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Vacuum caster questions and suggestions


#1

I’m setting up my own workshop for the first time, and I’ve never had
to really fiddle with equipment before and just used what was
available where I was (most of it being fairly old.) One of the
things I’m somewhat stuck on is the Vacuum Investor/Caster. Most of
these have vacuum pumps that require oil and I guess my first
question is how much maintenance do the require? The one time I’ve
seen checking the pump oil said to do it on a daily basis to see if
there’s water in it that needs to be drained (I’m assuming that means
if you use it on a daily basis.) I’m not likely going to be using it
on a daily basis and I don’t really want to deal with that level of
maintenance so I was wondering if it would be worth it to pay the
extra money and get one with an oil-less pump.

I’ve only found one oil-less Vacuum Investing/Casting machine from
Stuller and it’s quite a bit more than the one I was looking at over
at Otto Frei. If anyone has any other suggestions or advice, I’d
appreciate it.


#2
The one time I've seen checking the pump oil said to do it on a
daily basis to see if there's water in it that needs to be drained
(I'm assuming that means if you use it on a daily basis.) 

The vacuum pump manufactures say to change the oil after each use.
Each use condences water into the pump oil. Whan allowed to sit with
the water in the oil, it will rust and damage the pump. For that
reason, it is best to change the oil right after you use it, not just
before the next time you use it. Changing the oil after each use, the
pump should last a lifetime.

John
The Jewelry Equipment Dr.


#3

Wow! We’ve had the same pump for at least 25 years and only changed
the oil twice. It still works just great.

Jo Haemer
www.timothywgreen.com


#4

Here we go again…

I can’t believe the lack on sometimes. Now keep in mind
I’ve been using this equipment for almost 40 years. Perhaps after all
this time I’m just misinformed. Please consider the following…

The only vacuum pump available is from the heating & air
conditioning folks who use vacuum pumps exactly like the ones sold
in all our vacuum casting/investing units. Most vacuum pumps sold are
used in this industry. They change the oil every time because they
have to clear out the refrigerant in any cooling unit (refrigerator
or freezer). This oil is now contaminated. Sometimes the vacuum pump
is left on for a full day to make sure there in no Freon (or the new
stuff) or moisture left in the system. The program next to me at the
Southeast Technical College is the HVAC (Heating Ventilation Air
Conditioning) Program so I know this first hand! I see the use
everyday in their shop as well as the jewelry facility. I’ve also
compared noted with both instructors about vacuum pump use.

The pumps are quite hearty. I have three that I use every week. I
change the oil once or twice a year. I have a good filter on my
casting unit to prevent plaster dust from entering the unit. I have
had several casting flasks blow out over the years that would put
quite a lot of investment into the unit, so a filter is absolutely
necessary.

The pump I invest with and vacuum silicone molds does not have a
filter. This pump new in 1985 cost me $350.00. It still works like a
champ after all this time with the oil change schedule I use. And yes
sometimes I empty it with a bit of water that drains into the water
jug I use to collect it. Keep the oil level at the recommended level.
Sometime there is a small window to see the oil. Others may have a
dip stick just like a car.

You don’t need the jewelry supply vacuum pump oil either. If you
live near any farm store, vacuum pump oil for milking machines works
fine. It’s either that or an HVAC supplier. We continue to get
screwed by jewelry suppliers telling us that theirs is the only
product to use, when much of it is just renamed. Jewelry source
vacuum pump oil is very expensive and has to be shipped UPS because
of its hazardous definition.

To dispose of the oil, bring it to an automotive service station.
They recycle it just like motor oil. In the old days they would heat
their garages with it in some type of oil burning system.

The one change I would make in any of the vacuum systems in our
jewelry world is to increase the hose size in any of the units. I
have changed out all the vacuum hose to large diameter sizes and
improved every vacuum process from casting to investing and mold
making with this change. The problem I see is that these units are
designed by people who have never lit a torch or made any jewelry.

Best Regards,

Todd Hawkinson
Southeast Technical College


#5

Well folks, I expected responses like this.

Wow! We've had the same pump for at least 25 years and only
changed the oil twice. It still works just great.

I am in the business of repairing this equipment, so I know some
people in the jewelry casting business often go months, sometimes
years without changing oil. For those of you… recommended equipment
maintenance isn’t for you. I don’t make money selling vacuum pump
oil. On only make money repairing broken equipment.

You don't need the jewelry supply vacuum pump oil either. If you
live near any farm store, vacuum pump oil for milking machines
works fine. It's either that or an HVAC supplier. We continue to
get screwed by jewelry suppliers telling us that theirs is the only
product to use, when much of it is just renamed. Jewelry source
vacuum pump oil is very expensive and has to be shipped UPS
because of its hazardous definition.

Todd’s right. We buy the oil from Grainger. There is a Grainger
store in nearly every city. Sometimes it is less expensive at
Johnstone (a HVAC supplier), also in most cities.

To dispose of the oil, bring it to an automotive service station.
They recycle it just like motor oil.

Good suggestion. That’s what we do. Allow them to assume it is used
motor oil from your car.

The one change I would make in any of the vacuum systems in our
jewelry world is to increase the hose size in any of the units. I
have changed out all the vacuum hose to large diameter sizes and
improved every vacuum process from casting to investing and mold
making with this change. The problem I see is that these units are
designed by people who have never lit a torch or made any jewelry.

Well Todd, most of the units jewelers are using in small shops use
1/4" ID semi-rigid plastic tubing. I know the man who made that
design decision many years ago. Contact me ofline if interested.

John
The Jewelry Equipment Dr.


#6
Here we go again.... I can't believe the lack on information
sometimes. Now keep in mind I've been using this equipment for
almost 40 years. Perhaps after all this time I'm just misinformed. 

Yes possibly you are in your understanding of the mechanics of the
water and oil issue. There are several reasons that the vacuum pump
manufacturers want you to change the oil at a regular interval. The
first reason is that water has a vapor pressure that is several
orders of magnitude greater than the vacuum pump oil. This means the
pump will not be able to pump down to its specified base pressure if
there is any water in the oil. How does this aspect effect us? Not
at all, none of the vacuum systems I have ever seen used in jewelry
equipment can even begin to hold vacuum pressure as low as the pumps
that are supplied with the equipment can achieve when in good
working order with fresh oil. Since one of the main uses we have for
them is in pumping on a water and investment slurry you would be
unable to get below the vapor pressure of water even if you had a
oil free pump. And when used for vacuum assist casting the seal you
get between a hot flask and the silicon rubber gasket is not not so
good either, it has a fair amount of leakage by it and then there is
the tiny but real amount of gas that actually passes through the
investment. So both uses are not really critical of how low a
pressure you can achieve. The much more important aspect is the
pumps volume capacity or throughput, how many cubic feet or cubic
meters of gas it can pump in a given period of time. This is usually
expressed in cubic feet per minute or cubic meters per hour. This is
an area where having water in the oil will cause some problems as
the internal pressure in the pump drops below the vapor pressure of
water the water in the oil will vaporize and take up some of the
space in the pump cavity and reduce your vacuum pumps throughput.
But the biggest problem is that the water will rust the pumps
internal surfaces. The internal parts of the pump have incredibly
tight tolerances and very smooth surface finishes and that rust just
does a number on those surfaces. It will degrade both the ultimate
vacuum pressure and the throughput of the pump. The problem is it is
a slow degradation so it is almost impossible to notice the
performance degradation.

So what should one do about water in the oil? The biggest single
thing you can do is run the pump with the intake closed off (pumping
on the bell jar or just put a rubber stopper over the intake port)
for 30 min before investing and 30 min after investing. The pumps
run quite hot, the heated oil will have less ability to condense the
pumped water vapor and much of it will pass right through the pump
and be exhausted into the atmosphere. If the oil is cold as it is
when you first turn on the pump the water vapor will all tend to
condense and stay in the pump. It is critical to close off the
intake or you will also exhaust most of your oil out of the pump as
vapor, these pumps are not meant to run at atmospheric pressure for
any length of time. If you follow the above advise and change your
oil once a quarter or even twice a year you will make the pump last
a long long time and maintain its throughput near to its factory
specs.

James Binnion
James Binnion Metal Arts


#7
You don't need the jewelry supply vacuum pump oil either. If you
live near any farm store, vacuum pump oil for milking machines
works fine. 

Wow, Todd, thanks for this info. Last time I bought oil, I tried to
get it locally because of the hazardous fee, but I asked for it by
the jewelry name, and could not find it, of course. Milking machine
vacuum oil! Yay! Thanks for the tip on putting in larger hoses, too.
I might try that as well.

M’lou


#8

I’m new to your forum, a 28 year vacuum pump professional and really
impressed with the level of knowledge expressed. Mr. Binnion sounds
like a vacuum engineer with a lot of experience. In regards to the
water in the oil, in my industry it’s common to set up an “auto-purge
or manual-purge” in many different vacuum applications with a
condensable load using oil flooded pumps. Once the pump is brought up
to operating temperature and switched on line the key is trying to
keep the vapor above the dew point of the pumps exhaust temperature.
Obviously with the rotary vane technology most of you are using its
impossible to keep the pumps exhaust temp above 212F (without
damaging the pump) and water vapor will condense before the stream
makes its way out of the pump. When purging the pump before you shut
down a slight ballast or air leak that acts as carrier gas attaching
to the condensable load will help to purge the pump of water (Mw =
Ma/MWa (MWw) Pv/P-Pv). Most of the vacuum pumps your using today may
already have a gas-ballast valve as part of the pump that attempts to
inject the carrier gas during the start of the compression cycle.
When setting up your purge (it can be at the pump inlet; a pinhole
through that rubber stopper Mr. Binnion describes) air remember that
2-stage pumps without an internal oil pump should not be operated
above 10mm continuously (both mass flow and heat become an issue),
single stage oil flooded pumps can purge to 28.5"HgV. Hope some of
this may prove useful.

Mw-amount of water (lbs/hr)
Ma - amount of air (lbs/hr)
MWa - Molecular weight of air
MWw - Molecular weight of water
Pv - vapor pressure
P- operating pressure

Scot Desiderio
Houston Vacuum Pump & Compressor
www.scotdesiderio.com


#9
Well Todd, most of the units jewelers are using in small shops use
1/4" ID semi-rigid plastic tubing. I know the man who made that
design decision many years ago. Contact me ofline if interested. 

Perhaps just casting one ring at a time or only small flasks I could
see that this would be adequate. I usually go to vacuum casting for
volume, production and safety. I like to spin cast anything under 100
dwt.

This is one design I have changed with a good deal of success. It’s
simple; when you move more air you have better castings. Keep in
mind the investment is designed to allow air flow through it. The
vacuum sucks the metal in the flask right? If it was just gravity we
wouldn’t need the vacuum. Spin casting spins and forces the air
through the flasks as the metal also pushes the air through. A strong
enough pump is also important. With investing and mold making it does
not make that much difference.

I lucked out with my current casting pump. I friend of mine works at
3M and did quite a lot of work with an old electron microscope. It
ran off a powerful vacuum pump. I guess you have to look at some of
that 3M stuff under a vacuum. Anyway he requested and got a new
microscope without a vacuum. He said the pump created a small
vibration that did not help his research. I got the Vaughn pump for
$75.00. The diameter of the hose was 1". This is the strongest pump I
have ever seen! In just about new shape also. When building my
casting system I hooked it up with larger hose as well as brass
plumber’s pipe for a metal blow out trap. It also came with a
charcoal filter which works great for investment dust. With the
muscle of the pump and the larger diameter hose the vacuum casting
has never been better. There is no need for the perforated flasks or
the additional wax lines added to regular flasks for vacuum casting.

Even on the units sold as jewelry vacuum casting machines I feel a
larger diameter (perhaps not 1") hose would improve the quality of
casting. If you think about what you are trying to do, increased air
flow would only help. And in my experience it does.

Best regards,

Todd Hawkinson
Southeast Technical College


#10

M’Lou,

Here in Minnesota I get my oil at Mills Fleet Farm.

The brass hose fittings that still screw into the pump but have a
larger hose diameter I got at Menards.

Best Regards,
Todd


#11
This is one design I have changed with a good deal of success.
It's simple; when you move more air you have better castings. Keep
in mind the investment is designed to allow air flow through it.
The vacuum sucks the metal in the flask right? 

Yes it does pass very small amount of air through the investment but
the volume is tiny. Even the most anemic of the pumps used for these
machines has many times the throughput needed to pull all the gas
that can pass through any casting flask. The real need for pumping
speed is in rapidly pumping the the hoses and investment trap and
other spaces in the system and overcoming the losses of any leaking
components and the biggest culprit leakage around the flask gasket.
This is where larger pumps will shine.

I got the Vaughn pump for $75.00. The diameter of the hose was 1".
This is the strongest pump I have ever seen!

Typically vacuum pumps if they are a general purpose design like one
that would be in a pice of scientific equipment have a rough
relationship between the intake port diameter and the pump
throughput. One with a 1 inch port is typically around a 5 CFM pump.
I like the way a 5 CFM pump works for a well designed small casting
system. I have built several with this size pump.

Even on the units sold as jewelry vacuum casting machines I feel a
larger diameter (perhaps not 1") hose would improve the quality of
casting. If you think about what you are trying to do, increased
air flow would only help. And in my experience it does. 

The main problem I have seen with the casting machines offered by
most of the jewelry suppliers is that they are using pumps designed
for refrigeration service and the inlet port is either a 1/4" or
3/8" inch flare fitting. These are the typical sizes of the fittings
and hoses used in refrigeration service gear but the pumps are being
throttled by the small diameter of these fittings. These pumps are
typically the least expensive and smallest size for their pumping
capacity you can buy. You are absolutely correct that using larger
hoses will help these machines but you also need to upgrade to
larger valves and larger fittings and if possible replace the flare
fittings with a larger diameter fitting. If you only change just the
hoses you will gain only a small improvement in performance as the
other fittings and the valve will still act as limiters to the
pumping speed.

James Binnion
James Binnion Metal Arts


#12

My initial comments were to point out that it is good to change the
vacuum pump oil often because of the damage water can cause. I think
that Mr. James Binnion’s knowledge and suggestions are very
relevant.

So what should one do about water in the oil? The biggest single
thing you can do is run the pump with the intake closed off
(pumping on the bell jar or just put a rubber stopper over the
intake port) for 30 min before investing and 30 min after
investing. The pumps run quite hot, the heated oil will have less
ability to condense the pumped water vapor and much of it will pass
right through the pump and be exhausted into the atmosphere. If the
oil is cold as it is when you first turn on the pump the water
vapor will all tend to condense and stay in the pump. It is
critical to close off the intake or you will also exhaust most of
your oil out of the pump as vapor, these pumps are not meant to run
at atmospheric pressure for any length of time. 

I think this is practical advice that is helpful without spending a
lot of money.

There are, however, a few people who want to spend extra money to
upgrade their equipment. Increasing the size of the air lines and
fittings will help. We often make these changes here. Purchasing a
larger vacuum pump will also help. Todd is lucky to have found a
really good vacuum pump for $75. Perhaps the vacuum pumps Scot
describes would be a good choice. His explanation of the physics of
getting the moisture exhausted out of the pump is so interesting.

I will just continue to service broken equipment. Based on my
experience, I may also offer suggestions on equipment maintenance
that might reduce the chance of equipment failure.

John
The Jewelry Equipment Dr.


#13
The main problem I have seen with the casting machines offered by
most of the jewelry suppliers is that they are using pumps
designed for refrigeration service and the inlet port is either a
1/4" or 3/8" inch flare fitting. These are the typical sizes of the
fittings and hoses used in refrigeration service gear but the pumps
are being throttled by the small diameter of these fittings. These
pumps are typically the least expensive and smallest size for their
pumping capacity you can buy. You are absolutely correct that using
larger hoses will help these machines but you also need to upgrade
to larger valves and larger fittings and if possible replace the
flare fittings with a larger diameter fitting. If you only change
just the hoses you will gain only a small improvement in
performance as the other fittings and the valve will still act as
limiters to the pumping speed. 

Right on Jim.

The inside hose hole diameter should not change with the brass hose
fittings. Follow the path from the pump. A good hardware store
should have all the matching parts, wire reinforced hose also. Most
times the pump will have an intake hole size that cannot be
increased. Use that hole diameter and change out the hose diameter to
whatever the largest hose will allow. You’re lucky if the hose intake
size is larger that one quarter inch and can have the size increased.
A ball valve that is a bit larger will work fine if you need one. If
the size going smaller that would restrict air flow is what you want
to avoid.

Here’s another money saver. I have always made my own vacuum casting
tables. A quarter inch thick piece of aluminum works great.
Combination investing and casting can be done with a little more
work. If you drill a hole almost the size of the brass hose fitting
that goes through it you can just screw the brass fitting into the
plate without threading the aluminum. The brass will thread the
aluminum. Just go slow and straight. Match the size of the system.
You get one shot at straight. On each corner I drill and screw in
longer bolt and put springs under it to a counter top of platform of
some sort. Most units have these springs under for support and to
bounce the investment to help it fall.

Best regards,

Todd Hawkinson
Southeast Technical College