Forge design questions.

S

ShadNuke

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Aug 7, 2016
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First off, Hiya! I'm new to the forum, and have been lurking for the last year or 2, and just finally opened an account. I have a few questions regarding the forge that I am currently, or well, have been designing and trying to build for the last year and a bit. I am finally getting around to getting things put together. We moved 8 months ago, halfway across Canada, and I think it's time!! I am going the whole build route, the forge itself, and a burner. I will be using an old helium tank, which is roughly the same size as a 20 pound propane tank. I'm still looking at thee type of burner, but they seem simple enough, I just have to decide on which route to take. I can't, but I will haha. And pros and cons on burner styles would help me make up my mind! Now, the real dilemma I'm having, is the shape of the forge chamber. I was thinking of making it rectangular, and using 2 4x4x12 blocks of wood, side by side to form out the chamber, putting me around the 300ci range, and i could get away with a single burner. Would that be sufficient, or would I be better off going with a round chamber? The flame could then flow in the cylindrical shape, causing a vortex to heat more efficiently? Does that seem like a logical thing, or am I off base here? Either way, the plan can accompany either design. I will be using 2800° castable refractory, so shape really isn't a concern, I'm just wondering if there are benefits to different shapes. Is there anything I am missing, or am I on par with the basics? Any input would be greatly appreciated!

Thanks in advance!
Kevin
 
Two major things -
1) Go with a round chamber.
2) Don't use wood to build anything in a forge.


The stickys has a whole section on forges and burners.

The short of it is that a 4-5" ( 100-125mm) chamber about 10-14" ( 250-300mm) long is perfect for most knives. The minimum workable size would be 3X8" ( 75X200mm).

Any rigid metal object can be used as the shell. The ability to weld on the shell is nice, as this allows some simple add ons like legs and a work shelf at the ports. A helium tank is pretty thin, but should work if it is the only cylindrical object available. A section of pipe 8"x14" is the best thing. You may be able to get one for free if you check the places that use pipe. Check out the water company, plumbing suppliers, plumbers and contractors, machine shops, etc.

The shell is lined with refractory wool insulation. The most popular type is called Ins-wool. This can be one tor two inch thickness ( 25-50mm). Two inches (0mm) is best, but it isn't required.

The wool is coated with a refractory cement. The most popular type is called Satanite. This is about 1/4" to 3/8" thick ( 6-10mm).

For max efficiency, the satanite is given a thin coating of a reflective refractory called ITC-100. There are other similar products, but most use that one.

The ends can be round pieces of the wool that are coated with satanite and have a metal plate welded or bolted over them .... but the simplest way is to use 1" (25mm) thick refractory board called Ins-board. Another way to close of the forge is to just make it as a tunnel and stack fire bricks at the ends. These can be arranged in whatever placement needed to make the work shelf, and ports. The same type fire bricks can also be used to make the stand/base for the forge if welding on legs isn't an option.
 
Thanks for the reply. I know the basics of the build. No wood is going to be used in the construction of the forge, safe for using as a mould to form the internal chamber. I have designs in mind for the doors and such. I'd like to know the differences between a round or square chamber. The heium tank I have is very close to the same wall thickness as a propane tank, just a bit thinner. I have a propane tank I can use also if I decide to. The forge is just goin to be used for heat treating, which I just realized I forgot to mention in my initial post. I'm going to make my own bricks with the castable refractory to work as doors for the front and back, if I even decide to make a rear door on my heat treating forge. If I decide I want to make swords later on, I will look into designing things differently on the next forge. I just don't have the space for anything bigger than making knives... yet.
 
Use a round chamber to get a rolling flame, it heats more uniformly than a square chamber. The commercial forges use Ins-board as it's easier to cut and fab for than messing with a blanket of insulation, but you get hot spots and cool spots.

I prefer a blown design, easy enough to dial it back for low temp work and ramp it up for welding, with a lot of adjustment in between, and it runs more efficiently than a venturi style burner.

I've built several forges over the years and have settled on two basic designs, both blown. A vertical for damascus work to save your lining, and a horizontal for general work. The horizontal is basically a tube lined with ceramic blanket and coated with Satinite and ITC100, with open ends. There is a flat bar frame work on the ends where I stack fire brick to adjust the size of the openings to suit my needs.
 
Ugh, I hate castable in a forge. I'm sure nice forges have been built with it, but the two I've used took forever to come up to heat and used a lot of gas.
For a large dedicated welding forge maybe, since you may be running a welding heat for half a day or all day... but for general purpose and an average amount of damascus work, my vote would be for blanket.
I do have to re-line my forges every year or two, but that's not a big inconvenience to me.
 
You can use castable refractory, but it still needs an outer layer if insulation refractory. Casting bricks from solid refractory is a bad idea. You get a brick far too dense and it will have no heat shielding on the outside. Fire bricks are insulating, not just refractory. casting a solid block of refractory may have serious cracking issues without a large oven to cure and fire them in. Curing a cast liner is equally difficult without problems.
A cast liner forge needs more insulation wool than a regular wool/satanite forge. The refractory liner is wrapped in several inches of insulating wool to allow it to heat up completely and be a thermal mass. These type forges will take much longer to come up to temperature and use more gas to get there. It will require a blown burner. The plus is that after it finally comes up to temperature and fully soaks, the temperature is more constant. A cast liner forge without PID control or other accurate temperature control is a waste of time. I will tell you that making a cast liner forge is not an beginners project. You need a fair bit of forge design and building experience before you try that.
 
3"x8" is not the minimum workable size. 2.5" is a perfectly workable diameter for everything but the biggest choppers. 2" tends to be a little small for most hunting knives.

Don't forget the possibility of making your forge from insulating firebricks. IFB don't need special linings or casting, and insulate as well as anything commonly used.
 
Ok, then how about casting my own bricks? I have a bag of castable refractory that I'd like to use. As for burner, does anyone have any experience building their own? I have to do this as cheap as I can. Being on disability, making less than $10k a year, and having bills and everything else doesn't make it any easier.

Edit: Completely missed a post there! I can get the fibrefax for dirt cheap at a local al supplier here in town, so I can line it with that, then use the castable. I'm not worried about how difficult or complicated it may be, I'm mechanically inclined enough, and plan on letting it set for quite awhile, a little more time without a forge isn't a huge concern. I live in the desert in Canada, so the low humidity will help along curing.

I should also mention that it is pretty much only going to be used for heat treating. I'm not able to do any metal pounding, I will strictly be doing stock removal.
 
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Casting bricks isn't an option either. You would have dense refractory bricks that would need a kiln to be fired in. You would also have bricks that are nearly the opposite of what a soft firebrick is.

A castable liner is designed to absorb heat and get hot, then radiate that heat back as a very even heat source. It has a layer of insulation surrounding it.
A cast liner forge is much more complex than a standard wool and satanite forge.
 
When I was just getting started, the first gas forge I built was under the guidance of a local machinist/smith. He had a big bag of Mizzou castable refractory that I bought a portion of, to make that forge. That forge was so slow to come up to heat, and used so much gas to get there... I didn't know any better. Neither did he, years later I used his castable liner gas forge to draw some steel out at his shop, and it was pretty much as terrible as that first one that I built. In both cases, there was just refractory inside the steel forge shell. No outer layer of blanket. The first time I put 2" of kaowool into a forge it was a revelation. All of a sudden the question became how to reliably run LOWER than a welding heat, rather than how to barely get up to that hot.

Save yourself a big learning curve and just build a kaowool, durablanket, cerablanket, fiberfrax, whatever blanket forge. Just use a blanket with a 2300f rating at least. It costs a bit more but will last a lot longer.

Seriously, any money you may save by building with the castable you have on hand vs. spending less than $100 for the blanket will be offset completely in probably your first two propane fills. Over a year or two of smithing, the gas savings will probably be several hundred dollars at least.
 
Why not consider sending your blades out for heat treat? Rob Ridley and his wife Marilyn do a great job and the cost is very reasonable. That's Canadian Knifemaker Supply for a web site.
I don't believe you will do very accurate blade hardening in a forge.
Frank
 
I see. I guess I will hold on to the castable for when I decide to make a heat treating oven, and I'm gonna go all out making the elements and running it with the PID all all that jazz. So for the size of forge I want to build, it's gonna be one of 2 sizes. So between 320 and 600 cubic inches. What kind of btu range will be best to shoot for?
 
Heat treat is something I want to get into. It's also not worth it for me, because I will eventually building a proper electric oven. Once I finally get my actual heat treat oven built, and can get all the info into the computer, I can help the local guys here, and do small batch heat treating for friends.
 
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