Step 1: Motor Mount
I took a 3/4" birch plywood and marked the center with lines perpendicular to one another. I then marked a 12 1/4" circle and cut it on a bandsaw using a circle jig I also made. I left 1/4" on the diameter as I knew when I mounted it to the motor it would not be true.
I take care of this in the next step
Step 2: Trimming the 12" Disc to Size and True to the Motor
I then turned on the motor and cut the disc to size using my lathe tools.
Note that I made a box with base from 3/4" plywood assuring the Disc would be tall enough to clear the 3/4" threaded rod for the table.
Step 3: Balancing the Disc
So I mounted the motor to the table, and spun the disc by hand, marking the top back of the disc once it stopped spinning. I did this 6 to 10 times and taped a washer on the back of the disc near the marks as these showed the disc section that was the lightest. I.e. The heavy part of the disc will stop at the bottom more than not..
I then turned on the motor and still had vibration, but less than the initial test. I repeated the above tests, adding one more washer. I then moved these washers toward the center of the disc and checked vibration. Once I was happy I epoxied the washers on and attached the sanding disc and checked vibration again. Zero vibration... I could run the motor with disc on my bench with no movement.
Step 4: Sander Table Mount and Back of Sanding Disc Shield
I also cut a 12 1/2" disc and made a wood shield to cover the disc. I then painted the box and back shield.
Step 5: Adding Dust Collection
Step 6: Adding the Table to the 3/4" Shaft
I created a larger table to mount on top covering the entire 12" disc.
Step 7: Adding the Switch
Step 8: Finished Sander
I now have a Jet 12" that was damaged for $146... $600 sander and just minor repairs needed...