Sometimes I need to scribe an arc on metal with a compass. I made the compass on the left to hold a fine point marking pen or a lead pencil. It can also function as a divider with replaceable points made from common finish nails. I need another compass for a second location. Someone in my family gave me the black plastic compass on the right. I want to adapt it to hold a marking pen, too. This Instructable will describe a simple adapter anyone can make without any welding.

Tools

Drill
Saw
Digital caliper

Materials

5/16 inch dowel rod
1 x 2 wood

Step 1: Open the Pencil Holder

The plastic on this compass is molded to fit around a wooden pencil with six flat sides. If I loosen the two brass knobs sufficiently, I can slip a 5/16 inch dowel rod into the pencil holder.

Step 2: Drill Holes

Drill one hole in the 1 x 2 wood that is 5/16 inch in diameter.

I used a digital caliper to measure the part of the barrel on the marker pen just behind the tip. It is tapered and ranged from about 0.360 inch to about 0.388 inch. Te second photo shows my drill index. A 3/8 inch bit falls nicely in this range. I drilled a 3/8 inch hole. The pen will press into the hole and fit like a wedge.

Step 3: Fit to the Compass

I cut the dowel rod and slipped the 1 x 2 wood onto the dowel rod in the pencil holder. Then I pressed the pen into the 3/8 inch hole. I did not glue the dowel into the 1 x 2, but I could have.

Step 4: Use

A marker pen leaves a more visible mark, especially on sheet steel, than does a soft lead pencil.

Problems to consider are: 1) The point of the compass can slip, and 2) The tip dries out quickly. A patch of duct tape at the center of the arc would help. I try to keep the pen capped until I am ready to use it. It also helps to make a few strokes on paper until it begins to write well.

Step 5: Minimum Radius

This compass attachment works well if the radius is not too small. Even when folding the attachment around to shorten the distance between the point and the tip of the pen, the minimum radius is about one inch. I could have drilled the two holes in the 1 x 2 a bit closer together. I could also have angled the hole into which the pen is press mounted.

I looked at a common steel compass like many of us used in grade school. It is weak and flimsy. Its adjustment is loose. The 5/16 inch dowel almost fit into the pencil holder, and would if the dowel were shaved with a knife and some sandpaper. Still, I would not recommend such a compass unless it is dependable.

This is a quick and easy way to scribe an arc or a circle on a piece of steel more easily and make it easier to see than is the case with a lead pencil.