Electric HT oven build

G

gmwoodturner

Joined
Jan 23, 2013
Messages
2
I am about to build a HT oven using this information http://www.freewebs.com/knifemaking/HEAT TREATMENT FURNACEbackup2.pdf If any one could tell me what they would add or change it would be greatly appreciated . The one thing I am going to change is to add a PID controller and every thing to go with it ( not sure what the every thing is )and probably from AUBER as it is the only place I have found. I am open to any and all suggestions as my daughter and my self are new to this great hobby.
Thanks for any all the help.
Gary & Tiffany
 
That looks like a good build for a medium size oven. The stickies have a section with forge and HT oven info. There are some good HT oven wiring diagrams there. http://www.bladeforums.com/forums/showthread.php/782574-PID-CONTROL-FORGE-Salt-Pots-and-more

From Auberins you will need:
A PID controller. Get one that has programmable steps.
Two SSRs, and heat sinks for both. I would get 40 Amp ones, but 25 Amp units would work. Wire the load contact on each one to a coil end, and connect the line contacts to the two 220VAC power leads (one to each). Have the SSR control wires from the PID go to both SSRs. If you only use one SSR, the coils can electrocute you even if the SSR is de-energized.
A 12 gauge type K Thermocouple and a terminal block for it. You will also need type K thermocouple wire to connect it to the PID. The wire is polarity sensitive, make sure it matches the TC polarity and is hooked up the the +/- PID connections right. The TC should stick through the top center of the oven. It should stick in about 3".

Also, put an interlock in the door. It is a NO roller type microswitch that is in series with the + SCR control wire between the PID and the SCRs. It should close when the door is shut. When the door opens, the SRCs are de-energized, and the coils have no current in them.
 
A friend built one to that design and I did the control stuff on it. Then I built one myself with a couple of changes. I've built 5 now, each one, hopefully, slightly better than the last.

There are a only really a few things I did differently.

I made it slightly bigger; there's about 1/2"-3/4" of brick below the bottom element on mine, which lessens the chances of nudging a workpiece (or rack) into the element. I also made it slightly longer, using 5 bricks in the main structure, not 4, to give an 18" working length. I'm pretty sure you could go to 22.5" with 3 kW of elements if you use Thermal Ceramics JM23 or K23 IFBs. These are only made in Italy and the USA, so are expensive here in the UK. I'd expect them to be readily available in the US. The ones Andy Gascoigne used look like one of the denser, cheaper, types also available here. Thermal mass is greater, which slows down heating, and the insulation value is lower, limiting the maximum temperature. I used the cheap ones in my first HT oven, but spent the extra on the later ones.

There was some confusion over the elements used: Andy used 2 elements, each rated for 14A and 115V, connected in series for the 230V supply in the UK. BCS later also listed a 14A, 230V element, and at least one builder made the mistake of connecting 2 of these in series and couldn't reach temperature. Make sure you get the right ones.

I regard a ramp/soak controller as pretty much essential. I use an Omega CN8723 http://www.omega.com/ppt/pptsc.asp?ref=CN7800&Nav=vhpa04 mainly because they are available here.

The Auber ramp/soak controller looks like it'll get the job done, but I've not used it myself.

Using a slow ramp effectively eliminates overshoot and radiative heating issues, very important when tempering. It's also very useful for stainless steels with 2-step heating recommendations to Austenitizing. A more basic PID controller is fine if you are only going to be using the oven for Austenitizing "simpler" Carbon steels and use another oven for tempering. I use type N thermocouples, rather than type K, because they are more stable at high temperatures, and use Mineral-Insulated assemblies, rather than bare-wire. I'd suggest talking to Omega; they seem very knowledgable, very patient and give good advice, at least here in the UK.

I use a "Fotek" branded SSR bought off ebay (cheap). I use a 25A and I've had no problems. I've seen recommendations for oversizing the SSR (40A), but I'm pretty sure oversizing its heatsink is more helpful than oversizing the SSR itself. By all means oversize both if you wish.

I use a 2-second switching cycle for the controller output. I've had a datalogger on the oven and 2 seconds is noticably better than 5 seconds. I can't measure any advantage in going down to one second.

We have a 230V "hot" leg here and the Neutral is at ground potential; things are different enough that I can't really advise on electrics for the US.

I use a door switch. It seems safer and is pretty cheap.

Make sure the door can be opened and closed with one gloved hand if there is any chance you'll be treating multiple blades. I use Ceramic Fiber blanket for the door seal and getting the compression with one-handed closing took a little thought and experimentation.

There have been one or two element failures reported on BritishBlades (one of them was on one of "my" ovens). The jury is still out on whether the stainless steel allthread is the best way to connect the elements, or if it's better to take the element "tails" out through the brick and use a ceramic terminal bock on the outside. Either way, it's probably wise to make it as easy as you can to change the elements: I fully welded the frames on all of mine and now consider it a mistake.

I cut the element grooves in the first two with a saw and finish-filed them with allthread. It wasn't difficult, but got pretty tedious. I used a router on the later ones; much easier and quicker. Neater too.

It's worth following the various links from http://www.britishblades.com/forums...ng-a-Heat-Treatment-Oven-Information-Database

Many of the ovens linked to are largely based on Andy Gascoigne's design.

I hope you enjoy building yours as much as I did mine.

Tim
 
Thanks for chiming in Tim. Yes, there is a big difference in US 220VAC and British/European 220VAC. The plans in the stickies have been corrected for USA specs.
I've seen your builds, and they look like good ovens.

The rule for HT ovens is that you can't over-build one...... But you can certainly under-build one. If you have a choice of heavier gauge coils, thicker walls, beefier SSRs, etc. go the route that will last longer. Flimsy hinges, doors that are hard to open/close, and not making the legs/feet high enough are also things to consider.
 
And plenty of insulation. I recommend a layer of KAO Wool between the bricks and the walls. This helps insulate as well as allows for some expansion of the bricks. I personally recommend exiting the back of the oven with the ends of the elements to make your connections. I tried a couple times with internal connections and exiting with stainless and had problems. None since changing to exiting the rear to a shielded terminal block. I used a metal electrical enclosure with a bunch of 1/4" holes drilled in it mounted on the back of the oven and mounted a piece of high temp micarta in it, then mounted the terminal block to that. I stacked my bricks and pinned them together with stainless tig wire. I also made the top of the oven so it bolts on. That way if I have a problem I can remove the top, KAO wool and top bricks and replace the element or whatever.
 
Thanks for the replies you have been very helpful . We have every thing ordered and will post some pics when its all done
Have a great day and don't forget to smile
Gary & Tiffany
 
You must log in or register to reply here.
Back
Top