Rotating Ball nut

[drumsticksplinter]

If you keep the motor mount and ballnut mount as separate parts then it's easy to change the size of the motors

I think that this might be the way forward, I already have 3 x 4Nm nema 23's, which I deemed not powerful enough to spin the ballscrews... Now that option is out of the window then I might look favourably on these again, especially given that I can source the drives much cheaper for them.

They've also used one ballscew on the X-axis, which is pretty poor as it will cause the gantry to deflect/rack when the tool is near the ends.

That is a major concern because even moving the gantry by hand there is some racking and I've tried to create a fair balance for rigidity vs weight! I think now I have a clearer Idea of what I need to achieve with my motors and a rotating nut design...

Would I still have problems with my ballscrews being slightly bent if going for then rotating nut? I think I can straighten them up to almost perfect with a garage press, I know its going to be a long and tedious procedure and making sure I don't make them worse along the way.

[GEOFFREY]

I think you may find the best straightening device would be something like the one we used to use to straighten motorcycle fork tubes. If you don't know what that looks like, it is a bit like a very large bearing puller, but with only two legs at 180 degrees, each with a hooked end to hook over the item to be corrected. Find the centre of each part of the bend and apply gentle pressure to that point, with the leg hooks positioned to suit. The device does not need the legs to be pivoted. I hope that makes sense. G.

[m_c]

Geoffrey, that's essentially the same as using a press with some V-blocks.

[GEOFFREY]

Geoffrey, that's essentially the same as using a press with some V-blocks.

Yes, but with far more control and the screw could be supported on centres or V-blocks to enable regular rotating to check for staightness. G.

It may be possible to carry out that procedure with the ballscrew in place on the machine, something that would be impossible with a press. It could also be helpful to others who have bo access to a press. G.

[Jonathan]

That is a major concern because even moving the gantry by hand there is some racking and I've tried to create a fair balance for rigidity vs weight!

I'd be wary of compromising rigidity for weight at all. It's much more important to have a strong machine than a fast machine.

Would I still have problems with my ballscrews being slightly bent if going for then rotating nut?

The rotating nuts would still work well with a bent screw, however it will shorten the life of the ballnuts since you'll be applying a radial force to the nut which is not something they're designed for. How much it shortens the life depends on how much it's bent and the length, since that determines the force the nut has to apply to straighten it as it moves along. It's hard to quantify. How bent are they?

[drumsticksplinter]

I'd be wary of compromising rigidity for weight at all. It's much more important to have a strong machine than a fast machine.

I don't think I have compromised the rigidity really, the machine will be fairly heavy duty, but I've tried to use aluminium components where I can to help keep the weight down. I agree that the speed isn't as critical as a strong structure.

Well, I had one ballscrew that was very bent, of course when I bought them they were all packaged up, so it was hard to see that they could have been bent. I've managed to straighten the worse one almost completely, but am awaiting loan of a friends press to enable me to straighten it further. I'm wondering if I could get the ballscrews under tension whilst on the machine by tightening both ends, and hopefully pulling it straight?

[Swarfing]

You will have a problem pulling it straight. the only reliable method is to press it out. small amounts at a time, mark and check against a flat surface.

[drumsticksplinter]

You will have a problem pulling it straight. the only reliable method is to press it out. small amounts at a time, mark and check against a flat surface.

Sorry I meant once I had pressed it as straight as I can get it, then put it under tension whilst on the machine and in use.

[drumsticksplinter]

I' ve come up with an idea to house the bearings for this rotating nut idea:



If I went with Jonathan's suggestion of 7207 angular contact bearings with an OD = 72mm, then I could probably machine this housing myself on my lathe. Once bored out I should be left with a 6.5mm wall thickness, do you think this will be sufficient? After all, the main force acting on this setup will be axial. I was looking at the 7008 bearings with 68mm OD, which would leave me with more wall thickness, but I can't seem to find anyone selling them?? Failing that, would I get away with deep groove bearings?? There seem to be a much better size selection.

[Jonathan]

6.5mm would be suffcient and you could use 7007 bearings, but not 7008 for the reasons given earlier. There's no problem in terms of strength from using the smaller bearings, since compared to the bearings you'd use if spinning the screw the ratings are still far greater. Don't even think about deep groove bearings. They're not designed for axial loads. When you put an axial load on one, the balls are forced to ride up on the side of the ring, which isn't ground to such a good finish, so they wear out very very quickly.

How much have you found that spindle mount for? I could probably get a block of aluminium and bore it for you for less money...

Have you thought about how to make the shaft? I'm guessing your lathe is big enough, but bear in mind it needs machining very accurately for the bearings and nut with good concentricity. It also needs thread-cutting for a locknut.