Should we grind 3 jaw lathe chuck hard jaws?

[Parlo]

The jaws are not square... they are worn more to the outer tip than the inner. It is actually pretty obvious looking at them.

There is no discernable wear in the guide slots... which I thought was pretty amazing on a chuck this old... I've measured the full length of the guides, if there is any wear, it is evenly distributed along the full length of the guides.

Also, if the guide slots are worn, there would be some amount of play between the jaws and guides... there isn't any play.

I would guess that this chuck saw a lot of use chucking short parts... but that is just a guess.

-Bear

Looks like the chuck was well made. To save the performance of setting up a toolpost grinder, do you think grinding them individually on a surface grinder would work, dialling them in on a ground rod. It would also keep the grit off the machine.

 

[graham-xrf]

Looks like the chuck was well made. To save the performance of setting up a toolpost grinder, do you think grinding them individually on a surface grinder would work, dialling them in on a ground rod. It would also keep the grit off the machine.

I am truly with you on this. I can fully understand why you would like a more direct way, but I am assured there isn't one that can match having the chuck rotating.This is because in the same way any surfaces turned have to end up concentric to the lathe spindle, pretty much exactly, so too are the ground surfaces when doing a jaws fix. All the clearances in the train of forces are taken up. If the jaws are then forced to have a concentric contact (by grinding), everything is compensated for.

For a chuck to work at all, there has to be some clearance in the scroll. It would be about 0.010" to as much as 0.018", but the arrangements in the spiral are that when it is under tension, pulling the jaws in, contacting the "outer" faces of the scroll, the jaws should be correct when clamped onto something round.

I too, have not got it quite together when it comes to a toolpost grinder, or some bodge getup ghetto version knocked up by mounting a long shaft die grinder. I much hoped to duck the grinding scene somehow. In the end, I will have something, but it is a project down the line. Quite a few other more pressing stuff has to be taken care of first - like moving (two) lathes to their new shop space.

Without any grinding..
There is a way without having to grind anything. Joe Pieczynski has a dodge work-around that lets you get near exactly true, no matter how messed up is the run-out.

 

[Parlo]

For a 3 jaw chuck, I imagined clamping onto a ground bar so the jaws were under load evenly ( as all 3 will naturally touch ). Then indicating the bar and grinding the jaw that is on or near the low spot by 80% of the runout. Then repeat until the clamped bar runs true. Of course, as in all jaw grinding, this is only guaranteed at the ground bar diameter. p.s. I would grind all 3 jaws equally first.

If you wanted a really quick and much simpler work around. Take a piece of round stock - pereferably tube, about 1/8" larger diameter than the part to be machined, nip up in the chuck, bore to slip fit, slide in the part and clamp tighter. The 1/16" thin wall will not need a slot to close up. It simply uses the same part of the scroll when bored and when clamped. It's just like changing the ends of hard jaws into soft jaws.