View Full Version : how to true up old 3 jaws?

04-27-2003, 12:35 AM
on my old reed I have a 3 jaw that is out about .20 some of the time, but not always all of the time. I have become happy with my 4 jaw. very happy. does anybody have a cure for correcting or reparing old chucks that seem to be good "sometimes". I have taken the chuck apart and cleaned and inspected.

04-27-2003, 01:07 AM
You need to know what the problem is first, before you can correct it.

Out 0.2 whats? If inches, the jaws were put in in the wrong order, or it is more worn than I have ever seen. That would not be a sometimes thing.

assuming that is a mistype for some smaller amount, the jaws might be in the wrong slots, also no a sometimes thing.

There might be stripped teeth on the back of the jaw, or a really worn or chipped spot in the scroll. Those could be "sometimes" things, but are probably uncorrectable.

Or they might be worn bellmouthed. That lets the work shift "sometimes", but does not measure out as far as it "acts".

Bellmouth can be ground back into straight, although it won't be perfect everywhere.
For grinding, I don't like the 3-hole ring method, I prefer drilling a hole in the end of the jaws. Then put in a pin in each, and close onto a thin ring to load the jaws.

Now grind, sparking out until no more grinding occurs, and the jaws are uniformely marked by the stone.

The bad part is that they now have a radius on them.

I have heard of surface grinding them, but I don't think it is as precise.

One issue that many don't know about is loose scrolls. Loose on the center post so they can shift radially.

No matter what you do to center, grind jaws, etc, a loose scroll will hurt you by shifting under pressure.

You can take it apart and check for looseness. If it rattles on its bearing, you can shim it with a long thin strip the right thickness. I have done this, and it saved the chuck.

04-27-2003, 01:16 AM
I am sorry I meant to say 0.20 out.

04-27-2003, 12:46 PM
Oso, you know way too much about 3 jaws. Thanks.

04-27-2003, 01:33 PM
That's 2/10" ? If so, that is indeed a LOT. Check for the jaws-in-the-wrong-slots problem or some of Oso's other ideas first....

[This message has been edited by SGW (edited 04-27-2003).]

John Foster
04-30-2003, 09:49 PM
Surface grinding the jaws, while not perfect, can bring them back in very nicely. This method is about the only way to true jaws from a chuck with a streched scroll. The problem with a streched scroll is that as each jaw comes to the streched part, that jaw will be off. Grinding the jaws in a chuck with this condition only makes the jaws right when chucking something at that diameter.
I use an angle plate, lay the plate on the side, hold the jaw against it, turn on the magnet and clamp. Grind the first jaw, starting with the one with the tooth closest to the end, until it is nicely cleaned up. Using a depth mic, check the measurement from the ground surface to the first tooth. Remember, the teeth are at an angle so always check at the high point of the thread. If you have a jaw with 3 threads per inch, the spacing (pitch) is .333 and each jaw will have the first tooth .111 further in, so add .111 to the first reading and that will be the distance to the first tooth on the second jaw. Grind until you have that dimension. Repeat for the third jaw adding another .111 and you have restored the accuracy of the JAWS, not the chuck.
If the jaws are a little loose in the chuck you can even shim the jaw, a little bit, so that you grind a slight taper to the jaw and they will grip more evenly.

David Hafnorske
04-30-2003, 10:33 PM
I think you mean .002 or 2 thousandths or perhaps .020 or 20 thousandths, since .20 is almost a 1/4 inch, which seems very extreme.

05-01-2003, 12:16 AM
Yeah, surface grinding is "precise".

I guess I should have said that it does not adapt the jaw to the chuck at all. You have to cheat it to do that.

And I sure did leave a stretched scroll off my list....dang... http://bbs.homeshopmachinist.net//biggrin.gif

05-01-2003, 01:03 AM
A stretched scroll sure doesn't sound too good. I was thinking about the problem, and I came to the realization that the scroll actually has to make point contact with the teeth on the jaws, not full line contact, as the curvature of the scroll changes along it's length. It may match one tooth on the jaws exactly at one point, but never will all the jaw's teeth have full contact with the scroll. There's bound to be some damage happening, sometime, with these high contact pressures.

05-01-2003, 11:53 PM
after doing some grinding on jaws of skinner WELL WORN 6in.,( so much bellmouth ,it was turning horizontal flats ( didnt beleive what i saw,but had it explained to me by someone who could think better ),i had axially true but variable repeatability 0f .008 tir ..i indicated the zero point , & HELD the bar there while i closed the jaws around it ...not too hard to get +/- .001......
picked up a 4 in china chuck on sale for $36..after carefully fitting a backplate , it repeats under .001 throuout ( NOW , i dont want to USE it!!!!).....if u have poor repeatability , u will end up as i did w/ the skinner!, ...the scroll has wear /abuse...the thickest shim u can stand around scroll bearing will help!!
best wishes

05-02-2003, 12:01 AM
forgot to mention that the 6+ in skinner 3 jaw & 12 in 4jaw knuckle buster ,10 in 4jaw knucklebuster w/ in/out scroll, came w/ 1895 14 in reed ...spindle a very unusual 2x7tpi...

best wishes

05-02-2003, 01:03 AM
My 6 in 4 jaw Skinner knucklebuster was so bad that I had to shim the jaws to hold parts in it. Had a partly stripped jaw screw too.

Got a new Bison, and always intended to make the old one into a drillpress vise, but never did. Its under the bench somewhere.

05-02-2003, 09:47 PM
I think many three jaw chucks are damaged by overtightening, hence the stretched scrolls.

06-11-2003, 12:01 AM
thanks for all the advise. when time permits and 4 year boy takes a day off. yea right! will re-check using all advise givin. thank you.