On Notching, or the Joys of the Hole Saw
This might surprise some of you, and it was certainly a surprise to me, but my favourite thing about the framebuilding experience so far has been learning about all the machines in Mike Flanigan's shop. I have never been into this sort of thing before, preferring to use the simplest tools possible for DIY stuff. The "shop atmosphere," with its various lathes and saws spinning menacingly, has always confused and intimidated me. I am not sure what changed now. Maybe the part of my brain that's responsible for this kind of stuff is just now maturing, but suddenly I am like a kid in a candy store. The machines are super-useful, physically easy to operate, and are proving to be excellent teaching tools when it comes to mechanical concepts I find difficult to grasp. For example: notching!
Building a bicycle frame is mainly about joining together pieces of tubing, which involves a lot more work than merely brazing or welding the joints. For instance, before the tubes can be joined they need to fit together properly.
Think about it: When we get a tube, its edge is cut straight across. But if we want that edge to join the rounded surface of another tube, we need to sort of scoop out the center of that opening, to scallop it. This is what's called notching - also known as mitering or coping.
And because the tubes join at an angle that is almost never 90°, the notch must be done asymmetrically, to fit the precise angle. In theory, I understood the concept, but in practice I had a hard time imagining concretely how this was to be done. How were builders able to determine the shape of the scalloped edge with such precision and draw it on the edge of the tube? Too embarrassed to ask this question, I tried to read up on it. Unfortunately, the more I read, the more confused I got.
But the mystery was cleared up in a matter of seconds once it was time to notch my own tubes. Mike has what's called a horizontal milling machine, which can be fitted with all sorts of tooling - including hole saws of various diameters.
A hole saw is literally a round saw that makes holes in things. They are available in a variety of diameters. You choose the saw that matches the diameter of the dominant tube - the one to which you will be joining the tube you want notched.
After attaching the correct saw and installing the to-be-notched tube in the clamp, you then set the angle of the joint, according to your bicycle frame's geometry.
And that's it. As you turn the crank, the hole saw makes its way through the edge of the tube and notches it. Basically the saw forces the shape of the dominant tube through at the correct angle. Watching this happening I experienced a sudden flash of understanding and it was immensely satisfying.
Of course, by far not everyone who builds bicycle frames has this type of machine handy, and the low-tech notching method involves using lugs to make guide marks, then a hacksaw to make the cut. But even if I never have access to such machines again, operating them has done me more good than I can express.
I love the clean look of a notched frame; the way everything fits together perfectly and makes total visual sense before you get it all filthy with flux and leaky brazing marks. But also, watching the tubes fit and actually getting how and why they fit is wonderful. My head is bursting with the sudden understanding of concepts I've previously struggled with, and that is an exciting feeling to have. Whether it's framebuilding or any other subject-matter, it is never too late to learn new things.