Build a forge
KNIFE MAKERS CAN HAVE A GO AT MAKING AN LPG BRICK FORGE OR A TANK FORGE THAT RUNS ON USED ENGINE OIL
Des Thomson shows how to build two types of durable forges, one powered by LPG, the other by used engine oil
When my wife and I were in Japan in 2006 we visited a traditional Japanese knife maker on the island of Kyushu. He is one of a handful of traditional knife makers left in Japan, with the family business stretching back for 250 years. The interesting thing was that he heated his forge using used engine oil. The burner was a very simple oil-atomizing arrangement using compressed air blown over a nozzle. The oil was directed into a brick furnace in which he heated his knife blanks. You can see a video of his furnace on The Shed website. The furnace ran with no apparent smoke or smell and achieved temperatures in excess of 1100°C. It
inspired me to try to make my own.
There is always something fascinating about viewing the red-hot interior of a knife-making or blacksmith’s forge (Image 1).
In this article I will explain how I made a brick forge that runs on LPG, and a tank forge that runs on used engine oil or LPG.
Forges should only be used where there is good ventilation
Hazards
There are a number of hazards that you need to be aware of if you want to run any forge.
The high temperatures can cause serious burns and hot metal will easily cause a fire if it dropped on combustible surfaces. Forges should only be used where there is good ventilation. Carbonmonoxide levels can rise wherever there is combustion, so don’t use them in enclosed spaces.
Eye, body, and hearing protection is just common sense. A red-hot piece of slag from hammering hot steel could cause loss of sight if it gets into your eye. An old blacksmith that I spoke to recounted how he had several months off work due to a piece of slag dropping into his boot. By the time he had cooled it off in the water trough it had turned into a third-degree burn. A leather apron is also good insurance. Don’t take any chances.
Brick forge
This brick forge has been built at the Halswell Menzshed shed and is constructed from lightweight insulating firebrick. Known as ‘K26 bricks’, they available from Certec in Auckland (certec.co.nz/).
These insulating fire bricks are rated to 1426°C and measure 230x115x75mm. They are commonly used to line foundry furnaces, forges, and kilns.
They are soft bricks and can easily be cut to size with an ordinary wood saw, drilled to create burner openings, or routed to create channels. They have very low thermal conductivity and low thermal shock characteristics; however, because they are soft they will wear when used on the floor of the forge. Welding flux will also eat them away. To prevent this, you can apply a layer of fireclay or use a hard firebrick or unglazed porcelain tile on the forge floor.
Before starting construction of the actual forge I stacked the loose firebricks in the shape I wanted so that I could determine the overall dimensions (2). I figured that an internal space of 115x 115x345mm (width x height x length) would be a good size for most heating and forging work.
Before I committed to the forge interior size I also wanted to test-fire the burner that I had made to ensure that it would have sufficient output to get the forge up to temperature. I used the same burner in a metal-melting furnace where it will easily melt a crucible of bronze or aluminium.
I cut a burner hole in one of bricks and with the aid of a temporary burner support fired it up. Within five minutes the interior of the forge was a cherry red, reaching 1280°C. The home-made burner was more than adequate (3).
The insulating properties of the bricks are amazing. With the interior of the temporary stack of bricks red hot, the exterior was still at room temperature, although there is some heat transfer after extended firing (4).
Angle-iron frame
The first step in constructing the furnace was to use 30x30x3mm angle iron to
fabricate the top and bottom frames that would hold the bricks (5). Angle-iron feet were also cut and welded in place (6).
I made the top angle-iron frame removable in case it was ever necessary to disassemble the furnace for any reason. The four corner angle-iron supports have small locating lugs and M8 nuts welded at the top (7 and 8). This enables the top frame to be bolted in place using an M8 x 50mm long bolt in each corner (9).
As we have a sheet-metal guillotine and folder at the shed I decided to finish the exterior in Zincalume to give a professional looking finish to the project. We get end-of-roll off-cuts of this from a local roofing supplier, and it is very handy for all sorts of small projects. Panels to fit the inside of the frame were cut by Menzshed member Graham Weal (10).
The corner angle-iron frame supports the front and rear doors. The doors were made slightly larger than the interior of the furnace so there is some overlap. The doors are made from 25x25x3mm angle iron. Each door is mounted on a hinge pin made from 12mm bar. The bar is mounted in two lugs welded to the corner post (11 and 12). When operating, the forge must have one or both doors lifted above the hearth to allow the forge to breathe. A short piece of 15mm pipe sits above the top hinge lug so that the gap between the door and hearth can be
As the burner that I made can be used on the forge and metalmelting furnace, I made it removable
adjusted (13). The doors are able to be swung right out of the way if required.
For heating longer work in the forge, it is good to have adjustable work supports. I made these from 12mm plain rebar and they slide in 15mm water pipe. I bent the rebar supports up first and then used them to hold the 15mm water pipe in the correct position while they were welded in place (14). To get nice tight bends on the rebar we used a carbon arc torch to heat them up (15–17). This runs off the arc welder producing temperatures in excess of 3000°C between the carbon electrodes. It is a great tool for spot heating but you need to be careful not to melt the steel you are heating. As the burner that I made can be used on the forge and metal-melting furnace, I made it removable. The burner is held in a pipe clamp, which in turn is supported by a simple cross made from 15mm pipe. The cross piece is slid over a 12mm bar welded to the top angle-iron frame. All the sliding parts are clamped by 8mm bolts (18).
All the parts were then given a coat of black manifold paint ready for assembly (19).
Assembly of the furnace
No mortar is required when installing the bricks. Just butt them together. If required, the bricks can be very easily cut with an ordinary wood saw or hacksaw blade. I used an old blade in the bandsaw to cut them (20).
The Zincalume base and side panels are held in place by the bricks (21–24).
To keep things simple I decided to use just one burner. On the internet you will find knife-making forges with two, three, or even four burners. It really comes down to what sort of work you will be doing. With doors at both ends of this forge, longer work can also be accommodated.
To cut the hole for the burner I used a hole saw working from both sides of the brick (25). The brick is very soft and cuts easily. An 8mm deep counterbore was done with a larger hole saw (26). This forms a recess to allow Kaowool to be packed around the burner tube to seal it.
I then formed a flare on the inside of the burner hole using a sheet-metal reamer that I made. The purpose of a flare at the end of a burner is to slow down the gas velocity to allow the flame to burn at the tip. If the gas leaves the burner at too
fast a rate the flame cannot consume the gas fast enough, and it will blow out. The principle of this is very easily demonstrated. On my metal-melting furnace burner I have a one to 1∕14-inch
union that acts as a flare to slow the gas. Without this in place the burner will not run out of the furnace. Put it back on and I have a beautiful blue flame that will burn outside the furnace (27).
The optimum flare taper is 12:1, so I drew this out on some paper and cut a piece of sheet metal to that shape and used it as a reamer on the brick (28).
A flare is not actually necessary on a burner used inside a forge but it will help to give you a very stable flame.
The insulating brick for the doors was cut to 30mm thick on the bandsaw and then a shallow groove was filed in the top and bottom edges. A 6mm round bar was fitted into the groove and tack welded in place to hold the brick into the door frame (29–31).
Making the LPG burner
If you research online you will find a lot of very good resource material on burner
I decided to use a butt-weld fitting rather than a threaded socket because it gave a very smooth inlet for the air
design. Ron Reil has some excellent information on making burners and forges (ronreil.abana.org/Forge1.shtml).
You should not attempt to make your own burner unless you are sure that you have the skills to do it safely. LPG is potentially very hazardous. The burners I describe here work very well and I have not had any problems with them, but you build them at your own risk.
It is very important to check all joints in the gas line for leaks with soapy water. If bubbles form, immediately turn off the gas and fix the problem. As LPG is heavier than air it will drop down and collect in any low-lying places such as a drain or pit. All you need is approximately two to eight per cent of LPG in the air and a source of ignition and an explosion can occur.
Jet assembly
The first step in making the burner is the jet assembly. Access to a lathe will make building this much easier. This is made from 12mm OD x 1mm wall-thickness mild steel tube x 150mm long. This needs to have a ¼BSP brass nipple fitted at one end and at the other have an M6 thread to enable a MIG-welding tip to be screwed in.
Depending on the wall thickness of your 12mm tube, an M6 nut can be hammered into the end and then Easyflo-ed in place (32). Alternatively you could Easyflo a steel plug in place and drill and tap an M6 thread in the lathe.
The brass nipple needs to be drilled out to 12mm diameter for half its length so that the 12mm steel tube can be Easyflo-ed into place (33). I use 45-per-cent silver Easyflo as a good general-purpose rod.
A ¼BSP ball valve can now be fitted to the nipple using thread tape suitable for LPG. A 0.6mm MIG-welding tip is screwed in at the other end. This completes the jet assembly (34).
Burner tube
The burner tube can now be made. I got all the fittings for this from Steel and Tube.
The first step is to turn one end of the 200mm long 20NB pipe nipple to be a neat fit into the 40x25NB concentric reducer. I decided to use a butt-weld fitting rather than a threaded socket because it gave a very smooth inlet for the air. The inside of the 20mm tube was also given a small taper (35 and 36). It was then clamped and welded (37).
I then turned up a 20mm OD x 12mm ID steel bush to hold the jet-assembly tube. It is important that the jet-assembly tube is concentric with the burner tube. By using a length of 12mm threaded rod through the assembly, I was able to hold everything in alignment as it was welded (38).
To adjust the air–gas mix, I made a sliding choke with a locking screw. With this, all the burner parts were complete, and it could then be fully assembled
Tuning the burner
To get the burner to run you will need a heavy-duty adjustable regulator (39). The gas demand is too great for a standard LPG regulator for a barbecue. I also recommend that you get a gas supplier to make up the LPG hose and fittings for you. It is a small cost compared with the possible consequences of a leak.
Once you have assembled the regulator, hose, and burner it is time to check for leaks. Set the gauge to 100kpa. With the ball valve on the torch closed, turn on the gas cylinder to pressurize the hose. Check all the joints with soapy water. A bubble will indicate a leak. If all is well, move on to lighting the burner. If you want to be able to use the burner outside the furnace then you will need to screw a 25x20NB socket in place to act as a flare.
To get the burner running in the furnace, you will need to mount the burner tube approximately 25mm into the burner hole. At that point it should be just at the beginning of the internal