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)
No comments:
Post a Comment