Rotating Ballnut - design ideas

[Jonathan]

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

Yes that method is used on some expensive commercial laser cutters. They use large servomotors with a hollow shaft containing the ball-nut running on a tensioned ballscrew. They get feedrates measured in meters per second.

There are few stepper or servos available with hollow shafts. The common ones have too little torque or the bore is insufficient. Anything above 16mm would be useful. It may be possible to dismantle an existing motor, machine a new shaft with the hole through the middle and use bigger, angular contact, bearings. For the size of my machine (1.7*0.74*0.4m) it's just not worth it as the 3nm steppers can already run it more than fast enough.

The advantage with using a standard motor, other than simplicity, is the timing belt allows you to change the drive ratio and reduces resonance. Currently mine is on 1:1 ratio, however it does go faster with a larger pulley on the motor. I elected to leave it at 1:1 however as 10m/min is plenty and I prefer to keep the resolution than get 15m/min+, but at least I have the option which wouldn't be the case with a hollow motor. In short I'm saying you have to be very careful with the selection of the motor, which makes it even more difficult to find a suitable one.

[Gary]

This is something we are developing, but it will not be for hobby use.
It will start with a servo system and the 2525 and 3232 ballnut,
We will then be moving to the 4040 and 5050 ballnut.

The only steppers that could have been used for this design were the IOS motor from IMS, but these are not made anymore.

Yes that method is used on some expensive commercial laser cutters. They use large servomotors with a hollow shaft containing the ball-nut running on a tensioned ballscrew. They get feedrates measured in meters per second.

There are few stepper or servos available with hollow shafts. The common ones have too little torque or the bore is insufficient. Anything above 16mm would be useful. It may be possible to dismantle an existing motor, machine a new shaft with the hole through the middle and use bigger, angular contact, bearings. For the size of my machine (1.7*0.74*0.4m) it's just not worth it as the 3nm steppers can already run it more than fast enough.

The advantage with using a standard motor, other than simplicity, is the timing belt allows you to change the drive ratio and reduces resonance. Currently mine is on 1:1 ratio, however it does go faster with a larger pulley on the motor. I elected to leave it at 1:1 however as 10m/min is plenty and I prefer to keep the resolution than get 15m/min+, but at least I have the option which wouldn't be the case with a hollow motor. In short I'm saying you have to be very careful with the selection of the motor, which makes it even more difficult to find a suitable one.

[D-man]

Hi Jonathan, any chance you could knock 2 of these units up for me?

Im running the 2020 ballscrew and need someway of using a screw of 3000mm length without whipping and this looks ideal

[Jonathan]

Hi Jonathan, any chance you could knock 2 of these units up for me?

Im running the 2020 ballscrew and need someway of using a screw of 3000mm length without whipping and this looks ideal

Yes I can, depending on how soon you need them.

A rotating ballnut would be ideal with the a 2020, 3000mm long screw. Since (I gather) this is a woodworking machine, you would probably want to gear it 2:1 so the gantry moves 10mm for every motor revolution.

Here's one I made recently, version 3 I suppose. The significant change is using sealed angular contact bearings (turns out they do exist) and threading the shaft to enable a pair of locknuts to pre-load the bearings.



Will post a video when I have time.

[D-man]

I would like it to cut Ali is there a reason you think it's just a woodworking machine? What have I done wrong lol?

If you wanted to test the R2020 screw and nut I don't have a problem sending some over as long as you send them back when your done :-)

[JAZZCNC]

If you wanted to test the R2020 screw and nut I don't have a problem sending some over as long as you send them back when your done :-)

Sure he will but in 5yrs time then they'll be bent and worn out. . lol

[Jonathan]

I would like it to cut Ali is there a reason you think it's just a woodworking machine? What have I done wrong lol?

Er, not sure really... maybe the MDF bed. That wont stop it working, but my router bed is currently MDF and it is rather limiting.

If you wanted to test the R2020 screw and nut I don't have a problem sending some over as long as you send them back when your done :-)

Thanks for the offer, but it wouldn't really be practical/worth it .. I'll just wait and see how it goes on your machine.

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.

I used 3x because that's what's in irving's motor calculation spreadsheet. In reality I got a lot better than that - if you include the inertia of the pulleys in the calculation it's quite a lot more accurate.

[boldford]

I used 3x because that's what's in irving's motor calculation spreadsheet. In reality I got a lot better than that - if you include the inertia of the pulleys in the calculation it's quite a lot more accurate.

Many thanks for the pointer. That's really useful information which may save me some grief.

[jcb121]

I'm just wondering, that timing pulley you used? was it around 26 teeth 5mm pitch?

[Jonathan]

I'm just wondering, that timing pulley you used? was it around 26 teeth 5mm pitch?

For the one in post #61 I used a 30T HTD 5m pulley. Clearly it depends on the ratio you require, but if you can keep the pulleys small that helps a lot with reducing the moment of inertia of the rotating assembly.