Rotating Ballnut - design ideas

[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 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.

[wiatroda]

looks really good
What's the gear ratio between motor and pulley/ballnut??
Is 10mm screw pitch?

[Jonathan]

It is 1:1 and 10mm pitch, but clearly one could use it with any pitch and to change the ratio just change the pulley on the motor. The mount for the motor is long enough to accommodate different pulleys without changing the belts. It will go faster with a bigger pulley on the motor, but with a 10mm pitch screw there's not much point ... you're just loosing resolution.

[andrewbond]

So I know I may be over simplifying and havnt researched this much but...

Would it be possible, and much simpler to use a hollow shaft stepper/servo and use feed the screw through the motor and mount the motor directly to the ballnut with a flanged hub

Andrew

[JAZZCNC]

So I know I may be over simplifying and havnt researched this much but...

Would it be possible, and much simpler to use a hollow shaft stepper/servo and use feed the screw through the motor and mount the motor directly to the ballnut with a flanged hub

Andrew

Ye that would work great.!! . . . . Now You try finding an affordable motor with a 25mm hollow shaft.!!!!!!