grinding a three jaw chuck

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curtis cutter
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grinding a three jaw chuck

Post by curtis cutter » Sat Apr 14, 2018 2:25 pm

I have been looking at the concept of grinding three jaw chucks that may be out of true. I understand the basics I believe in how it is done but I do have a couple questions though.

1) Is there a particular desired diameter that it is done to? I mean would you want to do it at say a 1" diameter or 2" or where, assuming the end result will be grinding to a diameter.
2) It could possibly create a sharp edge, do you break this edge at all or just let it be?

No, I am not touching my jaws right now. No need for a quick response...... :) Just learning.

Gregg
Gregg
Just let go of it, it will eventually unplug itself.

Harold_V
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Re: grinding a three jaw chuck

Post by Harold_V » Sat Apr 14, 2018 3:49 pm

The diameter you choose makes little difference, as the chuck will run dead true only at that diameter, unless you're fortunate. However, in general, the chuck should be improved, with pretty much any location running improved. The greatest benefit is that the jaws, assuming you load them correctly prior to grinding, will be dead parallel to the spindle centerline when used. You'll be surprised to find long pieces inserted in the chuck tend to run true, whereas prior to grinding that likely wasn't true.

Needless to say, the profile of the ground jaw surface will mate with the part being gripped only when the part is the precise diameter that you grind, that assuming the part being gripped is round. If you alter the edges of the jaws, unless you do it in perfect symmetry, the piece will be offset to some degree, giving up some of the precision you had hoped to establish. I would recommend you do not alter the edges, although when gripping pieces larger than the chosen diameter they will be inclined to indent the work piece. Smaller pieces will be gripped by the center of the jaws, so most likely would not display any deformation unless you tighten the chuck severely.

Armed with the above thought, you may wish to choose a reasonably large diameter to do the grind. The choice is yours.
If you do not understand the use of soft jaws, it would pay for you to do so before electing to grind the jaws. There are certain things you must do if you hope to improve the chuck. Grinding hard jaws is no different from boring soft jaws, the chief difference being that hard jaws are typically not disposable, whereas soft jaws are intended to be.

Hope some of my ravings help. :-)

H
Wise people talk because they have something to say. Fools talk because they have to say something.

curtis cutter
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Re: grinding a three jaw chuck

Post by curtis cutter » Sat Apr 14, 2018 5:35 pm

Thanks Harold.

I am not at the point where I feel my weaknesses are benefited by any work on the jaws. I am still at the point where my tooling, speeds, materials and general skills need further development. I was just looking for some clarity to confirm my thought processes of the act of grinding.

Basically, I cannot blame the machine yet... That will be a long time out. :)
Gregg
Just let go of it, it will eventually unplug itself.

John Hasler
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Re: grinding a three jaw chuck

Post by John Hasler » Sat Apr 14, 2018 7:06 pm

Harold writes:
> If you alter the edges of the jaws, unless you do it in perfect symmetry,
> the piece will be offset to some degree, giving up some of the precision
> you had hoped to establish. I would recommend you do not alter the edges,
> although when gripping pieces larger than the chosen diameter
> they will be inclined to indent the work piece.

Suggestion: after grinding the jaws at your chosen diameter run them out to near maximum and grind again, taking off just the sharp edges (you'd have to make another spider). This should reduce indentation without compromising concentricity. It might even improve it since indentation may not be uniform.

I've not tried this yet.

Harold_V
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Re: grinding a three jaw chuck

Post by Harold_V » Sun Apr 15, 2018 1:03 am

John Hasler wrote:
Sat Apr 14, 2018 7:06 pm
I've not tried this yet.
It will have marginal benefits. It does eliminate the sharp edge that would dig in on larger diameters, but there's still one there, it's just less pronounced, and will have been moved slightly inboard, towards center. Worth doing for those who might hope for the best possible condition.

I use straight bored soft jaws for gripping stock. They are used in place of hard jaws, and have a given radius machined on the gripping surface. The benefit has been straight gripping (parallel with the spindle centerline) and better conditions in regards to concentricity. I haven't been too concerned about any indenting, as there's usually no need to protect the gripped surface. When it matters, I machine soft jaws for the specific application, which eliminates the problem entirely.
H
Wise people talk because they have something to say. Fools talk because they have to say something.

earlgo
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Re: grinding a three jaw chuck

Post by earlgo » Sun Apr 15, 2018 8:36 am

When I straightened the ATLAS 5" 3 jaw chuck jaws, the original jaws faces were flat. So when I ground the jaws, I left a very tiny bit of margin at the edges. Now I am thinking, from the above comments, that this may have been in error. However, the grip and concentricity improved noticeably, even though some would still think it intolerable.
Jaw grinding setup small.JPG
ATLAS 5" 3 jaw
Yes, there is a ring at the back of the jaws to clamp onto. The ring is gripped by the last thread of the jaw so I could grind the entire jaw face. The resultant jaw faces should be good for anything that will fit thru the 7/8" spindle bore. Beyond that ?? I used an ancient 22k rpm Dumore electric die grinder.
--earlgo
Before you do anything, you must do something else first. - Washington's principle.

John Hasler
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Re: grinding a three jaw chuck

Post by John Hasler » Sun Apr 15, 2018 9:02 am

Harold_V wrote:
Sun Apr 15, 2018 1:03 am
John Hasler wrote:
Sat Apr 14, 2018 7:06 pm
I've not tried this yet.
It will have marginal benefits. It does eliminate the sharp edge that would dig in on larger diameters, but there's still one there, it's just less pronounced, and will have been moved slightly inboard, towards center. Worth doing for those who might hope for the best possible condition.

I use straight bored soft jaws for gripping stock. They are used in place of hard jaws, and have a given radius machined on the gripping surface. The benefit has been straight gripping (parallel with the spindle centerline) and better conditions in regards to concentricity. I haven't been too concerned about any indenting, as there's usually no need to protect the gripped surface. When it matters, I machine soft jaws for the specific application, which eliminates the problem entirely.
H
It would be pointless with soft jaws, obviously. However I have a three-jaw (with one-piece jaws) that is so bellmouthed as to be useless but otherwise in good shape.

Harold_V
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Re: grinding a three jaw chuck

Post by Harold_V » Sun Apr 15, 2018 4:19 pm

John Hasler wrote:
Sun Apr 15, 2018 9:02 am
Harold_V wrote:
Sun Apr 15, 2018 1:03 am
John Hasler wrote:
Sat Apr 14, 2018 7:06 pm
I've not tried this yet.
It will have marginal benefits. It does eliminate the sharp edge that would dig in on larger diameters, but there's still one there, it's just less pronounced, and will have been moved slightly inboard, towards center. Worth doing for those who might hope for the best possible condition.

I use straight bored soft jaws for gripping stock. They are used in place of hard jaws, and have a given radius machined on the gripping surface. The benefit has been straight gripping (parallel with the spindle centerline) and better conditions in regards to concentricity. I haven't been too concerned about any indenting, as there's usually no need to protect the gripped surface. When it matters, I machine soft jaws for the specific application, which eliminates the problem entirely.
H
It would be pointless with soft jaws, obviously.
It isn't pointless, as what you suggested provides an improvement, depending on the application.
However I have a three-jaw (with one-piece jaws) that is so bellmouthed as to be useless but otherwise in good shape.
That's reasonably common for small chucks, often the result of reefing on the chuck wrench when gripping by the very tips of the jaws. It often results in a sprung chuck--distorting the slides. That condition can be corrected nicely by grinding the jaws, but special care must be exercised to ensure that the jaw tips are extended to their distorted position before grinding. If they aren't, while the chuck might be improved to some degree, the tips of the jaws won't provide loading, so they grip only at the inward portion.

Should you attempt to correct this condition, it's important that you provide the appropriate load on the jaws. That may be difficult, as you must grind the entire surface, so there's no place to install a spider. In that case, you can create a ring with slots milled to the appropriate width, located @ 120° intervals. The slots would engage the tapered sides of the jaws, and would be located at the outward edge, leaving the bore free, so the jaws could be ground full length. Aside from the benefit of freeing the jaw surfaces, this places loading on the jaws where it is needed most--at the tips, which spreads the jaws to a loaded condition.

It is important that you slots be created uniformly, so the jaws do not get loaded to either side.

An alternate solution is to drill small holes in the ends of the jaws (remember, they're heat treated) in which you place pins. The pins would engage a ring, which would load the jaws.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

John Hasler
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Re: grinding a three jaw chuck

Post by John Hasler » Sun Apr 15, 2018 4:39 pm

It's always been my intent to make the sort of spider you suggest. It will be easier now that I have a crude sort of rotary table. Drilling the jaws will become a possibillity should I ever finish my EDM drill. That would also open up the possibilty of mounting soft jaws (though I'm more likely to want them on my large three-jaw. It isn't bellmouthed and has little runout, but the non-reversible inverted jaws limit its usefulness).

Harold_V
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Re: grinding a three jaw chuck

Post by Harold_V » Mon Apr 16, 2018 1:52 am

A very functional spider can be made from a large hex nut. Use the flats to index the nut. Works great, and saves a lot of unnecessary setup. If you do make a spider, it's a good idea to favor one side, so you can bore deeper without hitting the nut. Drill and tap your holes as close to the edge as you can, leaving just a marginal amount of material between the threads and the end of the spider.

H

Edit: One more thing. It's relatively important that the tapped holes are in alignment with the center of the spider. If they are not, you run the risk of loading the jaws to one side or the other, which will result in jaws that don't grip properly, and won't yield the degree of precision you'd achieve if the spider was well centered.
Wise people talk because they have something to say. Fools talk because they have to say something.

earlgo
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Re: grinding a three jaw chuck

Post by earlgo » Mon Apr 16, 2018 8:18 am

H:
Sorry but I am having a problem visualizing your hex nut spider, unless it is to be gripped inside the jaws. In that case how would one grind the jaw faces. Maybe I missed the boat entirely on this one. Would you please post an illustration of some sort?
--earlgo
Before you do anything, you must do something else first. - Washington's principle.

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SteveM
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Re: grinding a three jaw chuck

Post by SteveM » Mon Apr 16, 2018 9:27 am

earlgo wrote:
Mon Apr 16, 2018 8:18 am
H:
Sorry but I am having a problem visualizing your hex nut spider, unless it is to be gripped inside the jaws. In that case how would one grind the jaw faces. Maybe I missed the boat entirely on this one. Would you please post an illustration of some sort?
--earlgo
Here's mine:
Image

That one works in this instance because it's not in the way of machining the jaw "shoes".

To use one like this for grinding, you would have to have it all the way at the rear, grind the jaws up to the spider, then remove the spider and grind a relief where the spider was. That would lose you some gripping surface on your jaws. Other downside is that if you are trying to grind to fix a bellmouth situation, this won't work because it's not loading the jaws at the front.

One of the nice things about soft jaws that I have experienced is that I will machine the soft jaws at one diameter, and if I use them at another diameter, they are still pretty concentric, and they grip along the full length of the jaws because they were machined parallel to the bore.
Image

Steve

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