Building a set of dual chamber "great" bellows

Blacksmith lessons Building bellows


The next project is a fairly big one, and will be one of the more costly in terms of materials we will cover in this series. This project is building a set of dual chamber bellows to drive air into your forge. It is worth noting that the dual chamber bellows are sometimes called the great bellows as well, and in older texts you will see this name more often.

Of course you can simply use a blower or fan system, as we described in the bucket forge lessons, but there is a certain romance and elegance to the great bellows that a whining shop vac simply can not provide. Another benefit is the bellows are very quiet, and rather efficient. They are a tremendous pleasure to work with, having an almost musical quality to the quiet sounds they make once you get the hang of using them.

This project can be scaled up or down, depending on how much storage space you have. Larger bellows will be easier to work with, though a great deal harder to move and store. If you follow roughly the proportions laid out here, you can scale these up to a set at least 12 feet long or as small as 12 inches and they will still work. The biggest difference between a medium and large sized set of great bellows is the number of pumps they will need to bring a forge up to a given temperature.

First let's have a bit of overview on the pieces of the project and how the great bellows works. This will help you better understand what we are doing, and why somethings are done in the way they are.

The great bellows is made up of two chambers, one on top of the other. In the bellows resting state, the bottom chamber is fully inflated while the top chamber is fully deflated. Pulling up on the handle of the bellows pulls the bottom board upward, driving the air in the lower chamber through the air valves into the upper chamber. This inflates the upper chamber and air begins to be driven out the nozzle of the bellows into the twyre pipe that feeds the forge from underneath. The top chamber is deflated as the top board falls free, driving the valves shut as the pressure from the bottom chamber drops off so the air is only funneled into the forge. The bottom chamber bottom board drops back down as you release the handle, and as it falls the vacuum inside it opens the bottom set of valves and refills the bottom chamber with air. Once it is refilled you can press down on the handle again, refilling the top chamber.

The benefit to having the two chambers in series like this is the bellows produce a steady stream of air blowing into the forge, where a single chamber set of bellows will only blow intermittent puffs, much like using your own lungs and a blow pipe. The linked chambers allows the bellows to “breath in” at the same time the upper chamber is still “breathing out”. There is another, less obvious benefit to this as well. Sometimes, a single chamber set of bellows may draw hot air back out of the forge, which can contain unburned flammable gas. If this gets inside the bellows, it may ignite. While it only very rarely will cause the bellows to catch fire, and probably never explode, the increase in internal pressure may be enough to blow the “lung” off the bellows, making a repair immediately necessary.

For the “lung” section of most old great bellows, a large sheet of leather was generally used. We are going to cheat a little here to help you save money, and if it is done correctly the bellows will still look completely proper for any historical re-enactment. Unless someone touches the actual lung surface, most people will think you spent a fortune on leather to wrap your bellows.

So now that you have a bit better idea of what the bellows do in action, lets begin the process of building them.

The bellows are made up of three “boards”, a nozzle, valves in two of the three boards and a lung. There also needs to be some sort of frame or stand to hold the bellows with enough space for both halves to be fully inflated at once, and a handle with a linkage to the bottom board of the bellows to allow you to operate them. Note that there are many ways to build most of these pieces, but I have used the most common shape and mounting system here, along with the materials that are the easiest to work with and find.

The middle board does not move, and the nozzle will be built attached to the middle board. The frame that holds up the bellows is also connected to the middle board at each outer edge. The top and bottom boards are hinged to the nozzle and supported by the lung around the outer edge. The bottom board will also have a projection at the rear farthest away from the nozzle for the handle to connect to. In larger sets of bellows you may want to add “ribs” along the lung to help it open and close without puffing over the sides of the boards as well.

For our project, I suggest ½ inch plywood for the three “boards”. This is a good compromise between strength and weight and is much easier than trying to carefully laminate planks and cheaper than using pre-cut tongue and groove material. Plywood also is airtight so you don't have to worry about that issue. If you like, you can cover the top board with planks glued on the top to make your bellows look more authentic.

The drawings here will help you get a feel for the proportions that are generally seen, and these evolved over many generations of smiths using the bellows daily. You can change the shape and proportions to suit your needs or materials at hand, but this layout is known to work. You are on your own if you decide to do a giant letter K shaped set of bellows or something of the like. I will interject here that a coffin shaped set of bellows works fairly well. Also, if you scale your bellows to fit width wise between the legs of your wooden forge, the forge legs can be used as the mounting bracket, making the footprint of the whole system much smaller.

Usually, your best bet is to draw a full sized template on a large sheet of cardboard or several sheets of newspaper taped together. This will allow you to layout all three boards so they are the same size. You can also cut out one board and trace that as your pattern. Once you have your pattern, trace it on your plywood with a sharpie pen or something else you can easily see.

You will need some way of cutting these out of the raw sheet of plywood, such as a jigsaw. You can also use a circular saw and cut away all the material outside your layout lines, then use rasps or a surform plane to shave the edges smooth. Do NOT attempt to cut a curved line on a table saw! This will more than likely catch the blade and kick the panel out and more than likely it will hurt you. It could also pull your hand into the saw or outright kill you if a thrown piece hit you in the head with sufficient force. Play smart, anything that cuts wood cuts you better.

Usually, once I have the boards cut out, I clamp them together and smooth the edges with an angle grinder fitted with a sanding disc so the boards are all the same size. You can also use a surform or rasps, and it is not even all that critical that the boards are all the same size, it just makes the final product look a bit better. A clean, square edge does help when it comes time to install the “lung” but is not critical.

The next step is cutting holes for the valves. A very large hole saw would work, but drilling a hole large enough to pass the jigsaw blade and jigsawing the hole to size will work just as well. If all you have is a skill saw, you can plunge cut squares, but these may be fairly tricky to do. The holes do not need to be perfectly round either, again it is a purely cosmetic thing, but in this case in a place that will not be seen in normal use. The valves only need to be an inch or so larger than the hole in all directions, and can be made from scraps of the main sheet of plywood. It will help to have one straight edge on the valves to allow you to mount the hinges so the valve opens and closes cleanly and efficiently.

The next part to make is the nozzle. Historically speaking, these are usually shaped into a nice smoothly transitioned shape, but they can be left blocky and square without changing how the bellows work. I usually build up the nozzle out of 2x6 stock with a layer of 1/2” ply across the top. The drawing will help explain the layout. Mostly, you are trying to make the air flow smoothly out of the top chamber into the forge inlet pipe. At times I have made this section more complicated, but this simple version works very well and is much quicker to make than the version that requires cutting through the center board and carving out the bottom board.

Normally, a tapered snout is mounted to the front of the bellows, so it can just be wedged into the inlet pipe. This can be made from sheet steel, or even a can cut down the side and reshaped. This can be fairly complicated to fit up, but a pattern is provided here if you feel you have the sheet metal skills to make this part. Another option is to build a “cheater” adapter mounted to the snout. This is a box with a hole in it mounted to the snout. The hole is the same size as the outside of your inlet pipe and allows the pipe to simply slip in. You will probably want to stuff something like a rag in around the pipe if the fit is not very tight, to get as much air into your forge as possible. Both are illustrated here.


Next comes putting the valves in. Your valves can be square or the tombstone shape shown, no one sees them in use. I like the tombstone shape, as it allows me to more easily get my hand inside the bellows and feel around should something get in there that has to be retrieved. The valves mount on the top side of each board, and are usually covered on their bottom face with some kind of felt or other fuzzy cloth to help them seal somewhat when the valve is closed. An old washcloth can be glued to the face of each valve with decent results. By wrapping the cloth up around the valve and only gluing on the back or top side of each valve, you won't have problems with clumps of glue preventing the valve from sealing.

For the hinge here either a strip of thin leather or vinyl is very easy and fairly fool proof. Simply tack it on as shown, and you could probably even use glue (though put in a few tacks, a failure here is gonna be a pain in the butt to fix later). I usually also add a strap on each valve to prevent the valve from flopping over backwards and staying open or getting caught. Almost anything flexible can be used as a strap, even old shoestrings or a bit of canvas. The valve should be able to open about 30 to 45 degrees, so the board above pushing against it will close it without it catching.

A note on the valves; you do not need two valves in each board, you can have one, or have two holes and one large valve board. I use two out of tradition and have to admit I am not sure why most sets of bellows have two valves. Most likely it is a compromise between getting air into the chamber quickly and the weight of the valve itself. Too heavy of a valve will not open easily and admit air and too small of a hole will make the restricted air move from chamber to chamber very slowly. Having two moderate sized holes is a good compromise. I have seen a few old 18th century sets with three smaller valve holes that appeared to use one large valve board across all three holes.

Once you have the nozzle and valves built and installed, you will need to put the hinged top and bottom boards in place. Again, a flexible strip hinge here is an easy and effective way to hinge the board, and seal up the connection. I generally put the hinge strips on the outside of each assembly for ease of maintenance. A little construction secret here can save you a lot of extra time pumping at the bellows. Put silicone caulk on the face of each piece and the underside of your hinge strap before assembly and let it dry. Once assembled, this silicone will act as a gasket. Don't worry, you will have another use for the rest of it very soon so it won't go to waste.

With the boards in place, now comes the fun part. This is also going to be one of the most costly parts of this project. We are about to put the lung on your bellows, and this can be a very time consuming, frustrating step.

The lung is a flexible, airtight sheet of material that wraps around the outside of the bellows, and is nailed to the boards in each layer to help seal the chambers. Historically, the lung was usually leather and the joints along the edge were sealed with hot pitch a short section at a time as the lung was nailed on. We are going to take a whole bunch of modern short cuts here then dress the whole thing up so it looks like you went to the full effort of doing it right.

Instead of leather, I would suggest finding a mottled brown, leather look cloth backed vinyl. This material can often be found at fabric stores, but be ready for some sticker shock. It is usually about 10 dollars or more a linear yard. Usually it is about 60” wide though, so you may be in luck and only need a single yard if your bellows are small. Using a flexible tape measure, check the distance around your bellows board perimeter. Don't forget to give yourself at least 1 inch of overlap on the ends where the nozzle is. A bit more would be even better.

A less airtight material is canvas, but this can serve fairly well, and if sprayed with something like Thompson's Water Seal, it can be quite serviceable. At our local Field's Fabrics, vinyl remnants can often be had for less than the price of canvas, sometimes as little as 10 dollars for a 60” wide section that is 3 yards (9 feet) long. If you can find these deals, you will save a considerable amount on this project.

Once you have your material laid out and all the wrinkles have been pressed out of it, you can caulk the edges of your boards with silicone. I try to mount my bellows lung with the silicone still wet, so it bonds to the lung material more effectively. You can work in sections if you do not feel comfortable with trying to do this all in one fell swoop. Generally, I fold the edge over at the start and staple it to the side of the nozzle box. Then I work backward around the bellows, stapling to all three boards evenly. Something very important must be taken into consideration here; the bellows must be fully open at this step! This will limit the travel of the bellows and if you make the mistake of having the lungs closed you have just made a very strange coffee table. Usually, I cheat a bit here and staple a strip of heavy cardboard or nail a piece of light scrap wood across the back of the bellows boards to hold the boards open. You have to be sure that the open position is just a bit less than the full width of your lung material, otherwise you wont be able to mount the lung and seal up the bellows. With 60” material, I generally set the open width to 58” outside surface to outside surface but only on a very large set of bellows.

A good rule of thumb is to make the bellows lung as wide as the boards plus or minus a bit. So if you have a set of bellows that are made to fit inside your colonial forge, the width of the board will probably be about 22” so the lung material might be 28” or so. If you are building a narrower set of bellows like this, you have the option to buy half as much material in length. This is then cut into two pieces half as wide and a seam is sewn down the center back of the lung. Many large leather lunged bellows have seams, so this is not uncommon.

Remember; your frame needs to be able to hold the fully expanded height of your bellows. If you opt to hang your smaller bellows underneath your forge, you will have to limit your bellows lung height to just slightly less than the length from the underside of your forge to the ground.

If you have opted to build a larger set of bellows, the full width of the material may be useful (for example, if you have built each board from a single sheet of plywood, so the bellows are 48” wide – note that this is a very large set of bellows and they will be quite heavy to try to move). You will have to adapt these instructions, like many of the other lessons here to meet your needs.

Once the lung is in place and stapled down all the way around, I trim off any excess material, usually leaving about 1inch extra, which I fold under and staple on the top and bottom of the bellows. This will help form a better seal and the folded edge gives the lung a nice, finished look. If you have used canvas, a bit more work may be needed to make the edges look good.

Now you have a couple options to dress the final look. One possible thing to do is outline the edges of the boards with a thin strip of screen door molding, nailed on ever few inches with a dome headed furniture nail. The strip will have to be soaked in water overnight to allow it to flex around the curve of the bellows without breaking. Another option is to simply put a row of nails in the edge of each board to hide the staples somewhat. If appearance is not a concern to you, simply leave the great bellows as they are.

Now that we have completed a set of bellows, our next installment will show you how to mount them in a frame and set everything up to put them to use. We are nearly ready to start forging!