Well I bought one also. With the Festool sanders you really need to secure
smaller pieces down to keep them from moving. Clamping from the sides was
desirable as the clamps would not have to be moved to sand the entire
surface. Festool sells a pair of side clamping clamps for about $90. I was
reluctant to take the plunge and kept putting that purchase off. I have
been using rubber router pads but they were not quite enough any more.
I bought the Vacuum Clamp from Woodcraft this afternoon and to add to what
Robatoy has mentioned, the clamps do work well on porous Oak. To check the
holding power I set the vacuum clamp, which is about 5.5" square, on my TS
top and turned the clamp on. Then I placed a 2' x 4' piece of 1/2" Baltic
Birch up on the clamp except I sat the panel on the clamp on the corner of
the plywood panel. All but the corner of the panel was hanging ff the edge
of the TS top. Pretty impressive.
Next I set the clamp on a steel top work bench, turned it on, sat a 5' long
piece of 1x6 Mesquite on the clamp and again at the very end of the board.
I grabbed the opposite end of the board and moved it up and down. The steel
top of the work bench flexed but the clamp held fast.
Pictures of the last test.