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.)