On 2008-08-13, Joseph Gwinn wrote:
In early June 2008 I bought an old Vigor Burnout Furnace (model CA 1065)
for $95 from a seller on Craigslist. The furnace appears to have been
built in 1979, but does work.

O.K. I picked up a small one at a tool flea market at a
collector's gathering. The heated area was about 4" wide, by 3" high,
by about 6" deep, with a counterbalanced raising door with a mica window
for viewing the interior when hot.

But it came with *no* controller.

IIRC, I think that it cost me something like $25.00 a few years
ago.

The furnace came with a so-called "infinite" control, which is an
electromechanical contraption with ten intensity settings (from mostly
off to always on). What varies is the average duty cycle of power on
versus power off. There is no feedback control. This is far too crude
and clumsy to heat-treat metals, so I decided to add an electronic
furnace controller.

Since I already had one of the Omega controllers and a stainless
steel enclosed thermocouple, all I had to do was to make a housing for
the controller and the SSR (a 20A 240V one which I also already had).
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.

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.

So I bought a 1/32 DIN PID controller and solid-state relay (SSR) from
Omega Engineering. I'm still using the thermocouple that came with the
furnace, but not for long, as the original thermocouple is old and
oxidized and a bit off. But it works well enough for now, and will hold
to within a degree or so of a temperature, far better than is required
in heat treating of say O1 oil hardening steel or A2 air hardening steel.

The big problem was that the original design didn't have a big enough
heatsink on the SSR, and there wasn't enough thermal isolation between
the control box and the furnace box, so the controller and SSR both got
too hot when the furnace went to max temperature (about 990 degrees C).
The controller would get to 50 degrees C, which is right at its upper
temperature limit, which is asking for reliability problems. The SSR
would get almost to 60 degrees C, which is also in its range, but asking
for trouble.

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? 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. :-)

At this point, I have declared victory, as 35 or 36 degrees C is low
enough that reliability won't be much reduced.

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.

I got the controller for half price from someone in this
newsgroup in advance of need -- just knowing that I would eventually
need it. As it turns out, I was right. :-) Same for the SSRs -- I tend
to stock a few for future needs.

I've so far taken mine up to 1850F (1010C) and that took about
an hour, so I don't expect it to get much hotter. :-)

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---