Depends entirely upon the size, composition and purpose. If you need to hold small parts, threaded 1/4-20 on 2” centers for a 12” square plate is what I would use.
If you go larger, and want something more flexible, 18” square or larger, and 5/8” through hole on 2”-4” centers. I’d use a minimum 1/2” thick piece of aluminum. Through hole fixture plate also needs “feet” to lift it off the table.
If you go larger, and want something more flexible, 18” square or larger, and 5/8” through hole on 2”-4” centers. I’d use a minimum 1/2” thick piece of aluminum. Through hole fixture plate also needs “feet” to lift it off the table.
I'm going to order the Crummy plate. Really nice and the price seems reasonable for what it is. Jody uses one on some of his videos. It is14 x 16 x 1 with either 5/8" through holes or 1/2-13 threaded holes, 2" on center. With the latter, I could also pick up one of the Amazon mill hold-down kits.
https://crummywelding.com/shop/ols/prod ... ture-plate
https://crummywelding.com/shop/ols/prod ... ture-plate