Thread: Chuck jaws not parallel

Some food for thought, before you proceed. (Have done this quite a few times at work on cylindrical grinders.)

1) Most jaws are case hardened. If you grind them, you might find yourself doing it again in a year.

2) Everytime you grind the jaws, the diameter of stock you can hold will decrease.

3) When you grind, do not grind parallel. A very fine amount of backtaper should be ground. (More metal removal towards back of jaws than at face.)

4) If you can't grind them better than 2-3 micron (0.001mm-0.003mm), don't waste your time.

5) Yes, holding raw or unmachined stock in a 3 jaw chuck will decrease its lifespan significantly. (The stock's not round, so one of the jaws has to move further to compensate. This means scroll wear.)

6) As Metalhead has said, a 4-jaw really is the way to go. A bit iffy to learn at first, but once you've got the touch, you'll wonder how you ever did without it.

7) Steel ring seconded. Standard practice at work when grinding the inside of the jaws.

8) With the steel ring on the chuck, put a dial indicator on the inside of the jaws and see how much you're going to need to grind off. Write down how much the error is. Put a different size steel ring on the chuck and try to apply the same amount of tension to it. If the difference in runout to your first set of readings is more than say 10 micron, you're probably suffering scrollwear as opposed to jaw wear.

9) As previously mentioned, if you do end up grinding the jaws true, mark a witness on the chuck near the first jaw. Always do the chuck up from this point in future. In the case of a floating chuck, this will help ensure repeatability. (Put a piece of machined stock in a few times with the same pressure on the chuck key and eventually it'll run true without you having to hit it.)

Hope this has helped.

just curious again,if your going to do your method of holding a piece of ally how do you stop your piece of ally falling out afterall you said items held in chuck wobble ?

Originally Posted by Article99

Some food for thought, before you proceed. (Have done this quite a few times at work on cylindrical grinders.)

1) Most jaws are case hardened. If you grind them, you might find yourself doing it again in a year.

2) Everytime you grind the jaws, the diameter of stock you can hold will decrease.

3) When you grind, do not grind parallel. A very fine amount of backtaper should be ground. (More metal removal towards back of jaws than at face.)

4) If you can't grind them better than 2-3 micron (0.001mm-0.003mm), don't waste your time.

5) Yes, holding raw or unmachined stock in a 3 jaw chuck will decrease its lifespan significantly. (The stock's not round, so one of the jaws has to move further to compensate. This means scroll wear.)

6) As Metalhead has said, a 4-jaw really is the way to go. A bit iffy to learn at first, but once you've got the touch, you'll wonder how you ever did without it.

7) Steel ring seconded. Standard practice at work when grinding the inside of the jaws.

8) With the steel ring on the chuck, put a dial indicator on the inside of the jaws and see how much you're going to need to grind off. Write down how much the error is. Put a different size steel ring on the chuck and try to apply the same amount of tension to it. If the difference in runout to your first set of readings is more than say 10 micron, you're probably suffering scrollwear as opposed to jaw wear.

9) As previously mentioned, if you do end up grinding the jaws true, mark a witness on the chuck near the first jaw. Always do the chuck up from this point in future. In the case of a floating chuck, this will help ensure repeatability. (Put a piece of machined stock in a few times with the same pressure on the chuck key and eventually it'll run true without you having to hit it.)

Hope this has helped.

Thanks for that Article, I learnt something from it to. As you rightly said once you have gained the experience from clocking up jobs in the 4-jaw you wonder why you ever bothered with a 3 jaw. I must say if I find my 4 jaw too cumbersome for a small round bar I often use a collet which can save some time and is no less accurate most times.

Cheers
MH

A three jaw has its uses. We use floating 3 jaws on the grinders at work. Which means the work can be run true to 2 microns usually. (The chuck is mounted to the face plate with three bolts that are done up lightly.)

Myself, I can't stand collets. Have had some very strange experiences with them already. Had one job for 100 off. Carbide no less, so just the material was worth a few grand. Anyway, the piece was held in a collet and clocked true at either end. No bends in the stock, it was pre-ground. Anyways, 100 off later, and the customer sends them all back saying there's an average of 20 microns runout. Put them all back up in a chuck and watch the dial indicator go nuts. After a bit of arseing around, it was determined the taper they dropped into was running out. (Makes little sense, seeing as each piece was clocked true before grinding.) After nightmares like that, I only grind in a chuck.

Pipeclay, after using a soft ring of some description on the outside of the chuck to take up the slack and grinding the inner jaws, it should hold a piece of round stock on the inside to grind the outside of the jaws quite well.

im just a bit confused originally the problem was not being able to hold something on the inner of the jaws without it being wobbly,why would the jaws hold a ring externally any better for grinding

Technically, they're not going to. By using a ring made of something nice and soft, like copper, alu, hell, even a big rubber band might do the trick, it takes the slack out of them so they can be ground reasonably accurately. If the jaw's out any more than a quarter mm or so, time for a new set of jaws or even a new chuck.