Next to bending tube, welding is high on the list of interests to anyone wanting to fabricate motorcycle frames and related parts. Makes sense…after you have the parts bent up, you need to weld them.
First issue is what welding process to use? We will limit this article to materials and material thickness found on custom motorcycles. Metals used in motorcycle can vary widely in thickness. “Gauge” sheet metal is a thickness less than ¼”. 16 Gauge is about 1/16” thick and higher numbers are thinner. 18 gauge is thinner than 16 gauge. Sheet metal ¼” and thicker is referred to a “plate”.
Oxy / Acetylene torch rigs are often used for aircraft frame construction with chromoly steel tubing. A proper mixture of fuel and oxygen makes a neutral flame that protects the weld from oxidation. The flame heats the metal until a puddle is formed and filler rod is hand fed into the puddle forming the weld. Dark goggles are used so the weld puddle can be seen in the bright light of the torch flame. Other benefits of an Oxy rig are the ability to heat metal for bending or attaching a cutting torch for cutting steel.
For the small shop and home shop, arc welding processes are the usually the best choice for welding. Arc welding includes Stick, MIG and TIG. Rather than go into a big discussion on how to weld or how each process works, let’s just try to find out what is best for doing the job. Consider that you need a welder capable of at least 100-140 amps to fillet weld 1/8” wall tube (lower amps will be used but this is a good number to start with). This does not include things like motor mounts, axle plates and steering necks, which are much thicker and may take more power. Proper joint fitting and multiple passes will allow a smaller welder to do heavier work (within limits). “Duty cycle” is an important consideration as this is the amount of time you can weld in a 10-minute period before the welder needs to cool down. If the welder you are using has a 50 percent duty cycle at 120 amps, you can weld for 5 minutes and then quit for 5 minutes to let the welder cool. At 200 amps, this same welder may have a 20 percent duty cycle allowing 2 minutes welding and 8 minutes cooling.
Stick welding is a low cost process compared to MIG and TIG. Stick welding for practical motorcycle fabrication is limited to welding steel. Stick welding is faster than TIG but slower than MIG. A bit of skill is required in holding the angle of the welding rod and feeding as the rod is consumed by the weld. Stick welding works well on less than clean metal but makes a bit of smoke and spatter in use and works well outdoors since there is no concern for shield gas. A stick welder with 225 amps output makes a good small shop welder capable of doing heavy gauge metals to thicker plates and shapes. Stick is not good for sheet metal work. There are small rods for sheet metal but results are not usually good. Stick creates a slag cover over the weld that needs to be chipped off (can be easy to a real pain in the butt). There is also spatter to clean off that usually has to be removed with a chisel or sander.
MIG welding is a higher cost process. The same machine can weld steel and aluminum by changing wire and shield gas. Flux core wire eliminates the need for shield gas and functions similar to stick welding, as it is not sensitive to breezes or less than clean metal and also smokes and spatters. Using a mix of CO2 and Argon reduces smoke, spatter and helps improve penetration when shielding solid core wire. Argon alone is used with aluminum. Switching between different thickness metals and aluminum may require different wire sizes and type, rollers, contact tips, torch liners and gas mixtures. Expect to get a machine with at least 210 amps output for bike fabrication. The smaller MIG welders in home improvement stores just don’t have enough power to weld even the thinner frame tubing. MIG is good for sheet metal work especially for making quick tacks. Simply pointing the wire and pulling the trigger makes a quick spot weld. There does however tend to be some excess weld buildup with MIG on sheet metal causing a lot of extra finish work. MIG also produces a bit of spatter that requires a chisel or sander to remove. Flux core makes a slag similar to stick that needs to be removed. Shielded gas / solid wire MIG makes nice clean welds that require little other finishing (other than removing spatter).
TIG welding is a higher cost process and usually the highest since the power supplies are complex and there are a number of accessory items required to start welding. TIG welding is slower that stick or MIG. There is little or no smoke and spatter with TIG. Welds are very clean and generally require no finish work. Penetration is not a strong point of TIG welding and proper joint preparation is very important. For most metals, straight argon is used for a shield gas. With straight Argon, welding can jump from most varieties of steel, to aluminum, copper, brass and a number of other metals will little more than machine adjustments and a selection of filler rods. Since TIG uses a shield gas, breezes or a fan can cause issues with the shield gas which will ruin a weld. Stick and MIG welding are accomplished by holding an arc or wire length, angle of electrode or wire and travel speed. TIG also involves these elements plus feeding filler wire with one hand and controlling arc start and amperage with a foot control (or torch mounted finger control). These extra elements make TIG a bit harder to master than Stick or MIG.
Welding the thin (about 1/8”) wall of motorcycle frame tubing is probably best done with TIG. The process is slow and very controlled. With proper application, the other arc processes will weld this size material but will require extra special attention to making good welds and will have somewhat larger bead deposits. A welder should be selected by capability and not cost. Amperage and duty cycle are two main elements to consider.
More to come…