Truing up chuck jaws

I just got a 3-jaw chuck fo my Taig lathe. It comes with two Tommy (?) bars, a hex key, a washer and a set of instructions. The jaws are aluminium and unfinished, the instructions tell you how to turn the jaws true.

Before I started I measured the runout using a 1/2" drill rod. It came to


I did as the instructions told me. I used the washer provided, chucked it up at the back of the jaws, made sure it was nice and flat and carefully turned

0.002" off the jaws. The I filed down the little nubbins at the back of the jaws where the washer was being held during the procedure. I re-measured the runout: This time it was 0.004". I inspected the jaws: There was evidence of "clean-up" on all three, the filing seemed satisfactory (I touched up one of the jaws just to make sure).

I was puzzled by this poor result. I could not think of an explanation. Then I measured the washer and it turns out to be out of round by 0.004".

I have a number of questions:

1) Is 0.004" TIR satisfactory for a small 3-jaw chuck? I suspect that far from it but I do not want to be unreasonable. 2) Is the washer the most likely culprit? 3) How to rescue the situation? The obvious solution (assuming the washer is the culprit) is to find something tthat is perfectly round, chuck it up again and repeat the procedure. Presumably the object will have to be thicker than the filed-down portion of the jaws so it is held by the turned down portion but I may be wrong. 4) What object to use for that purpose? The best I can think right now is to get a piece of aluminium bar and turn and face it in my 4-jaw chuck and then part it off at the correct thickness. 5) If one used an object that is too thick or repeated the procedure a few times the turned down (and hopefully true) portion of the jaws will become smaller and smaller. What is the minimum size of the gripping portion of the jaws to provide secure workholding?


Reply to
Michael Koblic
Loading thread data ...

Even though the washer is out of round, any three points on its outer edge define a (perfect) circle. So, unless the washer is so wonky that one of the jaws wasn't touching it, I don't see how it would make any difference.

Reply to
Ned Simmons

Most of the time .004 TIR is much better than what is needed. Consider if you hold something by your four jaw chuck and deliberately have it not centered by say .100. Now without removing the work from the chuck, you machine the piece until part of it is .500 dia and another part is .250. And you part it off. The part you machined is round and the fact that it was not centered exactly when you started did not make any difference.

Of course if you have to remove the part and then grip it say from the other end, well that is a different story. Now you have to mark the part and one jaw so you can put it back so the TIR does not matter.

I think the washer is the culprit. Although the three points that touch the washer are three points on a circle, the center of that circle is not on the axis of rotation.

Note that you can turn something perfectly round whether or not it started out as round or square and this can be done in either a 4 jaw or your three jaw chuck. So you can make your own washer. Or use a bolt and a couple of nuts to hold the washer and turn it to be round.

You can make a washer with a large hole, and hold it with your three jaw chuck touching the hole, not the outside. Then use a boring bar on the inside of the jaws. That way you do not have to file the little nibs off. Then check to see what the TIR is. If it is not as good as you want, then use the washer again and file the little nibs off.

I personally would not try to get the TIR lower, until you try checking what the TIR is with different sized round things. The TIR can be perfect at one diameter and off at another if the scroll in the three jaw chuck is not perfect.

=20 Dan

Reply to

If the center of the circle is not on the axis of rotation, it is because the jaws are not equal distances from the axis. Otherwise it will be on the axis, regardless of what's being chucked.

What might not be on the axis is the center of the washer. But the location of its center is irrelevant.


Reply to
Bob Engelhardt

Check runout before removing the washer, should be dead nuts on. Open and close jaws back on the washer, recheck. The washer has nothing to do with your runout. The three contact points is true, (as someone else said) no matter how much runout you have before cutting. When all three jaws are trimmed, should be .000 runout.

Reply to
Rick Samuel

O.K. Did you tighten the screws holding the jaws as you installed them? Did you press each jaw outward as you tightened the screws? (Otherwise, they are likely to shift a bit under load.)

The two-piece jaws for larger chucks have both a groove along the length to keep the top jaws parallel to the master jaws, and a projection from the master at right angles to the jaws and a matching cross groove on the underside of the top jaws so they always have the same length projection -- no depending on the fit of the screws as the Taig chuck does. (For this reason, on the Taig, I mount the jaws, turn them to dimension and use them without ever removing them.)

Hmm ... IIRC, the washer contacts only the hard jaws below the top (soft) jaws, not the soft jaws so there are no "nubbins" left -- unless you are turning a step to both hold and support a disk-shaped workpiece.

Not too bad -- depending on the age and quality of the chuck. I would expect 0.001" or better on a brand new quality chuck (like the Austrian made chucks for my Compact-5). But those have only one-piece jaws -- either hardened jaws with steps already made, or soft jaws which go all the way down to the scroll plate as one piece.

First thing is whether there are multiple holes for the tommy bars. If so, try with each one (using only the master jaws to clamp with at first) and see which one gives the best concentricity. Then

*mark* that tommy-bar hole in the body so you can always use it for the final tightening. (In the case of chucks which tighten with keys, if there are three sockets, one should be marked with a "-0-" or something similar by the manufacturer. Some have only one socket so there is never any question.

Mine (an older one) only has tommy bar holes on the scroll plate, none on the body, so I usually put a Crescent wrench around the jaws close to the body to hold it from turning when I use the Tommy bar on the scroll pate.

But once you have bored the jaws to the proper size for the current workpiece, you should get well under 0.001". It is only when you move the jaws to grip a different diameter that you can get significant change in runout -- especially if the scroll plate is not truly concentric -- or is loose on the projection of the body so it can shift from side to side as you tighten.

It could have been a contributing factor -- as could shifting of the jaws between the truing and the clamping of the test workpiece.

Make it so it will fit behind the soft jaws in contact only with the master jaws. You may have to unscrew the chuck from the spindle to get it into place properly.

Huh? You mean the contact *length* along the faces of the jaws? Not if you do the clamping only with the master jaws and turn the full length of the soft jaws. For most things, you *want* the full length of the soft jaws in contact with the workpiece.

The exception is when you are making soft jaws to hold a disc, so you turn it true only for a short distance (a little less than the intended final thickness of the workpiece, so you can face both sides in the chuck.

And you should have multiple sets of soft jaws. Either buy more, or machine some aluminum in your small mill (which should be large enough for this task) to make extra jaws. Make one set for general purpose -- a stepped set for standard ID gripping, and a reverse stepped set for larger OD griping. Depending on the precision you need, you can often get away with turning the existing jaws end-for-end.

Whenever you make a set of jaws, use a number stamp set to mark the jaws for position 1, 2, or 3, so when you put them back on, they will be on the same master jaws.

That depends on a lot of things, including the material of the jaws (aluminum in this case, but hardened steel for hardened top jaws for larger chucks, or mild steel for soft top jaws for similar sized chucks. Softer jaws, or softer workpieces need larger contact areas. And when you are holding a long workpiece and turning some distance from the chuck jaws, you need the full length of the soft jaws' faces to minimize tilt in the jaws.

There are other ways to hold the jaws preloaded for turning. Do web searches and find them.

Good Luck, DoN.

Reply to
DoN. Nichols

Unless the shaft you are chucking is the same diameter as the washer you used to cut the jaws, you will get an error caused by the scroll that moves the jaws in and out.


Reply to

You could stuff the test bar in so it contacts only as much as the washer did.

Personally the way I use a 3-jaw, all that matters is that the jaws are parallel when tightened, so the work doesn't wobble. And if it does anyway, like the stamped head of a bolt, I'll support the end with the tailstock. You've probably achieved parallelism already unless the jaws tilted. They shouldn't have if the washer was back near the scroll.

Plan the job so you can make the finish cuts on all surfaces without loosening the chuck. It doesn't matter for roughing as long as you leave an allowance larger than the runout.

I doubt you will ever get the 3-jaw to run true enough that you can reverse the work and make the cuts from both ends meet invisibly. That's difficult even with a Set-Tru, 4-jaw or collets, and a good reason to turn between centers. You could make a gnomon with extra metal in the ends for the center holes and then part them off later in the 3-jaw. Or turn to a step or groove from both ends, a little runout won't show across it.


Reply to
Jim Wilkins

They were tight already.


No. The soft jaws cover the whole thing. There are nubbins...

This a is a US-made new Taig chuck.

There are three pairs of holes. I did play with them but it did not seem to make any difference.

This is seems to be the recurring motif. I did not appreciate that this is a feature of scroll chucks.

Not possible. See above.

Again, not possible.

Done that. Mainly on account of being sure that I would roger the first set of jaws and would need the spare. I was not going to touch anything though until I understood the current problem.


OK. Bigger is better.

Not too sure that I understand what that means. I shall hit the books.


Reply to
Michael Koblic

Thank you and all the others.

The critical bit of information seems to be that 3-jaw scroll chucks do not necessarily hold true at all diameters thus trying to improve on the 4 thou of runout would probably put me on the flat portion of the diminishing return curve.

I am not set up for turning between centres yet - that comes next and will invove cobbling some sort of centre for the headstock and a dog (apparently the cognoscenti like a hose clip for this). This will invove turning things to a point so a compound slide will be in order. Another issue is facing off longish cylinders (the inner diameter of the spindle is only 5/16") and I shall be needing a steady rest.

But all of that had to be put on hold as I have not had a decent arrangement to grind lathe bits. I finally finished it today.

This is worse than having a baby.

I wonder at what point one returns to some semblance of productive work rather just continue making tools for tools...:-) It has become a running joke in the family.

Reply to
Michael Koblic

Not sure about the construction of the TAIG chuck and how it operates given the mention of a tommy bar and looking at the TAIG site but all the 3 jaw self centering chucks I have are marked with a master pinion which gives the least run-out when used. The chucks I have all have 3 pinions which operate the scroll and one produces the least run-out. As the TAIG is a scroll chuck, from what I read, then if you have more than one hole for the tommy bar to tighten the chuck, then maybe you want to mark and always use the same hole to ensure consistancy, that may improve you run-out. Maybe also tighten using different tommy bar holes and see which produce the least run-out.

Reply to
David Billington

Tight -- but perhaps not preloaded when tightening. There is some slop in the screws fit into the soft jaws which can allow them to shift if you don't pre-load the jaws towards the outer diameter while you (or the factory) are tightening the screws.

O.K. Take the chuck off the lathe.

Adjust the jaws to the point where the hard "master" jaws are level with the OD of the chuck body. The soft jaws will stick out beyond that point.

Now -- using a small flashlight, look in through the threaded aperture in the back which screws onto the lathe's spindle. You should see the ends of the master jaws sticking out about 1/16" or a bit more. Past those you will see the soft jaws closing to about the right size to hold a 1/16" diameter workpiece (minus what you have removed in truing the jaws).

Now -- turn something (a plug) which will just barely slip in past the threads, and about the length of the master jaws. Close the

*master* jaws tightly onto that, and re-mount the chuck on the lathe spindle.

Now -- use a drill bit to slightly enlarge the hole in the center where the soft jaws almost meet -- and then use a boring tool to reach down through there and enlarge the hole a little. If you intend to hold something of a known diameter, drill a little undersized for that and then bore to barely fit that. Then loosen the jaws, remove the plug which held the jaws pre-loaded, and tighten the jaws onto your workpiece. If you are going to hold a disk instead of a shaft which will fit through, bore to leave about 1/2" of the jaws near the body (which should clear the screws which mount the jaws), and to the diameter of the disk to be held.

I thought so, but I was writing for more than just you. Note that I covered "quality" above as well -- The Austrian chucks for my Compact-5 sold for more than your Taig cost complete with both chucks. Each one comes with a certificate of accuracy -- when brand new, and my examples were more accurate than the certificate promised.

Hmm ... you don't really want to call them pairs, since depending on the size of the workpiece, any one of the scroll plate holes may be used with any one of the body holes. And the hole which matters is the body hole (which mine does not have), not the scroll plate hole. You use the scroll plate hole which is the closest upstream from the preferred body hole as the jaws just start to touch.

A scroll-backed (universal) chuck is mostly for quick setup where the concentricity is not too critical -- e.g. when you are going to turn all the features in one setup and then part off from what is held in the chuck.

If you want *precision*, you take the time with an independent

4-jaw chuck to do the setup with a sensitive dial runout indicator.

But another use for a 3-jaw is with either reversed jaws (assuming one-piece chuck jaws), or with soft jaws turned to make reversed steps to hold a larger diameter disk shaped object. If you want to handle even larger, make a set of extra-long soft jaws for the

3-jaw you have and bore them to a close fit so you can clamp the jaws onto the workpiece with very little motion of the scroll plate. You don't really need precision centering for this, because you will be using it to face the workpiece not to turn the OD.

You can also reverse the jaws on your 4-jaw chuck, so the jaws look somewhat like this (use a fixed-pitch font like Courier to avoid

*serious* distortion of the image): +--------+ +--------+ | | | | | | | | +---------+ +---------+ | | | | | | | | | | Jaw #1 +---------+ +---------+ Jaw #3 | | | | | | | | |____________________________| |____________________________| +---------------------------------+ +---------------------------------+ | Chuck Body | | | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.......~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

so you can grip larger diameter stock -- though not as large as you can with the 3-jaw with custom soft jaws. Just be careful that the custom soft jaws are not so long that they will hit the bed.

See description above. The preload filler contacts the hard master jaws, not the soft jaws which you are turning to true them up. (Preload because it holds the master jaws in contact with the scroll plate the way they will be in normal use.) Note that the advice which you got in one of the other followups yesterday, which suggested a large ring and backing the jaws out against that is not good for when you are trying to true the jaws for normal gripping, as it preloads against the wrong face of the scroll and the jaws' matching teeth. However, it is

*good* advice if you are turning the OD of the jaws, or turning steps on the jaws to grip the ID of a workpiece, since in this case the jaws and the scroll plate will be preloaded in the direction in which they will be used.

Note that if you want real repeatable precision, you want collets, and the good collets for a Taig spindle (not the standard one, but the special one) are the WW style drawbar collets, which are size limited -- 3/16" max for pass-through, and 1/4" for short gripped stock.

The standard collets which come with the lathe will handle larger workpieces, but are not as precise. If you want precision for anything larger than 1/4" with this lathe, you will have to use the

4-jaw and a good dial runout indicator.

Hmm .... I wonder whether they make an ER-16 or ER-25 collet nosepiece for this machine?

Yes -- it *is* possible -- if you follow my suggestion at the top of this for a plug for preloading the *master* jaws while you are turning the soft jaws.


Remember that you can also *make* new soft jaws -- use an unmodified set as a pattern -- and you can make them over-long to grip larger diameter disk workpieces.

And if there are no markings beside the jaws on the chuck body (there are none on mine) use the same number stamps to mark the jaw ways, starting with a randomly picked jaw as 1, and increasing as you move in the direction that you turn the scroll plate to tighten the jaws. Note that you will not be able to stamp the master jaws -- they are too hard. But at least my chuck won't turn far enough to take the master jaws out -- you need to remove the circlip on the back and withdraw the scroll plate to do this -- and you should not need to do so.

O.K. Now there *are* markings -- I went down to do that while verifying the capacity of the WW series collets. I used 1/16" number stamps which are a bit small, but are certain to fit on a part of the side of the soft jaw which is not likely to be turned off.

Instead of tightening the jaws -- or the master jaws as I have suggested above -- onto a washer or a plug, you can do other things:

1) Put three pieces of the same thickness between the angled faces of the jaws as you close it. I would suggest 3/16" or 1/4" HSS tool bits, as they tend to be rather precise in dimensions. 2) Drill three holes to form an equilateral triangle around a large center hole, tap for screws (say #10 or maybe 1/4") and place those screws so they project into the holes for the heads of the screws which hold the soft jaws to the master jaws. Place these into the holes and tighten the jaws. You may want to make it with several sets of screw holes for different jaw positions. You bore the jaws through the center hole. 3) Three larger holes in a triangle with a center hole bored to intersect the three so the tips of the jaws stick through for turning while the holes' walls press on the angled faces of the jaws while you are turning.

I hope that this helps. DoN.

Reply to
DoN. Nichols
[ ... ]

Very good advice.

Yes. How close to 0.001" you get is a function of how much you spend for the chuck -- and the precision of the lathe spindle setup too. The Taig is likely to shift a bit every time you remove and replace the chuck. Try it with a piece chucked up and see what happens.

Interesting -- but I guess that it will work. For the Taig, I use a small dog which came with my Unimat SL-1000.

Also -- ideally, you will want a live (ball bearing) center for the tailstock end. That way you don't have to keep lubricating the center.

Yes -- you will.



The primary purpose of *any* home shop is to make tools for the tools. Any actual products other than this are a matter of luck -- or testing the tools which you made. :-)

Enjoy, DoN.

Reply to
DoN. Nichols

[ ... ]

No pinions on the Taig 3-jaw. The scroll plate is bare on the back of the chuck, with three holes for Tommy bars drilled in its edge for tommy bars. The chuck body (thin compared to one for pinions) may also have three holes for Tommy bars. (Mine is an old one, and does not have the holes in the body, so I have to grip the jaws. :-)

Actually -- the closest approximation of the "same pinion" in the Taig is the same hole in the chuck body. The scroll plate should use the nearest hole approaching the master hole in the body to maintain an approximation of the same pinion effect.

The chuck has two-piece jaws, with the master jaws being hardened, and captive unless you remove the snap-ring and scroll plate (which should be avoided unless necessary). The top jaws are aluminum, with a groove to maintain alignment with the master jaws, but without a cross-groove to maintain the position of the soft jaws along the master. They depend on the fit of the screws which secure the top jaws and nothing else.

Yes -- pay particular attention to which tommy bar hole in the body gives the most consistent runout. It will be more sensitive to that than to the tommy-bar hole in the scroll plate.

Enjoy, DoN.

Reply to
DoN. Nichols

I saw this today, and bookmarked it to post as relevant to this part of this thread:

scroll (way) down to where you see:

====================================================================== Part Two - Extra Long Soft Jaws ======================================================================

(It looks like it is about 2/3 to 3/4 of the way down the web page.)

Anyway -- this is what I was talking about for making the long soft jaws. You can also see the difference in the way the jaws are made by comparing the first photo with your jaw. It has more features to assure alignment. By the end of the page, you can see it being used to make a disc like what you would use for your sundials.

Enjoy, DoN.

Reply to
DoN. Nichols

This is good stuff! Duly bookmarked, thanks.

Reply to
Michael Koblic

OK, I understand the procedure. Clearly this could have been done neither as per the factory instruction nor using the kit provided.

I am still not clear about the purpose of doing it this way. I understand the purpose of pre-loading the jaws (as per the info I have found so far) is to prevent the cant of the jaws outwards at the tips if the workpiece is inserted only part way.

Let us assume that the soft jaws are loose on the screws attaching them to the master jaws. If one pre-loads the master jaws as you outlined and then tries to turn the soft jaws they will not be supported and the cut will be made in a way that will leave the surfaces in no relationship to what they would be eventually when tightened on a workpiece. If, however, one follows the procedure as outlined in the OP, the base of the soft jaws is tight against the washer. This pressure will push the jaws tight against the screws and transmit the pressure further to the master jaws which should thus become pr-loaded in a manner identical to yours. Thus everyhting should be nice and tight and identical to a workiece with a diameter identical to the diameter of the washer being held in the jaws. The jaws are fixed and turning will remove the material form the right places.


I went back to this coincidentally this afternoon before reading your post and quickly realized that this is the case. The reason I did not get any helpful result before is that I did not appreciate the difference between the holes. In fact after correcting for this I could get the run out as little as 0.001" using a specific hole on the body which is now clearly marked. Problem solved!


You also feel the death approaching that much more quickly...

Holding large objects is one thing, turning them is another. I do not care for the noise the little fella makes when I order him to do this.


I did that and then turned and faced a 4.5" washer. The bottom cleared the ways by about 1/8" and getting to the side of the disk was a little tricky. Not to mention doing it at 575 rpm...


I started up on that. Inadvertently. The first parting tool I made just did not cut it. It spun the toolpost and shaved the jaws before I realized what was going on.

But the advice on turning the OD jaws is noted. I also found some good stuff here:

formatting link
related to what the shape of the jaws should be etc.


Scratching has worked well so far. Engraving also a possibility.


No. My head just exploded. OTOH the second (re-worked) parting tool works just fine. Once you get past the squealing and screeching of the bigger OD down to about 3/8" :-) Gotta stay positive...

Reply to
Michael Koblic

Did I not mention it? It is on my Lee Valley wish list...

It nearly burned down today. See my other post.


When I look at the list of things that are needed and compare it with the work rate so far it is clear that this year will be written off unless I start throwing money at the various issues. OTOH that would be nowhere near as educational...

Reply to
Michael Koblic

Well ... it would have been possible to put the washer in there behind the soft jaws and in contact with the master jaws.

That depend in part on the rigidity of the chuck and the jaws. Your top jaws are aluminum, so they will give more with given force than the steel ones will.

Then first tighten them somewhat onto a cylindrical part, and then tighten the screws mostly. Then tighten a bit harder onto the cylindrical part, and firmly tighten down the screws the rest of the way. (I guess that I should suggest a torque limit, but do you have a torque wrench which measures in inch pounds anyway?) I typically go by the spring of the Allen key's handle as an indication of torque.

Right -- which is why the first step is to make sure the jaw's screws are tightened while the jaws are preloaded as above.

Ans as soon as you loosen the chuck to remove the washer, you also let the jaws shift, because you did not take pains to tighten the jaws' screws under preload first. :-)

[ ... ]



Nope -- you are too busy setting things up to notice that. :-)

Keep the extension of everything you can as short as possible.

That is what the soft jaws are for. If the washer (or other disc) is thinner than the steps of the jaws on the 4-jaw, you take a new set of soft jaws for the 3-jaw, and turn them (after preloading) to a step just deep enough to allow facing both sides. You can actually make it a little deeper, and just expect to machine off a bit of the height of the soft jaws as you do the first workpiece. Remember -- the parts of the soft jaws are expendable. :-)

I would swear that mine will go slower than that. How many belt steps are on your pulleys?

And you could replace the motor with a three phase or a DC motor and connect an appropriate controller to it to get speeds perhaps 1/6th your current speed. That would take you down to 100 RPM or a bit slower. My 12" Clausing will go down to 210 RPM in direct drive, and if I engage the back gear, it will go down to 35 RPM. The corresponding highest speeds are 1600 RPM and 270 RPM. Having a variable speed motor would be somewhat the equivalent of having a back gear -- except that it would not boost the torque the way a back gear does.

Clamp some heavy paper between the toolpost and the table or compound and it will increase the holding power by fitting into

Of course, this is written from the perspective of someone who is after 0.0001" runout -- and is working with a machine capable of that. :-)

But it does have good information -- including the ring and pins in the jaws' mounting holes which is shown in the first photograph. It looks as though the pins are a permanent part -- under the heads of the outer bolts holding the top (soft) jaws to the master jaws.

O.K. Be sure to use surfaces which are not critical -- perhaps the outer ends of the master jaws. A carbide-tipped vibrating engraver (Burgess Vibrograver or equivalent) will probably do.

Those are not all used at the same time. They are three different ways to preload the jaws without getting in the way of the boring operation. It helps to find the web pages illustrating the techniques. I've seen (1) and (3) above illustrated in web pages.

Keeping the amount or projection down to a minimum helps a lot. Making sure that the sides of the parting tool are parallel and perpendicular to the axis of the workpiece helps. Having the parting happening as close to the chuck's jaws as possible helps. Getting lubricant to the bottom of the groove helps. And ideally, not having a compound as part of the stackup will help minimize the flex of the system.


Enjoy, DoN.

Reply to
DoN. Nichols


Ouch! I'll look for that. But it is rather late tonight, so I might not see it until later today. (It is now approaching 1:30 AM. :-)


And the more you learn, the less damage you will do to purchased tools. :-)

Enjoy, DoN.

Reply to
DoN. Nichols

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.