Blacksmith Lessons – The Side Draft Forge Hood, part 2

Part one is here.

In our last lesson we covered laying out your pattern in cardboard. Now it is time to start building the real deal. The hood can be built from any spare sheet steel you have about, ranging from 16 gauge up to 1/4” thick plate stock. The thicker the material, the heavier it will be and the more difficult it will be to install and move your forge after it is in place, so keep that in mind. Ours is made of 1/8” plate from the scrap yard and cost us about 50 dollars US when we built it. All the steel does not need to be the same thickness either; if you have a mix of thicknesses, you can still build the hood.

If using mixed weight steels, your front, top and back take the most heat and strain, so you may want to use thicker stock for those. The base takes the least stress, so you can use lighter steel here. As the box heats up, the back wall may flex and make a heck of a bang, throwing dust and ash about and surprising you. Thicker steel or reinforcements (covered later) will help to prevent this.

Once you have your steel, take your cardboard patterns and either a marker or chalk and lay out all your parts. If needed, you can join two or more smaller pieces to make a section large enough for your use. If you do have to join plates, make sure the joint is airtight with either close set rivets, or a weld. Be aware, a single long, straight weld is likely going to warp the steel, so you will have to weld a few inches, move to another section of the joint and weld there and so on until the entire joint is welded to reduce warping. Rivets look great (very steampunk!) but a weld is easier to get airtight.

Once your pieces are laid out, cut them to shape using either a cutting torch or plasma cutter, or if in thinner steel some kind of metal cutting shear. Thinner steel will be harder to weld and may require some unusual modifications to the techniques given here to work. If needed, clean up your cuts a bit with an angle grinder or file, but be careful to not cut too much metal away. Having extra metal is far better than too little, as you can always grind or file away, but adding a bit is more tricky.

These instructions assume you are welding all the parts together. You can, however, use sections of thin 16g steel cut into long, 3 to 4 inch wide strips, folded to 90 degrees lengthwise and a LOT of rivets to rivet the entire hood together. Pop rivets are not as strong as hammer set rivets or roofing nails clipped short (which make workable make shift rivets and are less expensive by far). And this has not been tested with pop rivets either, it may well not work!

With your pieces cut out, you are ready to begin assembly. Start with the base plate and the back plate. Lay the base plate flat on a welding table or concrete, and place the back plate in place. It will need to be braced to keep the joint at 90 degrees the entire length. Tack it in a few spots on the inside and outside with 1” long tack welds. We welded ours entirely from the inside along the lower joints, you may as well, or turn the assembly over and weld from the outside. The inside welds allow the box to sit flatter on the forge table however.

With this weld complete, check the joint for square, and adjust if needed, then weld in one side piece. Again tack in a number of places before welding to help reduce any warping or shifting. You may want to tack the other side in place as well and test fit your front piece just to be sure everything fits. It will be easier to correct a problem now, with everything just tacked, than once it is welded in firmly.

Our hood has a section of light weight angle iron along each upright corner to make it look better, and add some strength to the structure. It is welded in place from the inside, tacking it in through a couple holes drilled at an angle through the corner joints. Once our corner covers were tacked in place, each corner was welded from the inside full length in one pass with enough heat to penetrate somewhat into the angle iron. You do not need to be this aggressive about it, as long as the joint is airtight. It can also be “skip welded”. That means you can weld a section, move to another and come back once the first weld has cooled, so long as the joint is clean and solid.

Next we suggest adding the extension to the face of the front. We welded this on from the inside as it looks better. With that in place, we welded the front on. Here is where it begins to get tricky, as you will have to lay the tacked assembly on its face and nearly climb into it to weld the bottom joint and the lower part of each upright joint. And you may have to support the front with bricks and connect your arc welder clamp directly to the assembly to keep the whole from rocking with the front shroud already in place.

Once you have the lower assembly together, you may want to weld in a pair of bars from the front to back plates to keep the walls from flexing. Angle iron works well here, with one section mounted on each side of the top of the intake opening. Putting a single bar across above the opening is not as effective, and heat will probably cause it to flex over time. These support struts are not critical, and are completely optional.

If you have decided to put in the angle plate that directs smoke upward, put it in and tack it in now. It does not need to be fully welded into place. Just make sure the front edge at the inlet is welded flush with the bottom so there is no gap.

Now that the lower assembly is complete, the upper half will now be assembled. This will be a little tricky, but not horrible.

First, tack the collar and check to make sure your pipe will flex and fit over it tightly. Then weld the collar into place and tack the top plate at 90 degrees to the back plate. Make sure the collar is on the outside! This plate will make lining up the rest of the plates a lot easier. Lay the assembly on its back, and fit and tack the front upper into place, then each upper side. If they are curved, you may have some fun trying to match the curves so the joint it tight and easy to weld. A heavy pipe in a heavy vice is one way to allow you to form the curve; just hold the steel over it and strike beyond the pipe with a wooden or rubber hammer. You can also put it across two bricks and stand on it, or if all else fails, drive over it with a car. Be creative but safe!

With all the parts tacked in place, weld all the uppers from the outside. There are some long welds here, so take your time and do a good job, as these will show. Once you are done with the welds, you can take an angle grinder and smooth your welds if needed.

Your side draft hood is now done! Now you (and probably a friend) will have to move it into place on your forge bed. Watch out for pinched fingers! If you like, the hood can be raised a bit with a layer of brick, and this can aid in creating a better draw, but is not critical.

Once the hood is in place, mount your riser pipe up to your stack (or straight up through a collar in your roof if in a shed or the like) making sure all is firesafe and sealed (another much larger topic!!). With that, your hood is ready to use.

We will cover making the “flip up hat” part in the next post, though you may want to make it and mount it before you install your hood. It allows you to suck up much more smoke when you first light the forge.

A few notes on using the side draft hood-

Before lighting the forge, put a ball of paper in the intake of the hood and light it to pre-warm the air and start the draft.

With the forge first lit, when the most smoke pours out, you may need to place a thick plate of steel on the forge angled up to the intake hole on the hood to get the hood warmer and suck up most of the smoke.

Once the hood is warm, the draft will be more effective. As soot and ash seal any tiny holes in your stack and hood, it will draft better. The more airtight the hood and riser pipe are, the better the draft. In fact, the draft can get quite strong. In the right situations, ours will suck up a piece of paper left on the forge table (much to our surprise). The trad off here is it will also suck any heat out of the room. You may need to make a cover for the intake for times you are not using the forge to prevent it from drawing all the heat out of your shop in the winter.

Once the hood is drawing well, you may notice the flame rising above the forge bending at a steep angle into the hood! Just because the heat is not rising directly does not mean the infrared coming from the forge is not... if your ceiling beams are low and wood, you may need some kind of heat shield still.

The forge hood will get warm in use, and in heavy use it will get hot. Be careful about brushing against it, as it can burn you. Also make sure it is well away from walls and nothing flammable is leaning on it.

A larger intake opening will still draft, though not as well. If you only have an 8” exit stack, you can still have a 12” by 14” intake with a tall chimney and good connector pipe. It will not draw as effectively, but it will still draw.

On some days, when the atmosphere is right and the hood is warm enough, you may hear a faint rumbling as air is drawn into the opening. This is normal and not a problem. If it is quite loud, something is amiss or something else in the shop is causing it. The large, flat walls of the hood may be resonating in sympathy with something else in the room. Try sticking a large magnet to the side, off center, and see if that stops the sound.

That about covers it for now, images and part three coming soon!

Blacksmith Lessons – The Side Draft Forge Hood, part 1

In previous lessons, we covered building a few different forge beds, and more are coming soon. This lesson will help in getting the smoke and heat from your forge moved to some other place, preferably outside.

There are many types of forge hoods, including some positive pressure down draft hoods that we may cover in another lesson (uncommon, but used in some welding booths and chemistry work). This lesson is on building the side draft style forge hood we use in the shop. This project will require some fairly large sheets of steel, a fair bit of layout work and the ability to cut said steel using either a shear, saw or torch/plasma cutter. You may have to see if a friend has some of the gear needed for this build if you want to follow the plans below closely and do not have the equipment. It is intended to be electrically welded, but there are other ways to make this hood without a welder, which we will cover.

To create a pattern, we suggest cardboard boxes to build a mock up first, then use these pieces as pattern parts to layout and cut your steel with. You will need several large cardboard boxes, opened up, and tape to assemble the pieces. Of course, some method of cutting the cardboard will be needed as well.

The version of the hood we use also has a flip up “smoke hood” for the rush of heavy smoke when first starting the forge. You do not need to make this on yours, but it is quite helpful. Do not make it solidly mounted, however. It will get in your way more often than you would like to imagine.

We purposefully left out dimensions on the side draft forge hood drawings; you need to build this hood to stand on the table you have, so you will need to create a pattern based on your own system and measurements.

To start with, you will need to layout your base. The edge of the base should be very near the edge of your forge bowl. Ours is 1 1/2” from the top of the bowl, and sits against the projecting flange of the bowl. At first this may seem strange, but it needs to be close to make the draft effective. Cut a piece of cardboard that will lay on your forge table next to your firepot. Of course, make sure the forge table isn't still hot or the forge lit! Make this bottom plate as large as feasible, ours is 32” wide by 24” deep. This will allow a large expansion chamber inside the side draft and will help your smoke draw more effectively.

The next part to design is the back of the unit, which should be about twice as tall as the chamber is wide, or a bit more. In ours, and in the illustrations here, the side walls are straight for about as long as the box is wide, then have a sweeping taper to help the rising smoke gain velocity as it enters the flue pipe above. This is not critical, but it does look more graceful than flat angled upper sections. With the back made, tape that section to your base plate.

Next up come the two sides. Use measurements from the bottom and back to lay out two sheets of cardboard, cut and tape in place. Then comes the front.

Of note here; the opening in the front should not have as much volume as your riser stack pipe does. The larger the opening, the less draft the stack will have. If you have an 8” diameter stack, your front opening should be around 50 square inches or less. So an opening that is 10” by 6” may work, while an 8” x 6” opening will draw better. The problem here is that your fire pot will be larger than your intake in this case and some smoke will escape into the room. If at all possible, use 12” pipe since you get about a 112 square inch opening to work with. That is a nice 10” by 11” intake area, and is roughly what ours is. Once the hood warms and the stack begins to effectively draw, we have very little smoke in the room.

We made our opening with an arched, rounded top in the cut, which was outlined with the smoke lip (more on that in a bit). It is set to be centered with the forge inlet, so if your bowl is not right in the middle of your forge table (front to back) you may have to adjust where your opening is. Tape this in place with the other cardboard pattern pieces.

Now you will have to make the front upper part. This is the most complex part of the side draft forge. It is very similar to the upper part of the back pattern piece, but since it leans back, it is slightly elongated. There is a lazy way to do this however. Cut your two side uppers, with a long angle cut on one edge of each along the length. The upper edge should be the diameter of your inlet pipe, while the bottom edge will be the length of your side pieces. If you cut your back piece with sweeping uppers, you will need to measure how long the curved surface is using a piece of string or a flexible tape measure to get the length of your uppers. With these pieces cut and taped into place you can lay your uncut front upper against them, reach inside and trace the correct shape onto the cardboard then cut it to shape and tape it in place.

All that is left of the basic box is the top plate. Take the measurements from the uppers, and cut a piece. We will make a mount for your flue pipe that is slightly oval, as it will be easier to mount firmly and will make the stack draw better. Cut a strip of cardboard about 2” wide that is as long as the inside of your flue stack (use string or a flexible tape to measure this). Tape it end to end to make a circle, then squash it a little bit to make an oval, and put it on the top plate to draw the cut out hole. Cut the hole, then tape the collar in place.

The main body of your pattern is done! There are a couple more pieces to make though. A few more small strips are used to outline the front of your intake opening, and this is not needed but does give the hood more strength and makes it look better. It also increases the draft, since the opening is now extended closer to the forge fire. We also added an angled plate internally to help funnel the smoke up the expansion chamber and increase the draft. This is a piece as wide as your base, and several inches longer than it is deep (ie, in our, the base is 24” by 32”, so our plate is 32” square, and made of lighter steel than the main body of the hood). This is not a critical part however.

Now your pattern is complete! In our next installment we will cover the actual construction and discuss theory further.

(note, pictures are still in the works - they should be up soon)