the nut is below preload threshold, my mistake.
But having preload rated against cutting force (ie force resulting in the cutter itself) is not enough! don't forget that you have also a motion!
There is force needed to achieve proper acceleration, and this add to the cutting forces.
Once you are above cutting forces, you need to have highest rigidity possible: shortest and strongest spring possible.
Having a very very high preload itself is not always the way to go: higher wear, need more driving power due to loss.

[Jonathan]

Yes of course you need to include all the forces, cutting and F=ma. Taking my machine as an example, mass of gantry is about 40kg, a is 1m/s^2 at the moment (it will do 3m/s^2 but that doesn't help) ... so that means you have up to 40N extra from the acceleration.