Fire pot dimensions for coal forge

[Donal Harris]

I’ve burned the heck out of my brake drum fire pot and have to build a new one. 

Does my math look right?  I found several plans online, but most seemed to be metric and not inches. I can function with metric weights, temperature, mass, and volume, but not length. 
 

I want the pot to be 13” long and 10” wide at the top, with the bottom 6”x6”, and 5 inches deep. My tuyere is some sort of pipe with an external dimension of 4”. Not sure of the internal dimension of the pipe, but it looks to be at least 3/8”. The plate for the pot is rather thick, maybe 3/8” as well. I will weld 1/4” flat bar around the pot as a lip to hold it in the table. 
 

What do I plan to heat up in the forge?  Like most, nothing much larger than 3/8”, but I do on occasion have to heat things up to 2”.  An example of that would be a bale spike. Before I bought “The Undisputed King of All Anvils”, I took a bale spike, heated it up about 16” from the tip, and beat it to 90 degrees. I then cut it off and mounted it in a stump. It was what served as sort of a horn for me. 

[anvil]

Your dimensions are good. Actually, a 4" depth works well. 

I would add a clinker ball to the bottom. It's pretty necessary for directing your air flow and dumping ash without having to break down your fire. Check out Centaur Forge for clinker balls and handles.

[Donal Harris]

I was concerned my depth was not enough. I forget who said it, but in one of the other solid fuel threads, someone said 6” for coal was the minimum and even that might be a bit shallow. 
 

Ad for the clinker breaker, I had planned to go without one, because I couldn’t figure out how to make it so I could swap out the clinker breaker when it burned away. I was just going to weld up a small grate out of some square bar I have. But on the Fiery Furnace website I saw a welded fire pot. The pot was bolted to the tuyere. The clinker breaker goes between the two.
 

I should probably just buy a fire pot. All the plate and pipe is free and it gives me a reason to drive down and see both my Dad and FIL. If it weren’t for that, I would just buy one. 

[dave in pa.]

I make mine 10 inch square at the top, 3 inch square at the bottom, 4 inches deep.

I use 1/4 inch plate and have the first one that I made 14 years ago.

It has been used HARD (everything from 1/4 stock to 3 inch stock) and barely shows it.

I have used it with bit. coal, coke, and corn and it works well with all of them.

I have made many, given away several, and have extras... but I'm not sure I'll ever need another one the way this one is wearing. 

[Donal Harris]

My math was wrong. While it may have been correct in 2D, it was not in 3D. I had cut patterns in cardboard and I could quickly see it just wasn’t going to go together. There must be some way to have a rectangular top and square bottom, but it won’t be this one. 
 

I cut the bottom so it is 5”x6” and then cut the short sides to a length that work. The fire pot is still rectangular in shape, but only just barely. I am really glad I did a mock-up prior to cutting anything. As is I think it will be too large. 

[anvil]

I can't help you with your 2D/3D solution but it's pretty simple to lay out with chalk an a bit of geometry on my table. 

[Frosty]

I use cardboard and tape to make patterns like that.

Frosty The Lucky.

[Irondragon Forge ClayWorks]

Have you seen this one. It's about as good as a firepot you will find. Why reinvent the wheel?

http://www.bamsite.org/firepot.html

[Donal Harris]

I saw that one. If was the basic plan I was trying to go with. I just didn’t like the depth.
 

Last year or perhaps late the year before that I had sent a PM to Charles Stevens, who lives fairly near me, about my brake drum forge. I was having problems with clinker. He asked for measurements and pictures. After seeing them he felt my pot wasn’t deep enough to do the mods he was going to recommend I do. Thus my desire for a 5” deep pot and not 3” as the BAM plan seems to be. He was supposed to drop by the next time he went to Norman to visit his daughter, but my wife was having none of that. She told me until I cleaned up “that mess of an area”, no one was coming by to see anything. She is not too pleased with the pile of scrap I have collected, even though compared to the size of most scrap piles I have seen, mine is practically nonexistent. 
 

As for laying it out in chalk, not an option for me. My dad can do that sort of stuff in his head and doesn’t need the chalk. Me, folding boxes in my head is just not my stick. I have to have something I can actually touch. 

[Glenn]

 Make a cross of cardboard with the arms the width of the forge bottom.  You know the size of the top opening you want so label each of the measurements N, S, E, & W respectively and cut say a 1 inch strip of cardboard and glue it to the arms at the depth of the forge sides, to form a T.  Fold up the arms so the edges of the T's meet.  This will form the skeleton of the forge.  Measure or transfer each side to a cardboard sheet and you have the pattern.

It is easier to make than to describe.

 

[Donal Harris]

I cut the plate today. Didn’t finish welding all the pieces together, but did finish the hard part, which is welding up the sides. The camera on my phone doesn’t work, so I have no photos yet. 
 

Before driving down to my Dad’s to use his shop, I borrowed my wife’s phone and took a couple of photos of my totally trashed forge as it was before I loaded it up. The first shows the pot as it looks below the table. The second is from the top. You can see it was in very bad shape. The friend of mine who built it for me had welded the top of the drum to the bottom of the table. I was pretty sure that was going to fail pretty quickly once it was in use, so I asked him to secure it to the table by placing flat bar under the drum and welding the drum to the table. 
 

It worked well for a month or two, but after that I began to see smoke coming out from beneath the table as I used the forge. You can see from the first photo that what I expected to happen did happen. The lip of the drum cracked. Had the straps not been there the pot would have likely fallen to the ground after the first few times I used it.
 

He is a blacksmith (and an engineer). Since he is a blacksmith I had assumed he would just use solid pieces of flat bar for the straps, and not do what he did, which was take small pieces and weld them together. That was a bad move. One of the joints broke after I had been using the forge for about a year. The other three joints, plus his welds to the bottom of the table itself held, otherwise the pot would have fallen. I used it like that for about two years (cracked bracket and lip of brake drum. “Cracked” lip, not busted lip. The gaping holes formed much later.)

The second photo shows what the inside of the pot looked like. You can see quite a bit of daylight, but the photo doesn’t quite show the extent of the damage. A few hammer blows would likely knock the bottom out. 
 

My plan is to use the same table, except this time, drop the pot in from the top, with the pot supported by the lips around the pot,  I don’t plan to weld the pot to the table. I don’t think that is necessary. The table is in reasonably good shape. I will have to clean it up and cut the round hole into a square and repair a couple of cracks, but not much else. 

[Donal Harris]

Finished it yesterday. I decided against a clinker breaker. It isn’t that hard to just remove them with a poker. I still need to weld up a grate. I had forgotten to bring square bar with me.  My Dad is great, but is very funny about his scrap. Asking to use even a piece from his trash barrel will cause him to sort of scrunch up his face, so I thought it best to just wait until I got back home. Until I have time to make a grate I will just place a couple pieces of scrap square bar across the opening.

The pot isn’t quite square, but it sits flush with the top without rocking. I hope the welds hold up to the heat. I am not a welder. 

[Chris C]

Well Don, that looks great to me.

[Donal Harris]

Thanks. I debated replacing the top of the table as well. It was pretty rusty and had cracked in two spots around the old pot. But It seemed to be reasonably sound, so I just welded the cracks.  I also cut a piece out of far side bed rail to allow me to put longer pieces in the fire without them being tilted. The bed rail on the near side I had cut out shortly after I first got the forge. I am a little concerned that having now cut away part of both side rails I may have weakened the top. It may begin dipping down in the middle. I wish I had braced it. 

I have to say that after having done all of this, next time I will probably just buy a cast iron pot. I didn’t this time only because I wanted a reason to get my Dad out into his shop. I am thinking there aren’t going to many of those times left. 

[anvil]

The purpose of a clinker breaker is to easily remove ash and clinker fines without having to take your fire apart to do so.

Also it looks like your air inlet is not horizontal. I suspect your air pipe will fill up with ash whenever you dump it. 

[Chris C]

9 hours ago, DHarris said:

I wanted a reason to get my Dad out into his shop. I am thinking there aren’t going to many of those times left. 

Good for you, Don.  One of these days you'll look up and he won't be there.  I wish I'd spent more time with my Dad while he was still alive.

Oh, and just flip that cart upside down and weld an angle iron brace along the bottom under those openings.  That should reinforce it nicely.

[ThomasPowers]

Using high grade coal that produces little clinker I've used expanded metal as a grate before.  Picked up a strip at the scrapyard that was just the right width and cut a section.  I had to replace it every few weeks of hobby smithing.  More often if I was welding billets; but cost me under a dollar and lasted long enough to get around to doing it another way.

[HumanAfterAll]

Hey, DHarris. Nice design. I'm looking into making one for myself too but am concerned with the air flow. How do you control yours?

Also, this might be a stupid question for all you more seasoned on the craft, but why did your pot depth is 5" and not 3" as suggested on Bam's design. Why and what's the difference?

[Glenn]

The design of a fire pot should be based on the type fuel you are using, the type work you do, the size stock you use, and other factors.  Early on I found that moving the sweet spot of the fire up or down to match the height of the table was important to me.  That way you knew if the stock was resting on the forge table, the metal was in the sweet spot.  

None of the forges I have built were welded together before being used for a while.  Changes were easily made until I got what I wanted.  Only then did they get finalized.

[HumanAfterAll]

5 hours ago, Glenn said:

None of the forges I have built were welded together before being used for a while.  Changes were easily made until I got what I wanted.  Only then did they get finalized.

How do you keep the pieces together?

[Donal Harris]

I may brace it later, Chris. Still not sure. As for my Dad, I wish he would come up here every now and then, but Highway 81 only goes the one way apparently.

Human, my brake drum pot was maybe 3.5” deep. It was Ok, but it didn’t seem to work well for anything thicker than an inch. Charles once said coal with a bottom blast needs to be deeper. I went with 5” just because it seemed a good compromise between 4” and 6”. If I find 5” is too deep, I can weld up a grate from 3/8” square stock that is just a bit larger than the bottom of the pot. That would raise it.

i wouldn’t really refer to my pot as having any sort of design. I did the math and even built a mock-up out of cardboard, but what I actually ended up with in steel didn’t quite match what I had envisioned. I had wanted the pot to be rectangular and not the square I ended up with.  I don’t know this is true, but logically it seems as if a square pot would require more coal than a rectangular one.  I had also planned on having a clinker breaker, but at the last minute decided against it. Our club has pretty clean coal. I can usually go a couple of hours before it is a problem. It is also pretty inexpensive. I buy it two 55 gallon drums at a time. 
 

As for the tuyere, I went with 3” because I had always felt my old one was too wide. If the 3” proves to be too small, I will just build something to divert part of the air. I don’t want to go any narrower than 3” because a friend has one which is 2” and it is easily clogged. Clogging is also why I went with quite a bit of distance between the air intake tube and the ash dump. I won’t have to dump the ashes as much.

On my next one, assuming I don’t go with a cast iron pot, I will do like Glenn suggests and not weld the pot completely until sure I like it. 

[Glenn]

Pieces of my forges were temporally tack welded in place to see how they would work. Cut or break the tacks to make adjustments.

The forges that I use most at this point are the 55 Forge with the supercharger, a 13-1/2 inch diameter brake drum with a 1 inch deeper extension and a table, and a 4 inch diameter cast iron pot with a 1 inch deeper extension and a table.  The 55 Forge has gone through so many modifications I quit counting.  The brake drum forge has had two different size tables and two different depths of the table, and the 4 inch diameter forge was the third built and has remained about the same configuration.

Each produces a different type heat and different shape heat.  They are just 3 different tools so you can choose which one fits the job at hand.

[Donal Harris]

If you use a hair dryer, plug it into something with a switch you can easily turn off. That is what killed my pot. I had a switch I could click with my foot, but the switch was weak and broke after a month of use. Reaching all the way under the table to turn the hair dryer on and off was a pain and I was lazy. Even the lowest setting put out quite a bit of air. After three to four hours of use, the bottom of the pot would have quite a bit of color. The first to go was the weld on one side of the pot. The table itself at that spot also warped up a good inch. 
 

You can expect to replace the hair dryer every couple of months, or at least I did mine. But then they were all the cheapest I could find at Dollar General. I purchased a perfectly usable Champion #40 blower at the same time I began blacksmithing a couple of years ago. I just never hooked it up. The friend who made the forge for me used pipe that was larger than the flexible tubing I could easily find, so I just never hooked it up. This new pot has a smaller air inlet. I will be using the blower and not the hair dryer now. 

[Welshj]

12 hours ago, DHarris said:

The friend who made the forge for me used pipe that was larger than the flexible tubing I could easily find, so I just never hooked it up. This new pot has a smaller air inlet. I will be using the blower and not the hair dryer now. 

There are ways around it. It's probably not the best example, but I dropped down from 3.5 inch pipe to a 2 inch pipe- using an angle grinder with a cutoff wheel, and a mig welder. Just took time. Cut, bend, weld, cut bend weld.

[Donal Harris]

That would work. I was just going to use flashing and duct tape.