# Truing a lathe 3-jaw chuck



## Philipintexas (Oct 22, 2012)

I have a relatively new lathe that came with a "precision" 3-jaw chuck. I immediately noticed it is not completely true in that if I cut a diameter, reverse the cylinder in the chuck and cut the other end, they are not on the same axis. I realize a 3-jaw is never to be assumed to be true. Changing the 3 cam-lock mounting positions, and the 3 chuck-to-backing plate positions results in .0025" - .005" total run-out. The chuck does not mate to its mounting plate very tightly as I can easily separate them without any mechanical bind. So it occurs to me that I might true the mounting plate in the lathe and cut a tighter shoulder for the chuck body to mate with the backing plate.  There is plenty of material on the backing plate. I know I'm being anal, but that's what it's all about in this hobby. 
My question is; (1) Should I try to improve on the total runout or leave well enough alone? 
(2) Has anyone been successful in improving a 3-jaw in this way?


----------



## dave-in-england (Oct 22, 2012)

A standard three jaw chuck by it's very design is not going to be super accurate for repeat operations.

If you need good and consistent repeatability you should think about buying a collet holder and collets for the lathe.

dave


----------



## syrtismajor (Oct 22, 2012)

Three jaws can be a pain for accuracy. Personally I use a four jaw as they are adjustable until each part you're working on is running true. This is incredibly helpful if you're working with parts that are not hexagonal or round, it also gives more grip on rough castings.


----------



## bb218 (Oct 22, 2012)

I agree with Dave that collets are better but if you open up clearance on the piliot you could loosen the chuck bolts and indicate a round part then snug up the mounting bolts. At least the chuck should be really close at the diameteer of the round part you selected.


----------



## n4zou (Oct 22, 2012)

bb218 said:


> I agree with Dave that collets are better but if you open up clearance on the piliot you could loosen the chuck bolts and indicate a round part then snug up the mounting bolts. At least the chuck should be really close at the diameteer of the round part you selected.



That's what I did. I undercut the back plate shoulder and doubled the number of bolts holding the chuck on the back plate. This allows moving the chuck slightly to get nearly perfect alignment for that diameter.


----------



## lensman57 (Oct 22, 2012)

Hi,

I have consistently had the same problem with any chuck back plate combination. If your lathe is of a Chinese origin then you are correct in wanting to true the back plate on the spindle as these back plates are generic. Both the face of the back plate and the mounting boss-register , if it has any that is, have to be faced and trued on the spindle. Then try to mount the chuck in different sequence to see if it makes any difference, if you achieve a consistent .002" run out you consider yourself very lucky. I once machined the boss register of a back plate for my SIEG C1 about 0.002" undersized and then used the play in the mounting screws to bring the run out to within .002". This proved to be a futile effort as the next time I mounted a piece to machine, the run out was back to 0.004" due to the inaccuracy of the chuck itself. As suggested a 4 jaws independent chuck or a collet sysem such as ER32 or ER40 are the only options if accuracy is your aim.

Regards,

A.G


----------



## lennardhme (Oct 22, 2012)

After truing the back plate try rotating the chuck in its mounting holes & checking each position with a DTI. You will probably find one possie better than the others. If no good THEN drill out the holes. Remember drilling out the holes will require the chuck to be indicated each time you use it, or at least each time it is remounted.
Leonard


----------



## jerryc (Oct 23, 2012)

If you check your machine specs, .002/.005 TIR is about industry standard for a 3-jaw chuck. For closer work you can go to a collet or a 4-jaw chuck.The choice is yours. Collets are usually within .001 or so, some brands like Hardinge seem to run within .0005 while the ones sold by somebody like _Horrible Freight _not so close.


----------



## kvom (Oct 23, 2012)

For best accuracy use a 3-jaw chuck with 2-piece jaws.  Make soft jaws for the uppers and bore a cavity to match the work.  No runout for as long as the chuck is not dismounted.


----------



## rake60 (Oct 26, 2012)

I have a method, but it isn't foolproof.

I take the largest washer my 3 jaw can grip and bore it out to make a perfectly round I.D.
Then I chuck OUT on that I.D. and dust through the I.D. of the jaws with the boring bar.

It works great with the 3 jaw chuck that came with my Grizzly G4000 9 X 19 lathe.
However, if the I.D. and O.D. gripping directions in the scroll are not the same the results 
could end up being worse. 

If that would happen, just chuck up a piece of round stock in the I.D. of the chuck back 
behind the raised teeth and dust through it again. That would have you back to the original 
condition.

Rick


----------



## Tonnetto (Oct 27, 2012)

There is another option, but it's a little pricey. There's a 3 jaw chuck on the market called a Buck Chuck, it's essentially a standard chuck with a floating 2 piece baseplate that has 4 set screws, once the piece is clamped into the chuck you work the 4 screws like a 4 jaw chuck to bring in near perfect runout.


----------



## goldstar31 (Oct 27, 2012)

Tonnetto is probably refering to what we Brits call a Griptru chuck which has adjustment screws in addition to retaining screws. And they are pricey!

In addition, there is an old dodge of elongating the screw holes a bit and knocking the chuck into alignment and then tightening up. 
It doesn't work for three jaw chucks which have the 'backplate' integral with the chuck body itself. I can add that ALL mine are. You then have to think in terms of soft jaws which will be machined each time a holding dimension is changed.
Again, soft jaws are not cheap or easily available and the workaround is to insert soft( probably brass) between the jaws and the work and cut them to hold precise accuracy as for proper soft jaws.

So back to the comment about accuracy and the fact that most decent chucks will hold no better than about 3 thous- and that is when they are new, unworn and unstrained.
When chucks are worn or whatever, there is no solution to regain accuracy by grinding or whatever other than buying a new one.

There may be a bit of disagreement with my last comments but you need a  very fancy tool and cutter grinder to bring the chuck to truth. My grinder will do it but I can't


----------



## jerryc (Oct 29, 2012)

A simple cheap tool post type grinder will often do the job. Place a washer the approximate OD of the stock you will use and grind to fit.


----------



## goldstar31 (Oct 29, 2012)

Indeed you are quite correct but you will only be accurate at the diameter of the washer.
Then you want something of a different diameter- and you are back to square one.

I've done it. Initially I used a valve grinding compound and then I used a Bosch POF 50 router which is or was a tool post grinder. I then used the spindle from a Quorn t and c which I built up long long ago. Now I have still the Quorn, added a Kennet and made a Stent and bought a Clarkson mk1. 

I'm glad that someone likes the post. As a somewhat old and very jaundiced old fart, is there a do dah which will say NO?


----------



## lensman57 (Oct 29, 2012)

Hi,

As a total novice, I can true a piece of work in an independent 4 jaws chuck within a minuteat the longest, using two chuck keys in push and pull manner. This is with somethig as small and fidlly as my Taig or Sherline lathes. The 3 jaws have their use for convinience and once a truing cut has been made the work stays there for the duration of the operations where possible. I have spent quite a lot of £££ and wasted a lot of time looking for the illusive accurate 3 jaws that does not exist or none that I could afford. The best that I can come up with is about 0.003~0.005" with a routine set up. I suggest that if accuracy is required either a collet system 0.0015~0.002" or a 4 jaws independent be used. The only advantage of the expensive 3 jaws chucks over most of the import variety is a slightly betetr level of accuracy and much better construction and materials that will help the chuck retain its accuraccy longer in it's life. 

Regards,

A.G


----------



## goldstar31 (Oct 29, 2012)

There is a subtle difference between a chuck with a worn scroll or bell mouthed jaws and a knackered set of guides and a chuck which will hold a TIR of perhaps 0.003"be it English, Chinese or from the Greasy Limpopo. Sorry, there is!
Again, this collet thing is somewhat confusing. I have, I think four different sets and the arguably best set are standard  Imperial Myford issue in the Good Old Days! They aren't and they are. They are fine for holding Imperial round bar but only if the bar is precission ground or I have turned mild steel down to an exact size. Round bar can be anywhere 3 thous or more down ie 1/2" bar may be 0.497" and this will either rotate in the collet or break the collet if tightened up enough. It's accurate stuff but only if it has exact stuff. The other set is cheap and nasty Chinese and not nearly so repeatable. The other sets are 8mm watchmaker's and a set of ERX's for a even cheaper and nastier( you claim) Taiwanese Unimat clone. Oh, and I have just remembered that I have two more sets.
Silly me- but it is worth mentioning to avoid someone hearing his precious collet go crack.

I sort of know about chucks but broken collets- not really my thing


----------



## rkepler (Oct 30, 2012)

If the error change with the change in cam and pin mating then the chucks isn't mounting to the camlock taper face correctly.  Try blueing the face of the spindle and see how good the contact is with the backplate, correct the backplate to make it consistent.  I've used Plastigage on the face of the spindle to see if I was getting 100% contact, just hold it on with a dab of grease or oil.  Mark the pins with the cam numbers so that you always use the same mounts and the chuck should return to the same position on the spindle.

Once you're sure of the backplate mounting on the spindle take a pass on the face of the backplate, and a little cleaning pass on the outside to make things square and concentric.  I don't worry a lot about the register being a press fit with the chuck back recess but feel free to do so yourself.  It should mount cleanly, make sure that the boss on the backplate doesn't bottom out in the chuck.  If you want take a little time and blue the interface between backplate and chuck back, you should have a decent and even transfer.

When you mount you can tighten up the screws a bit then knock the chuck to center if there's a little clearance in the back.  A lot of chucks have the jaws ground true to the outside of the chuck so this should help bring the jaws to center.

If the jaws don't come to center after this try cleaning the insternals of the chuck, paying close attention to the fit of the scroll to the center boss - if it's loose nothing will ever center reliably.  If things look good clean them up and reassemble.  

If there's still error characterize the error - is it the same on the same test piece?  Does it change with the side of the test piece?  If it's about the same all around you could regrind the jaws, if it's all over the map the shuck was likely overstressed at some point (likely a gorilla using it) and there's not a lot you can do to fix it.

I do most of the above as a matter of course so I don't really know if it's improved any chucks.  But most of my chucks can be mounted and function as I want them to immediately so something must be right.

(BTW: .002-.003 is likely in the chuck spec, if you really want something better that you can slap on and make work you'll want to investigate a draw-in chuck system such as a 5C collet chuck.  If you have soft jaws or an adjust-true chuck you can make a particular diameter have a low error, but will have to adjust again when you move to something else.  There are a lot of things that you can try, the above is just a summary.)


----------



## modelman1838 (Oct 30, 2012)

Hi 
I had cause to remove my 3 jaw and replace with the 4 jaw about six years ago and the 3 jaw has only been back once when I needed to machine a 6 spoke flywheel by locating on the inner cast rim, the 4 jaw was then back straight away. As lensman 57 says it take no time to clock up true using 2 keys and in model engineering any excentricity is too much, so why not have it spot on every time.
Hugh


----------

