Squaring up a Rotary Table / 4th Axis

[awemawson]

Using my 4th axis on my CNC mill the other day, I noticed that it's axis is not quite parallel to it's mounting face 

Slipping feeler gauges under the base at various points showed that it was far from flat, so somehow it needs re-working to get the base flat and in one axis parallel to the axis of the rotary part.

I'm sure that the proper way is to scrape it flat and parallel, but it's a huge lump with all sorts of bits sticking out making this approach none too easy.

So in the MadModder tradition I'm trying to think 'outside the box'. Were I to make a base on which to stand it, with three 'levelling screws' then I'm sure that I could fairly easily achieve an acceptable degree of accuracy. BUT the three screws would not stand up to the pressure of use and undoubtedly sink into their beds under load.

So supposing I made up the base and levelling screws, achieved the desired position, then removed it and spread Titanium Putty on the base, and allowed it to sink down to the screws and hence back to level - would that work? I've heard of similar ways of aligning lathe cross slides and bringing them back up to height after a re-grind using Mogalice. After all the 4th axis would still be held down as always to the mill table, so there is no issue with bond strength only compressive forces, and the stuff claims 105 kPa

Or am I mad to consider this approach?

[mattinker]

I would say that it is a perfectly sound not mad solution!!

Regards, Matthew.

[PekkaNF]

Lot of that stuff has only one issue: Too much adhession when you don't expect it and too little when you are counting on it!

I don't know how unconfortable (and expensive) lump of metal that is and how big is the error, but would milling it closer to flat be an option? Like level the 4:th axis foot up on the table and mill it flat?

Does it has a trough hole? Can you mount a ground bar trough the arbor of it exactly trough the centre (like taper/collet/ground hole)? If you can, would it possible:
1) Put precission ground bar trough the axis
2) Put the prisms on the milling table and clamp the bar on these - axis sorted.
3) use engineers jack to level the foot, might be iffy

Or 3:rd tep: if you don't need all the head room: Mount plate on the foot with aforementioned putty, and mill it's bottom flat and parallel to axis. A little oversize plate would allow screw holes or other mounting options and offending appendises could be sawed of after leveleing.

If there is no precision hole troughn the spindle, I'm pretty sure you have a angle plate and 4:th axis table should be really perpendicular to rotary axis.

just a tought. I know that milling really does not make a flat surface, but if you are can put feelref gage under mount, it would improve it a whole lot, maybe even eneough.

Pekka

[awemawson]

Pekka,

Milling would be my preferred solution as it would be adequately accurate for my usage. After all anything super critical should be between centres not just gripped in a three jaw.

It is a very beefy lump that I can barely shift around, but certainly the idea of mounting it upside down on a shaft resting on large V blocks is a distinct possibility, with as you say jacks under the hold down lugs. I suppose I can use an engineers level to set the 'none axial' axis level.

[sparky961]

I really like your idea of using a secondary base.  Have you considered some combination of screws and or shim blocks between the two, instead of the adhesive filler?  After you have it set where you want you may be able to bolt (or tack weld) the two together.  This had the advantage of re-adjustment (a bit harder if you weld) if you don't get it quite right the first time and doing away with it all together if your situation changes in the future.