Grinding chuck jaws?

Anyone had any luck regrinding chuck jaws? I have a smaller 3 jaw scroll chuck that I cannot get the runout less than 20-25 thou (no matter how I position the piece). Runout on the body is less than 1-1.5 thou, its just the jaws themselves giving me most of the trouble. You can see where the jaws aren't making consistent contact down their length, so they will have to be trued. I've watched a few videos on regrinding, and while it seems fairly straightforward, I don't have access to a tool post grinder.

Here is my question. Would it be possible to first try and true up each jaw individually on a milling machine? Specifically, skim the face of the jaws just enough so they are at least consistent down the length of the face? I know this still poses the problem of them not being "true" in relation to one another, but would at least give me starting point. Of course, getting the jaw located correctly to skim may be a little easier said than done, so any advice would be greatly appreciated.

I realize I'll likely end up having to replace the chuck altogether, but I would at least like to try cleaning it up enough to use.

This has been repeated so many times. you need to clean your chuck very well, chips free - then charge the jaws in the pushing down attitude so they are tight to the scroll, tight to the direction they would hold a part with the part not in the chuck then grind the jaw faces.
How to charge the jaws is in many different ways. Custom made jig to hold jaws outward is one common way. (see some of these on you tube) or search grinding chuck jaws on PM.

Some /a few chucks may have a jaw screw so one can use to hold a circle ring under. Many jaws are hard and have little or no way to grasp in order to hold to the open position..

With no other way a trick is to set a small C clamp to each jaw, stretch a number of rubber bands from each C clamp to a locking device that might be a stout rod, wire or even a rope tied behind the chuck. Yes be sure the chuck does not have a sharp corner edge.

Agree with (Iwananew10k) next post. find the number and have jaws in the correct slot is a must. so they might run their best / or if regrinding so you can put them back is the sane as they were ground.

Still the scroll may be sloppy where it sets in the body and may move about each time you close it on the jaws, some time a shim might be set around circle the scroll to reduce the sloppiness to .002 or so.. Some times the jaw slots are wasted wide so change when holding at different diameters.

I have tried grinding each jaw one at a time with a surface grinder and was very lucky to get perhaps .007 so don't think that is very good.

You may get it ground .003 in one certain diameter and the it may be off .010 and another diameter because it is just wore out.
I ran clunker lathes that being quick to go to centers was the key to good work. Rough to plus a little than finish between centers.

Or run everything in one chucking then part off your part so only the left stub in the chuck is off.

are the numbered jaws in their respective numbered slots?

if not, you must reinstall them and do so in the proper manner-

starting with jaw 1 in slot 1 and applying light pressure rotate scroll until the scroll catches the jaw- STOP.
then do the same with jaw 2 and rotate until it catches- STOP
then jaw 3....

once installed tighten scroll to be sure the jaws meet.

re-check the chuck.

do take the opportunity to clean any chips or debris while you are at it.

Thank you for the suggestions, but I have tried all the normal tricks (mentioned above) with the chuck. First thing I did was strip it down, clean, and regrease, hoping that would help. I've even tried swapping the jaws into the other slots to see if the runout improved, without any success.

You are on the right track.

Years ago I had a 3 jaw Buck (adjust true?) chuck that i trued up by chucking a 1 inch diameter part near the back of the jaw. I picked a diameter that I use the most. Then using a indicator measured each jaw for its runout near the part I was holding.

I ground each jaw individually--using a surface grinder-- holding it perpindicular to its sliding surface. Worked well for me.

If there is jaw slop this may not work. But, then other approaches may not work.

Lost

QT: [I don't have access to a tool post grinder.] You might make a holding jig to hold a die grinder. be sure to cover the lathe works very well if doing and grinding, jig the die grinder far forward, anything to keep abrasive out of the works, And you should clean abrasive out of the chuck when done.and clean the lathe... *Actually It may be better to just buy a decent chuck.

Perhaps someone ground the jaws with them not being under pressure and so only grinding properly might make it fair to good.

I am basically a pack rat and can't quite seem to be able to throw anything away, especially tooling. I have tried on many occasions to improve the runout on three jaw chucks and I have had some success. In my opinion however, it is just not worth the trouble. Use the three jaw for anything that is only a one operation project or use a four jaw if you want accuracy on a multiple operation projects. If the scroll is worn then you are just chasing rainbows. You might get it true in one spot on the scroll but never right on the entire range of a worn scroll.

^^^ Yup, sell it on craigslist (with disclaimer) for a welding rotator.

Before I had a toolpost grinder, I used a carbide boring bar, turned up the speed to 1200 and did a bunch of skim passes on the jaw faces. Worked pretty well for me.

I do 'em on the surface grinder and can get runout down to .001" with no problem. You can also put a bit of toe on them. The trick is determining how much to take off based on a tir measurement of a good shaft. I always seem to confuse myself. It's stupidly simple, just like me, but I finally wrote a utility for it at the bottom of this page- Conrad's Free Stuff Windows will complain about it- I should recompile the thing so it doesn't, but had almost forgotten I wrote it.

It should work OK if you skim them on the mill with a carbide cutter.

I have a 6" Buck 6 jaw that I bought in 1969 and has been on 4 different lathes. The standard jaws are worn (naturally) and I have not been able to make them work properly anymore, but the reversed jaws have had little use and still work well. I have a high quality tool post grinder as well as a surface grinder and can do all the recommended operations. The moral to the story is that sometimes things get worn to the point they are unsalvageable.

I also have a 12" Chinese 3 jaw chuck that never held work true. Although it is not that old, I decided to regrind the jaws. I set up the tool post grinder to lightly touch the jaws and ground from the front back. Near the inside end of the jaws I got a scream from the grinder and a shower of sparks. The jaws are reversable, bolted to the permanent inside pieces. The inside jaws have a small gripping area that stuck out beyond the main jaws, so I was holding work on little patches of jaw and the work could tip however it wanted to. I ground everything to the same diameter and things got a lot better.

Bill

In a pinch, a dremel or die grinder can serve as a good-enough toolpost grinder. At least for truing a 3-jaw chuck that's .020 out.

Thread a rectangular piece of aluminum with the same thread as the nose of the dremel. Clamp that bar in your aloris toolholder and you can (roughly) true the jaws. Use a ring to keep the jaws in compression.

Perhaps experienced folks get tired of replying to some questions, the comments I read in this thread are pretty funny.

This is a workable device to load the jaws for grinding. It is a thin piece of metal perhaps 1/4"thick.

Chuck Jaw Truing Fixture

When making this piece, have it so the jaws are loaded as the at just entering the center hole of the body, this will retain the factory radius on the jaw tips.

When actually grinding the jaws, the loading plate will need to be adjusted as to how far the plate is from the chuck body toward the jaws tips, this will control the actual jaw contact on the work piece.
Think about what I just said!
If the loading plate is placed right at the jaw tips, they will be held back slightly during grinding...and will contact the work more at the tips. So,... the plate will need to place at a point along the jaws to obtain even jaw contact on a work piece-tested with Prussian blue.

Basic machine tips.
Run the spindle around 200 rpm or so.
Dress the grinding wheel so it has pretty narrow contact with the jaws, 1/4" or less seems to work well on smaller chucks.

How well this works depends on the condition of the chuck, particularity the fit of the scroll on its internal pivot, I often repair that first off.

The condition of the chucks is important to what I use lathes for, I they are out, it wastes considerable time.

A pulled from scrap Buck 6 jaw, reconditioned. A chuck with trued jaws will hold a test bar true within .001" of the reading at the jaws, eight inches out .
The chucks supplied to this machine from the factory, will hold the above both 4 and 3 jaws. Thats how to get things done and make money.




That 6jaw was a nice addition to that Monarch ee, but first, the scroll pivot had to be repaired, as can be seen by that brass bushing on the scroll pivot, I used brass, because it was already the right size, I usually use steel pipe.
Most of the time, repairing the scroll pivot is good enough, other wear point can be worked also for a complete repair.

It is always entertaining reading stuff from those that have no experience and no clue, like "worn spots on scrolls", not worth the time because they cant do it, My opinion is, if you cant fix a chuck, I am sure you would struggle changing a tire.

Many, many will tell you that there is something magic about the numbers on the slots. There is NO MAGIC. The radial position of the jaws is determined by the scroll. The three slots are cut to fairly tight tolerances as radial paths and they are at 120 degree spacing from each other. I doubt that you can measure any errors in those two factors in your shop. And that is all that they need to be: along radial lines and spaced equally.

The ONLY requirement for inserting the chuck jaws is that they be in the CORRECT SEQUENCE. 1-2-3. 2-3-1, and 3-1-2 will ALL work. But 1-3-2 and other variations will not work and the error will be a big one that is easily visible.

They do NOT have to be in the matching number slots. The numbers are there to make getting them in the correct sequence easier and faster.

If you use one of the other sequences I said will work, the biggest differences you will see in the centering will be in tenths and will be more due to other factors, not the exact slot the jaws are in. ONLY the sequence matters. And trying those other slot assignments would not be expected to cure any problems.

Jason280 said:

Thank you for the suggestions, but I have tried all the normal tricks (mentioned above) with the chuck. First thing I did was strip it down, clean, and regrease, hoping that would help. I've even tried swapping the jaws into the other slots to see if the runout improved, without any success.

Click to expand...

And yes, I have done this. An old and rather worn 3 jaw chuck came with my SB lathe. It had issues. Runout exceeded 0.010" at some diameters. BUT not at all diameters and that is an important point. So I tried grinding the jaws. I did it on the lathe. And I used a Dremel tool with an extender for the tool holder that I made. Here is the action shot. I love the fireworks.



Here is the overall set-up:



You can see that I just used a tool holder and some hose clamps and some padding to hold the Dremel.

And a close up:



Here you can see that I drilled three holes in the tops of the jaws with a carbide bit. The exact locations of the holes is not important. The only requirement is that the ring is outside of the inside jaw surface that you are going to grind. It loads the jaws as they will be loaded when holding a part: well, approximately anyway.

As for the results, it was not a great success. Yes, the faces of the jaws were renewed. They came out dead straight. But, because the jaws were somewhat loose in the slots, those jaw faces are NOT square to the chuck face: they present a slightly tapered angle to the work. That is OK because for most small work pieces that is the angle that they will sit at anyway. Longer pieces will be held better by the back end of the jaws but this was going to be a compromise no matter what.

Subsequent tests showed little or no improvement in the concentricity. Before grinding there were different readings at different diameters and the same range of readings were found after the grinding.

I tribute this to the fact that although the jaws were worn, the damage to the scroll is a greater factor in the runout. Consider, the teeth of the jaws are curved to approximately match the curve of the scroll. But it is not an exact match and their curved radius must be smaller than the smallest radius that they will encounter on the scroll. So the contact on the scroll is at a slightly extended point for each jaw tooth.

The jaw teeth will be loaded each and every time that a part is chucked. So their wear will be more or less uniform, tooth to tooth and jaw to jaw. You can not load one tooth without loading all the rest of them. BUT, each part will have the jaw teeth at a different place on the scroll. Even with nominally identical parts or stock, the position on the scroll will be a bit different. Some places on the scroll will get a lot of wear due to more parts or stock of a particular diameter and others will get less. And we are all familiar with the uneven action of tightening a chuck with single pinion: we are told to use all three for the best concentricity. But that first pinion will cock the scroll plate to one side and produce unequal pressure/loading at one jaw vs. the other two. This may be due more to a scraping action as the scroll plate cocks sideways, than to actual pressure.

As far as run out is concerned, the larger share of the uneven chuck wear/damage will be on the scroll. And that means that grinding the jaws is not going to improve the runout in most cases. The scroll would need to be reground.

I purchased a new three jaw chuck shortly after those photos were taken. And I included a true adjust feature in the back plate that I made for it.

In case you are interested, here is a picture of the parts of the Dremel extender that I used:



The nut and collet are Dremel. The extended holder is shop made. The forces in grinding the chuck jaws are very light so it worked just fine in spite of the somewhat large extension involved. However, that construction prevented me from dressing the stone before using it so I was probably grinding with the high spot on that stone. I made sure that I could run the entire length of the stone past the inside edge of the jaws so, no matter where that high spot was, it did dress every part of the jaw face. Besides, Dremel bearings are not that great anyway. I made good use of the law of averages here.

Turning with cbn insert worked well for me.

If off by .020-.025 consistently, identify the bad jaw (if there is one), and take it down selectively.
With care, you could use a bench grinder. Fix bell mouth wear while you are at it !

When reduced to a repeatable error of .005" over several diameter work pieces, true up the three jaws. A soft metal round bar charged with Clover lapping paste could do that.

There is no magic in scroll chucks, just simple mechanical facts.

When chucks are made, the jaws are ground in place loading the jaws similar but not exactly like the the plate I linked to above.

If it bothers you, the scroll itself can be inspected using a proper sized ball bearing in its grove on a surface plate

The main problem of repeating error on the same size part, is the wear on the scrolls pivot, that is on the front body half of a quality chuck. Once that pivot wears more then .002", the repeat problems increase.
Also, the front body half wear pads that the scrolls face runs on... pointed out in this photo.


To repair the wear pads, the chuck halves mounted in a 4 jaw or face plate, the amount removed from the wear pads, is also removed from the body edge to maintain scroll end play.
The end play can be measured using a piece of lead wire between the scroll face and wear pad, about .002" is good.
The wear described is caused by the pinion gear camming the scroll face into the pad. The Buck pictured uses only one pinion, so the wear is concentrated at that point.
So, as the chuck is used, the pivot wears, at the same time, the scroll begins to cock as it wears into the wear pad, this is the main cause of not repeating on a same diameter.

The clearances needed on all the parts of a scroll chuck, adds to the overall problems that are not a problem on 4 jaw independent chucks. Some chucks have looser jaws and slots for example but still can be accurate, the problems always point to the internal wear points.

I can repair a scroll chuck, and re-grind all the jaw grip surfaces in 5hrs, generally ending up with a chuck that will repeat within its full range .002" or so, its worth the time to me considering the price of good chucks.

This may be helpful for beginners.

When tightening the chuck, rotate the work clockwise to help take up the jaw slop, use only one pinion, the one that bring s the work most true.
Keep in mind scroll chucks, because of their internal clearances can shift under hard, reversing or high speed use, causing the work to run out.
Be aware when using a worn chuck, that it is not causing a bind when using a tail stock, or pull on a bar to get it to line up with the tail center. If that is the case, use soft metal pads on the chuck jaws to relieve that pressure, or the work will come out of the lathe pre-bent.

My home made grinder, for chuck jaws.

I spent 50 years in tool and die job shops and NEVER saw anyone GRIND a chuck jaw. Use a carbide boring bar or a sturdy carbide tool bit. The tip WILL BREAK DOWN. CHANGE INSERTS OR RESHARPER a few times. Use dykem before the last few cuts.You will be able to see and hear if your tool breaks down,just as you would see " spark out" on a grinding wheel.I have recut some very raunchy 3 jaws this way out of necessity. And yes I have acsess to a toolpost grinder. I still use carbide,not up for debate I HAVE DONE IT MANY TIMES ON MANY CHUCKS .THIS DOES WORK.Edwin Dirnbeck

EPAIII said:

Many, many will tell you that there is something magic about the numbers on the slots. There is NO MAGIC. The radial position of the jaws is determined by the scroll.

Click to expand...

And what locates the scroll to the spindle centerline? The chuck body. Putting the jaws in the correct slots means the scroll TIR is in the same location that was in when the jaws were ground. Just like how using the marked key hole will shift the scroll the same way that it was when it the jaws were ground.

My good 3 jaws have a marked key hole, numbered slots and master jaws, and the hard jaws are numbered to the masters.