Yeah, you can make an impromptu vac caster out of a big shop-vac,
but it’s not exactly a substitute for a real one.
Procedure: Get a BIG shop vac, with 2.25" (or larger) diameter
hosing. The more power it has, (and the greater the CFM) the better
Get a flat plate of steel, about 1/4" thick, or thicker. (The one I
used in college was about 3/8") about 8" square. Get a large cast
iron plumbing “T”. I don’t remember the size on the one we had, but
it was roughly comparable to the 2.25" hosing from the vac.
Weld one of the sides of the “T” to the bottom of the plate, in the
Drill a largeish (1/2"-5/8") hole through the center of the steel
plate, right over the center of the welded-on “T” joint.
Put a plug cap on the other side of the “T”.
Rig some way to attach the hosing from the vac to the center leg of
the “T”. I don’t remember exactly how ours was done, except that
there was a lot of duct tape around the joint.
Put the “T” joint in a vise, or clamp it so that it sticks out over
the edge of a table, with the steel plate up (and flat) and the arms
of the “T” pointing down. The center leg (with the vac hose should
The idea is that the “T” forms a blowout trap in case you get metal
blowing through your flask. It goes through the hole, and winds up
in the bottom of the ‘down’ arm of the “T” joint. Meanwhile, the vac
is pulling from the center leg, which ends up half way up the wall.
That way you don’t end up feeding molten metal into your vac. in the
event of an accident.
(Weirdly enough, modern investment/casting machines have done away
with the traps on their casting plates. I’ve had to rebuild 3 of
them with blown vac lines, and metal all over their guts. The old
ones have the traps on them.)
In use, you need to get one of the silicone casting pads that go
with the ‘for real’ vac casters (the ones that use flat plates, not
the pit vacuums.).
Wet it down, put it on the plate. Then get a pad made up out of 4-6
sheets of newspaper. Tear out a hole in the center, and soak the
whole pad in water. Place it over your silicone pad. Make sure
there’s water between the silicone and the steel, and the silicone
and the paper. Make sure the paper’s soaked.
Get your metal molten in a hand crucible, and have your minion pull
the flask out of the oven. Set it nose up on the pad, and flip on
the vac. Pour metal into flask. Pray to the casting gods… The
paper won’t burn, it’ll just give a good seal, and char a little.
This will work better if the top of the flask is straight and flat,
and the steel plate is flat as well. It will also work better if you
use wax web, or put a couple of strips of sprue wax up the inside
wall of the flask, to help the vacuum get at as much of the flask
volume as possible. Remember to leave 1/8"-1/4" of empty headspace at
the top of the flask to act as a plenum chamber to help spread out
the vacuum as well.
Shop vacs can’t pull as high an absolute vacuum as a real pump can,
but they can move a lot of air, and for pulling air out of a
flask, that’s more important than ultimate vacuum. This is definitely
a frankenstein rig, but I cast knife guard sized things in bronze on
one successfully all through college.
PS–> for anybody in California: I’ve got a couple of huge old
welch vacuum pumps that I’m thinking of unloading. Contact me offline
if you’re interested. I’m in Santa Barbara, but I go up to San Jose
regularly. They’d be in the $400-500 range, depending on size.