From DIYinfo.org
[edit] Trammel Heads
Have you ever looked up in awe, at something like this Gothic rose window, in the Strasbourg Cathedral. You would have to wonder how the hell they did it? No computers or calculators in those days.
(Click on image for a larger view and source details.}
Well their main setting out tool was the trammel. The two trammel heads on the right when clamped to varying lengths battens can scribe arcs, divide lines, divide the circumference of circles and thereby define angles etc etc.
(Note, the two in the photos were NOT used on Strasbourg, they just look like it :-) I know, I should clean up my tools, "so much to do, so little time".
I am not real sharp at mental arithmetic, (not much better on paper actually) but with a trammel I can do remarkably accurate setting out.
If you look at the photos of the trammel heads you will see that the points are not central to to the round steel pointed bars. The point is centered on one edge of the round bar, so that by rotating the bar slightly, very fine changes to the distance can be made.
Lets say that I am working out bush, and my calculator is stuffed. I could work out a stair by drawing it out full scale on a concrete floor, just a big triangle, total height, total going.
I would measure the diagonal length, and then I have to divided that length into the number of steps, say I have to divide it by 17 to mark out my strings. I do a rough mental division and get within 10mm. (Of course you guys that live in imperial countries will have already worked it out to the nearest 64th of an inch by this time :-).
I get my trammel heads,and set them on a bit of skinny timber about the right distance apart, and mark off from one point to the next, pivoting around alternate points till I've done 17. Sure enough I was a bit out so I adjust the trammel head and do it again, until it is spot on.
Easy peasy, how to divide a line into a given number of segments without even a tape measure. Trial and error. But accurate.
Now for the 99.9% of you that have read this far and don't have your own set of trammel heads, here's a tip. A length of batten with two nails driven through it is can be a trammel. Just bend the nails slightly to get fine increments. Easy. You can use it for stairs or Gothic windows:-)
[edit] Author
Bill Bradley
