Machining thin disks

Machining thin disks is not an uncommon problem for the hobbyist.

Problem: Machine a 4" diameter by 1/4" thick disk so that both faces are parallel and the circumference is machined. No central hole so using an arbor isn't an option. A chuck with soft jaws is not available and 4" is pretty damn big for a pot chuck. The spindle hole on your lathe is too small to accept 4" stock.

Approach: Obtain a (roughly circular) slug of greater thickness - for concreteness we'll say 1/2" thick. (This could be cut from a 4+" bar or bandsawed out of 1/2" stock.) Mount in 4jaw and face flat. Now turn a flat-bottomed shallow hole in the material. Make this hole 3-1/2" diameter by 3/16" deep.

Remove slug and mount using the inside chuck jaws to grip on the inner surface of this shallow hole. Face flat and machine the circumference to the desired 4" diameter.

Remount slug with the shallow hole facing outward gripping such that the hole can be machined away and the disk brought to the required 1/4" thickness. Use parallels or a chuck spider of some sort to get the disk mounted square.

The approach described wastes stock but does offer a way of getting the part made if one doesn't have the proper thickness plate or has some scrap disks lying around. It isn't applicable to every disk cutting application but it's a clever idea to salt away in your mental bag of tricks.

Excellent method and a clear description of the various stages.
I have had a similar need in the past though my method is not so elegant or as universal.
I tack welded the slug of mild steel( low carbon) to a suitable piece of tube then faced and turned the OD as required.
Cut/grind off the tacks and reverse in the chuck to finish the required thickness.
Tack welding lugs would and have served the purpose too.
Downside ... needs additional equipment and skill.
I very much like your approach.
Thanks again for your most useful post.