A vacuum table top holds the 1/4" mdf sheet, you see tool marks becasue he laid the parts right on the 1/4" and the air pulls right through the 1/4" MDF AND holds the parts in place. The tool marks are his profile cut through to insure a good bottom edge(verse onion skin). This vacuum through the 1/4" MDF method holds the larger partsbeter than smaller parts. You would see marks for clamps and such, since you don't he was just laying parts right on the 1/4" MDF. This is actually the best way whenever you can get away with it.
Even if a regular old 1/4" MDF sheet works fine for vacuum through the spoil board there are few hints for sucking vacuum through the 1/4" MDF to make to even better for holding even smaller parts:
1) Paint or seal the edges of 1/4" MDF spoil board on all 4 sides. This blocks any air from being pulled through the edges giving a better pull to through top of the 1/4" MDF.
2) Skim cut(surface) the top and bottom of the 1/4" sheet about 1/32" before using it , especially if there is a shiny side to the 1/4" MDF . This allows more air to be pulled through the top of the 1/4" MDF.
3) Try to find whats called Light weight MDF for the 1/4" spoil boards and then do the above.
4) Use a down cut bit for all the "profile through cuts", it tends to pack saw dust in the cut line that actually keeps the cut part from moving, gives the top edge a better cut and if you cut through a bit into the 1/4" MDF your bottom cut will be perfect as well. Plus, up-cut bits tend to grab and move the work pieces and give the top edge a furry feel.
I stopped using up cut bits on wood except for 3D bas-relief carvings years ago. I was taught to use an up-cut, but now I go against the the old timers and only use up-cut for metal and plastic now.
You may not have to do any of this, but doing so might enable you to secure parts even better, possibly giving you the ability to hold much smaller parts than you could if you used the 1/4" MDF right off the stack.
I always like to the "pull the vacuum through the 1/4" spoil board" method best. Because through cuts can be made without touching your main table and you don't need pucks or any kind of clamps etc to hold the parts.
I would play with this set up. If your vac is powerful enough and you can get it to work, even just for larger parts, imagine rarely ever having to clamp or screw parts down, it opens up a different world.
You may not be able to hold really small parts unless your pump is fairly large, so try my hints above. Ocne you do you will get a feel for what part sizes work with this method, if any. For small parts their are tons of other hints for clamping or using zone vacuum to make it work. Other than vacuum for the smaller parts I use a 3/4" spoil board to protect my table and use little screw's, other people use T tracks with clamps, etc. Some guys onion skin the parts that need be cut through so the parts don't move, I got away from that as it tends to leaves extra work. I want a clean cut on the bottom edge as well as the top.
