Rotating Ballnut - design ideas

[m_c]

It's always best to eliminate play, but there are costs to that, as you're discovering.

Most accurate way to machine bearing bores so they're in perfect alignment, is ideally do them in one go, but for that design wouldn't be too easy.
However, as the double row bearing gets clamped in position, you could machine it's bore to allow the bearing to move around a bit, so when you finally assemble it, the bearings get lined up via the shaft, and it's just a case of clamping the doublerow in place.

[wiatroda]

Very nicely done

[Jonathan]

Very nicely done

Thanks :). I hope it works well after all this... I modified irving's spreadsheet to include pulleys and used it to model this setup. Looks like, with the 3Nm motors I'll get at best 7500mm/min rapid and 2000mm/min with 50N cutting force. Both of those are a lot lower than the prediction for the Y-axis, 18100mm/min and 8200mm/min. All of those figures are with a 3x safety factor on the torque, so fingers crossed it might be better!

I'm currently making a mandrel from 35mm steel bar to hold the shaft in the rotary table/4th-axis to mill the pulley teeth on to it.

[wiatroda]

Thanks :) .... mill the pulley teeth on to it.

What belt profile? How wide pulley going to be ? I had to buy 3M HTD pulley recently , but I would give a try to cut my own .

[Jonathan]

I'm using XL belts, purely because I already have lots of XL timing pulleys which will enable me to experiment with the ratios. HTD would be better in theory.

I will use a form tool to cut the pulleys. The profile is a simple trapezium which I have ground on to a piece of HSS. You can make HTD pulleys accurately with a ballnose cutter ... or some people use a drill but I think that's a bit crude.

If you do decide to try it you might find my program here useful:

http://www.mycncuk.com/forums/showth...ating-programs

[Jonathan]

Today I cut the pulley teeth into the first shaft, and whilst that was running made the second shaft.

Pictures:



The belt fits (yay!):



I will cut the pulley teeth into the second shaft tomorrow, then I just need to make the part that connects the shaft to the ballnut, and the ring that pre-loads the bearings...and maybe some bits to act as bearing seals.

[boldford]

. . . . .All of those figures are with a 3x safety factor on the torque, . . . . . .

Was the 3x figure a guestimate or based on based on "real world" experience? Your reply may help me determine if some unlabled steppers I have will work reliably in my first router.

[AdCNC]

Looking good fella!

[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]

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: