Here's how we chose to hold the smokebox front: Using a 6" 3-jaw chuck with the removable jaws off, putting 1/2" square stock in the jaw alignment slots to hold the part away from the jaws, we grip the inner ring of the smokebox. Since this is a casting, the inner ring is tapered and with a generous radius at the bottom. So it's hard to hold onto and you can't clamp in a straight manner when the jaws are in the radius (thus the 1/2" standoff blocks).