I wanted to fit a 4-jaw chuck to the 7x12 mini lathe to provide more versatility to the machine. The mini lathe has a spindle flange for chuck mounting with a register shoulder and 6 mounting holes (1 hole is shared as a mounting hole for both the 3-jaw and 4-jaw chucks).
In looking around at various vendors to see what kind of quality and prices were available, I decided to watch eBag for a while to see what might come along.
Since an adapter plate would be required, I looked for some disks cut to size which would be suitable for fabricating an adapter, and found some 6061 disks 3/8" x 4" diameter.
After receiving a used Dunlap 4" 4-jaw chuck, I was able to determine what fasteners to use and the appropriate layout dimensions for machining the adapter plate.
Drilled and tapped 4 thru holes to match the lathe's spindle flange holes. Mounted the plate to the flange with 4 studs and nuts and skimmed the face of the plate. Cut out the recess for the spindle flange register. Remounted the plate facing the normal way with the studs and nuts. Skim the face of the disk and true the circumference. Mark the spindle flange and adapter plate edges at the one hole that's common to the 4-jaw and 3-jaw chuck, so the chucks can be mounted in the same positions repeatedly. Cut the plate register to match the existing recess on the back of the 4-jaw chuck. Drill and bore a 7/8" center hole in the back plate for long workpieces to pass thru. Drill and tap the 4 chuck mounting screw holes. Clean the plate and 4 mounting studs, then blue Loctite the studs and chuck mounting screws in place.
Handy tips for hole layouts:
For 4 holes in a bolt circle, multiply the bolt circle diameter by .707 to get the center distance spacing of the holes.
For finding/checking the centers of 2 holes (or pins) add the inside spacing distance to the outside distance and divide by 2.