It’s pretty important when building a frame that you know where where the butts are.
A frame tube normally has a thicker part at each end. Although the outside of the tube looks the same along it’s length, the internal diameter varies. This essentially allows you to make a tube which is very light (thin in the middle) but still maintains strength (thick at the end).
When you join tubes by welding or brazing, the tubes get to an incredibly high temperature. Since the addition of heat will (in most cases) weaken the material, it’s important to minimize as much as possible, the area affected by the heat. The area that gets this hot is sometimes know as the HAZ (Heat Affected Zone).
If we were to weld two tubes together that had very thin walls (0.5mm for example) it would be harder to minimize the distortion that can occur when welding or brazing. To make things easier, we use tubes with thicker walls at the end. In a typical tube with wall thickness in the middle of say 0.5mm, you can expect the thick part at the end (the butted section) to have a wall thickness of about 0.8mm. Although we’re still talking about pretty thin-wall tubing, this additional material makes for a much better weld.
There’s a pretty good description of how butted tubes are made here: Reynolds Butted Tubing and here’s a wee diagram to help.
That’s not why I started writing this though!
The main point of the post was to illustrate how you determine where those butted sections are.
It’s important that you know where the butts are as when you cut the tubes down (not all bikes are the same size!) you want to make sure you still have a butted section at each end. Sometimes you can just trim one end to get your required tube length complete with butted section, other times you might need to trim some off each end. It’s also pretty useful to know where the butted sections are when your adding braze-ons.
One way is to simply read off the dimensions from the supplier spec sheet. Like the wee picture above. Unfortunately the spec sheet might not always be available and in my opinion it’s always better to double check these things anyway.
Another way is to clean out the inside of the tube with a good degreaser and hold it up to the light (a florescent bulb works well). If you squint and peer through the tube while running your hand down the length of the tube, you can usually tell roughly where the butts are. While this works ok, it’s not very accurate and doesn’t tell you what the actual wall thickness is.
My way is to use a little device that allows you to very accurately determine both the wall thickness and the position of the butt.
Its a rigid, U-shaped device that’s long enough to take at least half the length of most tubing we use to build with. One leg of the U-shape has an arm attached to it and attached to the arm is a standard dial indicator. The moving part of dial indicator rests on a small sphere attached near to the end of the other leg of the U-Shape.
The indicator is set to zero while resting on the sphere.
We then slide the tube we’re measuring onto the leg and let it rest on the small sphere. The indicator then rests on the tube and if we take a reading from the indicator, this gives us our first wall thickness measurement.
When we slide the tube further on the leg of the U-shape we notice where the dial changes and gives us a new reading. This gives us the new wall thickness and the exact position of the butted transitions can be marked on the outside of the tube. The process is repeated for the other end of the tube.
This little device took about an hour to make and should have been part of the ‘Just Making Stuff Up Series’!