Hi All. (warning, long read)
My friend and I are planning our next forge. He works at a pulp mill and has picked up some pieces that we thought would make a fine, tough (but heavy) shell. They are solid steel (mild, I think) U-shaped ... things. The plan is to either use them as is for the front and back skeleton of the shell and weld sheet steel to the outside. The inside will be layered with ceramic blanket insulation and refractory cement. The whole thing will make an upside down U shape, and we'll use hard kiln brick for a floor. The brick pile forge floor we have been using is wearing well, and I like the idea of being able to lift the shell and replace the brick, since we have plenty now, with any that get too worn out from thermal cycling and/or flux etc.
These steel things are pretty big. They are about 2" thick, and the "wall" thickness of them is about 3.5 inches. The full radius of the semi-circle part at the top is about 6", so the inside radius (the space that would remain after insulation) is about 2.5"
After some thinking I realized that those steel skeleton parts are going to cunduct heat to the outside quite a bit. Do you think a 1/2" of refractory cement on them will do enough insulation, or do we need to consider narrowing the inside cavity and put 1" thick kaowool over the entire interior including the skeleton pieces and then put refractory cement over that? They will only be the front and rear "wall" of the forge.
We are considering a ribbon burner, or dual ribbon burner setup, possibly two 6" ones about 3/4 of the way up, where the radiused part of the shape is. We figured that if we did 2 6" burners, on opposite sides and overlapping by about an inch, the burners don't come too close to the outside, with a length of 20".
The legs of the skeleton pieces, from the radiused part downwards is 7". That seems really tall to me, so we have discussed cutting them down to 5" tall. That gives us a forge front height of 9.5 inches to the top of the curved roof. The width of the opening is 7.5". (With the thickness of them, that means the entire front face would be 14.5" wide.)
My volume calculations @ 20" long tell me we'll have ~1246 cubic inches of internal space with the 7" legs. With the 5" legs that brings us down to about ~946 cubic inches of heated volume. I've done a pretty detailed spreadsheet to calculate the volume of internal space, the volume we'll have to insulate with wool and refractory, and the approximate BTU we will need to reach "forge welding" temperature. I've used info I found on an old thread here (maybe from Frosty) that suggests 450BTU/Cubic inch is the minimum, and 540BTU/Cubic inch is the ideal ratio to achieve the temperatures we're looking for. That means, using the above measurements, I think we want about 673000BTU for the taller size, or 511000BTU for the shorter size.
If anyone is a math wiz, I'd love to have someone look at my sheet and make sure I'm calculationing correctly. Math and I have never been fast friends. Anyone with the link can comment. Link is: https://docs.google.com/spreadsheets/d/18t1R3EFrTOLovLSqt2eC2y53TRge55txo_V6TnoVyKQ/edit?usp=sharing
The forge we have now is 4.5Hx7Wx27.5L We have trouble forge welding in it, as it's set up with a single burner in the center. That burner is a blown DIY pipe burner using a leaf-blower as the blower. It's a 1" pipe burner and we run it about 5lbs for regular forging and about 9lbs when we attempt forge welds. ( I know that doesn't mean a whole lot, because of mixture/MIG tip diameter/relational measurements etc) Because the heat is all in the center, it's hard to get a "short" heat on a portion of a piece. I don't know if this is a classic problem with gas forges themselves, or with large space gas forges that are underpowered. We can forge weld, it just takes lots of luck to get the right amount of heat in the right place, as all the heat is concentrated about 13" in the center.
My ideal goal would be to have a forge that allows us room to make oddly shaped things (I've forged ladder rungs, weird hooks/spirals for hangers and lots of other strange stuff that uses up all and more of the space we have currently), allows us to easily forge weld for various projects (including eventually 'damascus' billets) and will allow us to put things only a little way into the forge to get a "short" heat, ie, heating tong bits up without having to heat up the entire thing 10" down the reins. I figure this will make the stress of keeping things from getting all bendy where you don't want it bendy easier. (We also make *a ton* of scale, again, this is something I want to understand better and improve)
I'll post another discussion with pictures of the current forge to see what suggestions people have for making some of our challenges easier with what we have until we decide on a concrete plan, source the parts and actually get a chance to build it. This project likely won't start in earnest until the summertime, it's cold now and I don't want to try and cure refractory etc. in sub-zero (celcius) temperatures.
