Advice please, from anyone who has replaced a broken spring on a
centrifugal casting machine? Any special precautions, when removing
the broken spring? Such as, is a clamp on the spring necessary, when
removing the broken spring from the housing? Will the spring unwind
quickly,or was all the stored energy released when the spring broke?
thanks, Andy
Andy,
When replacing the spring, make sure you lay it into the opening as
the old one is positioned. I put one in backwards once and had a hell
of a time rewinding it and putting it in correct. If the spring is
open without any housing, you will need a vice grip or two to place
it into position and release the grip that keeps it wound. Make sure
the little tab on the spring that grips the machine is in the right
position.
The newer ones are self contained and don’t come out of a housing.
They just drop in. They last for many years. As they age you just
have to add one more rotation to the tension. This depends on the
feel of the spring strength. As long as they are not broken, you
should be able to get years out of one spring.
This is not for the Neycraft machine. I’ve never used one of those.
This is the broken arm most commonly seen one. Don’t know the brand
names.
Good luck,
Todd Hawkinson
Southeast Technical College
The sprong will have lost its energy wut removing and refitting a
new one may well require a clamp to keep the thing located where you
want it. Have never done it on a casting machine but have on a
similar clockwork machine and got it out of synch because the broken
one doesnt really tell you how much spring tension there is when at
rest.
Nick Royall
Andy,
The old spring will be under some tension. Note the direction of the
old spring, then be careful removing it. The new spring will have a
clamp or wire tie on it. Do not remove that clamp or tie until it is
in the housing. Sometimes I can do the operation without locking
pliers like Vise-Grips, but always have them ready, if needed.
John
The Jewelry Equipment Dr.
When I had problems with my spring, I had to get castings done so I
threw a silicone pad on the vacuum table and vacuum cast. Well that
was 20 years ago and I have not seen my centrifigal machine for
years. Sure did free up space in my workshop.
Bill Wismar
Well Bill,
I can tell you one thing for sure, it you have every tried to cast
14 or 10 white gold with a vacuum with any amount of quality you’re a
better caster than me.
Even the best casting houses that do heavy production depend on spin
casting for top quality. I’m not talking about onesy twosy casting
here either. If you can produce thirty to sixty white gold rings in
one flask without any porosity let me know your secret.
In my little casting world, vacuum casting is turned to for volume,
productivity and profit. With spin casting everything is guaranteed
to turn out. Vacuum casting requires a bit more heat to do the same
job, which can complicate the process & product.
Did you also know that if you take two identical waxes and cast one
spin and the other vacuum they will have different weights? In
today’s market that can spell a bit on money.
Best regards,
Todd Hawkinson
Southeast Technical College
(We teach both spin and vacuum casting here)
Even the best casting houses that do heavy production depend on spin casting for top quality. I'm not talking about onesy twosy casting here either. If you can produce thirty to sixty white gold rings in one flask without any porosity let me know your secret.
In my little casting world I have far less failures using vacuum
casting than with spin. I am a small shop, like most of us here so it
works better for me. I was at Stuller a year ago and they vacuum cast
everything except for platinum. I placed an order with my New York
casting house today and ask them what they do. All vacuum cast with
trees from 50 to 75 pieces. Platinum spin cast.
Maybe you should look into the new vacuum machines as a viable
option to teach your students.
Hi Guys
Modern commercial vacuum casting machines are very good and control
the atmosphere, very fine, solid castings can be produces using a
vacuum caster.
However the cheapest I’ve seen is about 40k.
The company where I get some of my castings done ( the really fine
ones), use machines with an argon atmosphere. They don’t use spin
casters. They do masses of work on a daily basis. They have more
than one machine also. I saw an example of a sprue tree with hundreds
of white gold rings on it. They’re not lightweights, it’s all about
quality for them 
Regards Charles A.
I placed an order with my New York casting house today and ask them what they do.
Did you talk to the person who actually does the casting? The best
ones I know have a spin caster tucked away in case of a real
difficult project. These would probably be old farts like me though.
The casters I have trained always start out spin casting. For the
community here, a large expensive vacuum casting machine is out of
budget. Even so, the casting experts need a lot of practice to
master the basics of spin csting or the complexity of more
complicated equipment.
Always best intensions & regards,
Todd Hawkinson
Southeast Technical College
I keep an extra spring on hand, just in case. It’s really a drag to
have a kiln load of flasks and have a broken spring. I’ve replaced
the spring before, but it was long enough ago that I don’t remember
how it was done, only that it took about twenty minutes and didn’t
require any tools that I didn’t have handy.
As far as vacuum versus centrifugal casting, I use both. I vacuum
heavier pieces without a lot of intricate detail, and throw those
things that have thin areas and of course, all platinum castings.
White gold especially in thin or intricate pieces can produce
non-fills using vacuum in my experience. No right or wrong here, I
just use what works for me. Porosity seems to be a function of
temperature more than anything else, for me anyway, although gating
thick to thin and back to thick again is often problematic. Get the
metal too hot for the piece being cast and you will get porosity,
regardless of pretty much anything else including casting method and
the most careful gating. Old metal is a concern, but I do a lot of
casting using customer’s old gold, and as long as I don’t get it too
hot, it works just fine, most of the time. Adding a tiny piece or two
of Re-Cast-It from Roseco helps a bunch.
Did you also know that if you take two identical waxes and cast one spin and the other vacuum they will have different weights? In today's market that can spell a bit on money.
I read this in an ad for a centrifugal machine years ago, didn’t
believe it and did an experiment to find out. Two waxes with
identical weights, one cast using centrifugal, one using vacuum. They
both cast to the same identical weight, weighed to the thousandth of
a gram on a carat scale, or at least within a file stroke or two.
Metal is pretty much incompressible, meaning that unless it is
subject to the forces inside a black hole, the specific gravity will
be the same regardless of the casting process used. Even for forged
metal, the only differences are in crystal structure, so the specific
gravity remains identical, at least for our purposes. The only
difference that might be measurable would be caused by porosity. Air
pockets are a lot lighter than metal, so a lot of air pockets or an
open crystal structure might cause a very slightly lighter casting.
But my guess is that a casting that is measurably lighter would be so
full of holes that it wouldn’t be usable. Bottom line, don’t look for
savings in metal costs by using one method of casting over another,
there are none. Use the method that gives you the best castings.
Dave Phelps
Modern commercial vacuum casting machines are very good and control the atmosphere, very fine, solid castings can be produces using a vacuum caster. However the cheapest I've seen is about 40k.
40k will buy you a argon shielded induction machine. Ya got to plan
on a lot of casting, and don’t forget the consumables, they WILL eat
you alive.
Don’t forget the installation, tend to be heavy suckers which need
water cooling and 3 phase power, argon, and a vac pump.
For a small shop a spin machine is good, or for less $ just throw a
silicone pad on your vac table. I use both, spin delicate stuff and
vac larger heavier work. Few problems and none worthy of 40K 
jeffD
Demand Designs
Analog/Digital Modelling & Goldsmithing
http://www.gmavt.net/~jdemand
Metal is pretty much incompressible, meaning that unless it is subject to the forces inside a black hole, the specific gravity will be the same regardless of the casting process used.
This is a fun video, coin shrinking using very high voltage.
HBL coin shrinking
Enjoy.
Regards Charles A.
This is a fun video, coin shrinking using very high voltage.
Yup. Fun. But a little deceptive in that it seems as though the coin
gets smaller and thus more dense. But look closely and you find that
although the diameter shrunk, along with the various details of the
coin, the thickness of the coin increases. The total volume occupied
by the coin does not change, and the density or specific gravity
remains the same. It’s useful to remember that specific gravity
(density by it’s new name) is considered a diagnostic and unchanging
property of most solid and liquid materials. Thermal expansion and
contraction change it, but not much else. The density of a given
alloy is, what it is, and that can be trusted to remain the same time
after time, even if exposed to some experimenter’s high voltage
discharges. (I think the way those work is generating high eddy
current in the metal, which in turn generates high magnetic fields,
and the interaction of these magnetic fields is what gives the
compressive forces… I could be wrong on that, but that’s how I
understand it, remembered from a similar demo quite some time ago.
The coin is, in essence, sort of trying to become a sphere… Getting
thicker in thickness while reducing the diameter of the disk is a
start in that direction.
In casting, the only thing that will vary the apparent specific
gravity of the casting is when the casting is not perfectly solid.
Porosity or voids within the casting will change/reduce the density
of the whole casting just as accidental inclusions of other materials
in the cast metal can also change it. But this is not a change in the
specific gravity of the metal itself, only a flaw in the casting that
throws off the measurement thereof.
Peter
The only difference that might be measurable would be caused by porosity. Air pockets are a lot lighter than metal, so a lot of air pockets or an open crystal structure might cause a very slightly lighter casting.
Even in worst case scenario this is on the order of a tenth of one
percent and that would be a horrible amount of porosity it would be
like swiss cheese.
James Binnion
James Binnion Metal Arts
Jeff,
Another way to improve your vacuum castings is to melt your metal in
a protective gas atmosphere with tight temperature control. This can
be done with a relatively small investment in an electromelt
furnace. They have a sacrificial carbon crucible that releases CO2
gas as it heats up, thereby shielding the metal from the oxygen in
the atmosphere. Additionally, it allows me to control the heat very
closely to the ideal casting temperature.
I use mine when I am vacuum casting, and it gives me great results.
But like you, I use spin casting for the small stuff.
Craig
Even in worst case scenario this is on the order of a tenth of one percent and that would be a horrible amount of porosity it would be like swiss cheese.
It can be more than that, Jim, but not in the form of porosity. I’ve
seen shrinkage cavities fully enclosed within a casting that are
sometimes surprisingly large. If the flask temperature and metal
temperature and improper spruing combine just right, you can get the
outside surface of the casting remaining fully true to the mold’s
shape, while the sprue solidifies too soon, and then the molten metal
within the casting proceeds to shrink, which literally tears the
metal apart in the center portion of the heaviest part of the piece,
and sometimes elsewhere too. I’ve seen rings where the sprue at the
bottom of the shank couldn’t keep the upper shoulders fed, so the
entire shoulders of the ring can be, essentially, partially hollow
(I’m not taling little tears, either. how about oblong cavities 4mm
long and 2mm wide and deep along the center of the shoulder or a
ring. Saw one like that in 18K white gold just last week. That’s a
significant difference in the weight of the ring… Which might not
have been swiss cheese, but it still needed to be recast (I just
didn’t want to weld my entire remaining coil of 18K white laser
filler wire into that rings shoulder to fill the cavity, and we still
didn’t know if the other shoulder had done the same…
Peter
It can be more than that, Jim, but not in the form of porosity. I've seen shrinkage cavities fully enclosed within a casting that are sometimes surprisingly large.
Yes but these are operator error and not based on the type of
casting machine. If the gating is wrong you can have all kinds of
nasty fill problems in any form of casting.
Jim
James Binnion
James Binnion Metal Arts