On 2008-08-15, Joseph Gwinn wrote:
In article ,
"DoN. Nichols" wrote:

On 2008-08-14, Joseph Gwinn wrote:
In article ,
"DoN. Nichols" wrote:

On 2008-08-13, Joseph Gwinn wrote:

[ ... ]

Hmm. 4*3*6= 72 cubic inches, versus 13*6.5*7= 591.5 cubic inches, a
volume ratio of 8.2 to one. Did you get a burnout furnace, or a melter?

I have no idea. There were no labels -- manufacturer or model
number -- remaining on this one.

It could be either I suppose. I assume that one needs both kinds of
furnace, or the logistics of getting molten metal into glowing-hot mold
become intractible. Unless the melting is done with a torch and
crucible.

Well ... I intend to use this purely for heat treating. It is
too small to handle a crucible of molten metal safely. You press down
on a counterweight on a lever to the left and the door lifts from in
front of the heat zone.

There is a shelf in front of it -- slate or perhaps asbestos
board. Too discolored with age to tell for sure. About 1/4" thick,
FWIW.

But there were no provisions for runoff of wax, and the
orientation is similar to a brick on its flat side, with the smallest
area towards the door.

There are no provisions for wax runoff in mine either. I don't think
they worry about the wax, and just let it burn off. I have read of
burnoff furnaces with a smoke vent, but I suppose that would only be for
large furnaces.

Hmm ... no vent here, unless the 1" hole in the back was
supposed to be one. But it is not fully at the top of the back.


If the original controller were still present I might have a
clue from the range it was intended to cover. But all there was was a
pair of stiff wires insulated in asbestos coming out the back to connect
to my controller. It works nicely for heat treating small workpieces at
least.

Probably a 14 guage Type K (chromel-alumel) thermocouple.

Nope -- it is the wiring to the heating element. *Nothing*
which came with it was for measuring the temperature -- other than
perhaps the 1" diameter mica window in the door with an optical
pyrometer. :-)

Chromel and alumel are basically a form of stainless steel.

O.K. I've got the alloy definitions in my old CRC _Handbook of
Chemistry and Physics_ (otherwise known as the "rubber tables". :-)

[ ... ]

While I was at it, I added a locking miniature toggle switch (the kind
where you pull the handle to enable switching) between the output of the
controller and the input of the SSR, so I could use the controller to
track the cooldown temperature without having to reset it to cool down.

OK. I didn't do that.

I also added a larger LED to watch the cycling of the heating
element.

An easy thing to add after the fact -- as long as you have some
extra front panel space available.

I made space by removing the original moving-coil pyrometer. The 1/32
DIN controller easily fits in the hole left by the pyrometer.

The pyrometer does seem to work, and I'll calibrate it and use it as a
backup for the fancy electronics. It may benefit from a new
thermocouple.

O.K.

The oven was on legs about as tall as the aluminum box which I
selected for the housing, so I put that box on standoffs on the
right-hand side legs to minimize conducted heat from them (and not much
heat there anyway).

The box was sufficient heat sink for the 20A SSR, since I could
expect no more than 15A load from the wall anyway. I did use white heat
sink goop when I mounted it to the chassis, of course.

This would *not* work on my furnace - the box is steel (poor heat
conductor) and itself gets quite warm at max temperature.

O.K. Make an aluminum box to mount on the side, with standoffs
and perhaps an intermediate sheet of aluminum between two sets of
standoffs. Use the original controller box for storing accessories
which won't be harmed by the heat. :-)

That would certainly work, but would stick out very far to the right.

O.K. Can you punch a couple of holes in the side and mount a
small computer fan in there to circulate air whenever it is running?
Perhaps add a thermostat to keep the fan running for as long as there is
significant heat coming through the wall?

I took my mechanical inspiration from pictures of current furnaces with
digital controllers.

O.K. I just went with available Bud Minibox chassis (I didn't
at that time have a shear large enough to accompany my 24" brake to make
my own.)



[ ... ]

So, for the SSR I got a large heatsink from Omega, which dropped the
temperature to 36 degrees C.

The controller was a bit more difficult. I ended up making a set of
spacers from 0.5" diameter gummy aluminum rod on the lathe, and cutting
two aluminum plates with clearance holes, all to space the control box
away from the body of the furnace, with two parallel plates between,
thermally isolating control box from furnace body. Now, the PID
controller temperature is 35 degrees C max.

No tall legs on yours, I presume?

None whatsoever. There is a ventilated skirt which is an extension of
the vertical walls of the furnace. I have it setting on a wood bench,
which does not get all that warm.

O.K. How tall is the skirt? Not tall enough to use it as the
mounting point for the controller, I presume.

About 2.5". One could mount the controller in the skirt, but it would
make using the controller quite awkward, as it would be so low, and
would expose it to damage from dropped hot objects.

O.K. A bad idea, then.


Overall, it sounds like a
Blue-M muffle furnace which I had a work with the same duty-cycle type
controller. That one had the controller in a box to the right as part
of the overall oven housing. It did have a heavy-duty thermocouple
connected to an analog meter for readouts of the temperature.

No doubt this has long since gone to a surplus sale. :-)

Sounds like the Vigor for sure.

Proably made by the same company -- whether that it Vigor or
Blue-M -- just for different markets. The Blue-M came from a lab supply
catalog.

I think that this was an industry-standard design, made by everybody and
sold by the pound.

[ ... ]

Agreed. I was quite amazed at the Omega controller. About two
thirds of the way up to set-point it shut off and measured the coast of
the heat on up, so when it reached near the set-point it had a total
overshoot of 1 degree F.

That's a self-tuning PID (Proportional Integral Derivative) controller
for you. These used to be expensive, top-drawer industrial controls,
and now they are jellybeans. Mine cost $99 new.

Hmm ... I think that they were $200.00 when I got mine, and
since I got mine at half price, I got it for the same that you did. :-)

Yes. By the way, the self-tuning feature may benefit from some
training. Once or twice, instead of simply turning the power off when
done, set the setpoint temperature to a little above ambient, say 50 C,
and let the controller follow the temperature down, for the experience.

O.K. A good idea.

There was someone on this newsgroup who had several left over
unused after a project closed down, and who was offering them here. I'm
glad that I got one. I wonder whether we even have the same model? It
would take me a bit of work to verify the model now. Mine has the
plug-in strip along the top which says that it is displaying in degrees
C, but I've switched mine over to display in degrees F, since that
matches the information in _Machinery's Handbook_ more closely.

What I got is a Omega CN7523 controller driving a Omega SSR330DC SSR
which is mounted on an Omega FHS-7 heatsink.

Mine is too hard to get to at night. Easier when I can open the
shop door -- as I intend it to be when I'm using the oven. And a good
fire extinguisher handy, too. :-)

[ ... ]

I did have to make a bushing to go around the thermocouple probe
to minimize airflow through it. The original hole was something like 1"
diameter -- way too much for the skinny probe which I had, so I turned
up a bushing of lava, and then cooked it in the oven itself (which turns
it from whiteish-gray as machined to pink. :-)

In my Vigor, the hole fits the ceramic sleeve insulating the
thermocouple reasonably tightly.

One can also make bushings from K23 foam firebrick and glue it into
place with furnace cement.

O.K. But I *had* the machinable lava. It machines quite easily
in the lathe (very dusty), and when it is fired it turns pink and *very*
hard.

[ ... ]

It may be time to do another one, with a larger hole for an
alternative thermocouple which I found out in /dev/barn/01 a couple of
weeks ago with an analog meter. That would give me a totally power-free
way to monitor the temperatures.

The traditional moving-coil pyrometers are self-powered, and are
optimized to work with a thermocouple. They are accurate enough for
heat treating. Some pyrometers were designed for mounting in a steel
panel, and are inaccurate if not in the panel, so if the panel is
aluminum it may be necessary to provide a piece of sheet steel as well.

Right -- the steel panel shunts off some of the magnetic field
which of course changes the sensitivity somewhat. IIRC, this was
originally mounted in a non-magnetic alloy of stainless, so there is no
problem there.

Enjoy,
DoN.

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