3 jaw chuck

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earlgo
Posts: 1794
Joined: Sat Jan 29, 2011 11:38 am
Location: NE Ohio

Re: 3 jaw chuck

Post by earlgo »

And speaking of worn out 3 jaw chucks... I used the 3 jaw from the 1958 Atlas to grip a chunk of stock on which I wanted to cut a M12x1.75 thread. Since I couldn't thread it normally, I bought a threading die from the hardware store. It, of course, cut the thread in 1 pass and the resultant torque spun the chuck on the stock, leaving this telltale mark. One can see that the part of the jaws under the scroll did the gripping whereas the front part of the jaws barely left any marks on the stock. The jaws are 1-1/4 long.
slip grip
slip grip
Time for some serious repair or replacement.
--earlgo
Before you do anything, you must do something else first. - Washington's principle.
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Harold_V
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Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: 3 jaw chuck

Post by Harold_V »

That condition translates in to a lot of trouble, as when a part is machined without a center, the effective gripping area is at the rear, allowing the part to oscillate as the pressure of the cut moves between the jaws. Makes it almost impossible to observe the three times the diameter projection when machining without support.

Assuming your chuck has master jaws, there's nothing quite as nice as running soft jaws for routine machining. The problem of rear gripping is easily addressed, and the jaws tend to run far better, even when various diameters are gripped. While I have a set of hard jaws, they almost never are inserted. The three jaw I use came with two sets of hard jaws, as well as a master set. For that, I will be eternally grateful, as I practice what I preach.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
earlgo
Posts: 1794
Joined: Sat Jan 29, 2011 11:38 am
Location: NE Ohio

Re: 3 jaw chuck

Post by earlgo »

You are correct as usual, Harold. Unfortunately the 5" 3 jaw chuck that came with the Atlas lathe has 1 pc jaws and like the rest of us post 60 year old things, some parts become sloppy and loose. :shock: Had it not been so cold or I so lazy, I woulda/shoulda put on the 6" SHAR chuck and done the right thing. It has 2 pc jaws that are right, but it weighs 23# as opposed to the 6# Atlas chuck, and I just didn't want to heft it. This is all on me, but demonstrates succinctly what happens when a chuck is worn out.
--earlgo
Before you do anything, you must do something else first. - Washington's principle.
SteveM
Posts: 7763
Joined: Mon Jun 27, 2005 6:18 pm
Location: Wisconsin

Re: 3 jaw chuck

Post by SteveM »

The soft jaws also have the advantage of gripping over a larger area, as the grip is more akin to an ER collet than a chuck with small gripping surfaces on each jaw.

Steve
tim9lives
Posts: 18
Joined: Tue Dec 20, 2016 5:04 pm

Re: 3 jaw chuck

Post by tim9lives »

Just another possibility is that if the jaw backing plate was originally off another screw on spindle from different lathe, then it’s possible you May need to recut the backing plate. When I upgraded to a 9a from 9c, I wanted to use a few of the 9c chucks on my 9a. They were all out around 3 thousandths. I had to recut the backing plates for the chucks to run true on my 9a.
Screw on type chucks can reliably be used on different lathes. They are matched to one lathe. But...of course we are only talking about scroll chucks. I’m guessing a 4 jaw independent jaw chuck could be swapped back and forth between multiple lathes with same spindle thread pitch.
SteveM
Posts: 7763
Joined: Mon Jun 27, 2005 6:18 pm
Location: Wisconsin

Re: 3 jaw chuck

Post by SteveM »

tim9lives wrote: Thu Dec 19, 2019 8:22 am I’m guessing a 4 jaw independent jaw chuck could be swapped back and forth between multiple lathes with same spindle thread pitch.
It is possible that a 4-jaw, while it would not show axial runout (where a part was off the same near the chuck as away) because you can adjust that, could show a wobble on one lathe and not on another. Facing the backplate would cure that.

When fitting a backplate, I do the equivalent of marking the rear surface of the backplate like you were scraping a part to a surface plate master. You put the ink on the shoulder of the spindle, thread the backplate on, take it off and scrape where the ink transfers. Keep doing until you see contact all the way around. What that does is ensure maximum contact so that as you put the chuck on and off, it doesn't wear down and cause the chuck to stop threading on at a different rotation. It also ensures that the chuck is not being cocked sideways by only contacting on one side.

Steve
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