8" Forge dimensions

Atlas Knife Company

Atlas Knife Company

Joined
Feb 16, 2010
Messages
3,668
People keep asking me to make bigger forges, and I'm finally ready to start on prototypes. Having not built many larger forges, I'm not sure what works best and I'd really like to skip a couple years of the multi-year development process by picking your collective brains a little first.

I'm working on an 8" diameter forge. It will be made using soft firebricks instead of wool. What I'm not sure of is what size diameter I should use for the chamber. The Atlas works well with 1" of insulation, so I was thinking of 1½" on this bigger forge giving me a 5" chamber. I was thinking of a 3" round front opening aligned with the bottom of the chamber, with a 3" × 2" oval rear port. Overall length, 14½", with 1½" front wall and 1" rear wall giving a 5" × 12" chamber. This would give 235 cubic inches of chamber, so my 100K BTU burner would provide the 400 BTU/cubic inch recommend for forge welding.

Any thoughts?

Charles
 
Charles, I wish I had stopped at your table at Blade. I totally meant to do it and just never found the time. I know little to nothing about forging but, I am going to be attending the Intro to Bladesmithing course at the Moran School this November. I have been back and forth on buying one of your mini forges as my first forge. My biggest concern is the small diameter of the chamber. This larger forge sounds ideal but, I think I would want to have a slightly larger front opening; maybe 4" x 3" rectangular. I'm just thinking that if it will be hot enough to weld with, then I want to be able to easily get stack-ups in and out. If you need a complete newb to test out a prototype, let me know.

Bob
 
I've played a bit with building forges, but actually using them is something I'm weak on.

Building something round with bricks seems a lot like hard work.

If you use 3" x 4 1/2" x 9" bricks, you can build a 7 1/2" square with a 1 1/2" square hole through the middle, then core drill it out to whatever inside diameter you want.

I have built a couple of IFB forges with 6" x 6" x 13 1/2" chambers. They've achieved welding temperature and more with a 0.6mm (.023") MIG tip for the gas jet (albeit fitted in a 1" commercial Venturi mixer) that measures just under .030" using drills as Go/NoGo gauges. It gives just under 100K BTU/hr at 60 PSI according to the BTU calculator linked to from the ABANA/RonReil pages, so about the same as your burner.

I'm pretty sure the size of the openings has a much greater effect on the temperature reached than does the chamber volume. Mine were 3" x 3" at the back and 3" W x 4 1/2" H at the front, and it worked best with the back one plugged and the front one restricted.

IFB is horrendously prone to flux damage. Not as bad as fiber blanket, but still pretty bad. I think you'll need a good layer of something like Mizzou or better if you are looking to make it weld-friendly.

I found that cheap no-name grade 23 IFBs work pretty well for a welding forge. I managed to melt the Morgan Thermal Ceramics JM23 that I'd used to restrict the forge opening, where the cheapies remained intact. If you are going with the K23/JM23 IFBs, I'd definitely advise that coat of Mizzou or similar.

I was trying to build a one-forge-fits-all, able to go from HT to welding temperatures:

http://www.britishblades.com/forums/showthread.php?144072-New-forge-and-burner
 
My question would be why a round front port and an oval rear port. Square will work fine for the front and rectangle for the back. I think the chamber size is OK for a small forge. 1.5" firebrick walls would be fine.
 
Stacy E. Apelt - Bladesmith said:
My question would be why a round front port and an oval rear port. Square will work fine for the front and rectangle for the back. I think the chamber size is OK for a small forge. 1.5" firebrick walls would be fine.
Click to expand...

Partly for ease of manufacture, part asthetics. It's actually a bit easier to cut out an oval than a rectangle, and a circle is much easier than either. Although, I'm starting to think a semi-circle might be the best solution. I'll make a few different front plates and see what works best at a couple hammer-ins.

Charles
 
Well, here's the first prototype. I'm happy with the 4" opening in the front, but not ecstatic about the rear oval. I really like how the legs turned out, and the two part front makes brick removal easy. I might end up with the front top stainless, depend on if the mild warps when heated.

Final dimensions
12" front to back, 12" to top of chamber, 13.5" to top of handle
5" diameter x 10" long chamber
1" front and rear, 1.5" side insulation
4" × 3" front port
4" × 2" rear port


View attachment 566522 . View attachment 566523
 
Last edited:
Well, it works, but a couple minor adjustments should help. It sputters badly when first starting. I don't know if this is normal, or I don't have enough back pressure. I do have some issues with the burner port. Cutting an angle and round is a bit difficult without an 8" wheel on the grinder. It was shooting tiny flames out around the port, which you can see in the video. I'll weld these for now, but think I know how to prevent it in the future. I don't like the back port. It shoots a LOT of heat out the back, far more than I'm used to with the Atlas. I'm thinking of reducing it to 3" x 1". Do you think this would be adequate, or would you want it larger?

The good:
Nice, even heating of the inside of the chamber with just a little hot spot.
The leg design is rock solid on the ground.
The flame, although tilted backwards, swirls nicely out the front
No more than 6" dragon's breath, and it was a nice blue.


[youtube]SqognuF81Oc[/youtube]
 
Play with the burner position in the tube, and the gas pressure a bit and see if that fixes the sputter problem. Some rough sounding noise until it gets hot and smooths out is normal.

A 3"x1" back port should be fine. And/Or, you could include a firebrick that is shaped as a plug for the back port when not needed.
 
Pretty neat! How long would a forge like that run on a standard 20 pound BBQ propane tank?
 
You mean one of these? Yeah, it really does!

842211-01-main-180x191.jpg
 
New video uploaded. I think this is a bit better. The dragons breath out the back is reduced and the swirl is greatly enhanced. I still want to move the port to the bottom instead of the top.

[youtube]866u3raSm2c[/youtube]
 
Charles,
I had someone from our forum send me a suggestion that made me slap my forehead. I consider myself a thinker...but this thought never occurred to me.

Place the burner tube/port vertical, entering the chamber at 3:00 ( coming up from below). This way the flame travels all the way around the top and side before coming to the bottom. It also makes the problem of the tube sticking out sideways disappear. The hose will just drop straight down. It significantly lessens the footprint of the forge, and eliminates the potential flipping over of a forge if the hose gets yanked accidentally.

I am redesigning plans and drawings to reflect this change, and will post that thread soon ( hopefully). I may build a small forge with this change as a demo.
 
Hehe, I've been talking to Ed Caffrey about the same thing. It would enter the forge more around 4:30, angling up. Here's a rough mock-up on the plans showing my thoughts. The chamber is shown by the red circle and the burner can be upto almost 7" long. I have plenty of room to do this with my burners, but blown and side-arm are considerably longer and may pose a problem.

While tipping over is a concern, my reason for doing it this way is to get the rubber hose out of the possible path of any heat. Thanks for thinking of me, though.


View attachment 568960
 
Last edited:
Looking good!

I like that it is built for making knives... clean design and looks like it is going to work nicely.
 
I am very interested in this forge, especially with the larger openings. I have some round bar stockpiled, even though I have never forged, that was too big for the opening of your other forge but will slide right in this one. I will be staying tuned.
 
You must log in or register to reply here.
Back
Top