Homebrew PCBs

Just a note about today's task.  I scrounged some tubing and made some
adapters, got some scrap rubber gasket material, and converted my UV
exposure box to vacuum hold-down.  I used a 1/4" wide strip of rubber
sticky-backed gasket material around the edge, drilled a hole in from
the front and up into the space within the gasket, and made a wooden
adapter (stopper) to connect the shop-vac to the tubing.

I didn't feel much "suck" on the end of the tube with my finger, but
the first time I hooked it up and tested it I saw the glass bow in so
much I feared it would break!  I just need to find out if I can run
the shop vac with low air flow for the 5.5 minutes it takes to do an
exposure.

What I decided to do is keep a stack of card stock (half-sheets) to
prop up the board to keep it just a little below the height of the
1/8" gaskets, so that the glass doesn't bend that much, but not so
much that the film isn't held tightly to the board.  I'll have to run
some tests with a dial indicator.

I can't find my vacuum gauge, though.  I was thinking of making a new
adapter that allowed some air to flow through to the vacuum to keep it
from overheating, but without knowing how much vacuum it's drawing I
have no way of knowing how much extra flow I can afford.  Anyone else
have any idea how much vacuum is needed to hold the film down, or
other tips?

Now that I have a temperature controlled laminator to put films on (it
wants 240F, not the 320F the laminator used to do) and a vacuum to
hold them down, maybe tomorrow I'll try making some spirals too :-)