Thread: Lathe 3-jaw Chuck Wear

My go-to 3-jaw chuck on the lathe is showing its age. Accuracy is not really a problem, but bell-mouthing is.

My good machining mate urges me to use his toolpost/carriage grinder to clean up the jaws, after making a suitable clamping ring.
He has done this several times on his AL1000, and uses this approach to finish off critical shafts.

However, as an alternative, has anyone used a surface grinder on the jaws, either as a group or individually ?

A rough measure of jaw height in the body using calipers seems to indicate a variation of 0.4 mm.
If this is the case, a group grind would need shimming.
Perhaps squareness on the magnetic chuck also becomes an issue.
The jaw tip shape would also change from circular (as it is) to flat.
Multiple chuck assemble/mount/check/disassemble cycles might also be necessary.
As my mate would say "too hard".

Just seeking opinions,
Thanks, John.

I've used a die grinder on my three jaw a couple of times, easy as. Don't now if it is feasible to do it on a surface grinder. Sounds like a rather difficult and time consuming way to do it.

How would you do this on a surface grinder? The clamping faces of the jaws on a 3 jaw chuck usually have a radius (like an ID), that you duplicate by using an appropriately sized clamp ring when you use the die grinder technique. How would you get this radius on a surface grinder?

Also, the clamping faces on the jaws need to be referenced to the teeth that engage with the scroll, so how would you set that up on a surface grinder?

I'm not saying it can't be done, just keen to learn if and how it can.

Graham.

Forget about using the surface grinder, go with the die grinder and clamping ring.

Originally Posted by Oldbikerider

How would you do this on a surface grinder? The clamping faces of the jaws on a 3 jaw chuck usually have a radius (like an ID), that you duplicate by using an appropriately sized clamp ring when you use the die grinder technique. How would you get this radius on a surface grinder?

Also, the clamping faces on the jaws need to be referenced to the teeth that engage with the scroll, so how would you set that up on a surface grinder?

I'm not saying it can't be done, just keen to learn if and how it can.

Graham.

If your grinding wheel size matched the radius on the jaw, could it be done by simply locking the table and moving the wheel in and out? As long as you took the same amount off each jaw the indexing shouldn't change.

Wouldn't that be (partial) cylindrical grinding? Not surface grinding?

Graham.

Taking a step back, you need to determine whether the bell mouthing is due to wear on the jaw surfaces, or wear on the chuck jaw grooves/ slot on the jaw. Grinding the inside of the jaws if you have slot/ groove wear could possibly make things worse, as clamping out on a ring will pull the jaws away from the position that they would be in if you were clamping on some bar stock.

The other thing with a 3 jaw is the scroll could be worn, so grinding the jaws in one place may not give you uniform gripping across the whole range of jaw closing.

Michael

Originally Posted by Michael G

The other thing with a 3 jaw is the scroll could be worn, so grinding the jaws in one place may not give you uniform gripping across the whole range of jaw closing.

Highly unlikely to have uniform runout regardless. Tolerance stacking is rarely that forgiving.

I'm with the use a die grinder group. Just make sure you do it at the diameter you most care about.

3 jaw chucks are a PITA WRT accuracy/repeatability anyway. I like the soft jaw ones you can take a cut over but I've not found one that's less than 160mm. If I had endless time to spare I'd be tempted to do what Stefan Gottswinter did in one of his recent Youtube videos.

PDW

You could do it on a surface grinder by holding then in a vice or use a right angle mag transfer block. The die grinder method is fine though.

Originally Posted by caskwarrior

You could do it on a surface grinder by holding then in a vice or use a right angle mag transfer block. The die grinder method is fine though.

If there's no wear in the jaw slides and the chuck body sliding ways, sure. If there is such wear, pre-loading the jaws and grinding in situ will *somewhat* ameliorate the effects of such wear. For a while at least.

I've done it both ways. My solution is to treat a 3-jaw chuck as something that gives approximate results and is best used when the tolerances are pretty open, you're going to do all the machining in one setup, or both.

Mainly I use a 4 jaw chuck (which can also have bell-mouth issues if worn/abused of course) or collets.

PDW

Originally Posted by caskwarrior

You could do it on a surface grinder by holding then in a vice or use a right angle mag transfer block. The die grinder method is fine though.

How would you reference the grinding to the teeth on the chuck? Or are you suggesting to just take an even amount off the jaw holding faces?

Graham.

Thanks guys for all the comments, grinding on the lathe it is.
This could be my excuse to acquire a small air operated die grinder, or a genuine Dremel to replace of my current Ozito.

The chuck came off a small Colchester and the jaws seem to have been ground to suit 2" diameter bar stock.
The jaws have always been Ok for my work, so I will grind at 50 mm diameter.
Any reason to be any different ?

John

I have ground at least 4 sets of Pratt Burnerd 3 jaw chuck jaws using a home made fixture, to hold each jaw individually, on a Tool and Cutter Grinder. The results were very good but it does take time.

Originally Posted by Techo1

I have ground at least 4 sets of Pratt Burnerd 3 jaw chuck jaws using a home made fixture, to hold each jaw individually, on a Tool and Cutter Grinder. The results were very good but it does take time.

Can you give a description of the setup? I would like to understand what you use as a datum?

Originally Posted by electrosteam

grinding on the lathe it is.

John

Hi John, you likely know but I will will mention it anyway.

Make sure that the chuck mountings and chuck are well cleaned before you grind the jaws and also ensure that you have marks on the chuck and spindle to remount the chuck in the same position after it has been removed.

A couple of days ago I ground the jaws on my 4 jaw chuck for my T&CG, I removed and then refitted the chuck and then carefully dialed in an alignment bar near the chuck jaws and then moved the DTI 6" from the chuck jaws
and checked the runnout..... a wopping .0002"....actually I thought the DTI was stuck

I would be interested to see how you make the jaw stop for your 3 jaw as I have been contemplating this, I have a new chinese 8" chuck with dodgey jaws that I would like to give a lick with the grinder.

cheers, shed