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Vacuum table problems


#1

Hello Orchid!

I am having trouble with my vacuum casting table I can’t seem to get
the thing to draw vacuum all of a sudden out of nowhere and I cannot
figure out what happened. The oil is fresh and free of debris and
water, the lines are brand new and tightly fitted at each spot. I had
it pulling vacuum earlier today and now cannot get it to pull more
than 5 pounds. What happened??? :slight_smile:

Sean Terry
Jeweler/Caster
www.seanthejeweler.com
www.mpcastingco.com


#2

Hi Sean Terry,

The most likely cause of not enough vacuum in your casting machine
is a leak. You can plug the hole on the casting table to see if the
gauge reads more than 5 (I assume 5" of vacuum on a 0-30" scale
instead of 5 pounds), indicating a table or bell jar leak. You can
plug or cap any connection on the casting side of the gauge and see
if you get more vacuum. Placing a small amount of vacuum pump oil at
a leaky connection or area will temporarily increase the vacuum
reading.

Make sure the pump has the right amount of oil and it is on a level
surface (the oil is needed to create the vacuum in addition to pump
lubrication). Most vacuum pumps used in casting machines are
refrigeration and air conditioning servicing vacuum pumps. The pump
manufactures explain that oil must be changed after each use or each
day so the water condensed from the evacuated air does not sit inside
the pump and rust the valves. This will casue low vacuums problem. We
see this problem a lot here at the Equipment Repair center.

I hope you discover an easy to fix leak.

John
The Jewelry Equipment Dr.


#3

Hi Sean,

So, what kind of casting machine is it? Vac + Cast? Vacbell only?
Rotary pump or vane? (Big huge thing, or something that looks like a
little airbrush compressor?) (Did it hurt to lift when you installed
it? If so, then Rotary.)

Step one: check your vent valve. (yeah, I know, but I’ve left it open
myself on occasion.)

step two: If it’s the bell you’re having trouble with, check both
sides of the rubber for crud that’s impeding a seal.

step 2A: get both sides of the rubber wet, and see if you get a good
seal that way.

Step 3: start pulling the lines apart. Pull the upstream connection
out of the vacuum gage. Stick your finger over it to seal it, and
see what you get.

There shouldn’t be much in the way of connections or hoses between
the gage and the pump. If it’s not pulling decently with nothing but
the gage connected, it’s either the pump, or one of the two or three
connections (or hoses) in between. Be suspicious of the hoses. I’ve
seen even new ones split. They generally do it in the deepest,
darkest corners of the machine, and always on the side that faces
away from where you’d be looking from.

If you’re still having trouble with nothing but the gage connected,
try listening for the leak. Seal the upstream end of the gage, and
listen for the hissing. (You may have better luck in the half second
after you shut off the pump.)

Best of luck,
Brian.

PS–> if it’s a casting table you’re having trouble with, you do
have a metal trap set up under your table orifice, right? (And you
didn’t just suddenly start having trouble after a pour, right? ) If
you had trouble starting right after a pour, the odds are good you
had a blowout that dropped molten metal into your lines and burned a
hole in them. For some reason, all of the tabletop vac casting rigs
I’ve seen lately have omitted the metal trap that’s needed to
prevent blowouts from trashing the machine. Not much of a problem to
add one, just a couple of bits of hardware store plumbing to add and
seal.


#4

Sean, troubleshoot your vacuum problems by getting a vacuum gauge and
inserting it is the vacuum line at each junction to see where you are
losing vacuum. It could be a crack in a hose, a split fitting, or
something like that. If you are losing vacuum at the pump, it could
be a bad diaphragm.

Good luck…Teddy


#5

Sean, you probably have a leak somewhere. Maybe the lines, maybe
between the rubber pad and the investing table or the bell jar,
maybe internally in the pump. If the sound the pump makes changed,
even just a little bit, maybe it started making a little bit of a
buzzing, whining or grinding noise, it’s likely the pump. If the pump
sounds the same, it’s somewhere in the plumbing or table itself.
Assuming you have checked the release valve and if you have a
box-like unit plumbed with translucent plastic lines, I’ll bet one of
the lines cracked next to one of the fittings. Cracks like that can
be hard to find, but I’ll bet that’s the problem. It’s usually where
the tubing attaches at the bottom of the investing table as that one
is subject to lots of movement. If you’ve ever had a flask blow out,
the one under the casting table or chamber is a likely place, the
damage to the tubing can take months to make itself known. The next
most likely place is the where the tubing is attached at the pump,
due to the vibration of the pump.

You can listen for a hissing sound, but it’s hard to hear over the
pump, and even harder to isolate. A little water on a brush or
something applied to the tubing will show where the leak is, you will
see the water dripping or making little droplets inside the tubing.
Be careful with this, if water gets in the pump it will surely kill
it if you don’t change the oil right away. The oil will look milky if
it is contaminated with water.

If it turns out to be a cracked line, make sure you use hose or
tubing rated for high-pressure (at least 300 psi) to replace it, not
just regular rubber hose from the hardware store. Hose not up to
high-pressure standards can collapse under vacuum. It is possible to
use the existing line if it’s long enough, but if it’s old and
brittle enough to have cracked once, it’ll do it again.

Dave Phelps