Motivation
If you haven’t been living under a rock for the last three years, you’re likely familiar with the various supply chain issues that have been plaguing just about every industry. One supply chain that has been hit particularly hard is Bicycles. Between skyrocketing demand, materials shortages, and COVID shutdowns in the places that make bikes it’s been quite difficult to get ahold of bicycle parts and frames lately. As someone who has a nagging compulsion (some would say addiction) to building up bicycles from frames, this has been frustrating to say the least.
Ah, but what if you could build your own bicycle frames? I am fortunate enough to have access to a decent TIG welder to do so, but there are a few skills that need to be mastered first. First off, I’m not that good at TIG welding. I’m not completely awful, but I have very little experience working with walls as thin as bicycle tubing- not to mention the complex shapes that tube intersections make. Rather than wasting a ton of time and money attempting this with expensive chromoly bicycle tubing, I’d like to start with some cheap low-carbon steel tube to practice TIG welding tube intersections. Before welding things together however, it’s important that the tubes fit together as closely as possible. This is called ‘coping’ or ‘notching’ or ‘mitering’ depending upon the crowd you roll with. A sane/rich person would do this with a milling machine or the appropriate jig with a hole saw attached. Unfortunately I am neither sane nor rich and milling machines and jigs that don’t fall apart are rather expensive so I will be attempting this with saws and files- you know, for fun. I won’t be going into this without help though as I can make templates in CAD for marking out the tubes and cut them with the Wazer!
Modeling in CAD
Let’s start by modeling the desired tube intersection that we want to practice with. I’m going with 56 degrees here since this is probably the most extreme angle that one would encounter when welding tubes for a bicycle.
First, we’ll model the pipe lengths and angles as simple lines.
Next, we’ll create the models of both pipes with the swept feature tool. In this case we’re making each pipe 1” in outside diameter with 0.049” walls to match the practice material.
Now we’ll use the outer surface of the non-coped tube to cut the cope on the angled tube
This leaves us with an accurate model of the coped pipe.
Note that the outer edges of the cope are impossibly thin and pointy. This is not easy to do with files, nor is it practical for TIG welding as the thin, sharp material will just cause issues with the TIG arc and possibly cause material blowouts. I will address this later, but we will proceed with the naïve implementation for now.
To create the template we’ll create a new part that is just a thickened sleeve surrounding the modeled tube. I’ve set this to 0.010” arbitrarily. After creating the sleeve, we’ll use a sketch to slice a small section out of it.
After creating this slice, we now have a geometry that can be unrolled as a sheet metal part.
At this point we can export the face of this to a DXF file for export and cutting on the Wazer.
Cutting Template on Wazer
I’ve chosen a 0.005” thick spring steel for the template as it should cut relatively cleanly and can be wrapped around the tube with no distortion to mark out the cuts.
Since this is thin material, I’ve left a small tab on the edge of the pattern. This clips off easily with a set of diagonal cutters.
Prepping and Welding Tubes
Once the template is cut out and cleaned up, we can now use it to mark out our tube. I’ve used some TESA 4298 strapping tape to pull it taught. This stuff is incredibly strong and tacky for our purposes (Fun fact: this tape is the same type that’s used for taping tubeless rims on bicycles. Stan’s, DT Swiss, and other bicycle brands are usually just selling you a rebranded roll of this stuff cut to size. Now that’s not to say that it’s a complete rip-off- they do cut it to widths that aren’t really available by default, but if you’re taping an 18mm internal-width rim, a ¾” roll of strapping tape will work quite well)
Once the tube is marked out, I used a hacksaw to make the initial cuts
Once the larger pieces of metal are cut off, I used files to get the pipe end into shape to mate with the side of the tube.
On my first attempt I was a bit overzealous with removing the flanks of the material and as a result there was a large gap between the two pipes that was left.
I was able to patch this up with the TIG welder, but as you can see, the results at the end were less than satisfactory.
After a few tries I was able to get the two pipes fitting nice and snug. You want to make sure that you can’t fit a fingernail in the joint before it’s welded. These tubes were scuffed with brown Scotchbrite and cleaned off with Acetone before being welded.
The final welded product can be seen below. This was maybe the third attempt or so. The welds are far less blobby and inconsistent than the first try.
That’s all for now- I believe the next task on the docket is creating dropouts to weld onto the frame which should be fun? In the mean time, let me know if you have any questions 
Till next time, -G