My DIY heat treat oven

Z

zhyla

Joined
Sep 11, 2011
Messages
538
Warning: several smart members have safety concerns about this oven. Do not blindly follow my design, it might get you killed!
In particular:
- The wiring connections to the coils need to be more reliably insulated could lead to shorting of a hot lead to the housing.
- The lack of a door switch to cut the power to the coils can lead to inadvertent electrocution.
- A GFCI wouldn't be a bad idea.
- The coils move around under temperature and could short to each other or the work piece.

I had posted some of this in the Hammer & Tongs subforum and someone suggested I posted it here.

This follows fairly closely D. Corneau's heat treat oven. The main difference is I used pop rivets to form a sheet metal case instead of welding. This was really easy and I am very happy with the results. It was fairly inexpensive too -- pop rivets are cheap and I used roofing flashing from Home Depot.

The electronics are a MyPin TA4 PID controller with a K-type thermocouple and 3x Kanthal A1 coils (in parallel). The current draw is around 12 A @ 120 VAC.

I haven't gotten this up to heat treat temperatures yet but initial testing seemed to go well. I did find that the heating coils like to move around when they heat up and had to add a few more staples to hold them in place.

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Do you have any info on the elements?

I would like to build a smaller HT oven than my previous ones (18", 22.5" and 42" chamber lengths) and have been struggling to find elements that will fit into the groove length I can realistically get in a 12" chamber, without going too thin on the Kanthal and running into burnout problems.
 
Did you stretch out your elements after you coiled them?
 
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Too late now, but the top is not the best place for the controller and electrical hookup. It is the hottest spot on the outside of oven.
 
timgunn1962 said:
Do you have any info on the elements?
Click to expand...

I'm unsure on the BF policy on posting links to commercial sites but I used the "79.5cm 31.3" Kanthal A1 Heating Element Coil Heater Wire 2500W" coils off of Amazon.

tattooedfreak said:
Did you stretch out your elements after you coiled them?
Click to expand...

I bought them pre-coiled and used them as-is. I got the oven up to 750 F last night and man those things writhe around a lot. I didn't expect that. I'm a little worried at full temperature they'll contact each other and blow a fuse. I should probably buy spares, these must not last forever if they expand and contract so much.

Stacy E. Apelt - Bladesmith said:
Too late now, but the top is not the best place for the controller and electrical hookup. It is the hottest spot on the outside of oven.
Click to expand...

Yeah... I have trouble designing things that look asymmetrical. The fan seems to keep the box cool. After an hour at 750 F the control box was just slightly warm (maybe 80 F). But definitely, putting it on the side is a better idea.

Kosa_PL said:
I see here a kiln for someone who want to commit suicide >.>
Click to expand...

I have no idea what you're referring to. It is certainly an appliance to treat with respect... and welding gloves.
 
What Kosha is getting at is the metal frame with no apparent ground, bulging coils in the chamber, NO INTERLOCK SWITCH on the door,..... and most seriously.... A really stupid ( sorry, but it is) mounting for the high voltage lines and coli connections.. Any of these can lead to death. The connection box is just crazy. I would not plug the oven in again until you changed the power/coil mounts to properly insulated standoffs.
 
When you get the elements, if coiled tight, you are supposed to stretch them out a bit, some say up to twice their length, this will stop them from moving and give you better heat control. Dont do it now though, you will end up breaking them. I have a pdf of building an oven and I know there is one on here somewhere that gives a very good breakdown.
 
zhyla said:
I have no idea what you're referring to. It is certainly an appliance to treat with respect... and welding gloves.
Click to expand...

If you have no Idea , what things can kill you at this kiln...
Knifemaking is not for you.

Go collect post stamps, Breed hamster, etc.

We do not want your life, on our shoulders.
 
Ok, first let me say I'm all ears about the safety issues and thank you for comments and strongly worded warnings. Strongly worded warnings are warranted when we're talking about something lethal.

Door switch: I'm planning on doing this I just haven't found the switch yet. Recommendations welcome. At the moment I manually switch the relay off (small black switch on front of control box) before opening the door.

Wiring box: I should have taken a photo of the final wiring. It's not much different but there is some shrink tubing in a key place to prevent shorting the end of the rod to the housing. It may not be obvious but other 3 of the rod ends are captured in slots in some firebrick pieces. Obviously I thought this would be a safe, reliable setup so I must be missing something. Is there concern the threaded rods will wiggle around? The firebrick will break down?

Grounding: The control box, wiring box, chamber box, and door are all well grounded to earth ground.

Bulging coils: Yeah, I'm not real sure what to do here. If I replaced the coils with stretched coils they may be too long. If I pin them down more they may stay put, or crack, or bulge out in a weird way.

I will edit the top of my post to warn people there are safety concerns with this build.
 
First, thank you for editing the post and adding the caution. It just might save someone's life one day when a kid Googles "DIY oven".

Glad that it is grounded, but there are still a lot of issues.

Door switch is any simple limit switch. Often called a contact switch. eBay has thousands. Here is an example:
http://www.ebay.com/itm/Micro-Switc...479?pt=LH_DefaultDomain_0&hash=item3f3faf0d5f

Shrink wrap ain't gonna' cut it on hot connections.
The connectors should be mounted on insulating board and be proper stainless steel stand-off connectors with 1/4-20 threads, washers and nuts....or even better, a high temp ceramic terminal block. You can make them or buy one. Here is an example:
http://www.ebay.com/itm/30A-380V-In...950?pt=LH_DefaultDomain_0&hash=item485b5caa96
http://www.ebay.com/itm/NEW-2-Pcs-2...838?pt=LH_DefaultDomain_0&hash=item3f3f70d8fe

The coils should be spread about one half to one coil thickness between coils. If they contact each other, the coil will draw too much current and burn out faster. If the coil is too tightly bunched, it can cause the whole thing to overload and burn up the wiring. The coils should be pinned down with "U" pins made from the same type nichrome wire, or stainless wire at a minimum.
 
There are, as has been pointed out, a few things that give cause for concern.

Much of the electrical stuff will be different on your side of the pond to the 230V mains supply system we have over here.

I tend to deal with much of the electrical safety side of things by using a door switch which drops out a contactor when the door is opened and completely isolates the oven from the mains supply.

I also fit an RCD on the supply (Residual Current Device: I think it's similar to your GFCI. As long as the current going out on the live balances with the current coming back on the neutral, all is well. If there is more than a 30 mA imbalance it trips and cuts off the supply. 30 mA is not pleasant when it's passing through you to ground, but it's very unlikely to kill you).

The RCD effectively means that I can be pretty sure nobody can be electocuted. The Contactor/doorswitch means that nobody will get a sub-30 mA shock while unloading the oven, hurl a hot workpiece somewhere and get burnt to death in the resulting fire.

Whilst the element connections on the top of your oven look scary, I find that using something similar can actually be a pretty good way of getting the elements connected in parallel, as long as appropriate safety precautions are taken. I tend to use Tufnol (a phenolic-bonded-paper insulating material) to provide the insulating supports at the ends of the studding (allthread) and use a fingerproof guard to prevent inadverdent contact.

The elements look a lot like some I got from ebay: small diameter coils of very fine Kanthal A1 wire. I'd got them because the price was so low they had to be worth looking at, but I did not use them.

Things I was concerned about included the fineness of the wire (I've had a few elements burn out when using 16 AWG Kanthal A1 (1.29mm dia). I've since gone to 1.6mm Kanthal A1 (about 1/16" or 14 AWG) without any further burnouts), the lack of "tails" (they seemed to be cut from a continuous coil, so would need to be straightened out for connection, with a high risk of minor damage during the straightening causing a thin spot which would subsequently burn out) and the small outside diameter (which needs quite a long groove; doubling the OD would halve the groove length needed).

The coils need to be stretched. The usual advice is to 2-3 times their unstretched length. There are several reasons for this, but the main one is to ensure the resistance stays in spec. If you imagine a coil with all the coils touching, it effectively becomes a tube. The resistance of the tube will be much lower than the resistance of a single strand of wire and the overall resistance of the coil will reduce. With reduced resistance, the current will increase. If you have lengths of single-strand element interspersed with lengths of closed-coil "tube", the lengths of single-strand will carry the increased current and are very likely to burn out.

The stretch needs to be sufficient to prevent adjacent coils coming into contact. If you are using staples to hold your elements, the stretch needs to take this into account as well. The gap needs to be enough that the staple cannot short out a coil. I usually stretch to 3x the closed coil length and use 1mm (.040") Kanthal A1 staples with either the 16AWG or 14AWG elements.

It looks like you could fairly easily get close to twice your current groove length with W-shaped, instead of U-shaped grooves, though longer would be better.
 
Stacy E. Apelt - Bladesmith said:
Shrink wrap ain't gonna' cut it on hot connections.
Click to expand...

This is completely true and I hadn't thought that thru. I'm not even sure what type I had lying around but it's probably only good to 350 F and that rod is probably getting hotter than that.

timgunn1962 said:
I also fit an RCD on the supply (Residual Current Device: I think it's similar to your GFCI. As long as the current going out on the live balances with the current coming back on the neutral, all is well. If there is more than a 30 mA imbalance it trips and cuts off the supply. 30 mA is not pleasant when it's passing through you to ground, but it's very unlikely to kill you).
Click to expand...

I could do this. I think I'm not as paranoid about electric shock as you guys are. As long as either the door is shut or the relay is open (door switch or manual override) there isn't an exceptional danger here. I've built much higher voltage equipment before, though I'm learning I'm not very experienced with high temperatures.

timgunn1962 said:
Whilst the element connections on the top of your oven look scary, I find that using something similar can actually be a pretty good way of getting the elements connected in parallel, as long as appropriate safety precautions are taken. I tend to use Tufnol (a phenolic-bonded-paper insulating material) to provide the insulating supports at the ends of the studding (allthread) and use a fingerproof guard to prevent inadverdent contact.
Click to expand...

I need to find some of this stuff and rework that wiring cavity.

timgunn1962 said:
The elements look a lot like some I got from ebay: small diameter coils of very fine Kanthal A1 wire. I'd got them because the price was so low they had to be worth looking at, but I did not use them.

Things I was concerned about included the fineness of the wire (I've had a few elements burn out when using 16 AWG Kanthal A1 (1.29mm dia). I've since gone to 1.6mm Kanthal A1 (about 1/16" or 14 AWG) without any further burnouts), the lack of "tails" (they seemed to be cut from a continuous coil, so would need to be straightened out for connection, with a high risk of minor damage during the straightening causing a thin spot which would subsequently burn out) and the small outside diameter (which needs quite a long groove; doubling the OD would halve the groove length needed).
Click to expand...

I have noticed one of the coils becomes red at one end of the loop when it is on and the others do not show color anywhere. I don't know what that means. I think these are maybe 1 mm diameter but I haven't measured them. They measured 40 ohms each (unstretched). I'm putting far less than their rated wattage thru them. I think if they fail it will be from the mechanical stress of stretching out under temperature.

timgunn1962 said:
The coils need to be stretched. The usual advice is to 2-3 times their unstretched length. There are several reasons for this, but the main one is to ensure the resistance stays in spec. If you imagine a coil with all the coils touching, it effectively becomes a tube. The resistance of the tube will be much lower than the resistance of a single strand of wire and the overall resistance of the coil will reduce. With reduced resistance, the current will increase. If you have lengths of single-strand element interspersed with lengths of closed-coil "tube", the lengths of single-strand will carry the increased current and are very likely to burn out.
Click to expand...

This really surprises me. These are off-the-shelf heating coils. I've never found any documentation on coiled A1 so maybe they're intended to be stretched prior to installation but it seemed like they were made to be used as-is. Now I have found at least one place selling Kanthal coils that state minimum and maximum stretched lengths.

I think what I'm going to do is put a LOT more staples to wrangle the coils into submission and see how it goes. My reasoning here is these coils are rated for 2500 W and I'm giving them each less than 500 W. If they fail then I'll have to route some stretched wires in there.

Thanks so much from the comments.
 
My main worry I actually have more experience with electronics then knife making lol. I would be afraid of the connections on the top there crumbling. You also have bare wire stuck between 2 nuts? You can use the crimp on wire loop connectors there as well. I would move the control box to the side or under it or something where its not going to be right on top. The wires you are using are those rated at the type of temp they will be seeing? Can you find another way of supporting those bolts ceramic insulators from high voltage lines might work if they will hold up to the temp. I admire the DIY nature of your build but you may have been able to order a oven used for around the same price you are spending doing this.
 
You definitely need to stretch the coils. Either buy a shorter element, or carve twice as many rows.
 
bigboysbushcraft said:
I found one for around 300 shipped if links are not allowed let me know.
Click to expand...

Yeah, but mine has a chamber twice as long as that.

Well, the contraption held together for one blade at least. I had one snag... for some reason the MyPin controller got to 1275 F and decided to shut off. Very strange, it would go 1273, 1274, 1275, 0000, (heat cuts out) 1274, 1275, 0000 (heat cuts out). After some screwing with it I found it didn't have a problem when set to Celsius instead of Farenheit. Annoying but at least it worked.

Finally drove all the water out of the brick, it doesn't steam anymore.
 
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