Chuck a mile out

I fitted one of my spare 3 jaw scroll chucks to the L5 and noted that a bar put in it was 10 thou out when rotated. One can only assume that it ran concentric when new so how did it get into this state, and is there any way of fixing it?

Hi Fizzy, presume its a Harrison L5? however which spindle nose does it have? Do you perchance have a similar chuck and have mixed up the jaws, they are usually serial numbered as well as 1.2.3.

For any fix you need to work through any elements where an error could be from the spindle nose to the backplate register and so forth to the chuck body etc

Hope this helps a little John

You have to fish for clues... Is the chuck body out by the same? Is the error still .01" and inch further out? Do different size bars have the same error?

When you take out the chuck jaws to check the sequencing, look at the scroll and the jaw scroll segments to check there ain't a piece of swarf wedged in there.

Look at the front face of the chuck whilst running and see if there's any wobble.

Edited By Mick B1 on 24/09/2017 11:10:16

chuck body runs true, jaws apear to be matched set, no swarf - had it apart and cleaned it, other chuck runs true. Just seems a shame to bin it. Dont know what different spindle noses there are but the hole down the middle is rather small - I know there are variants with bigger spindle holes.

Don't bin it. The jaws can be trued up with a Dremel type tool held in the toolpost if a suitable "clover leaf" or packing pieces are used to spread the outer ends of the jaws while grinding is done.

Have you tried it with different diameters of bar?

Yes Neil - problem is less apparent

If you want accuracy then don't use a self centering chuck, use a chuck with independent jaws that allow you to set up work accurately. You can use the scroll chuck to turn all the features that can be done in one setup and they will all be concentric. Once the workpiece has been removed from the chuck it needs to be set up in an independent chuck or accurate collets to continue This topic comes up regularly.

Martin C

Depends how accurate you need it to be. It's quite reasonable to expect a new 3-jaw to be concentric to a thou or two and some - even budget ones - can be better than that. Plus there's always the option to machine a set of soft jaws to achieve the same or better.

I think the OP's quite right to think something's more than usually out of line when a chuck that appears to be correctly fitted shows 10 thou eccentricity.

I wonder if the 'bar' used was not accurately round, or perhaps hexagonal? I have found examples of hex bar in which the AF dimensions were out. (AF = across flats)

I agree with the 'don't bin it' advice. There are many jobs where the concentricity of the chuck is not critical, and many off-the-shelf components where the parts are not concentric from new. Things like hydraulic pipe fittings, where the flats are not always in line with the threads or the bore.

But before you consider your next move, it would be useful to do a series of checks using (eg) silver steel bars of different sizes, noting the run-out of each. This would give you info to work out whether the scroll was bad in parts, or a jaw was distorted. It would be no use grinding the jaws to get it back on form, if the scroll was bruised on one (or more) places. The next time you used it with a different size, it would show a new eccentricity.

Regards, Tim

Tim Stevens has said it all really, but I will just add that the best concentricity on a 3 jaw when new will be within 2 to 3 thou, if you get one that is spot on, it will be by good luck alone. If you turn all the faces of the component you are making, it will be concentric with the bed of the lathe, but not concentric with the bar left in the chuck, so your finished component will be unaffected by the inaccuracy of the chuck. This is why you should always use stock which is oversize in all dimensions. If you consider the way a scroll chuck works, any slight movement in the pivot of the scroll, or the recess it seats in will result in unequal torque on the jaws, and give rise to run out. As Tim says above, if the chuck has been over tightened, the scroll itself can be bruised and that will bring inaccuracy at certain diameters.. Grinding the jaws is not an answer, one of our biggest chuck makers (can't remember which, but someone will) used to offer a recon service, and gave it up! The most accurate chuck you have is a 4 jaw independent.

Well I've struck lucky with my last two 3-Jaws then, both inside a thou

Far more accurate than I had any right to expect, but SCRUPULOUS cleaning of the jaws is required.

Neil

Yes, I got one o' them, too, leastways on the main jaws when new. Still OK to about a thou an' a bit after two years' abuse...