One of the problems with installing a 3 piece slate on a ProAm is that you lose the leveling system because the slates are screwed down with slate screws to the frame of the table, which means you can't adjust the level if the slates unless you can back off on the slate screws, in which you can't after the table is fully assembled.
So, rather than concede defeat, I took the table back to the Diamond factory and replaced the one piece slate with a 3 piece, then...back up to the customers house I go.
Once I got there, I set the table up in his garage, took the rails off so I could get at the slates...and steel plate joined the seams together with 2 flat steel flat bars 2" wide and 3/16"ths thick, drilling the slates every 6" on both sides of the seams and inserting 8x32x1" stainless steel machine screws. Once I was finished, we carried the slates upstairs and placed them on the table. I then proceeded to screw the slates to the flat bar and join the seams together. Once everything was put together, I filled the screw holes and seams with bondo, sanded off the excess, mounted the rails on the table...and dry leveled the slates. With that done, and they leveled up perfect with the Diamond leveling system, took the rails off, covered the slate with the Simonis 860HR cloth, re-installed the rails....and was done. Table plays great, and the leveling system did it's job, and the 3 piece slate is not mounted to the table no different than a one piece slate is, it's still free floating until the rail bolts are tightened down.
