In article ,
"DoN. Nichols" wrote:

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.

On my Vigor, the heater volume is 13" wide by 6.5" high by 7" deep. The
door swings to one side on vertical-axis hinges.

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?


But it came with *no* controller.

I have a Robertshaw "infinite" controller for sale cheap...


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.

OK. I didn't do that.


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.


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?

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.


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.


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.

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.


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. :-)

I don't think my Vigor will get quite that hot at present, although if I
do some more repairs it probably will manage. One fine day the heater
element will fail, and they are still available for about $60, long
after Vigor went bust. Vigor must have sold a lot of these furnaces.

Joe Gwinn