Truing up jaws in lathe chuck

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Harold_V
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Post by Harold_V » Mon Mar 13, 2006 12:06 am

Ernie,
For what ever reason, your picture isn't working.

Unless there's something drastically wrong with your chuck, I can't see any reason why you can't get it working to your satisfaction.

I'm not clear on if you used a given hole to tighten the chuck when you ground it, or not. That's very important for success, but keep in mind that the chuck may run true only at that diameter, with that hole. Using random holes to tighten the chuck once ground guarantees that it won't run true. Mark, and use, only one hole.

If the chuck is worn badly, so the scroll is free to move about the center pivot excessively, while you can get the chuck to run true, it will be prone to shifting under certain conditions, although, in general, it should still provide acceptable service. The tightness of the jaw slides is also a consideration, especially with the altered jaw you spoke of. If they're sloppy, the jaw is likely to shift to the side when tightened (possibly deflected by the altered surface), then return to center when released. Your chance of achieving a true chuck under that condition is not good.

It would help a great deal if I could see the picture. If you can't get it to work, please forward it to me via my email address, which you will find on my profile page. I'll get it posted for others to see, and try to come up with a solution to your problem. Please make sure it's not very large, no larger than 150 kb. I'm on a dial-up

Harold

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Metalman
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Photo

Post by Metalman » Mon Mar 13, 2006 12:19 am

Hi Harold
The photo will help, I sent it to your email address. I'll have to re-educate myself on posting photos :oops: , (I did it before but couldn't get it to happen this time :x )
Ernie F.
PS: I think I figured it out :D , thanks again.

Harold_V
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Post by Harold_V » Mon Mar 13, 2006 4:18 am

OK, Ernie! I've looked at the picture and don't think the jaw is contributing to the problem, assuming all three were ground identically. From all appearances, the back of the jaw was "killed", meaning it's possible someone else had already ground the jaws, and removed the small portion where they couldn't grind---the area where the "plug" that expands the jaws, had been placed. Otherwise, it's possible the jaw(s) had been damaged and relieved so the balance of the jaw was still useful. No big deal, the jaws should still be very serviceable.

Assuming the jaws fit the slides well, and the chuck is in general good condition as you suggested, and the three jaws are identical aside from not running perfectly true, I see no reason why the chuck can't be made to run well.

I'm going to assume that the jaws are identical----all ground in place, even if they don't run perfectly true-----but the relief on the rear of the jaws is uniform on all three jaws.

First thing you should do is chuck a round pin, using each of the wrench positions, to determine which one runs closest to true. Mark that hole, then once again grip that piece of 1½" round material you spoke of, holding it at the rear of the jaws, in the killed area. You want to make sure the jaws are properly loaded, such that the scroll is spreading the jaws against the slides, just as they'd be in use when machining. Tighten the chuck an appropriate amount, but don't over stress it. Avoid hitting any of the jaws when you do this----so you don't shift the scroll from where it wants to be when the chuck is tightened. Make certain the plug you use doesn't run crooked, which has the potential to load the jaws in a dissimilar fashion, and is usually the source of runout. Once you have the plug in place and properly tightened, once again make a setup to grind the jaws.

While you commented that you had set up an abrasive point in your tool post, you didn't mention how you powered the point. It's important that you use a grinder that has minimal runout if you expect decent results. It need not be a high precision spindle, but something like a die grinder can often have enough slop that the wheel will deflect, so you might struggle getting the jaws to run uniformly. If that's all you have at your disposal, use it, but give the wheel plenty of spark time so it's happy. Probably a good idea to take light cuts until all three jaws appear to be sparking uniformly.

Grinding wheels perform very best when applied at the proper speed. You'll want to use an aluminum oxide wheel, not a silicon carbide one. Vitrified wheels can usually be safely run @ 6,000 SFPM, so you'll have to crank up the small wheel quite fast in order to get it to perform as intended. Running it too slowly will work, but the wheel will behave as if it's softer than it is and slough off faster than it should. No harm if you have no alternative but to run that way-----not as long as you can get through the job.

If, by chance, it takes quite a bit to bring the jaws true, you may have to go back in after grinding the gripping portion and kill the area where the plug is held. If you're not worried about that part of the jaws, which are, in truth, no longer of any use to you for gripping, you can relieve them by hand, or you can find a way to steady the jaws and go in with the mounted wheel and grind the area below the holding surface while running the spindle of the lathe. With a little luck, your plug will have been held far enough back that you can grind without going below the already relieved area. Unless I don't see it clearly, the area already killed was done at an angle.

If I don't have it right, and the jaws are not identical, we need to talk further. It's important that they hold the plug uniformly.

Let me know how it goes, Ernie.

Harold

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SteveM
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Post by SteveM » Mon Mar 13, 2006 7:46 am

I have a 5" Pratt 4-jaw, and the front ends of the jaws are worn more than the rear. Should I follow the same procedure to true them up, or would grinding them outside the chuck be satisfactory, as it is not a scroll holding them in place?

Also, see my post on the capitive screws if you have any suggestions for that issue (please answer there to keep the thread on that subject intact).

Steve

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Metalman
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3 jaw chuck

Post by Metalman » Mon Mar 13, 2006 3:21 pm

Hi Harold
Harold wrote:
I'm going to assume that the jaws are identical----all ground in place, even if they don't run perfectly true-----but the relief on the rear of the jaws is uniform on all three jaws.
I think one of the problems is the rear of the jaws (where the relief was ground) are not identical. As you already noticed the were hand ground on an angle and #1 and 2 are close in height but #3 is about .012" taller. This I where I had the slug clamped thinking that if they all hit the round slug and I ground the jaws it wouldn't make much difference because the grip surface is what I was truing. Evidently they where not clamping equally. The second problem may be that I did not select the best wrench position of the three, (I have learned something here) and that caused an uneven clamp prior to grinding. I have done that and marked the best position.
I used a good 23000 rpm air die grinder clamped solid to the tool post and took very small cuts in case there was deflection. The stone was an unknown so I will order a good aluminum oxide wheel, what grit do you recommend?
Do you think I need to spend some time getting those relieved areas on the back of the jaws cleaned up, uniform, and as close as possible in height
prior to regrinding the jaws? I think the fact that I had to put a .002" shim under #2 was the result of #3 being to tall. I think the next attempt will work but I still have some doubt.
Thanks
Ernie F.

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Post by Harold_V » Tue Mar 14, 2006 1:14 am

Ernie,

It was not possible for me to tell if the relief (the killed area) you spoke of was done by hand, or not, but, yes, it would serve you much better if all three were uniformly ground, and parallel to the gripping surfaces. When they vary as you suggest, one may have the undesired affect of tilting the jaw in a way that is not how it would run under proper conditions. When you grind the jaws, you'll duplicate the improper condition, then it's the problem instead of the fix.

It's important that the plug be held as if you had a straight piece of material in the chuck, with parallel jaws, so the effort expands the jaws uniformly, and straight out against the slides.

If you don't have a surface grinder at your disposal so you can grind the killed areas to make them identical, you might enjoy reasonable success mounting a cup wheel on a spud arbor and running it in a vertical mill. While that leaves a great deal to be desired, it will work, and solve the problem at hand.

If you choose this route, mask your entire mill well, so you don't get any abrasive on the way surfaces, and take exceptional care when removing the paper, which should be taped down well at all seams to prevent migration of the abrasive that will come from the operation, including dressing the wheel.

Be mindful of the rated speed of the wheel, and run as closely to the recommended speed as is possible, but NOT OVER. I'd suggest a 46 grit wheel, aluminum oxide, with a hardness in the K through M range. Speed will have a serious affect on how the wheel behaves. so hardness may not be an issue. If you must run well under the recommended operating speed, selecting a harder wheel (higher letter in the alphabet means it's harder----M is harder than J, for example).

Do not diamond dress the wheel for use, but instead hand dress with a silicon carbide dressing stick, a slight angle inward, so the corner of the cup wheel does all the grinding. Redress when it starts cutting too hot, which will be obvious to you once you start grinding. Move as quickly as possible, and take light passes on all the jaws at the same time, until you have an identical relief on all three. You'd have the best luck by placing your vise on the mill paralleling the table, with the jaws in a uniform stack with the scroll side out the back side, where the wheel can grind them without touching the vise. Set the width of the relief you intend to grind with the saddle of the machine, then lock it in place and do all the feeding back and forth with the table, and raise the knee to feed the wheel into the work. Half thou passes will likely be as much as you'll want to take.

It doesn't matter that the relief is not circular, the plug you grip will find center, and the grip will be uniform across the three jaws (even if it's slightly eccentric. That doesn't matter in this instance). That is a major step forward in achieving acceptable results.

I find no problem with the grinder you spoke of, especially when you addressed the issues I raised in the manner that you did. Frankly, you thought it out quite well, and had you been somewhat experienced with soft jaws, you'd have figured out the things I've talked about----for they relate the same way.

Considering you have to regrind the killed area, you're unlikely to have to go back after grinding the jaws to further kill the same place, so the work you will do in preparation is not wasted.

Trust me on the setup. The chuck, if you follow the instructions, may surprise you when it's finished, even above or below the diameter at which you grind the jaws. I run soft jaws almost continually---with one set that is intended for general purpose gripping. I can chuck almost any size material and enjoy concentricity that was once only a dream, and the material runs straight, with no whipping on the end unless the material is crooked. The improvement in chucking is hard to believe. Do your work well, and you should be rewarded with a chuck that surpasses many right out of the box.

I think a 46 grit mounted wheel for grinding the jaws would be a good choice. If you go too fine, you'll get considerable burning. My experience with mounted wheels is somewhat limited, so I'm not sure how much flexibility you might have in selecting a particular one, but any wheel will work to some degree as long as it's not a silicon carbide wheel. They are generally shiny black or dull green. The black ones are for general non-ferrous work, and the green ones are intended for grinding carbide, although I've never seen a mounted point that was green silicon carbide. If the wheel you use is any other color, it's aluminum oxide and will serve well, assuming it's not too fine, as I said.

Because you can't grind all the way through the jaws (the plug prevents that), it might be in your best interest to do all the feeding on the inside, plunging the wheel instead of starting on the outside and feeding inward. That way you won't destroy the corner of the wheel and generate a slight taper in the grinding process. If, by chance, that doesn't work well for you, make sure to dress the wheel and go back once you have the jaws uniformly ground, to insure you don't have a taper or step on the back side.

Keep us posted on your progress!

Harold

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Metalman
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3-jaw regrind a success!

Post by Metalman » Wed Mar 15, 2006 2:37 pm

Thank you Harold
I re-ground the jaws on the 3 jaw chuck and it is more than aceptable 8) .
My first move was to true the heights of the jaws and create the proper points the jaws could grip on the plug. Crude as it my be I had excellent results stacking them in the vice, as you suggested, and ground them true in my drill press! I don't have a knee mill or surface grinder (yet).
By sliding along a make shift fence and raising the table, I was able to grind a nice flat through that hand ground area.
After reassembling the chuck I set up the grinder and the plug, and took very small cuts from the inside out. Big improvement. Not trusting the original plug I took some small fine cuts off a chunk of 1.5" cold rolled. faced the end, drilled a pilot hole and parted off a new (hopefully more trustworthy) plug. This time I clamped the new plug and used a dial indicator to make sure it was perfectly square in the chuck jaws, (within.0005"). I redressed the stone and took a few more ultra fine passes. Sure enough the Dykem I put on the jaw surfaces showed they needed a little more cutting. I'm very pleased with the results, no more egg shaped orbits :roll: ! I chucked up some 1.5" cold rolled which is the chucks inside maximum and the dial indicator moved a total spread of about .003". With some .625" stock it only varied .002". Who can trust raw stock for round and true anyhow. I can live with this from a 3-jaw chuck. Learned a lot here and I'm glad I asked.
Thanks again
Ernie F.
PS. Being the novice I have one more question: What do they mean in the catalogs when they print ".003" T.I.R." pertaining to 3-jaw self centering chucks? What does "T.I.R." stand for?
Here is a photo of the grinding rig.
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Harold_V
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Post by Harold_V » Wed Mar 15, 2006 3:28 pm

Way cool, Ernie. My hat's off to you for your creativity. What often sorts out those that can from those that can't is the degree of imagination they have in conjuring a setup when the proper machine is not at hand.

Your setup looks good to me. I've done something very similar on occasion. The drill press "grinder" is one I'd have never thought of, though, and it's a great way to go. You don't risk ruining ways like you would using a mill. For the task at hand, it was more than adequate.

It's entirely possible that the chuck has enough use on it that hoping for anything closer than the results you achieved isn't reasonable, but as you suggested, it's definitely close enough for general use. The one area you will have gained is that it should now hold stock quite straight, having eliminated the bell mouth it used to have. You should find that you don't have as much chatter as before when machining. If you find you need to do work within tenths, that, along with other things, is what 4 jaw chucks are for.

I agree with your comments about material. Cold rolled material is not known for roundness or being straight, although it is within reasonable limits. I've experienced centerless belt sanded material (stainless steel, US made) that varied more than you found after grinding your jaws.
Being the novice I have one more question: What do they mean in the catalogs when they print ".003" T.I.R." pertaining to 3-jaw self centering chucks? What does "T.I.R." stand for?
Total indicator reading. They do that so there's no argument on how well an object is centered. It's easy enough to raise the question of how the reading could be interpreted otherwise. When one discusses .003" T.I.R., it's clear they mean the object will run off center a maximum amount of .0015", which, obviously, is a plus +.0015" and a -.0015", for a total reading of .003". It's the honest way to represent error, as you might deduct.

I'm pleased to have been a part of your success. I enjoy sharing the things that I was taught by those that went before me. It's my way of repaying my self imposed debt to society.

Harold

10 Wheeler Rob
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Thanks guys for the good advice on trueing up 3 jaws.

Post by 10 Wheeler Rob » Tue Jun 13, 2006 2:42 pm

I read the posts with much interest, but I am amazed no one brought up that learnig to to set and use a 4 jaw often is the beter route to doing beter work. Even more so when you need to reverse ends in the chuck or remount in the chuck after doing some other work on the part.

Once I mastered seting up the 4 jaw, typicaly with less then .0005 run out, whaich is about as good as the head stock bearings in my 1927 South Bend. I very rarely use the 3 jaw. except for polshing and other non crical ops.

One thing I didn't see mentionsed was to not over tighten the chuck, wether it is 3 or 4 jaw, over tightning just distresses the jaws and tee slots and makes the jaws calp unparallel. It is the worst thing for chauck, except a crash that is.

If you do small work, get a good set of collets, they run very true and also have much less over hange form the head sotck bearing, which means les deflection and very little chatter also.

The other fact is the 4 jaws that come with most used lathes are in much better condition as well. Once I mastered seting the 4 jaw up and made a spring loaded pump center rod, I rarely use the 3 jaw for machining.

I had leared to look for the best tightneing screw several years a go, but one thing I often do when using a 3 jaw is mark an idex on the work if posiable so if I have to rechuck it later in the 3 jaw.

Bill Shields

Chucking

Post by Bill Shields » Tue Jun 13, 2006 5:22 pm

Ah yes, the old 4-jaw chuck..

VERY USEFUL except to hold hex bar...

Now I DO have a chunk of Octogon bar floating around...don't ask.... :oops:

Harold_V
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Re: Thanks guys for the good advice on trueing up 3 jaws.

Post by Harold_V » Tue Jun 13, 2006 8:11 pm

10 Wheeler Rob wrote:I read the posts with much interest, but I am amazed no one brought up that learnig to to set and use a 4 jaw often is the beter route to doing beter work. Even more so when you need to reverse ends in the chuck or remount in the chuck after doing some other work on the part.
Very true in principle, Rob, but that's not the whole story. There are times when a 4 jaw is more than appropriate, but you have to look beyond such a situation. Often times a 4 jaw becomes a liability instead of an asset.

Having worked in the shop for gain, and having had to perform such that the profit in a job wasn't lost to setup time, there are many reasons to not use a 4 jaw. That may help explain why they don't get the attention you suggest.

If one is machining more than one piece, and it must be handled for more than one operation, a set of soft jaws will almost always be superior to a 4 jaw. That includes holding power. Because soft jaws are machined to exacting precision (or they should be), the part is gripped by 100% of the soft jaw surface, spreading chuck force out over a broad area. The large gripping area minimizes crushing, or distorting a part with a thin cross section. The jaws, when properly machined, will be concentric within a half thou, and dead perpendicular. so there is no time wasted orienting the work piece. Think of the savings when tens or hundreds of parts must be chucked. Beyond that, perfectly finished pieces can be chucked without marring the finished surfaces, which is beyond the ability of a 4 jaw, even when using shims.
I had leared to look for the best tightneing screw several years a go, but one thing I often do when using a 3 jaw is mark an idex on the work if posiable so if I have to rechuck it later in the 3 jaw.
A good practice, to be sure, but not necessary when using soft jaws. Just make sure you always use the same socket that is used when machining the soft jaws. A permanent mark is always recommended, which many chucks have directly from the manufacturer.

Harold

10 Wheeler Rob
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Yes 3 Jaws are nessary, and a tirck to simulate soft jaws

Post by 10 Wheeler Rob » Wed Jun 14, 2006 10:45 am

Yes I do agree in the real world 3 jaws are very nessary to make $$$, my point was that for us hobiest the 4 jaw has a lot usefullness, in getting better percission.

Are theres souces out there to get soft jaws for alot of the old chucks us hobist have? And soft jaws typically machined the dame way with a disc clampted in the back and then the high spot ground off after?

I only have hard jaws in all fothe chucks I have, one trick i use a lot is to make a alunimum ring of 6060-T6, saw a spit in it and clamp the part through the ring. This protects the suface under the hard jaws fom damage and usually grips fine for lighter cuts, drilling and boaring.

I have also used the slpit ring to grip hex stock in round collets and the like with very good sucess. The aluminum really grips the hex stock good and dose not bur up the edges of the hex, even when the hex is brass.

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