How Close Does a 4th Need To Be Dialed In?

(note: never dealt with any of this, don't have any idea WTF I'm doing).

Making a sub-plate for the Sankyo rotary on the Speedio. Machined some 1" thick D2 to size today, off to grinding tomorrow. When it gets back, I'll be putting 4 countersunk holes in it to bolt to the table, and 3x M12s on it so the clamps can bolt directly.

Question I've got is: just how dialed in are most people getting a 4th axis? Along Y is easy enough to knock in, but I'm mostly curious about Z. Bolted directly to the table, the face of the rotary is showing 0.0004" of tilt. That could get better/worse when I put a surface ground plate underneath it, but I honestly have no f-ing clue if that 0.0004" is good? Bad? What the hell is a good or bad reading here?

And shims seem like sort of a PITA, so at what point do you go through all that rigamarole?

Not making rocket-ship parts here so no crazy tolerances (I like to aim for +/- 0.002" all around in general). How dialed in would a practical machinist make this (i.e. somewhere between a total hack and the Hermann Schmidt).

Do you have solid numbers for the actual squareness of your Z? .0004" could just as easily be the machine as the 4th itself.

That said, if the bottom of the 4th is immaculate, and the surface of the table is immaculate, I'd fix it down and run with it.

Parallelism to the table becomes more of an issue when you're trying to do longer parts on a tailstock or trunnion.

gregormarwick said:

Do you have solid numbers for the actual squareness of your Z? .0004" could just as easily be the machine as the 4th itself.

That said, if the bottom of the 4th is immaculate, and the surface of the table is immaculate, I'd fix it down and run with it.

Parallelism to the table becomes more of an issue when you're trying to do longer parts on a tailstock or trunnion.

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The machine is brand new. The inspection sheet from the factory says Mr Okuda measured the Z axis parallelism to be 5 microns... So now that I think about it, that's like 0.0002" right there isn't it?

.0004" over how big the face is? 6" - 8"? I would call it good, unless you plan on running very large/long parts all the time. I would think (my opinion here) it would be easier to shim the occasional long part when needed then to screw around now. If you find you always are needing to shim because your work envelope is growing, then maybe go back and try shimming the 4th itself.

Mike1974 said:

.0004" over how big the face is? 6" - 8"? I would call it good, unless you plan on running very large/long parts all the time. I would think (my opinion here) it would be easier to shim the occasional long part when needed then to screw around now. If you find you always are needing to shim because your work envelope is growing, then maybe go back and try shimming the 4th itself.

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Without a tailstock, the most I can ever imagine sticking stuff off here is 6" of mini tombstone on the Lang receiver.

Obviously, the 0.0004" is something I can easily live with. Hell, I'm guessing if I took the rotary off and put it back on the bare Speedio table, I would probably get a different measurement!

I'm more worried about what happens when I throw this on the sub plate - just picturing a world where I heave it all together and find myself more like 0.002" out and wondering WTF to do.

gkoenig said:

I'm more worried about what happens when I throw this on the sub plate - just picturing a world where I heave it all together and find myself more like 0.002" out and wondering WTF to do.

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Simply put, you fix the plate if it goes bonkers after install. As long as your grind shop does a good job and everything it clean when you assemble it, things should be fine.

Is the 4th hanging down or pointing up ,gravity and tool pressure is going to push down so pointing upa bit would be an advantage I reckon.

What you don't say ( and is of paramount importance ) is whether or not you use a center on longer parts. Frankly, it's a non issue if you do. One can EASILY flex most modest material sizes that much once they're hanging out in space that far. And a center will accomplish that with aplomb for you and never think twice. The OCD would hound me until I corrected it, if not using a center, though.

gkoenig said:

I'm more worried about what happens when I throw this on the sub plate - just picturing a world where I heave it all together and find myself more like 0.002" out and wondering WTF to do.

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Some aluminum foil from the grocery store, folded and placed as needed under the 4th or even under the subplate if you want, will fix you right up. I think you're worrying too much about this.

Matt@RFR said:

Some aluminum foil from the grocery store, folded and placed as needed under the 4th or even under the subplate if you want, will fix you right up.

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That's "fadal machine alignment 101". Everyone knows that's THE way to get your geometry set up.

I like to keep my fixutres using up less than a quarter of my tolerance, preferably 10%. Don't shim, scrape your subplate to make any adjustments.

All my 4th axis fixtures are held in place with a pneumatic tailstock, which makes getting the tailstock properly aligned a new problem. When doing production I only swap pallets while the spindle is stopped, never parts.