loading

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.

<p>good work and idea </p>
Thank you. After posting this I was in Harbor Freight Tools and saw a steel divider with a tube on one leg for adding a pencil and using it as a compass. Unfortunately, the tube was not large enough for a marker pen.
<p>good one</p>
<p>Thank you. Welcome to Instructables.</p>
<p>I like it. Much better than the things I have cobbled up over the years!</p>
Thanks.
<p>Why do you even need a Compass? I think a Caliper can do the work as good, Or even better!</p>
I suppose yu can do that. Did yu mean scratching an arc with the caliper or attaching a pen to it? I like to keep measuring tools for measuring because using them as other tools might compromise their accuracy by bending or weakening them.
<p>Both of them work...</p><p>My caliper is pretty old, And impossible to scratch. I've tried scratching my caliper, But it just ends up scratching the other tool...</p><p>I can easily use it to mark things on metal</p>

About This Instructable

1,846views

22favorites

License:

Bio: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying ... More »
More by Phil B:Easy Monitor for NordicTrack Skier Uses for Spent K-Cups Make a Conduit Bender 
Add instructable to: