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Occasionally I need to draw circles or arcs of a specific size, usually smaller ones. In the past I’ve used various ways to do this, often searching for the right size round thing in my shop or surrounding area. Much time can be burned up that way and after looking high and low, I still might not find the diameter I need.
For an exact size, my favourite was to simply cut a thin strip of wood to length that is equal to the radius of the circle and swing that around a finish nail driven into the centre as a pivot. This works well but lacks the convenience and speed of a purpose built tool, something that can be quickly adjusted and used over and over.
A while ago I bought a nice compass for sketching ideas on paper, to use at my desk. As expected, at some point it found its way into my shop and now has wound up in this debilitating condition:
It got broke! Such is the state of modern manufacturing when a flimsy cast part is used and can’t stand up to just a few hundred pounds of wood and shop tools piled on it.
With this tragedy and the immediate need to draw 3-1/2″ circles, I decided to make a compass for shop use. I thought about it, did some sketches, mocked up some parts and settled on the final design. While building it, I made a video of the proceedings:
To get started, I cut two strips of hardwood, leaving them long. I then cut them to length and kept the off-cut ends. These pieces were then glued in at each end of the “beam”:
After the glue had dried, I sanded the beam smooth on my belt sander platform, and trimmed the ends off:
Marked and drilled the 5/16″ hole for the pencil and the 7/32″ hole for the clamping screw:
To make it so that it will clamp tightly on the pencil, I cut a slot in the end:
This can also be done on a band saw or by hand if you are squeamish about doing it this way.
With the beam finished, I made the pivot pin. It’s a 1/4″ bolt, cut to length and sharpened to a point:
The finished pivot:
Some test circles:
Works great! Infinitely adjustable within its range and very easy to make and use. It’s already a valuable addition to the shop and I expect it’ll be used often, assuming I can find it. I’ll probably paint the beam a bright colour, yellow, for example, to make it more visible.
If you prefer an easy to print pdf version, Jay Bates took the time to make this one from the SketchUp file available above:
It’s a single page to print and take the the shop with you. Thanks Jay!