Chuck jaw holding problem.

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littlejack

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Hey guys:
I just came in from playing with the Atlas 10" QC54. I was playing with threading a piece of 1/2" stock, and found a problem.
When I put the stock into the chuck and tighten, the jaws only clamp tightly on the back end (drive end). There is a slight gap
between the stock and the jaw on the feed end. There is enough gap that it can definitely be seen with the naked eye.
I removed the jaws and checked them with a machinist square. They look to be square, no gap along the holding surface.
I installed them back into the chuck and checked for play. There was a minute amount, but not much. Would this be wear?
Can I have the jaws milled so they hold the piece correctly?
Do I need to buy a new chuck?
Anyone ever have this problem?
Jack
 
Typicley you regrind them with a tool post grinder. Scence most home shops don't have a toolpost grinder. You can make a mount and use a drimmel tool or die grinder.
 
Are you sure it's the chuck and not the piece you chucked
 
It is definitely the chuck. I did try 2 pieces of stock, and both had the same results.
Thank you all for your replies.
Jack
 
Jack,

As Dan said, the jaws are bell mouthed. This can happen from any combination of three reasons. Wear on the outer ends of the jaws. Wear on the slots in the chuck body. And wear on the keys on the jaws that run in the slots. You can also have wear on the jaw teeth that run in the scroll (the disk that the tightening gears drive), on the lands in the scroll, on the scroll pilot or bearing, and on the hole in the chuck body that the pilot runs in. But from your description of being able to see a gap between the workpiece and the outer ends of the jaws, that is probably your worst problem.

To fix the bell mouth, you need a high speed grinder mounted firmly on the carriage, and a fixture to load the jaws in the proper direction during grinding. You can't just put a steel or aluminum ring around the inner ends of the jaws and back them out against it because that will cock the jaws in the opposite direction to that in which they cock when gripping the OD of a workpiece. There are two common fixture types. The simpler one requires a ring with three holes at 120 deg. and a hole drilled in the flat front of each jaw, with pins in the holes in the jaws and in the ring. When you tighten the jaws, they will cock outwards just as they do in actual use. However, you do have to drill holes in the jaws, which I never liked the idea of.

The other fixture consists of a round plate with a hole in the center and three holes at 120 deg. just large enough to fit over the jaws. The three intersect the center hole just enough for the inner face of the jaw to stick past so that the stone doesn't hit the ring. This also loads the jaws in the proper direction and requires no whittling on the jaws. If the scroll pilot isn't badly worn, this should return the chuck to usability.

There was an article on making such a plate in the Downloads section but since we started resurrecting Downloads, I haven't come across it. I have a local copy of one of them that I can send you if you want it and will send me your email address in a PM.

Robert D.
 
As Wa5cab pointed out the method shown is incorrect for grinding a chuck you must load the jaws correctly in the right direction I use a method that is very simple you cut three equal size pieces of key stock and place them inbetween each jaw but not far enough down so they get in your way when grinding close the jaws down tight until all three are secure and start your grind. Feed slow and take your time. Ray

- - - Updated - - -

Here is a post with some pictures.

http://www.hobby-machinist.com/show...huck-Jaw-Alignment-Issues?p=102483#post102483
 
Thanks again guys.
All of this help is great for the beginner/novice/ME.
The method used by Ray "iron man" looks to be very easy to do. I will need to get a bigger Dremel. I will need to make a Dremel holder.

Just thinkin out loud, but would it be feasible to load the chuck as iron man did, and then run a very sharp hone of the appropriate size into
the jaws via the tailstock?
Just askin.
Jack
 
I dont believe that would work the last one I did I used a die grinder it does not take very long. Ray
 
Hey guys:
Range report on fixing the jaws of my lathe. I did as instructed by you fellas.
It took me a few hours to fabricate a holding fixture for my Dremel tool. I did pre-load the jaws in the closing manor. After getting
everything set up, I began. I ran the lathe on the slowest speed with the Back Gears engaged. I ran the carriage feed on the slowest
setting also. I crept up on the jaws until I began to see sparks in the rear, the highest points. After that, I backed the stone out and made
my first cut. Each cut from there on was .001. A few times, I re-run the previous cut without moving the cross feed.
After all was said and done, to get the back of the jaw the same as the front, it took about .012 off. After cleaning the jaws up, I made a
couple extra passes just for fun.
The jaws came out absolutely perfect. I tried different size pieces of stock, and it took very little effort with the chuck key to lock them into
place.
I thank all of you for your experienced input. It is very much appreciated.
Regards
Jack
 
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