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BigBoy1
05-18-2009, 04:13 PM
Now that I've gotten the 13" South Bend that I'm rebuilding back to a point where I can run it under power, I wanted to check the trueness of the 3 jaw chuck that came with the lathe. The outside edge of the chuck runs + 0.0005" out of round which is acceptable to me. Now if I chuck a hardened, ground 1" diameter shaft in the 3 jaw chuck and check for roundness of the 1" shaft, I get + 0.004". (The shaft is NOT the problem as it has been check and has zero out of roundness. At least it is zero with my instrumentation.)

Can and how do I adjust the three jaw chuck to eliminate this out of roundness? That is way too much "error" in the chuck. A value of + 0.0005" to + 0.001" is more what I would like to have. Thanks.

agrip
05-18-2009, 05:08 PM
BB

As to the centering of parts in the typical 3 jaw, .004 TIR aint bad.

The major dependencies go something like this.

How well the scroll spiral is centered on the hole in the scroll plate.
How well said hole fits the boss in the chuck body.
How well centered said boss is on the backplate.
How well centered said back plate is on the spindle.
AND
How well centered the jaw teeth are on any particular place on the scroll.
Lastly
how well the jaws were last ground true
Which starts the cycle alll over again

Suggest you find a way to spring for an "adjust true" type scroll chuck.

Independent jaws have their own problems, but can be set to low TIR.

Good Luck Ag

philbur
05-18-2009, 05:59 PM
You missed the main none fixable issue, which is the precision of the scroll itself.

If you want concentricity use a collet or a 4 jaw. A 3 jaw self-centering chuck is primarily for convenience not precision.

Phil


BB

As to the centering of parts in the typical 3 jaw, .004 TIR aint bad.

The major dependencies go something like this.

How well the scroll spiral is centered on the hole in the scroll plate.
How well said hole fits the boss in the chuck body.
How well centered said boss is on the backplate.
How well centered said back plate is on the spindle.
AND
How well centered the jaw teeth are on any particular place on the scroll.
Lastly
how well the jaws were last ground true
Which starts the cycle alll over again

Suggest you find a way to spring for an "adjust true" type scroll chuck.

Independent jaws have their own problems, but can be set to low TIR.

Good Luck Ag

agrip
05-19-2009, 01:27 AM
You are right I didn't mention the scroll.
They used to be cut with a lead screw and a rotary table so the scroll was just as good as the screw and decent gears for even pitch. Not bad.

Now CNC is still a lead screw and another lead screw or
a LS and a Rotab. Still not bad.

Yeah, the home shop can't do much with the scroll, but scroll is not usually where the trouble is. That is, if you don't allow galling and swarf imbedment, which the home shop can control.

Come to think of it, I also didn't mention regular cleaning of the guts in a scroll chuck is required particularly if swarf is gritty. You can pretend the chuck is agricultural equipment and fill it full of grease to keep crud out. But I ain't gonna have to wash the walls and ceiling or your shirt.

I've been using a 6 jaw adjust true, with no concentricity problems for a long time. Can run four and eight point stock just by pulling two jaws
Can run thin wall rings of any size and don't need to mess with pot chucks and expanding collets.

Good luck Ag

BigBoy1
05-19-2009, 05:31 AM
The three jaw chuck was completely dissasembled and much "gunk" was removed: hard grease, swarf, etc. All mating surfaces were checked for smoothness of fit and any burrs and tight fits were stoned down until a smooth sliding fit was obtained. It was greased and reassembled and that is when I made the measurements so the cleaning and greasing has all ready been done. Thanks for the tips posted above.

JMS6449
05-19-2009, 09:44 AM
In order to true the chuck the jaws need to be ground true to the C\L of the lathe.

Typical way is to use truing ring on the outside of jaws, with them cranked out to the ring. The error in this is any wear in the slots will be excentuated.
If the jaws are two piece, chuck a piece of soft material in the permanent
portion, and tighten. This overcomes any error. Now you can grind the id true
with the lathe C/L.

If you do not have a tool post grinder, do what i just did to fixed a "import"
6 jaw. Any decent die grinder(even a "HUSKY") will do a fine job if you take light passes after you true the grinding wheel, and spark out.

FYI, I have 42 years in the business, and this was the first time of truing a chuck. I hate to admit using the "cheap" air grinder. The results are what counts. BTW, chuck is now "dead true".

If you ahve any further questions, you can PM me and I can send a few pics.