Rotating Ballnut - design ideas

[Jonathan]

Thanks for the kind words...

I just mounted one ballscrew to the frame with a couple of G-clamps and some wood, so not the best but it holds. So with one ballscrew and 3nm motor I'm getting at least 12,000mm/min! It might do more but things are wobbling a bit as I have not fixed the X-axis linear rails down to the frame.

With the previous rotating nut setup (see post #1) using M20 threaded rod the best it would do reliably was about 2500mm/min.

The gantry hasn't got the spindle and Y axis screw etc mounted, so it's only about 40kg not 50kg, but still looks very promising. It's odd since for this motor and pulley (28T on motor, 30T on screw) I calculated 7300mm/min.

Will post a photo in a few mins.

[Jonathan]

Video here:

http://www.youtube.com/watch?v=mSNFD...=youtube_gdata

Whipping caused it to stall at 15m/min. That be less of a problem when everything is mounted properly.

Some pictures:





At 8m/min I couldn't stall it by pushing on the gantry. Above that I
can, but only by bracing myself against the wall and pushing with both
hands...otherwise it just pushed me along the floor.

After the video I tried adjusting the acceleration. It went up to 3m/s^2 quite happily.

[Swarfing]

Jonathan interested to know what cutter you are using to cut the teeth on your pulleys?

[Jonathan]

Jonathan interested to know what cutter you are using to cut the teeth on your pulleys?

It's just a trapezium shaped form tool. I ground it by hand on to a piece of 5/16" HSS. I mentioned it in post #37. Can post a picture if you want, but there's really nothing to it...

[Swarfing]

yes please

[AdCNC]

any more news on the ballnut jon?

[Jonathan]

any more news on the ballnut jon?

Not a lot. I made one out of 4 aluminium mounts for the ballscrew from 90mm aluminium bar. Now the blade on my metal bandsaw has broken (don't really know why :sad:) ... I thought all was lost until I tried parting the bar off on the lathe. Quite unnerving having a 500mm long length of 90mm diameter bar spinning round on the lathe, but it went well and actually parted off very easily with my 2mm parting tool. I sawed through the last 10mm or so by hand for obvious reasons. Coolant made a bit of a mess ... it's actually a faster and more economical way of doing it than the bandsaw.

I used dial indicator to get the ballnut as close to on centre as I could. This completely eliminated the whip I had before. Now using 42T pulley on motor (shaft still 30T) instead of 28T I got 18m/min. There was no significant vibration or whipping at this speed, which is 50% higher than the critical speed of the screw in a fixed-fixed configuration (1180rpm)...which would require a pair of angular contact bearings at both ends (or similar). I would have tried more but I was limited by the kernel speed in mach3. That's with the motor on 1/4 step ... I'll try it on half and see what happens.

In the end I will probably stick to just (!) 12m/min since that gets me better resolution and is plenty fast enough. It probably doesn't do the ballnut any good spinning round too fast? Centripetal force may cause the oil to be flung outwards?

I'd love to try this with a 25mm lead ballscrew and really see how fast it would go...or something like this:

http://www.zappautomation.co.uk/r202...347140d1aaa921

Would probably be measuring the feedrate in meters per second with that... would need an encoder to get useful resolution though.

[Jonathan]

I've just found this quote from John S. a long time ago:

One area where spinning ball nuts come into their own is on a CNC
lathe. ... a spin off is if you also fit a bevel gear to the nut block you can then drive this with another bevel mounted on a disengageable handwheel. This way you can
get to use a CNC lathe in manual mode without having to rely on
standing at one end turning the ball screw.

(Source: http://groups.yahoo.com/group/CAD_CA.../message/70038)

Backdriving could still be an issue, but you could add some friction to the handle and/or a lock easily enough to combat that. That's tempted me to convert my lathe ... I could probably still use my design for the router, perhaps with better (tapered roller maybe) bearings as the forces are likely to be greater. The tricky bit will be fitting the ballnut into the cross slide.

[Jonathan]

I have now mounted both X-axis ballscrews, like this:



Interestingly with the second motor the feedrate decreased from 12m/min to 10.5m/min. I think this might be due to me not yet being able to get the second ballnut on centre which is causing the ballscrew to vibrate a bit.

[Jonathan]

Update:

Can't really fault the rotating ballnut mounts. They're rigid and get a plenty high enough feedrate from quite small motors. The oil does keep flying out of the ballnuts, as predicted, so I occasionally put more in and they seem fine.
I should have incorporated dust covers as swarf does land in them. I did cut some aluminium covers on the router to protect the angular contact bearings:




More recently I have made two more mounts, this time for 20mm or 16mm screws and with polycarbonate covers, HTD pulleys, and just generally neater as I made them on the router which gets a better finish than the milling machine: