Fabricating a P-Channel?

Hi all! I've been delving into RMHs for a little while now, and I'm in the process of my first complete build in my workshop. But, like many others, I'm already planning my next one, because fire is fun (when played with responsibly) and I love building things. Now, when it comes to masonry work or carpentry, I'm good to go. However, I want my next RMH to be a batch box, and from everything I can find online, it looks like I'll have to fabricate my own P-Channel out of steel, which I'm not as comfortable with. I assume I'll need an angle grinder with a cutting blade and a welder. Is this correct? Or are there other, easier ways to make one? I've been on donkeys forum and the batchrocket site and haven't been able to find any posts about the actual fabrication of a P-Channel for batch boxes, but I am slighty computer challenged so I apologize if this is covered on a thread I haven't found yet. Thanks for any feedback!

Those would be good tools for the job. I bought a 10" metal-cutting blade for my table saw, and found that it did an excellent job of cutting 1/8" steel plate, even plunge cutting a 7" square opening in the middle of a plate. It was much more precise than you can get with an angle grinder.

Welding the parts would be ideal, but a welder is expensive and has a distinct learning curve if you are not going to be using it regularly for other things. I expect to assemble the P-channel (actually PvdB's latest version of the Walker pre-port injector) for my next RMH with bolts if I get to it before my friend the expert welder is back in town. I hope to shape some of it by forging, but most people will not have the tools or knowledge for blacksmithing.

I had a similar question myself.I have miter saw ,angle grinders, sawzall,etc, and even a welder I don't know how to use.
My plan is to use metal studs, tin snips and self tapping metal screws.
I am also looking into making it out of steel pipe and fittings,translating the rectangular cross section of the pchannel  into an equivalent interior diameter of pipe.

Thanks for chiming in Glenn, I've seen you all over this forum and your advice is very reassuring! I was worried bolts might affect the airflow, but it's good to know they won't. I'm planning on this batchbox being my main source of heat, so I wanted to make sure I do it right, you know? And good call on the metal blade, I already have a table saw but don't have a grinder.  Hadn't even crossed my mind. You mention 1/8" steel; is this the recommended thickness for making a P-channel, or could I go with a lighter gauge?

William Bronson wrote: I had a similar question myself.I have miter saw ,angle grinders, sawzall,etc, and even a welder I don't know how to use.
My plan is to use metal studs, tin snips and self tapping metal screws.
I am also looking into making it out of steel pipe and fittings,translating the rectangular cross section of the pchannel  into an equivalent interior diameter of pipe.

That's a good idea, I would totally do that if I had some metal studs! I was wondering if the pchannel had to be a rectangle as well, I have some old angle stock that I could probably rig up a square channel with, even if it's just a temporary measure to make sure the system works properly.  Does anyone see any reason a square pchannel with the proper CSA for system size wouldn't work?

The width of the P-channel is more or less dictated by the width of the firebox's port, which makes rectangular tubing the better choice to achieve the required secondary air volume, at least on my 6" batch-box. Making the lower section of the P-channel removable / replaceable is handy for when it eventually burns out. There are several ways of doing that. On mine, I made a sleeve of stainless steel sheet stock that fits over the main tubing, screwed in place. The secondary air "nozzle" section of tubing plugs into the sleeve and is held by a couple of screws. The sleeve connection is on top of the firebox, at the 90 degree elbow of the P-channel, thus removed from flame path stress etc.

Byron Campbell wrote:The width of the P-channel is more or less dictated by the width of the firebox's port, which makes rectangular tubing the better choice to achieve the required secondary air volume, at least on my 6" batch-box.

Ah! That makes total sense, thank you. Looks like I'll be figuring out a way to fabricate a pchannel without a welder after all... That'll be a fun challenge! I really like your easily replaceable channel idea; do you happen to have a photo of that setup? I'm trying to picture it in my head but I can't quite wrap my head around it (though I may need sleep.)

Peter van den Berg's latest version of secondary air injector, which he is apparently calling the "floor channel", is completely removable without tools, for easy replacement of the standing tube. It would probably not be a big deal to fasten with machine screws or bolts instead of welding.
Floor Channel

Here ya go Jeff, photo attached. The cast iron door on my batch-box was salvaged from a junk wood burning furnace, with no room for secondary air entry near the top of the door as normally done when running the p-channel on top of the combustion unit. On my stove the p-channel's air input end is positioned on the side of the firebox. However, next on my list is to make a floor channel replacement which has several advantages (easy replacement, warms up faster, promotes lower position of riser secondary / re-burn). See Glenn's link to the floor channel. I'll be fashioning mine so that the vertical "nozzle" tightly plugs into the main tubing, so hopefully no welding will be needed.

I figure that the curve of a pipe should be able to snuggle up into the port.
Thin wall EMT could even be smashed into an oval to fit the space.

This is all seriously great information, thank you guys! I'm feeling a lot more confident about my first batch box. Mayhaps I'll even start a thread next season about the build in my tipi...