What to Look for in a vacuum pump?

Hello, I’m am trying to set a bench for casting small charms and I
think a vacuum cast system would work best but however I have little
capital to draw from, only $1000. I’ve seen the one’s they have in
the magazine for 300$, but that combined with the 200$ vacuum table
and the 500$ silver melting furnace plus all the other little
expenses adds up to a little(lot) more than 1000$. If you think
there is a better system for what I’m manufacturing please share
your insight. I was thinking of room temperature silicone mold,
manufacturing about 200 molds, and casting 400 small detailed
charms. And then using molten wax from boiling water and syringe,
and then using satin 20 investment. And then vacuum casting it from
the vacuum table in rio grande, and then finding another cheaper
vacuum to use to fulfill the purpose of airing out the investment
and for the suction power for casting. And then melting the sterling
silver in one of those small electric melting furnaces.

Would anybody know of what you would need in a vacuum pump for this
purpose, I didn’t think the mercury or the level of vacuum was too
important, as long as it can suck the air out for the casting
process. From reading the orchid forums it seems that you would need
a two stage direct drive vacuum pump for this purpose. And from
looking on ebay, I think this vacuum pump would be suitable.
ebay link removed It is 4 cfm, 1/3 horse power, 2 stage,
20 microns, and it vaporizes water so it looks like moisture won’t be
a problem.

What do you think? Would the horsepower need to be more powerful or
what would be important when looking for a vacuum pump for this
application, investing and casting. And if I can get the vacuum
cheaper if I don’t invest(use some other method to get the bubbles
out, I heard satin 20 has automatic debubbilizer), what would be
absolutely nesseccary specs for casting silver in a medium sized
flask? Thank You for the replies!

Chris; A 3 CFM 2 stage is sufficient to both De-air your flasks and
cast you flasks, I have a 5.6 CFM Sergeant Welch Scientific, that I
bought at a surplus auction at Los Alamos labs almost 15 years ago,
and it’s way more than I need. I can boil water in less than 20
seconds and pulls around 25 microns, at 600 ft. alt. my vacuum gauge
hits 30+ in HG at 7000 ft it was pulling 28 and boiling water in
under 25 seconds. I never had to alter my water temperature or ratio,
and hardly if ever had air bubbles on my castings.

As far as the cost of a vacuum table for de-airing flasks, or
casting forget it, make your own all you need is a stainless steel
salad bar pot that is 6 in. Diameter by 8 or more in. deep, a 1/4 or
3/8 three way brass valve, available as a replacement for the Kerr
vacuum casting machine, also available at industrial suppliers. I
bought a 4 foot counter top, cut the hole around the cylindrical pot/
(now vacuum chamber). You can get the adapter rings for the flasks
and the silicone gaskets from Rio, or maybe Andy from Stuller will
chime in.

I don’t know how much the vacuum pump is on eBay but new they are
around $300.00, the pot from any restaurant supply is around $10.00.
The 3 way valve is another 10 bucks, you can use compression fittings
if you like since they are easier.

I did my system with flare fittings harder but I don’t worry about
leaks, (my old system I had to retighten every thing every few
months). For the table I drilled 4 1/4 inch holes in a 1/4 inch thick
aluminum plate, do this while it is sitting on the counter top,
exactly where you want it and drill al the way through the counter
top. Now if you are not going to use a side draw Vacuum Bell, drilled
and tap a hole for a 1/4 inch pipe nipple. I slipped a guy at Loews
home supply a few bucks to put extra long threads on a 1/4 by 6 in
iron pipe nipple almost 1-1/2 inches of thread on one end, the end
that comes through the plate. I ran this up through the Aluminum
plate and used Teflon tape to seal the threaded nipple (I don’t like

If you use a side draw vacuum bell you can forget plumbing a hole
through the plate and get by with no hole in your rubber mat, also
less chance for a leak.

The worst part of the whole process was drilling the hole into the
casting chamber (former hot bar pot) it took some finding but I
found a “bulkhead fitting” for 14 inch compression fittings (Not my
first choice) that works quite nicely. Cost was about $15.00. Lets
recap here Not counting the pump, Vacuum Bell, nor counter top, we
have a total of about $65.00 add around $30.00 for the counter top, (
surplus building supply co) or $50.00 at a home building supply. The
Kerr flask adaptors were about $35.00 each, (just get the sizes or
sizes you need). If you are going to use perforated flasks get a
vacuum bell that will accommodate at least two flasks, the less time
spent investing flasks the healthier you will stay.

The side draw bell is by far the easiest to set up for, No plumbing
through the Aluminum plate, Now back to the plate, the four holes
that we drilled earlier? OK we put 4- 4 or 5 inch by 1/4 inch
carriage bolts in them, (get them with full length threads) put one
in each hole and put a nut on each bolt and draw up tight.

Now enlarge the holes in the counter top and put some compression
springs between the plate and the counter top, enlarge the holes to
around 3/8ts inch (so the bolts will slide easily. Now put flat 1/4
inch fender washers under the counter top and two nuts on each bolt.
Or you can use some locktite thread sealant,(auto parts store). You
can get the springs and copper tubing at a hardware or home repair
place like Loews or Home Depot.

The side draw bell Jars are around $125.00 and conventional jars are
about 25.00 less, GO WITH THE SIDE DRAW I wish I had. I forgot the
most important component of all the 1/4 inch aluminum plate, if you
want to do this email me off line. I have a suitable piece left, I
bought mine at a scrap yard/recycling center. Bottom line I have less
than $200.00 invested in my vacuum Casting system and that’s counting
the Bell Jar but not flasks or perforated flask tongs, or pump. This
system will cast flasks up to 5X7 inch. The pump cost me 175.00 for a
$1700.00 pump that I used for 5 years then had if over hauled for
400.00 .

So add $250.00 for the rest of your system to your pump, and see how
much you have left over for you electro melt and Perforated flasks,
(expensive ones do work better) and silicone gaskets for the adaptor
rings and flasks. I believe the Kerr system sells for around
$1300.00 and so does the Rio Vic-12 system Sorry yours wont be in a
nicely painted and stenciled cabinet, mine sits on a kitchen sink
cabinet another 65.00 but it looks OK and is convenient we mounted it
on rollers so we can move it around in the casting room.

But I saved a lot of money and it only took about 4 hours to build

Kenneth Ferrell

I’ve been following this thread regarding vacuum pumps. Wondering if
anyone has tried using an old automotive A/C compressor. (Can’t
remember where I read this, thought it was in “The Complete
Metalsmith” by Tim McCreight).

I’ve been able to locate one and had the clutch welded to the pulley,
but need to mount and attach to a motor. Any feedback would be

Richard Dubiel

Dear Chris, If you ever happen to find the perfect vacuum you will
likely find nothing at all. Let us know if you achieve success. J.M

Hi Chris…

My two cents. I bought a factory reconditioned 6 cfm vacuum pump from
an air conditioning parts and supply wholesaler for $150. It works
great. They had others, but I opted for a big one. AC mechanics use
these big pumps to quickly evacuate and dry A/C refrigerant lines.
Thus the A/C industry is the best source for vacuum pumps… even
new ones and oil.

I’m going to look for a salad bar SS container as others have
suggested, but I have used a Number 10 can for quite a while. A boil
over of RTV being vacuumed gave it a nice rust proof finish. The tin
can easily handles 30 inches of vacuum without collapse. It was free
after eating a lot of pork and beans.

My table is a 12x 12 piece of anodized aluminum .125 thick from a
local glass shop and is covered with a piece of .125 gasket rubber
ww Grainger. Plastic tubing works good. I eliminated some of the
copper after I had a hot metal blowout. I use a silicone sheet from
Indian Jewelers supply for vacuuming hot flasks.

The vacuum casting system is really simple and can be very
uncomplicated and still work well.

Use a torch to melt the metal. As you watch the various stages of
melting, comprehension dawns.