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I Forge Iron

Variable Volume Propane forge


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Attached are a couple concept drawings of the forge I'm building. Actually I SHOULD be out building it instead of writing this:rolleyes:

Anyway, as some of you will know this is similar to Ralph Sproul's forge. We kicked the idea around quite a bit some years ago and he started building them while I worked the paycheck job and built the farm.

We had some different ideas how the basic concept should be made and now I have the time to build my version we'll see how I do.

Enough history.

The drawings (Sketchup) show most of the important details. The basic concept is a refractory table and a refractory lid on a jack.

Side walls are fire or kiln bricks and can be configured in any shape or size that fits the table.

Ralph likes his trailer jacks while I like the scissor jack but think it's a distinction without a difference. I simply had access to a number of scissor jacks being tossed at work and glommed onto some.

The forge table I should be out working on right now is 3,400f split fire brick over a layer of 2,300f insulating castable refractory.

The lid will have about 3/4" of 4,000f rammable refractory, insulated by the 2,300f insulating refractory. The lid could just as easily be full size kiln brick as well and I may end up going that way as the 4,000f rammable is no longer available.

The lid will have a nut and bolt arrangement to provide compression to hold the brick in place. The table will have just enough space to remove and replace brick as necessary.

The bottom view shows a little more detail of the lid guide and jack mounting flange. It also shows the extending stock rest.

This isn't how the stock rest will be made though as it doesn't allow for one on the opposite side.The fix is easy though, I'll offset the welded guides enough to put in a second set. The female tube will be offset the same amount on the stock rests so they match up with the table no matter which side you put it on.

It'll also make a place to hang tongs, etc. when the stock rest is extended.

Lastly on the really RARE occasion I may need a forge even larger than this one the rests can serve as table extensions with additional bricks for sidewalls and cover.

The spreaders below the forge will probably end up being a shelf though with a little adjustment a 40lb prop bottle could be made to fit under it.

The entire thing is intended to knock down to relatively small pieces for transport.

The table is 18"x18" with no real limit on height though a reasonable person would probably not stack more than two, maybe three bricks. The jack is pinned to the guide and lid so down force can be applied making it a stable setup.

As the guys with experience know already a forge with a 18"x 18" table is WAY larger than a person needs under almost any circumstances. Once in a great while a person needs a BIG HONKIN forge like this.

That's why I designed it to be variable. Most of the time something 4"x4"x6" with a pass through works just fine. Other times I need to get a corner of my snowplow in the fire.

This forge will let me change it when I need to and I won't have to either spend extra money heating a too large forge or keep an extra forge around for the times I need big one.

Anyway, that's the basic beast you'll have to play with at the 2010 AK. Shindig Bill.




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I like the design, especially for the ease by which the chamber can be reconfigured. How efficient do you think this forge will be? If I am thinking right, and I'm sure other's here will correct me if I am wrong, aren't the brick type forges less efficient to heat to forging temps? Not knocking your design, as I think it is brilliant, just asking questions for my own good. I'm planning on building a propane forge in the near future. Just trying to do my homework. I think this forge is a great take on others I've seen using bricks as the "shell".

-aaron c.

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I've been using a forge like this for about 3 years. The scissors jack works very well. I built the top out of Kaowool in a expanded metal housing. The front has a 1" opening so I can heat small/short items without raising it. When I need to put in a larger item I raise it up and put fire brick around the sides to block it in. I don't like the Kaowool because it's too fragile. I've bought some castable refractory and plan to make a shape like a quonset hut and put a semi circle moveable wall in the back to close up the volume even more as needed. I recently put in one of Frosty's tee burner. It works very well.

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Hey Frosty,

Nice job on the drawing! Looks like someone mastered Sketchup in very little time!

The variable volume idea is a great way to go - I do mostly hammered vessel work and they vary so much in size throughout the process that I needed a forge that could change with them.

The pics are of the forge I built a couple years ago. I took the idea from a large commercial set up I saw at an NWBA conference at Ponderosa Forge in Sisters Ore. They had two set end to end to do really long work (they were about 6 ft long combined). Their design was cantilevered and counter weighted so you could raise them easily while hot and reconfigure the brick. They could put an entire garden gate or railing section in it to heat it up and true it. It was something to see.

I haven't got around to putting the counterweight system on mine but it's working fine by just lifting the top and resetting the brick.

Mine is about 14" wide and 28" long. this gives me the room to get good full even heats over the large blanks but cool ends to localize the heat if I need to.

The curved top is lined with 2" Kao wool coated with ITC-100 and the bottom is full hard brick ( I believe 3,000 degree). It takes a bit longer getting to full heat but she's a cookin' devil when she gets up to stride!




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I intend to use my naturally aspirated burners with a slight mod, a Cross instead of a "T". I'll let everybody know how it goes build wise. I think there's going to be an advantage using a Cross as I won't really need the lathe and more importantly (for me seeing as I do have a lathe) it'll move the jet farther from the throat for better induction.

As drawn the non-scale burners are 1" which are good for welding heat in 700-800 cu/in. That's full width and length one brick high allowing for the sidewalls and one endwall.

Adding more burners will be a matter of putting them in sidewalls. I've been seriously considering putting them in the sidewalls anyway but if I want to open up the far side the flame would just blow straight across and out so the lid mounted burners.

Control will be 1/4 turn ball valves to individual burners fed by a larger diameter regulated "main".

In a later incarnation I'm going to figure out how to pre-heat the propane. I've been reading over the last couple years where high end air-prop torch makers are getting really high performance by preheating the prop. Much 15-20%+ better than preheating the intake air. That's another day though.

Yeah, bringing hard firebrick to heat takes longer but it's a lot tougher than available insulating refractories. I was going to do what I did with my little gasser 14 years ago and ram a 3/4" hard inner liner from 4,000f high phosphate rammable refractory and insulate it with Kaowool.

Unfortunately the refractory is no longer available and I figured i should build the prototype like I will the production models. Yes, I'd like to sell a couple of these here and there.

The sidewalls and lid will be lined with kiln brick which is only slightly heavier than Kaowool's fiberboard but better rated and a LOT cheaper. The lid and sidewalls won't be taking the beating the floor will so they should hold up well enough.

I'd line the lid with straight Kaowool but it's hard to keep in a flat hanging situation at heat. There are a couple tricks I may try depending on whether or not the sales guy at EJ Bartells is willing to give me enough Kaowool scraps from their fab dept.

Short story though is no, this won't be as fast to heat nor as efficient as a ceramic blanket lined forge. It won't be as slow and inefficient as an all hard brick forge either. I'm shooting for a happy medium but will settle for a workable compromise.

As to "mastering" Sketchup . . . Uh, not really, not by a long shot. I've been making drawings as learning exercises but if I just pick something to draw I tend to not learn the hard stuff. having a project I actually want to build makes a big difference.

Too bad it's the free download, only about half the features in the tutorials work. As is the copy function sucks, it works but poorly. I could REALLY use radial, spline, multiple, etc. copy like I'm used to in Cad software.

Oh well, it's worth WAY more than I paid for it so I can't complain. :D

It's good to see other versions of variable volume forges in use. I know Ralph is happy with his and says folk who've bought or built versions are happy.

He and I spent several months kicking it around, sent many drawings and messages back and forth. We came to the same conclusions on most things and our differences are more distinctions without difference when it comes down to function.

We both wanted flat lids with all the attendant headaches keeping a lining in them. We spent a lot of time trying to figure out how to use a dome or vaulted lid but couldn't come up with a good method of making the volume narrower and still get a good seal at the lid.

I don't know what Ralph's latest lid liner is but he's gone through quite a few, all with pluses and minuses to consider.

Think I'll E-mail him and see what he's using now. Might have to change the lid but better before than after I build it eh?

I wanted to build one of these simply because I never know what I'll do next. I've limped along for years with a little gasser I built for a friend who ended up dieing before he lit a fire. He wanted to make primitive knives and wood working tools so it was long and narrow with small doors.

What I really like is a brick pile forge but there are drawbacks. First all brick takes a lot of burner and time.

Then there are the stability issues, it can be all too easy to knock a brick out and not only ruin a brick but burn yourself or worse someone else, start a fire, ruin the project, etc. I really REALLY prefer a good stable holder for my fires whether they're in the living room heating the house or roasting hot dogs on a sand bar.

Being able to change configurations on the fly also appeals to me, not so much because I think I'll need it very often if ever but from my philosophy of, "it's better to have it and not need it than need it and not have it."

I considered hand raising the lid and skipping the design and construction hassle the jack represents. Then I considered how many times I've wanted to heat a large area of sheet metal and being able to lower the lid to less than an inch and hold it there made the decision for me.

I think a trailer jack might be less hassle, probably a lot less hassle actually but I got these free so there it is.

Cheap or free scissor jacks (well ANYTHING) rule! :D

Be happy to build you one Chris, not the first copy though. For what I'm going to charge a person deserves a perfected model. I'll also be building small forges and custom designs for locals at least. maybe sell a few sets of plans for the Variable Volume forge if there's interest.

I hate to be mercenary but I'd like to see some return on my investments other than the satisfaction of infecting others with the smith's condition.


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Frosty, you're killin' me. I just finished one just for forging and now think I need one just for welding:-) I like the expandable idea. Ever think of a vertical expandable?

Why just one direction? This one is 3D variable.

Of course you may be invisioning something different and I might like it. A LOT. So how about a sketch of what you're thinking.

At least I'm killin someone; my life isn't a total waste. :rolleyes:

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What I'm thinking is something resembling a tube within a tube. Short for forging and extended for heat treating where you would get a more complete/even heat. Guess it would work vertical or horizontal. In the horizontal mode, long stock would heat better, also.
One of the ideas I'm brainstorming right now is a longer horizontal forge in which I can weld longer pieces of cable. The picture of the stand is right in line with what I had in mind to attach a vice to.

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it's great to see your V.V. forge design. Really like the "jack" height adjustment. I've also been wanting to build one for a few years. I first got to work with one at Enrique's shop in '01. His didn't have the jack, the burner top was VERY heavy and he used the overhead crane to lift it for reconfiguring. :o And his had two "rows" of burners(6 total), three in each row. The burners were the Sandia style burners, that preheated the intake. You could run all or any combination of burners depending upon the need. I saw the incredible potential of this design, and have wanted one ever since. I'm glad I've waited because the burners are so much simplier to make now. AND now that I see your modifications.........ya got me thinking about building it again. Thanks for sharing and the inspiration.
Can't wait to see yours in action.

Happy Hammering

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Instead of the jack (although "free" has been my favorite price, even when it didn't work out) what about a hand crank similar to that on a small drill press?
Of course, now you would have to find an abandoned drill press. you could easily move it up and down. at least my non-machinist mind would like to think so.

Overall though, way cool idea

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Hi Frosty
I know that you have seen my forges, the VV is on a 18" X 36" table which gives me more options, I think, I only use that forge when I have an odd size project as the liner is less efficiant than my spanish lake forge. because of this haveing the large table makes dealing with large or long parts nicer (they don't sagg from bridgeing). You may not remember that the top is fiber board and I am really happy how that has worked, I rarely lay anything on the inside top so it is holding up well.
I have every confidence that your forge will be much nicer than mine just a few thoughts.

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Gotcha Bruce. A telescoping forge right?

I considered something like it a few years ago but thought something else would be easier and more practical. Two ideas actually.

The first alternative I thought of was a sectional forge. Simply put several sections of forge that could be put together for length or stored for space. Each section would've had it's own burner so the heat was always even.

Then Ron Reil countered that idea with the movable back wall which he put in his large four burner forge. It's way simpler though the forge is always large. It's just a plug that can be moved wherever you need it in the forge. It blocks off the unnecessary volume so you only heat what you want.

Paul, if I had a rack and pinion on hand I would've used it, or a linear actuator or a trailer jack or I could've built a spring or counterweighted lid.

There are lots of options for making something like this. The criteria I think important are safety, simplicity and reliability. I have an old Johnson Appliance 133a and it has a lifting lid. It's mounted on a round shaft with a spring to counter the weight. I suppose it worked okay when it was new but it's a PITA now it's old and roughed up.

For my purposes I wanted the thing to be adjustable on the fly so hand lifting was pretty much out. I also wanted positive down force or a locking mechanism so it couldn't move when in place so simple counter weights or springs were out unless I rigged a safety lock of some sort. Not a deal killer but another layer of complexity in what should be a simple device.

Still, you're right a nice rack and pinion would be sweet and it'd look cool too. :cool:


I thought about your forges while I was drawing this one up. The Spanish Lake will be a lot more efficient than this one too as it's lined with Kaowool. I may line the lid with Kaowool if I can get enough cheap or free as rems. There is a trick for keeping it in a large flat lid like this but it takes a lot of small pieces.

Ralph had problems keeping fiber board in his forge lids. If he so much as touched it when at heat it'd crumble and eventually it'd sag and fail regardless. As I recall he was able to get 2,500f and it wasn't quite cutting it. I didn't get around to E-mailing him today, I had to make an adapter for my mig welder and forgot.

Your VV is only missing the lifting mechanism to be the same breed of cat as I'm putting together. I'm tricking mine a little more by insulating under the table and making it easy to knock down for transport. Still, they're the same idea and built for the same purpose, large and odd shapes.

I just hope mine will do small scale stuff efficiently enough I won't have to have several forges taking up space.

I hope I have it up and running for the iron pour.


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I've seen pictures of E.'s big forge on his site. I'd have to be a LOT more ambitious than I am now to want one that size. Still if a paying patron comes along who knows? :rolleyes:

I haven't decided on my final burner configuration yet. I like 1" but it's a lot of overkill for the heat I usually need. I may go with 4, 3/4" burners in the lid.

If I need more volume or more heat I'll introduce more burners from the sidewalls.

This is one reason I'm using kiln brick in the lid. It'll let me move burners by moving bricks till I get just what I like. Later I may formalize one configuration or another or maybe just keep it easily changed. Time will tell.

If I had a "stock in trade" set of items I wouldn't need to make such a variable forge, I'd know most of my product would fit in 400 cu/in or 800 cu/in of X shape or . . . whatever.

Actually the burners were always really easy to make if we'd just paid attention to how the pros did it we would've been using things like Mikey or Rex make decades ago.

Why commercial forge makers still use linear inducers as burners is a mystery to me. The linear is THE reason the myth about a naturally aspirated propane forge not being able to weld exists. Heck, my first generation super simple "T" jet burner will melt steel if you turn it up a little. Welding is a snap in it.

I'm hoping the Cross will be as easy to use as I think it will be. Construction will be similar to the Sidearm but the intake air will be a lot better balanced. Mixing should be more even as well. Moving the jet farther from the throat will increase induction too so it should be even less susceptible to back pressure and breezes.

I have lots to play with and miracle of of miracles a PLACE to do them! WooHoo! :D


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  • 3 weeks later...

This is an update on the Variable Volume forge I've been working on recently.

It's still a long way from firing up but it's starting to look like the finished gizmo.

It's been pretty damp the last few days so the insulating castable refractory under the split hard brick floor is taking it's own sweet time drying. There's a light bulb on it right now to speed things along.

I'm still tweeking the burners and once they're working to my satisfaction I'll make the gas manifold, valving and heat shield.

I stuck the 20lb bottle under the spreader/shelf to show one will fit. I normally run a 100lber at home and a 40lber for demos.

Pic one shows it at four burner max volume at a single brick height. Going higher is only a matter of jacking the lid up and stacking more bricks in the walls.

Pic two shows it in a single burner, small volume configuration with the extending rest out. There'll be a tong hanger that sockets into the sq tubing on the rest eventually.

To go a second lift on the sidewalls the 3/4" burners will need to be replaced with 1" burners to handle the additional volume.

Hopefully I'll be posting pics of it at temp next.




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