# Jaw grinding



## fkrel (Sep 19, 2018)

I used this method to grind my jaws from u-tube      



    The runout on mine were .012 before grinding and .003 after grinding and I can live with that.


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## benmychree (Sep 19, 2018)

To achieve a satisfactory job of jaw grinding, it is necessary to restrain the jaws as they are tightened inwards; on one small lathe, I drilled the face of the jaws at the smallest step with a carbide drill and chucked down on a steel ring sized big enough to allow the grinding wheel to enter the jaws.  There are other ways of achieving this.


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## fkrel (Sep 19, 2018)

I seen a video of someone doing it like that I made the spacers for between the jaws and it worked good the picture is after I removed the chuck and dissembled and washed it out and reoiled and then checked the runout


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## wa5cab (Sep 20, 2018)

Ideally, the grinder should be mounted to something like the Atlas milling attachment in order to have the gripping surface be flat instead of radiused.


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## Kernbigo (Sep 20, 2018)

i use the method like in the video except i use square gage block instead of the home made steel spacers, works great


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## bpimm (Sep 20, 2018)

I cut this plate for my 4 jaw on my first lathe.


Installed


worked good, these jaws were so worn that a rod could be moved after it was tightened down, get it indicated in bump it and it was out by .010-.020 Grinding it made it much better, actually usable.

Could do a 4 hole version for a 3 jaw chuck.


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## benmychree (Sep 20, 2018)

This device works well with 3 jaw chucks too, thanks for posting the picture.  It works especially well for 3 jaw chucks that do not have two piece jaws, for them, one can cut short sections of allen wrench that extend past the allen bolts used to secure the top jaws, and chuck down on a ring of A diameter that will allow the grinding wheel to enter the jaws.


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## RJSakowski (Sep 20, 2018)

Another version of the grinding fixture.
https://www.hobby-machinist.com/threads/grinding-my-jaws.50747/


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## bpimm (Sep 20, 2018)

RJSakowski said:


> Another version of the grinding fixture.
> https://www.hobby-machinist.com/threads/grinding-my-jaws.50747/



Very similar, I cut mine on the plasma table and made the tool holder out of aluminum 




I used the long nose air die grinder from Horrible Freight, just zip tie the trigger lever and use the ball valve to control the speed.


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## wa5cab (Sep 21, 2018)

I have read numerous threads on re-grinding chuck jaws on this and other sites, and seen not a few videos on places like U-Tube.  I have yet to see the first time that anyone mentions that regardless of how you pre-load the jaws, none of these methods are capable of returning the jaws to like new condition.  If you look at the holding end of a new or nearly new jaw, you will see that the surface of the jaw is flat, and in a plane that is exactly perpendicular to the direction in which the jaw moves as it is being tightened.  If you look at one that has been re-ground using any of these methods, you will see that it is concave.  If you then close the chuck on a work piece whose radius is smaller than or exactly equal to the radius at which the final grinding took place, you will have line contact between the the three jaws and the work piece, and every thing is fine.  But if the work piece is larger than that radius, you will have two lines of contact between each jaw and the work piece.  And as the jaws are hard and the corner is sharp, there is a good chance of marking the surface of the work piece.  And the reground jaws will have a shorter service life than they did the first time around.

The only way to re-grind the jaws so that they look and work as when new is to be able, after completing the grinding while rotating the spindle, to then be able to move the grinder vertically while grinding exactly at 9:00 or to move it down and do the additional grinding exactly at the 6:00 position, while moving the cross slide.  This also requires having a method to adjust and then lock the chuck at exactly 09:00 or 06:00.  Unfortunately, using the indexing pin is not an option as with the threaded spindle nose, there is only about one chance in 360 of a hole being in the exactly right location.  Making a clamp that would lock the chuck in the correct position would be doable if not easy.  But properly positioning the jaw for the flattening grind wouldn't be trivial.


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## markba633csi (Sep 21, 2018)

I think in many cases this is a last gasp procedure especially in cases of extreme bell mouthing it makes the difference between a terrible chuck and one that's a lot better than it was even with the concave profile of the jaw tips
Also too with 4 jaw independent chucks the jaws could be removed and ground on a surface grinder, they don't have to be done in situ
mark


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## Bob Korves (Sep 21, 2018)

I suppose that once you got the 3 jaws ground to diameter, then you could take them out of the chuck and then grind the tips square, stopping when the curved depression almost goes away equally on all three jaws...  Good enough for a 3 jaw.


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## wa5cab (Sep 22, 2018)

Yes, that would work if you can do it accurately enough.


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## yendor (Sep 24, 2018)

From your description of the need to flatten the surface of the jaws it sounds like it  would work best if the chuck was mounted to a rotary table on a mill.
Then index the jaws and use the X-axis to flatten the surface rotating 120 deg to access each jaw surface.
Actually I suppose the indexing for position would be the first step.


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## wa5cab (Sep 24, 2018)

Well, it would probably be easier, assuming that the mill can turn up to a suitable RPM for grinding.


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## bpimm (Sep 24, 2018)

In my case the jaws were so worn that even if they were ground on a surface grinder they would still bell mouth, the jaws and the chuck body have slop in them so a square ground jaw could tip out at the bottom when it was tightened. The first thing I tried was truing them up on a disk grinder, I got them really close to square on the combination square I was using but they still wouldn't hold the work piece, still rocked after tightening. After grinding them in place they hold the work piece much better. I just took a jaw out and took a photo of the angle that the jaw ground to when tightened down.



Now granted the proper thing to do would have been to replace the chuck but the rest of the machine shows the same kind of wear so is it worth it to spend a bunch of money on a 108 year old heavily worn machine? not to me, I made it work and it can do some pretty good work now but I don't use it much anymore since I got the JET 1440, except when I need 4 jaw work, I really need a 4 jaw for the JET.


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## Bob Korves (Sep 24, 2018)

bpimm said:


> In my case the jaws were so worn that even if they were ground on a surface grinder they would still bell mouth, the jaws and the chuck body have slop in them so a square ground jaw could tip out at the bottom when it was tightened. The first thing I tried was truing them up on a disk grinder, I got them really close to square on the combination square I was using but they still wouldn't hold the work piece, still rocked after tightening. After grinding them in place they hold the work piece much better. I just took a jaw out and took a photo of the angle that the jaw ground to when tightened down.
> View attachment 276254
> 
> 
> Now granted the proper thing to do would have been to replace the chuck but the rest of the machine shows the same kind of wear so is it worth it to spend a bunch of money on a 108 year old heavily worn machine? not to me, I made it work and it can do some pretty good work now but I don't use it much anymore since I got the JET 1440, except when I need 4 jaw work, I really need a 4 jaw for the JET.


The jaw in the picture actually shows the opposite of bell mouth.  Normal wear on chuck jaws makes them have a bigger diameter at the upper tips of the jaws.  Yours has a smaller diameter there.  Actually, with the wear that likely exists on the rest of the chuck and jaws, those will probably be close to matching the center line of the lathe when tightened onto workpieces.


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## bpimm (Sep 25, 2018)

Exactly, this is after grinding them with the tightening jig shown in the earlier post #6, the 4 jaw, it was bell mouthed and now it clamps down good. Just posted this pic to show how much wear was in the chuck preventing it from being ground on a surface grinder and the benefits of this procedure on an old worn chuck.


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## wa5cab (Sep 26, 2018)

I suspect that even when new, chuck jaws would exhibit a slight clearance at the heel.  Because they must have some clearance between the jaw and the slots in the chuck in order for them to move.


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## wa5cab (Sep 26, 2018)

What I meant by that was that to make a satisfactory chuck, the maker must somehow be able to compensate for the fact that the jaw will cock slightly when tightened,  One way would be to grind all jaws square to one end, and then to assign three (or 6) jaws to a chuck body, install and load them in that body, and finish grind the noses.  However, that would take four setups as each set of jaws is usable on both ends.  Someone who happens to live close to a chuck manufacturer could probably find out.


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## bpimm (Sep 26, 2018)

The manufacturer probably knows the angle to grind the jaw to for each chuck which would probably good enough for a production situation.


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## Firstram (Sep 26, 2018)

bpimm said:


> a 108 year old heavily worn machine


Good for you! I have a state of the art 1909 16x36 Pratt & Whitney engine lathe that I wouldn't trade for the world, worn out would be an understatement! It did help win the war with a tracer attachment to make artillery rounds and the ways are all but gone under the chuck. Grinding the jaws helped quite a bit but ,turning a part around is not an option.


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## AmericanMachinist (Apr 17, 2020)

Thank you!   This is very helpful inspiration.


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## Mini Cooper S (Apr 17, 2020)

Thanks for the post.  I have been fighting jaws that splay out forever, your post gave me the idea and motivation to grind my jaws.  In fact, grinding as we speak.  Your never to old to learn something!

Thanks again
Richard


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