Rotary Table CNCing

[Robin Hewitt]

I was looking at sintered bushes with a view to taking a .375" ID .6" OD bush, opening out to 10mmID and turning it down to 15mm OD.

Sintered bushes come oversize and go to size when you drive them home. I did try machining one once and it was distinctly odd, they are a bugger to chuck and smear rather than cut

[ptjw7uk]

As I said I managed to machine to the inside of the offset to accept a needle thrust bearing and then realised that there would be no support at that end so I inserted a sintered bush and reamed to size on the outside a normal ball thrust could be used as there is more room.
There were several momenst during the whole thing where I thought I had taken on more than I could do with my limited skill set. I also ended up with several bearing revisions as the whole thing was designed from bad idea to bad idea, if I had to do it again I would just buy a Arc euro(John S) finished one!

Peter

[irving2008]

Never one to be phased by my incompetence or lack of skills (just a learning curve right?) if I decide to fit a thrust bearing at the lower end of the worm spindle I reckon there are two options:

1/ mill 5.5mm of the bearing housing and fit a 10x18x5.5 ball thrust bearing, then bore out the table body to 20mm dia at the right depth for about 6 - 7mm. Assembly would be put the worm in, drop the thrust bearing over the spindle, push to one side so that the offset bearing can be inserted over the spindle and into the table body. See pic...




2/ manufacture a new offset bearing, 5mm shorter than the current one and bored out 8mm not 10mm. turn down the worm shaft to 8mm and use 8x15x5 hrust bearings each end. Or maybe reuse the existing offset bearing but line with a 8mm ID, 10mm OD oilite bearing. This won't require boring out of the table body.

[ptjw7uk]

Irving, your option 1 sounds very similar to my escapades only unforseen problem I had was that with the inner bearing fitted the offset wouldnt(couldnt) turn sufficently to engage the drive. I cured it by using a carbide burr in my drill press to make enough clearance for the bearing.

Peter

[irving2008]

How did you do that? I've added a drawing to show how I propose to do it..

[BillTodd]

Irv, why add the thrust bearing inside the housing?

Would it not be easier to add a pair of deep groove ball bearings in your motor mount to take the thrust?

Or maybe reuse the existing offset bearing but line with a 8mm ID, 10mm OD oilite bearing. This won't require boring out of the table body.

A much better idea IMHO

[ptjw7uk]

The thrust bearings are for the worm drive so as to limit back lash. My reason to fit an internal one was that I wanted to rotate in both directions and so wanted no movement of the screw. If you are cutting gears I think you could get away only having the outside one as you could always go in the same direction.

peter

[ptjw7uk]

Hi Irving,
Just one comment, the inner thrust bearing is to take the thrust when the table rotates in reverse which was the reason I did it in the first place, otherwise in reverse the worm part will wear on the offset part and worm drives excert a lot of force and I foresore a lot of wear.

Peter

[irving2008]

I agree Peter, but my thinking is by controlling the thrust externally the worm doesn't come into contact with the base of the offset part - its held off it by a few thou so that wear can't occur.

[irving2008]

Well the machining of the stepper mount has taken a back seat for a while due to the weather - it being too cold to work out in the garage (and I've had a cold too). So I decided to buy a PIC programmer with the original intention of using Kwackers software and a 16F452 PIC but then I decided I wanted the stepper driver to be built in, not a seperate board and, hey ho, I like writing code. So I knocked up a quick breadboard using a 16F877 I had lying around and wrote a fledgling set of routines to read the keyboard, drive the display, create stepper pulses, etc. At the moment this is driving my L297/298 stepper board, but I have some IRF520 logic level MOSFETS and I plan to drive the stepper directly in Unipolar mode with a chopper current limiter. I have a ex-laptop 24v 65W (2A+) power supply that is perfect for the job. The PIC and display take less than 60mA so a straight 7805 regulator on a small heatsink will provide the 5v rail.