Maching Thin Disks
submitted by Marv Klotz
Machining thin disks is not an uncommon problem for the hobbyist. Thin flywheels for model engines are one instance. Crank disks are another.
I have my own tools for dealing with this problem, some of which I've shown at meetings and in the video of my shop. However, while persusing the HSM BBS, I came across another method that I'd like to pass along. (In fact, this wouldn't be a bad 'everyone contribute' subject for one of our meetings.)
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.
Approach: Cut a (roughly circular) slug of greater thickness - for concreteness we'll say 1/2" thick. 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 on 3jaw using the inside 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. (Although not specified in the BBS writeup, one would need to use parallels or a chuck spider of some sort to get the disk mounted square.)
If one had a large enough slug of cylindrical stock, one could machine one face and the circumference before sawing off, then mount with parallels to face the remaining face. Alternatively, if the piece were cut from 1/4" plate, one could superglue it to a sacrifical face plate to cut the
circumference. (I've used both these alternatives.)
The BBS approach 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.