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Heat Treating Furnace

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John Purdy25/09/2020 05:18:49
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212 forum posts
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Many years ago I started to build a small kiln/heat treating furnace (bought the fire bricks about 1980). Finally finished it. It runs on 240 volts and is controlled by a home built controller with a digital temp. readout in deg.C. I've had it up to over 750C and it maintains temp within 3-5 degrees which is close enough for the use I envision for it. Inside dims. are 6"x7"x7" (HxWxD). Before someone says why didn't you use a PID controller the reason is when I originally made the controller the cheapest PID I could find was just under $400 (and up from there). My controller and temp readout cost less than $50. Since I finished it a couple of months ago I have ordered and received a PID controller (REX C100), 40 amp SSR, heat sink, and K thermocouple for $17 delivered to my door!

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ChrisB25/09/2020 06:10:49
548 forum posts
192 photos

Most interesting! Nicely built John. What did you use for a heating element, and what resistance/current/power did you go for? I plan to build a similar oven with pid control, so this thread is of great interest for me.

not done it yet25/09/2020 09:51:48
5024 forum posts
20 photos

Looks good. Always satisfying to build your own (if only ‘eventually’!).

I bought a Paragon SC2 for my wife several years ago - for precious metal clay, enamelling and lamp-work.

I use it more often than she does. I see they are now over £850 for the basic kiln (without the lamp-working extras), so yours must have been very economical - at least on materials - but possibly not if you take into account your time and effort.🙂 Also I expect they have developed some better insulating materials in that time - but who cares that much.

ChrisB,

Stan of Shadon HKW and the Bar Z Bash has recently developed a couple of ‘ovens’ for heat treatment and ,of course, there are several other good building ideas on youtube.

John Purdy25/09/2020 18:49:06
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212 forum posts
75 photos

The heating elements are what started it all. I found some replacement elements (Ohm-Rite) for hot plates in a small hardware store in Summerside PEI when I was there in 1980. The ones I used were rated at 660 watts @ 120 volts and I have two in series for 240 volts. They are, I suspect just nichrome wire and are .022" dia and measure 15.4 ohms cold, and the close wound coils (.195" dia) as bought are stretched out to just shy of 23 " to fit the grooves in the side walls. How long they will last I don't know. The walls are made from light insulating fire bricks rated for 2300 deg F bought from a refractory supplier in Halifax after I found the elements.. The elements and the bricks followed me around this country from one side to the other for 25+ years before I figured I better do something with them before the fragile bricks were completely destroyed.

John

not done it yet25/09/2020 19:44:27
5024 forum posts
20 photos

John,

Just looking at it again, I am not so sure about opening the door downwards. Working over and close to that at around 800-1000 Celsius might need a fair amount of arm insulation or long tongs?

Alternatively, it may aid keeping the item hot while adjusting grip for quenching? I mention this because most of the smaller kilns have used, had the door open upwards, with counterweights to keep them positioned. The larger ones with side hinges were for duties where they cooled before opening (after firing ceramics). The only side opener I’ve used, where it is opened hot, is the little Paragon.

John Purdy25/09/2020 23:30:13
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212 forum posts
75 photos

NDIY

When building the sheet metal enclosure I stewed over how to have the door open, up, down or side wise. I had searched a number of kiln manufacturers web sites for ideas including Paragon and side wise lost out as where it was going to sit in the shop was next to the wall and, as can be seen in the pictures, there was insufficient space. I couldn't see how to do it opening upwards, so down it was. My first iteration didn't work, wouldn't stay closed, so back to the drawing board to redesign the hinges. I don't think the heat while reaching in will be a problem as, dare I say it, I have a pair of asbestos gloves for protection that I use for boiler making. With them on I can pick up a red hot boiler and manipulate it with no problem.

John

Bazyle26/09/2020 14:06:28
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5479 forum posts
206 photos

I like your arrangement of bricks. I had been toying with an internal dimension of two whole bricks only giving length, not much width and height for a possible small crucible. At top temp what is the power consumption? is it about 100% duty cycle and does the outside get unpleasantly (dangerously?) hot?
Also I count 15 bricks, so off to count my stock.

ChrisB26/09/2020 17:58:54
548 forum posts
192 photos

Hi ndiy, yes I had subscribed to "Shadon HKW" he's got nice properly built ovens and explains in detail how they are built. For heating element I was going to use Kanthal A1 wire as it looks like its fairly stable at elevated temperatures.

John Purdy26/09/2020 18:42:29
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212 forum posts
75 photos

Bazyle

If you go to my "kiln" album you will find my cutting schedule for the bricks. Bricks are standard 9 x 4 1/2 x 2 1/2 insulating fire bricks. I made a rudimentary wood mitre box to cut them using a fine tooth woodworking back saw, they are very soft and saw easily. When on it draws ~7.5A at 240V. The outer case is mildly warm after 15-20 minutes or so. Once the temp reaches the temp preset by the pot on the front it just cycles full on and off at about a 50/50 duty cycle to maintain that temp +-3-5 deg or so. A PID controller would keep the temp to a much closer tolerance.

Chris

I would have preferred to use Kanthal elements as they will last a lot longer but I had these ones, so will see how they last. If they burn out I will replace them with Kanthal ones

John

not done it yet26/09/2020 19:22:55
5024 forum posts
20 photos

The best muffle furnace I have used was a 3-phase Barford - it had crusilite elements and went up to 1400C - and possibly more.

I used to make toast, for lunch, by simply passing the slice across the opening.🙂

I also remember the time a lad was almost thrown across the lab when he touched an element with the tongs. A good job he hadn’t already picked up a crucible... That made him more careful (and precipitated a visit from the electricians to adjust the mercury switch on the door mechanism).

Most furnace use was burning off filter papers for gravimetric analyses, or losses on ignition of feedstock for the kilns (with the regular determination of volatiles and ash content in coal samples). The wired heating elements were embedded in refractory and needed insulation around the outside - probably a lot of asbestos in those days.

Kanthal supply crusilite elements but I hate to think of the cost. There were at least six, possibly twelve, elements in the Barford (can’t remember as it was 50 years ago).

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