Heat Treat Oven Wiring Schematic

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Kieran Klein said:
Ok that makes sense. I am sure I will come back to this thread about 30 more times before the whole project is done. Thanks!
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Here is a basic wiring diagram for a heat treat oven, this will get you up and running but obviously it will need to be unpluged to turn it off


 

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Ok while that all makes sense now. I do have a question on fuses. Can I use the little glass tube fuses ( at correct amps ) or do I use the shotgun shell size fuses (also at correct amps) I was a little confused which to get. Or if it even makes a difference.

I think answering my own question. Every oven I have seen seems to use the small fuses so unless I find out differently thats the route I am going to take.
 
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I'm running this schmatic. 220v. My DMM is reading the element resistance at ~16 ohms, which works out to 13.75 amps.

I'm pretty sure I got the wrong slow-blow fuses (10amp, 32 volt) because I already blew two of them. They should be 10amp 250v volt, correct?
 
Bussmann glass fuses are fine if they are the right type and in proper mounts. I prefer the ceramic type for power fusing, but both work. For heating coils you want slow blow (MDL for glass or MDA for ceramic). Of course, the larger cartridge type will work, too.

The fuse to the PID should be fast acting ( ABC,AGC,AGX) and no more than 1 amp for most PID types.
 
I am getting the ceramic 10A 250v slow blow fuses later today.

Also, is there a certain speed the oven should be heating up? At the rate mine was going, it could get to 1500° F in 10 minutes probably, but I'm assuming it should ramp a little slower to account for any thermal shock or something.
 
Rate of heating is a factor of chamber size, insulation, and current draw. 1500F in 10 minutes sounds good.
 
What should I use for the physical connectors to the coil? I'm sure I can get them from Mouser, etc, but wasn't sure what they're called.

How "high" should the high temp wire be? I see stuff rated for 200 C up to 540 C.

I need mine to run off 120 VAC. Any advice? I've got some of the Kanthal coils and I'm just hoping I can sink enough current off a 20A breaker to heat a small forge.
 
Stainless steel nuts, bolts and washers seem to be the most effective metod of connecting the wires to the coils.

It seems to be better to use twisted tails to get through the walls, then use the nuts and bolts. The doubled wire for the tails is twisted together for the pass through the wall, so it doesn't produce much heat in the insulating brick. If you use a 1/4" bolt, it has a lot of cross-section relative to the coil and the resistive heating is minimal.

Bend the tail to a U-shape, or almost a circle, and clamp it between a pair of nuts and washers. Do not cross the wires as it seems this may cause a hotspot. If you use a long bolt (I use 6mm allthread), the coil connects at the hot end and you can use a low-temperature "normal" cable clamped in a similar way (I use crimp-on ring terminals) at the cold end. High-temp cable is added insurance and I use it if it is reasonably cheap. The 200 degC silicone stuff seems plenty good enough and is cheap, if slow, from ebay.

I stopped using the stainless allthread to go through the walls (as shown in Andy Gascoigne's plans) following an element failure on one of the HT ovens I'd built and a couple of similar element failures experienced by other guys on British Blades. The twisted tails seem to have fixed the problem.
 
I use these little bronze looking things.

paragon-element-connection.JPG


There are in a couple online sources but I don't remember which one off the top of my head. A few google searches can bring you there quickly.
 
I have never seen any reason why the Brass screw terminals would not work, but the guy who supplied my last few sets of elements was pretty adamant that they have been a factor in enough of the failures he's seen to convince him to use the nut-and-bolt option wherever possible.

As his business is repairing industrial kilns, furnaces, etc, I thought about it for a bit (to try to work out whether there was a difference between "our" process/equipment and "industrial" processes/equipment) and accepted his recommendation.
 
Any tips on holding the heating elements in place? What I've seen so far is carving grooves into the brick for the elements to lay in and some kind wire ties poked thru holes from the outside to hold the elements in the grooves. I want to avoid the wires sticking out the sides of the forge due to electrical hazard.

Any reason I can't use a few dabs of mortar to fasten the elements to the walls of the forge?
 
Pinning the elements in place with U-shaped staples bent from either Kanthal or Nichrone wire works for me.

The first build I did, I'd forgotten to order material for this with the elements and bought some Nichrome wire. After that, I remembered and bought Kanthal.

The wire size i use for the staples is about .036-,040" (0.8-1.0mm) and I make them 3/4"-1" long (1 1/2-2" of wire). They push into the soft fire brick quite easily with a pair of needle-nosed pliers and do not go all the way through. The wire diameter is less than the coil spacing to avoid shorting any of the coils.

I try to get the pin/staple spacing so that there are 2 pins in each groove per brick and I add extra on the turns.

Making and fitting the pins is the most tedious part of making an HT oven, since I started routing the grooves. Before that, cutting the grooves ran it pretty close.

I have not tried mortar. I'd be concerned about it cracking and also about potential hotspots which might lead to element failure.
 
Do you drill holes in the brick to accept the staples? I keep reading how soft the firebrick is but mine just feels like brick. I just tried lightly tapping a small nail into it and it was a no go.
 
No. I just push them in. Works with all 3 types of IFB I've tried, but is easiest with the K23/JM23.

If you can't push them in, double check the type of brick you have. Weighing one is a good start. Or seal one in a plastic bag (to stop it soaking up water) and see if it floats. The JM23/K23 will float. Other IFBs probably will. Dense brick will sink like a stone.

Have you cut the grooves yet? If they cut OK, I'd expect to get the pins in easy enough.

Dense Fire Brick is, obviously, dense. It does not insulate to any useful degree (for HT ovens) and it's best you find out early if you've got the wrong ones.
 
Sounds to me like you may have the wrong brick. K23 will cut easily with a wood saw or file. The grooves can easily be made by rubbing a round bar back and forth. Angeling the grooves will help to hold the coils in along with the pins
 
If a nail won't push in with your fingers, then it is the wrong type firebrick. Hard firebrick is for lining fireplaces. Soft firebrick is for lining ovens. they are not interchangeable.
 
Gah! There's more than one kind of fire brick??? Ok I have some soft stuff on order. Glad I asked, thanks guys.
 
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