Thats not the idea with soft jaws. They are soft so you can machine them true for fixed diameters not so they will not mark the part.

The idea with soft jaws is to machine to fit the size of the part you wish to hold. So for example if you wanted to hold a thin 1 inch disk truly concentric hold a small bar right inside the jaws (sub flush) and tighten. Bore the tops of the jaws 1" dia a little under the tickness of your part. When you release the jaws you will be able to hold your disc true to it's circumference and as it is back against the recess in the jaws with no wobble. As the chuck has not been removed from the lathe and the recess was bored at that diameter everything will be spot on for as many discs as you need to machine but only for that diameter.. You need to do this each time for different diameters and shapes of part.

regards Martin

PS Jason beat me to it.

Edited By Martin Kyte on 11/05/2021 13:09:25

..Yes, what these men just said..

Some further discussion & drawings here including new tops for 'used-up' soft jaws;

Harold Hall Using Soft Jaws in the 3 Jaw Chuck

Firstly, you need to eye up the way to machine the jaws removing the least ammount of metal. They should be clamped in the direction that they will hold the job to take out any slop or backlash. I have dozens of soft jaws, and even when they are close to worn out, there is the possibility of turning the stumps into master base jaws with bolt on tops.

The photo is good it shows what the Taig chuck is all about, I would suggest making some more sets of 3 jaws in alloy or mild steel.they could be kept in numbered sets drilled s they could be attached to the lower steel jaws and for position accuracy a locating dowel could be used provided the lower steel jaw can be drilled and reamed.So two sets could be made with similar jaws to conventional 3 jaw chuck and another kept in hand as spareThe usual soft jaw use is for holding a part or a batch of parts very accurately,also good for holding thin washers,and despite the comments above, they can be used to prevent damage to the workpiece,for instance on a larger chuck ,a workpiece with a fine thread and larger than an available collet can be held safely and accurately in soft jaws. Industrialy it was quite usual to see soft jaws with special jaws welded on, EG like pieces of steel tube which had a large area of grip to hold thin tubular workpieces.

Howard T makes a good point about gripping a stub of something round in the back of the soft jaws while machining them to fit a job.

However if the job is a ring, and the jaws will grip it by pressing outwards on the inside of the job, then for the task of machining the jaws to fit the work, the round stub needs to be replaced by a ring. The ring goes around the outside of the set of soft jaws so that the jaws will be pressed outwards ( to take up any slack in the scroll) rather than inwards. I hope this is clear, it does make sense when you think about it.

The idea of keeping lots of numbered sets of soft jaws works very well. Soft jaws are essentially sacrificial items but the more sets you have the more likely you are to have a set which is nearly right. And the less you have to machine off to get a fit for a new job, so they last a long time.

Rod