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’!
I’m sure there was a “Gary’s Bike (part 4)” post. It had a drawing of the frame with wheels and bars etc. It was sort of minty green. It’s not here no more. Weird.
UPDATE: thanks to the google cache it’s back
Got all the parts for Gary’s bike gathered together.
Ready to be cut mitred and prepped.
Here’s a sneak preview!
In my day job (programmer), we’re pretty ok about diving into something and ‘spiking’ something out to see if an idea’s got wings or if it’s just dumb. It’s easy to do this with something like programming as the cost of change is really low. In other words if you spend a couple of hours working on something only to come to the realization that it’s really stoopid, then you can just roll everything back and start again as if it never happened.
All you’ve lost is that two hours. In fact there’s a pretty strong likelihood that you’ll have learned a significant lesson there somewhere so you can put it down to education.
With manufacturing or construction or pretty much anything that requires investment in materials and anything that can’t be easily ‘rolled back’, this concept of ‘spiking out’ is usually called prototyping and far from being being cheap, is usually very expensive. Usually at this stage, techniques, materials and tooling costs more than the production equivalent. This in turn means that without significant investment and without a good deal of risk, it’s hard to just play around and see what comes out.
I’m kind of rambling now but what I’m getting at is that I’m pretty fastidious at producing quality specs and plans before doing any kind of fabrication work. I’m limiting the risk mostly. The negative impact of this is that as well as limiting risk (and cost), I’m also limiting imagination and in turn, limiting creative design.
So recently I’ve decided to go against convention and just make something up as I go along. The something in question is the incredibly cool and inspiration bottle trolley. Normally my oxy/acetylene tanks live inside a wooden frame that’s fixed to the ground. While this is safe, it makes it real pane to move them around. So if I’m welding or brazing, the work comes to the torch and not the other way round.
While I could throw caution to the wind and go out and spend about Â£20 on a bottle trolley, I thought I’d build one. In order to limit the risk and negate the ‘cost of change’ penalty, the main criteria was that I need to be able to build said trolley out of crap laying around the workshop.
I’ve made a start on this and work will progress over the next few days when I’ve got a couple of odd minutes to spare (I don’t want to invest any quality time in this). Stay tuned for further fascinating updates
Currently playing in iTunes: Cowboy Song by Thin Lizzy
Somebody asked me if I had a way of visualising a full bike when designing a frame. The answer is yes. The drawing I posted earlier was a stripped down drawing that makes it easy to read dimensions. Itâ€™s also possible to produce the drawing with more detail. Even including a view with a rider if needed.
Hereâ€™s a pic of the frame with more detail: