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Chuck Jaw Grinding Fixture With a Twist (literally)

nc5a

Aluminum
Joined
Jan 4, 2013
Location
Alaska
I have to make 5 small hydraulic fittings for a manifold to test hydraulic pressures in an automatic transmission. The problem right from the beginning was my 6 1/2" South Bend 3 jaw chuck jaws were so bell mouthed it would hold the tiny fittings securely at the front of the jaws. So grinding the jaws was necessary anyway because they were quite hammered at the front.

I have ground jaws in the past using the washer trick at the front of the jaws and the ring that allows the jaw cap screws to tension the jaws. But I had been thinking about a new fixture design for quite some time. In addition to the grinding fixture which I thought was a new concept I improved the grinding dust collection/vacuuming setup.

First some thoughts on grinding fixtures. Every fixture I'd seen in the past put tension on the jaws at one or two narrow points of each jaw. None put tension on the entire length of the jaw gripping surface like a piece of stock does when clamped. So I came up with the following design. Initially I thought my idea was unique but have since found an old youtube video of a similar design.

The fixture off to the side was my first design but had a couple problems. The one in the center works very well.
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A piece of 1 1/2" X 0.065 wall 4130 cleaned up in the lathe then chucked in the super spacer. A 1/8" carbide end mill cut the slots very nicely and allowed a small window in front of the jaws. The importance of the window will be clear as you look at the photos and the vacuuming dust control arrangement..
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Cutting the 1/2" wide slot. My jaws are 1" wide so I used half the jaw width for my slot width and it's pretty much a good rule of thumb.
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After machining I checked the height of each slots with precision pins and a 0.0005" indicator and saw a 0.001" difference from one end to the other and consistent for the 3 slots.
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View from the front
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Top front view. You can see the small window in front of the jaws which helps the vacuum suck the grinding dust through the spindle and into the vacuum.
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The vacuum hose slid over the spindle. There is a rotating joint on the hose that turns with the spindle. The speed was about 200 rpm and there has never been a problem with it spinning with the spindle. I have a video of it but don't know how to load it on HSM
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Dressing the wheel
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Getting ready to grind. Started off with 0.0005" DOC but jumped to ).001" once I got a feel for how the grinder would handle it.
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The finished product
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A couple other photos for your enjoyment
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Big Buck

Plastic
Joined
Oct 27, 2005
Location
tc mi.
It would be interesting to know the TIR of the chuck at 3 or 4 different gripping dia.'s & how much grinding it took to clean up the jaws ?
 

nc5a

Aluminum
Joined
Jan 4, 2013
Location
Alaska
Prior to grinding the jaws the run-out with a precision pin in the jaws and tightened via the master square drive was in the 4 to 5 thousandths range. After grinding and cleaning the chuck the bell mouth was gone and the run-out was a pretty consist 3 thousandths. However when I identified and used the new master square drive the run-out dropped in half to 0.0015" to 0.0018".

Previous discussions about grinding chuck jaws has indicated that it doesn't improve run-out because it's only purpose is to clean-up bell mouthed jaws. I agree grinding jaws if done correctly will in fact fix bell mouthed or damaged jaws. But it also allows the material to seat more accurately/evenly within the 3 jaws thus improving run-out. I have noticed an improvement in run-out each time I have ground jaws.

Also, something I wasn't aware of until this jaw grinding operation is that the master drive lug identified during manufacturing or in the shop may not be the master after considerable use and especially after grinding the jaws. Just something to keep in mind.

Some of the prep prior to and after grinding the jaws. Have a clean chuck to start with. I had just cleaned mine two weeks before and it didn't have much time on it so I didn't bother. Lightly deburr the slots in the grinding fixture. Stone the angled surfaces of the jaws that rest on the fixture to remove any burrs. Don't use a lot of clamping force or you will distort the fixture. Disassemble and clean the chuck after grinding.

With the vacuum hose on the end of the spindle there was virtually no grinding dust on the rags protecting the machine after the operation was done. As you can imagine the spindle is just an extension of the vacuum hose so the suction is very strong at the front of the jaws.
 

nc5a

Aluminum
Joined
Jan 4, 2013
Location
Alaska
It would be interesting to know the TIR of the chuck at 3 or 4 different gripping dia.'s & how much grinding it took to clean up the jaws ?


I'll have to check the TIR at different diameters as I was wondering the same. I removed .009" to .012" on the ID, I actually lost count on my dial and since I don't have a DRO it's my best guess.
 

Conrad Hoffman

Titanium
Joined
May 10, 2009
Location
Canandaigua, NY, USA
In the past I've ground jaws on the surface grinder and they run true if you calculate and reference the slot correctly. What I don't know is the right shape for the contact area. My flat jaws obviously put a small flat on all parts. If you grind in place, you can do it at any diameter that you can fixture. At the outside they're nearly flat. At the inside you get a curve that puts two marks on the part for every jaw if the diameter is greater than the grind diameter. Or, am I over-thinking this? BTW, nice fixture and execution!
 

richard newman

Titanium
Joined
Jul 28, 2006
Location
rochester, ny
Don't use a lot of clamping force or you will distort the fixture. Disassemble and clean the chuck after grinding.

I think you would want to use the same clamping force on your fixture that you would use to hold your parts. That should give minimum TIR, especially if the jaws are ground at the diameter of those parts. Would require a sturdier fixture of course.
 

nc5a

Aluminum
Joined
Jan 4, 2013
Location
Alaska
I think you would want to use the same clamping force on your fixture that you would use to hold your parts. That should give minimum TIR, especially if the jaws are ground at the diameter of those parts. Would require a sturdier fixture of course.


Richard, there is no need to put a lot of clamping pressure on the fixture. After all you are only making sure all the scroll slop/backlash is removed and the jaws are seated as square in the slots as possible given the wear in the chuck. One of the key points is to have a clean chuck before you start grinding. That ensures the chuck operates freely and grit and grime isn't affecting how the jaws clamp on the fixture.

Granted a thicker wall fixture wouldn't hurt but even then you don't need a lot of pressure. Keep in mind the grinding stone and grinder that I used puts very little tool pressure on the jaws so there is no concern about moving the jaws when grinding. So, a clean chuck and light clamping pressure is the name of the game with this type of fixture.
 

dian

Titanium
Joined
Feb 22, 2010
Location
ch
good idea, but:

1. the fixture doesnt need any special precision.
2. the closer you simulate real jaw pressure the better. everything deflects.

(at least that how i see it.)
 

BoxcarPete

Stainless
Joined
Nov 30, 2018
Location
Michigan, USA
good idea, but:

1. the fixture doesnt need any special precision.
2. the closer you simulate real jaw pressure the better. everything deflects.

(at least that how i see it.)

Added bold to the quote because, yep, it sure does!

lightvstight.jpg

But you see the behavior is the same, unless you are one of those folk who get the pipe extender out and yield your chuck scroll on a regular basis. The important thing is to make sure that the force exerted by the cutting tool (toolpost grinder should not be much) does not put you on the wrong side of zero where we have backlash. Deflection resulting from X lbf on the jaws otter be the same either way.
 

dian

Titanium
Joined
Feb 22, 2010
Location
ch
not sure what you are saying, that the chuck will deform equally when clamping? even if you use the full lenght of the jaws you intoduce a moment so it will not. but it depends on the accuracy your after if it matters. i personnaly grind the chuck at a slight angle, so the front of the jaws contacts first.
 

BoxcarPete

Stainless
Joined
Nov 30, 2018
Location
Michigan, USA
not sure what you are saying, that the chuck will deform equally when clamping? even if you use the full lenght of the jaws you intoduce a moment so it will not. but it depends on the accuracy your after if it matters. i personnaly grind the chuck at a slight angle, so the front of the jaws contacts first.

Ah, I see what you're saying. But in that case, the moment generated will do the opposite of what you want, having the jaw tightness maxed out during the grind will result in a bell-mouthed shape when released. So, I would expect the straightest result without grinding at an angle should be from light tension just enough to keep the jaws in contact with the fixture.
 

richard newman

Titanium
Joined
Jul 28, 2006
Location
rochester, ny
the moment generated will do the opposite of what you want, having the jaw tightness maxed out during the grind will result in a bell-mouthed shape when released. So, I would expect the straightest result without grinding at an angle should be from light tension just enough to keep the jaws in contact with the fixture.

I respectfully disagree. The moment on the jaws from tightening against the fixture should cause maximum bell mouthing, which when corrected by the grinding will result in the tips of the jaws touching first when clamping work. Scroll pushes bottom of jaws inward, they pivot against the fixture, top of jaws go outwards, as allowed by slop.
 

BoxcarPete

Stainless
Joined
Nov 30, 2018
Location
Michigan, USA
I respectfully disagree. The moment on the jaws from tightening against the fixture should cause maximum bell mouthing, which when corrected by the grinding will result in the tips of the jaws touching first when clamping work. Scroll pushes bottom of jaws inward, they pivot against the fixture, top of jaws go outwards, as allowed by slop.

:dunce: :wall: :sulk:
 

Kevin T

Stainless
Joined
Jan 26, 2019
Super job and thanks for all the pictures. I have a big ole 4 jaw that I can't bring myself to throw away. One of the jaws is .03 out of position on part of the clamping face due to sitting in wet dirt for a couple of decades. I have always thought about grinding the jaws but the dust!!! I like your solution to capture and control the dust. Thanks for sharing this.
 








 
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