The main problem is machining them in place on the lathe. If your lathe is turning a taper or some other strange shape due to spindle alignment etc you will finish up with bell mouthed jaws. Apart from that aspect they can hold work rather accurately.

I wouldn't entirely agree with Jason's comment. In use the jaws are under compression and even if they aren't tapered down at the end to hold low diameter works there will always be a degree of point contact when holding round work. The only sliding wear they may get is when your putting work in them. The scrolls on the back will wear more quickly than hardened jaws and that is the main reason for hardening them. This is why hardened soft jaw carriers are available for some chucks.

I've seen mention of machining or grinding ordinary jaws in various mags over the years. The jaws are just dangling in space = nvg. They need to be clamped on something for the best results. One way of doing that is to machine a shallow recess at the back of the jaws, say 1/16" x 1/16". Then grip a washer or what ever in that while pulling the washer back against the shoulder. Then machine the inside of the jaws going as close as you can to the washer. Remove the washers and take off the slight shoulder that is left.

If your jaws are worn you'll probably find that the ways in the chuck are too. When ever I have bought soft jaws I've found that they need fitting to the chuck and suspect they are deliberately made a bit over size to account for wear in the chuck. The slot in the jaws is usually a touch to narrow and the front width of the jaw past that a little too wide. I've just used a set of warding files to correct that being very careful to identify which part is fouling on the chuck body. I find it easier to keep the file still and move the jaw as that way I can feel when the surface being worked is square to the file.

Harold's idea of using them as carriers for other jaws is interesting. I'd wonder about annealing a set of soft jaws, doing what work is needed and then case hardening them. The mess on them after case hardening could be cleaned off with emery cloth with virtually zero change in size.

John

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