# Repeatability with a three jaw chuck



## wareagle (Nov 9, 2008)

I have a simple trick for getting decent accuracy with a three jaw chuck. There are some limitations with this trick, and I will outline those in the post as we go. I will also make the statement that this may not work for you, and I would verify the results before working on an actual part. 

*Fair warning:* there are times and applications where a three jaw chuck will not suffice, so use the four jaw when needed. Accurately locating a part in a four jaw chuck is not a hard task, and I encourage anyone not proficient in this to practice it! You don't even have to make any chips as long as you have a perfectly round object.

Okay, if you are machining something in the three jaw chuck, and will have to remove the part for other operations before being placed back into the lathe, then this tip might safe some time and grief. I have been able to get repeatable results to within .002" TIR (total indicated run out). Your results might be better or worse depending on the chuck and other factors.

First, you will need to "key" your jaws in the three jaw. I have keyed only two in mine, one is blue and the other is red. A dab of paint would be the best option for this marking. In The picture below, note the red dot on the top of the jaw.







The next thing you will have to do prior to removing your workpiece form the chuck is to mark the work piece to correspond to the specific jaws in the chuck. I use a permanent ink marker (which is a misnomer; the ink is far from permanent unless it is on your walls or cloths) to mark the workpiece. Be aware that cutting oils or other fluids may remove the permanent ink, so use a suitable method of identifying your work piece. Below I have a fixture "keyed" to the jaw. _Note the blue line next to the jaw._






Once your workpiece is "keyed", you can remove it from the three jaw and do what ever is needed outside of the lathe. Once you are ready, simply place the workpiece back into the lathe "keyed" to the corresponding jaw and your set to go again. The one thing that will throw this off is having dirty jaws. The jaws must be clean prior to
the first set up and any subsequent set ups. 

I have done several tests using this method with my lathe, and the repeatability is usually .001" TIR, but I have never seen it more than .002" TIR. If you are machining something that can that much error, then you are set as long as your equipment produces repeatability inside the tolerances allowed for your task. To test you three jaw for repeatability, take a piece of bar stock and turn it down so that you have a good round surface that is concentric with the center line of the lathe spindle. In other words, a simple clean up cut will suffice. 

Take a dial indicator mounted to a stand (or other preferred method) and take a reading to see where you are at. You should be dead on, if not then take another cut and double check. If you can't get a dead on reading, then you may have another issue with the lathe that is outside the scope of this thread. 






Once you have verified your reading, key your workpiece to the corresponding jaws and then remove it. To make an accurate assessment of the repeatability, I have taken another piece of material (different diameter) and clamped it into the three jaw, followed by removal. Te test piece is then placed back into the three jaw and is keyed to the corresponding jaws. Once it is clamped back into place, take a reading with the dial indicator and see what the TIR is. Do this procedure a few times making notes of your findings. 

This will give you an idea of the repeatability that you can expect using this method. Again, I will warn anyone using this that it is not a substitute for taking the time to set up the work piece in the four jaw when accuracy has to be right on, but for the quick and easy tasks, this is an easy method of getting repeatable results in the three jaw. One area this will not work is when you have a work piece that needs to be located accurately, but hasn't been machined and keyed with the same chuck.

Hopefully this will save some time for those looser tolerance jobs. And I would be interested on the results from others.


----------



## Stan (Nov 9, 2008)

One thing I would add to that. You have to use the same pinion when you tighten the chuck. 
My experience with three jaw chucks is that each pinion gives a different amount of runout. You can check this by marking the pinions and then tighten a piece in the chuck, measure the runout and then loosen the same pinion and repeat the test again with each of the other two pinions


----------



## jack404 (Nov 9, 2008)

Stan i agree with that 100%, hence i tighten all three every time, since started doing that a fair few years ago i have had a lot less surprise "now how did that move?" or "why is that not centre?" situations


----------



## mklotz (Nov 10, 2008)

Yes, it would seem that it matters which pinion you use to tighten the chuck. Here's a description of my experiment on that subject that I wrote for our metalworking club. It's admittedly anecdotal but may encourage some of you to perform some experiments with your own equipment.


---------------------------------------------

I'd heard and read about this for years. Basically, the premise is:

---
On a self-centering three jaw chuck, there's one particular pinion (the
so-called master pinion), which, if used to tighten the stock being held, will
center it with better accuracy than would be the case using any of the other
pinions.
---

So, I'm futzing around in the shop today, not wanting to start another project
before going to MMM, and I decide to test this out.

Now, understand, I do not consider the 3jaw chuck a precision tool. Its real
advantage, if it has any, is its ease of use and wide range. If I'm turning
something where all the cuts can be made with a single chucking, thus
guaranteeing coaxial features, fine, I'll use it. If the work requires several
chuckings and/or reasonable accuracy, I'll use collets or the four jaw.

The premise goes on to say that good chucks will have the master pinion marked.
My Bison (Polish) 6" 3jaw has a small '0' (zero) stamped next to one of the
pinion sockets so, if this is all a myth, it's one that's permeated as far as
the Katyn forest.

So, select a piece of 5/8" stainless shafting that mikes the same all round,
shove in the 3jaw and tighten down using one of the unmarked pinion sockets. 
Set up the DI and read 0.002" TIR (i.e., peak-to-peak) error in one
revolution. Pretty good for a 3jaw but proof of my wisdom in using
collets for anything important. Loosen chuck and retighten using other
unmarked pinion socket. TIR still comes in at 0.002". 

Ok, now repeat the experiment using the manufacturer-marked pinion hole. Lo
and behold, now the TIR is down to 0.001". I'll be damned. Being the
untrusting soul that I am, I chalk it up to a fluke. Try the experiment again
varying the pinion-tightening sequence... master-x-x, x-master-x, etc. as well
as rotating the shaft slightly between each clamping. Same results each time.
Much as it annoys me, there seems to be some truth to the premise.

I'll leave it as a puzzle for our experienced members to explain *why* there
is a master pinion in the first place. I don't have a clue nor do I have the
patience to research it. Offhand, my guess would be that the master is the
pinion used to preload the jaws when they were ground on the ID but that's
just a SWAG.

Those of you who are trying to squeeze the last thou of accuracy out of your
3jaw might want to try this with your own chuck. According to the premise,
even if your chuck doesn't have a master pinion marked, there is one pinion
that will provide better centering accuracy - your de facto master. Once you
identify it, you may want to consider marking its location for future
reference. If you don't want to stamp marks into the chuck, get one of those
paint pens (like a Sharpie, but filled with bright colored paint), clean the
metal with brake cleaner and paint a spot/line, whatever.

One side note here. The books tell us, if chucking a heavy piece, to tighten
the 3jaw using ALL THREE pinions. This raises the quandry ... Do we tighten
the master pinion first or last? Personally, I don't care. I'd use the 4jaw
if I was worried about the part slipping or coming loose.


----------



## Stan (Nov 10, 2008)

Marv: Just for laughs, try a different size piece of stock (say 1 1/2") and see if the results are repeatable. 
My experience with the 6" Taiwan chuck on my lathe is that it is capable of .001", sometimes, so rather than change to my 8" four jaw I have frequently done a lot of stock rotating and different pinion tightening trying to find that elusive .001 spot.

Edit: I also have a 5" and a 4" 3 jaw that only have one pinion so there is choice and no confusion.


----------



## Mcgyver (Nov 10, 2008)

in any assembly of moving parts there is necessarily clearance - depending on which pinion the force comes from, I can see how the scroll and hence jaws come to rest in a different position as the clearance is taken up and the force finds its opposing force...ie if clearance is equal around the scroll before tightening, tightening will push it ever so slightly eccentric and the indexing of that eccentricity would (seems logical) depend on the pinion. This is speculation as I haven't worked in a 3 jaw plant, but perhaps the master pinion if there is such a thing is the one they tightened to grind the jaw in situ. Run out is both from scroll inaccuracies and clearance...the counteracting logic would be that it shouldn't matter as scroll error can vary over the scroll; ie what works at one dia wont at another

a bit academic imo and I wouldn't worry about it; i don't find the amount (ie whether its 2 thou or 5) of three jaw run out an issue because for most work it either matters whether there's run out or it doesn't


----------



## Richard Carlstedt (Dec 15, 2008)

Marv
You are correct, on good chucks, and those properly reground, the primary chuck key location is/should be  marked.
The reason is the scroll has about .001 clearance on the diameter in it's c'bore, and the force of tightening it, causes the scroll to offset slightly away from the bevel gear.
Repetative cranking on the other keys is a non essential job and can lead to undesired movement of the work.

Having said that,there are times when it can help !
Mark, the absolutely greatest Lathe machinist I ever met, told me all three jaw chucks were adjustable.
To prove it, he chucked piece of stock and 'gently' snugged it
It was off about .003 TIR.
He then hit the jaws with a leather mallet to get it to .000
and he tightend each jaw on the chuck. HOWEVER, he did it with the jaw key in perfect allignment to his Dial Indicator, so he could see any change. His theory (and I believe it correct, as I saw him do it) was that the scrolls wore loose and to use them to your advantage, and the scroll would not bottom if you "adjusted" the scroll and work at the same time.

Rich


----------

