Gas forge question

V

Von Gruff

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Nov 24, 2014
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I am building a gas forge from a 20lb gas bottle and in most of the builds I have seen there is opening at the rear as well as the main front door. Is it necessary to have the two openings if long stock is not going to be used. The main purpose for this forge is heat treating of knives (as opposed to forging long stock) and none of my patterns have blade lengths of more than 8 1/2 to 9 inches and that will all fit inside the forge.
This is the work in progress

I got another batch of knives heat treated today and it reinforced the notion that I needed a better way to do it than in the kitchen fire. I had been looking at gas forges for a while and decided to build one for myself. There are a couple of good videos of how to make them from the 20lb gas bottles so when I was in the city yesterday I went to the metal scrap yard but the only gas bottle on the yard, had the top cut off it but there was a stack of lazer cut 10 3/4 in dia x 3/16 discs there so I got two of them as well as a length of 1 inch black pipe.

This is the bits I bought home.



So the first job was to grind the opening out a little to fit the plate which was a simple mark and grind.



I decided to do the bottom as well because when they were doing them on the youtube it seemed like a bit of a chore to make the insulation conform to the semi rounded shape so I figured that a flat surface woudl be easier.



 
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Then it needed a little clean up to make sure the plate fitted nice and flat. A friend just down the road welded the plates in place for me, so that all needed a clean up. I gave it all a good going over with a heavy flapper disc on the grinder to get rid of the old paint and the bit of surface rust.



I set it up with a couple of blocks to hold it still and make sure it was more or less secure for the marking out of the next step. Having the flat plate on the ends made this easier so I was pleased that I hadn't started with a complete bottle, as I might have had to do it on the curve surface.
Finding the center of the plate both verticall and horozontally was a simple matter, so after leveling the line accross the front (and rear as well) I was able to do the vertical which allowed me to mark the top center mark where the handle will go.



I have seen various measurements for the offset of the gas inlet from 10degrees to 30 and as the idea is to make the gas swirl round the inside of the forge I went with 25 degrees. Marking a square on the board and a 25 degree line in, I could go up the line by half the dia of the bottle and get the measurement away from the center line that would give me a 25 degree offset.



So that can be marked on the top of the bottle ready to be drilled for the gas inlet.



That is where it will have to sit till the gas bits and the insulation get here in a few days. I will need to get the exact height of the insulation plus the fire brik floor before I cut the opening at each end, or can I do it with juts the one opening??.
 
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I'd say it's very useful to have the second opening. You can make it smaller than the front, but good to have it be large enough for a blade to fit through if you are doing anything requiring an extra bit of length.
 
My philosophy is to over build it. Why not make it do more than initially intended? It's much easier to do it now than later, won't cost much more if any except a little more wire a few more scraps and if you're lining it with insowool you can always use the piece you cut out to plug the hole when not needed. Thant's just me though. I just made mine out of a 30lb tank and have no intentions of needing to use the rear port. Well not yet anyway. One thing I've learned in the short time I've been making knives, is just when I think I have a plan, it changes, something will spark interest and now I'm doing something else.
 
You will want the end port. It can be blocked with a few fire bricks when not needed.

TIP:
Put the forge on a cheap rolling welders cart (HF/Northern/etc.). It will allow the forge to be moved about as needed, stored in a corner when not needed, and will give storage below for forging tools and supplies. You can also put a 20# propane tank on the gas bottle shelf on the back side.
 
Thanks for the answers guys.
A bit far from harbour freight here in NZ Stacy but I do intend to have it on a cart for the reasons you mention and will open up a port in the rear wall as well. I have seen it done where a angle iron is welded under the rear opening for a fire brick to sit in when the rear port is not needed. I saw a cable forged blade near 30 years ago and that has niggled away in the back of my mind ever since so chances are that something more than heat treating may well be in the forges future.
 
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