Fabricating the Accelerator

The accelerator consists of the following parts:

• MIG contact tip
• MIG gas diffuser
• 1/4"ID x 6"brass pipe nipple
• 1/2" male-female bushing

These are the unmodified parts as show below.

The outside diameter of the right end of the MIG gas diffuser is about about 0.005" smaller than the inside diameter of the brass pipe nipple.  If we cut the diffuser in half right at the gas holes, we'll have a fitting with the proper thread size for the MIG contact tip.  I picked up the MIG gas diffuser from my local welding supply store for about $5 when I picked up the MIG contact tips.  This saved me a lot of time and trouble.  I simply cut the diffuser in half at the diffuser holes (discarded the piece on the left) and bevelled the cut end. 

The pipe nipple has a burr on the threaded ends that stops us from inserting the gas diffuser directly into the pipe.  If we file off this burr, the gas diffuser will flop around inside the pipe nipple and give us problems getting the MIG contact tip parallel with the burner tube.

If we leave the burr on, we can press-fit the gas diffuser into the pipe nipple.  Voila!  Our assembly is fairly parallel with the burner tube. 

Now that we have the tip of the MIG gas diffuser press-fitted into the pipe nipple, we next silver-solder the assembly.  The heat from the torch and the flux dulls the brass finish so we polished the brass with the wire wheel on our bench grinder.  

We want to streamline the business end of the gas diffuser so as to improve the air and gas flow and reduce the turbulence in the burner pipe.  We screwed our MIG tip into the threaded end of the gas diffuser, inserted the assembly into the lathe and filed it down to a streamlined point.  We made sure we didn't file into the small hole in the end of the MIG tip.

Since we'd been successful with one accelerator tube and were now on a roll, we kept going and made another 3 accelerator tubes and MIG tips, specifically some 0.023" and 0.303" tips..

We want the brass accelerator tube to slide freely inside the burner tube.  This is done by screwing the threaded female end of a 1/2" male-to-female brass bushing into the bell assembly on the end of the burner tube.  But first, we have to enlarge the hole in the brass bushing so that the accelerator slides freely inside.  If you have access to a metal lathe, you can get a nice tight fit using a boring bar.  Otherwise, it means drilling the hole out with an odd-sized drill bit - something between a 33/64" or a 17/32" bit. 


Once we have the hole bored to size, we then drill and tap three holes in the collar of the brass bushing so that we can screw in 3 set-screws.  The set-screws will hold the accelerator tube in place once we have adjusted the flame for the maximum heat.

(Insert photo of bushing - before-and-after, tapped, and set screws.)

Here's a photo of the accelerator tube inserted in the brass bushing. 

Just to recap the steps, here's a photo of what we did each step of the way.

Voila, we have now finished the accelerator tube.

(Don't throw away those discarded brass pieces!  Save them for a brass melt.)

Preparing The Burner Body and The Choke Sleeve

The burner body is a 3/4"x 10" pipe that is cut from a 3/4"x 24" pipe (we have enough pipe left over for a second burner, eh!?).  We polished the outside of the pipe with emery cloth to remove the black finish.  The inside diameter is 3/4" and we have bored it out to 7/8" with a ream. 

We next have to cut three evenly-spaced slots at the threaded end of the burner body which will allow sufficient air to be injected so as to create a very flame.  Each slot will be 5/16" wide and 2 1/2" long.  In order to get the equal spacing around the tube, we need to determine the circumference of the pipe.  We wrap some duct tape around the pipe and cut the tape.  Next we peel the duct tape from the pipe and spread it out on the table.  Using a pair of calipers, we determine that the circumference is 3.117".  Divided by 3 gives us 1.04".  We next mark off this distance from each end of the tape and on both sides. 

We wrap the marked duct tape back onto the pipe and scribe the marks onto the circumference of the pipe.  We next fill the distance on each mark with black magic marker.  This will allow us to better see the lines we will scribe on the pipe between the two lines. 

The Burner Parts

The main components for the burner consist of

• The Burner Nozzle - a stainless steel fitting that fits onto the end of the burner tube.  We're ordering this part from Larry Zoeller at Zoeller Forge 

• The Burner Tube - a 10" piece of 3/4" black steel pipe threaded at one end.  Actually it's 24" long and threaded at both ends.  But we'll cut it in half and save the second piece for another burner.  Propane fuel is introduced at the left end of the tube, mixed with air as it is propelled down the burner tube and ignited at the right end.  Three 2 1/2" slots in the left end serve to inject air into the burner tube using the venturi effect.  

• The Choke Slider - a 3" long tube piece of 1" black steel pipe that slides along the burner tube and covers the slots.  The air can be adjusted to almost no air admitted into the burner tube, or a lot of air admitted into the tube.  

•  The Bell Reducer - a 3/4"x 1/2" cast iron bell reducer which serves to hold the burner tube and the accelerator in place.  

• A 1/2" Brass Female-To-Male Bushing which has been drilled out to a bit more than 1/2".  This fitting is screwed onto the 1/2" end of the bell reducer.  The 1/2" hole allows the 1/2" pipe nipple of the accelerator to slide in-and-out of the Burner Tube - after we have bored out the threads to 1/2"and then enlarged the hole so that the accelerator tube freely slides through the hole. 

• The Accelerator introduces propane gas into the burner tube and consists of a number of brass parts: 
• a 0.023" MIG welding contact tip.  This forms the gas nozzle which injects propane gas into the burner tube (not to be confused with the burner nozzle above).  

• a threaded MIG welder gas diffuser to hold the welding contact tip.  The threaded half to the left is cut off at the gas holes and discarded.  We bevel the cut of the remaining half of the diffuser This half has an outside diameter that is almost 3/8".    

• a 6" long x 1/2" outside diameter brass nipple.  It just so happens that the inside diameter is 3/8"  (Eureka - almost (but not quite) the same ID/OD diameter as the MIG gas diffuser!!).  We'll press-fit the gas diffuser into the nipple and braze it in place. 

So, putting all of these bits and pieces together will go like this:

Here's what all of these raw parts look like after we've finished working on them.

 And when we get the bits and pieces assembled.

We also need a whole bunch of other parts to connect the Mikey burner to a bottle of propane gas.  Without limiting the generality of the foregoing, these parts consist of

• a stop-cock to quickly turn the propane gas on-and-off
• connections to connect the stop-cock  to a 12'-15' propane hose. 
• a 12'-15' propane hose
• an adjustable propane regulator
• a tank of propane gas. 

We'll describe the purpose and detail of these parts as we fabricate our Mikey Burner

Burner Background - The Mikey Burner

About 18 months ago, I got interested in pattern making and melting metal to make castings.  I subsequently visited an iron foundry, read up on backyard metal casting through the Internet, and bought a number of books on the subject ranging all the way from pattern making, green sand molds, to making your own foundry and burners.  In March and August of 2010, I visited Alumalloy Castings in Scarborough, Ont and spent a few mornings watching them make castings from start-to-finish.  During this period of time, I made some patterns, acquired some green sand, made some flasks, got some sodium silicate and made some sand cores with CO2.

One of the books I got was titled "Gas Burners for Forges, Furnaces & Kilns" by Michael Porter.  Michael gave clear step-by-step instructions on how to build a naturally-aspirated (no blower) propane-fueled burner.  Manufactured from readily-available pipe fittings, the burner can melt aluminum, brass, and other metals.  The burner works so well that the backyard casting hobby has dubbed it the "Mikey Burner".  Here's a diagram of what I hope my assembled burner will look like.

A few weeks back, Joemac posted a video of his Mikey Burner on YouTube.  This inspired me to put my posterior in gear and try my hand at building my own 3/4" "Mikey Burner" using Michael's designs for the 1/2" burner.  I'm also incorporating a few modifications to the parts list to simplify fabrication of the burner using a bit of my limited knowledge of MIG welder parts.  

The main features that I like about the Mikey Burner (in addition to its simplicity and ease of fabrication) is that it is "naturally aspirated" - ie it doesn't require a separate blower and motor.  It makes use of the "venturi effect" whereby a large volume of air is sucked into the blower by the heat of the flame.  This volume of air (and hence the flame) can be controlled through a very simple choke sleeve that slides along the body of the burner.   

In addition to the flame being controlled by the choke sleeve, it can also be controlled by the "accelerator" - the part which introduces propane gas at 5-15 psi into the larger burner body.  

The propane fuel, which is at a higher pressure than the surrounding air, causes the air to be sucked into the burner tube where the propane gas is mixed with atmospheric air.  The ratio of fuel-to-air can be controlled by sliding the choke tube across the slots, or by sliding the accelerator in-or-out of the burner body through the bell reducer.  The air and propane gas are mixed and ignited at the far end of the burner tube.  The flame creates more pressure, thus drawing in more air until the end of the burner starts to roar like a jet engine.  Because of this high mixture of propane to air, high temperatures can be produced by the burner - sufficiently high to melt aluminum, bronze, and brass.

To give you an idea of how the parts go together, Joemac from the Casting Hobby discussion forum has put together a nice video of his modified Mikey Burner.


Here's how his 3/4" Mikey Burner works.
http://www.youtube.com/user/Joemac404#p/a/u/1/O2x_gFkicgA
Love that flame, eh!?