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I work among a group of tool addicts, and I fit right in because I am one too. My tool dependency manifests itself in a couple of ways; I hoard the wrenches for routers, but the real monkey on my back is layout tools. My grandfather was a tool and die maker and one of my earliest memories of him is being shown how a micrometer worked. When I was six, I really didn’t care if one of the hairs on my head was thicker than a piece of paper, but as I grew older I came to appreciate good tools as well as the importance of careful and accurate layout. When it comes to drawing circles, there is no substitute for a good set of trammel points.
Compasses work well if you need to draw a small circle, but when the radius is more than a few inches, they become fussy and inaccurate. You have to hold everything just right and hope that nothing slips as you swing the arc. When you set a compass down it takes up a lot of space, and bumping one of the legs can make a compass setting slip. About ten years ago my traditional trammels disappeared, and one of the other lessons my grandfather taught me, frugality kicked in. I saved about twenty bucks and bought a tri-scribe. It was a decision I’ve never regretted, and between the numerous tools I own for drawing circles, this is my favorite.
The two heads lock securely on a steel rule, or any other thin, flat object. They lock securely and easily, and lay flat on the bench. The pencil can be replaced with a steel point or a knife blade, and when not in use the two heads store in minimal space. It’s an easy to use, well-made tool that not many people know about. It has some other functions as well, you can use the pencil holder with the standard head of a combination square. If you need to draw circles, you need to have this tool.
–Robert W. Lang
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I agree that these can be great layout tools. I frequently help with theater set construction in my hometown and have fabricated a set of trammels from doubled up 3/4 birch plywood, which will slide onto any of my levels. This gives the option of almost 72" with my longest level. Since most beams are about the same size this allows me to use it on a tool of just the length I want without being too awkward. I uses to use string but that often stretches and looses accuracy. Trammels are extremely useful.