In article ,
"DoN. Nichols" wrote:

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.

Can't be slate at that temperature, so asbestos board is more likely.


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.

My Vigor has a 1" diameter vent hole in the top, with a little metal
cover that one swings to the side. I wondered what it was for.


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. :-)

Ah.


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". :-)

The Omega.com website has lots of information as well. As does NIST.


[ ... ]

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.

The Vigor had a neon pilot connected across the heating element, and so
I left it as it was.


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.

I think one can buy pyrometers on eBay and at ham swap meets. There are
now lots of Pyrometers on the dole, having been made redundant by
digital controllers.


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 thought of this, but couldn't find an 115 volt AC fans that were small
enough. I probably can run a 24 volt fan off the controller signal to
the SSR, but the fan would run only when the element was on. Perhaps
that's enough.


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.)

Doesn't Bud (or Hammond) still make steel chassis? Just like in the
1950s?


[ ... ]


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. :-)

Yes about the fire issue. I don't run this furnace if I'm not present,
at least not until I have some experience with it. The first thing I
did with it was to measure temperature versus time under power, to see
how long it took and how hot it got. This caused some tightening-up
exercises, which allowed the peak temperature to climb a bit. I've had
it up to 1,000 C or so.


[ ... ]

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.

What make and model is this material? It may be useful.


[ ... ]

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.

So, you must avoid the steel.

Omega sells the #14 Type K thermocouples for something like $20 each, so
I wasn't tempted to scrounge for one.

Joe Gwinn