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My question is, what are these discs called officially (disc sanding
attachment? sanding plate? etc) and what size would you recommend for
a 1/2 HP motor. Seems like 8" would make sense.

thanks

i
9" is standard, you can buy disks that size or cut them from 9 X 12
sheets, plans for a sander using a vee-pulley and plywood disk he
http://benchnotes.com/DISK%20SANDER%20/Disk_Sander.html