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Grinding & Truing 3 Jaw Chuck Jaws

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coherent

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#1
I was searching for more info regarding truing up 3 jaw chuck jaws and came across this comment under one thread...

"You may want to try corks instead of making special expander shims, worked well for me: http://hobby-machinist.com/index.php?topic=2495.msg16853#msg16853"

It doesn't lead any specific message. Does anyone know what he meant by using corks as means of putting tension on the jaws to remove play while grinding?
I've seen a few different ways to accomplish this. Using 3 shims seems the most time consuming. A using a ring to jam the jaws from the outside seems like the most simple solution. Does anyone have any recommended ways (be it corks or whatever) to accomplish this?
 

Ulma Doctor

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#2
i'm wondering how he proposed to use corks.
i'd be more inclined to grind the jaws with closing pressure applied to the scroll.
i believe the geometry and lift of the jaws will change if you are opening the jaws against a ring rather than closing the jaws against a ring

i used a ring behind the jaws (inboard the chuck) to accomplish the task on a Atlas TH42 i used to have
here is (the crappy) video of actual grinding as set up

 

RJSakowski

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#3
Using a ring on the outside loads the jaws opposite of how they are loaded in normal use. The jaws will pivot inward at the front and when you grind them, there will be a slight taper, creating what is known as a bell mouth condition. You want to load the jaws in the same manner as they are used.

If you have 2 piece jaws, you can remove one of the socket head cap screws and fasten a ring using the threaded holes. You then tighten the jaws as for inside clamping.

If you have one piece jaws, it is possible to load the jaws using the 120º faces on the jaws. Some have used gage blocks to provide the bias. I suspect that the corks were used in this fashion. Placing three corks between the jaws and tightening the jaws and then trimming away the excess before grinding would bias the jaws properly.
 

Eddyde

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#4
I was looking into doing this a while back for the worn chuck on my South Bend, but wound up getting a new (to me) lathe instead... Anyway at the time, found this simple jig that can be easily made by boring 4 holes in a disc.
chuck_jaw_fixt.gif
 

coherent

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#5
Anyway at the time, found this simple jig that can be easily made by boring 4 holes in a disc.
This may be a simple option for me. I have a CNC plasma machine and was going to cut some parts out of 1/8" steel plate
today. Looks like a jig like this would work great. Thanks Eddy. With a little measuring and a couple minutes drawing I can cut a jig pretty quickly. As usual I sometimes try to make things more difficult than they need to be and all it takes is someone to show you a simple solution!

I agree with other's posts regarding using a ring on the outside of the jaws would not position the jaws correctly for grinding the insides.
 

RJSakowski

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#6
This may be a simple option for me. I have a CNC plasma machine and was going to cut some parts out of 1/8" steel plate
today. Looks like a jig like this would work great. Thanks Eddy. With a little measuring and a couple minutes drawing I can cut a jig pretty quickly. As usual I sometimes try to make things more difficult than they need to be and all it takes is someone to show you a simple solution!

I agree with other's posts regarding using a ring on the outside of the jaws would not position the jaws correctly for grinding the insides.
I would caution about having the small holes equally spaced and sized to avoid applying an unbalanced sideways force which could distort the positioning of the jaws.
 

coherent

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#7
I would caution about having the small holes equally spaced and sized to avoid applying an unbalanced sideways force which could distort the positioning of the jaws.
Not sure I understand. Aren't the 3 jaws equally spaced? Do you mean "caution about NOT having the holes equally spaced and sized"? If I'm using cad and cnc everything should be fairly accurate and equal. I suppose I could even mount the jig and fine tune it with the CNC mill but didn't think for this purpose that would be necessary.
 

Eddyde

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#8
Yeah, I think that's what RJ meant, Having the points of the jig contact the chuck jaws evenly. I would simply tune the jig up with a file after plasma cutting. Also, I would be careful no to over tighten the chuck on the jig.
 

bill70j

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#9
Does anyone have any recommended ways (be it corks or whatever) to accomplish this?
Here is a simple method that worked for me. The pic shows the set-up and the jaws after grinding.
Ground Jaws.jpg
 

WarrenP

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#10
Seems biill70j way makes the most sense I have seen. In my opinion.
 

coherent

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#11
Thanks for all the ideas and comments.
 
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