truing 3-jaw chuck question

[Pete Burne]

I'm going through the 3 jaw chuck issues right now with a Hendey 12" x 24" cone head lathe (Hendey calls it a 12 x 5 which means a 5' overall bed lenght). I just retired so now I have time to tweek things in the way I went them. The lathe came with an 8" Cushman chuck that looked good but was not repeatable. Come to find out that the jaws were worn in the chuck body. If you inserted the work dead on it would clamp up dead on; but if you inserted the work at random it would give me .015 TIR (yuck).
I decided to buy a new chuck and bought an 8" SetTru Bison from Enco (on sale for $350). The problem I now have is that the Hendey has a 2" threaded spindle and Bison has only 2-1/4" or 1-1/2" threaded back plates. I bought their back plate and am now machining it to fit. Fortunately I can work with the Cushman if I take the time to carefully set it up (yea, I know, I'm retired; what else do I have to do). In a week or so I expect to get .0005" TIR out of the SetTtu. The lathe shows no signs of wear and is heavy duty so I think it will do it. We'll find out.
I also have an old 12" x24" Seneca Falls lathe (manual change gears) that I can get .001 TIR out of repeatably with a 3-jaw.

Pete Burne (in Connecticut)

[Thrud]

Pete

Good choice in chucks! Have fun with the new lathe. You may be able to rebuild the Cushman back to new but it would cost more than the Bison did.

[trap]

Dave:

Prawns ???? I catch jumbo shrimp here .... the little ones we call lobsters.

ok so i screw up with the words and spelling but don't blame it on the shrimp .... I are just stupid. My excuse is that hard landing i made after the fan on the roof left .... the govt says naw ..... i'm just dumb.

I also have a speech problem, and i don't stand very straight, but I can still out plow a mule and arrrugh i can still shoot a cannon.

[John Foster]

If the scroll of the chuck has been streched from over tightening as in using a cheater on the chuck key (I know you would not do that but maybe the guy ahead of you did) or by chucking too long a piece and having it climb over the tool you will never have an accurate chuck. That said, you can still true the jaws and eliminate any bell mouth that has developed and have a chuck that you can use for routine work.
Take the jaws out of the chuck, mount the one with the shortest distance between the jaw surface and the first tooth on an angle plate and grind it on a surface grinder. Use a depth mic and find the distance from the clean, grownd, jaw surface and the first tooth. Mount the next jaw, grind and check. You want the same distance PLUS one pitch (the pitch will be .333 for 3 teeth per inch, .250 for 4 etc. on the jaw) and repeat on the last jaw with depth plus 2 pitches. If the chuck is worn also you can put a little angle to the grind too, and this will let the jaw grip a little toward the front. You have to use a little "windage" on this step and guess how much to angle the jaw. This does not cure the repeatability problem as with a streched scroll the jaws are always at a different place on the "strech" and will not repeat. It does save a bad chuck for everyday use.

[Thrud]

trap:

I was only teasing you brother - besides I sometimes fart enough I need FAA numbers painted under my arms. Tip: always make sure you are using "blanks" - or someone could get struck from "jet blast"...

Prawns, shrimp, lobster - its all sea 'roach to me - pass the sea food sauce.

I have a speech problem too. I go to political rallies to heckle politicians during elections. I usually bring a shovel with me and stand at the front. Handing out air sickness bags is too crass for me. Last election the NFA (Canada's National Firarms Association) printed pro-gun slip covers made from garbage bags for the political canidates lawn signs. I liked that.