This Instructable shows how to make a precision center finder with fairly common tools found in the average home workshop.

A center finder made by DrPeper got me thinking about a design very similar to his, but that would not require a laser cutter to make.

Although you cannot see all of my center finder in the photo, you can see the pencil marks I made with it on the top of a plastic bottle cap.

Materials

1/2 inch plywood
Screw
1/4 inch aluminum rod

Tools

Table saw
Drill press
C clamps
Hacksaw
Grinder or file
Vise
Bandsaw

Step 1: Layout the Blank

The blank for this center finder is a piece of 1/2 inch plywood about 4 inches wide and 5 or more inches long. As shown by the pencil lines in the photo, mark a rectangle to be cut out of the blank.

Step 2: Saw the Blank

I used a table saw to make a straight cut that will be used to mark the center lines.

Step 3: Saw the Rest of the Blank

Then I sawed out the rectangle marked in step 1 and I cut the blank from the rest of the plywood.

Step 4: Prepare the Drilling Guide

I used the rest of my plywood piece to make a base for a drilling guide. I marked a center line. I needed a stop and used a screw from the underside. The head of the screw fits into a countersink hole so the underside is smooth and flush.

Step 5: Clamp a Straightedge to the Drilling Guide

I pushed the center finder blank against the screw stop. The part of the blank that will be used to mark pencil lines follows the center pencil line on the drilling guide. Two C clamps hold a straightedge piece firmly on the drilling guide. A special clamp holds the drilling guide in place on the drill press table. (The drill bit is inside the first hole to be drilled in the center finder blank.) Drill the first hole with a 1/4 inch bit.

This center finder will have a wooden edge. If you want a steel edge for better wear, attach a piece of 1/8 x 1/2 inch steel bar to the face of the wooden edge before placing the center finder blank against the straightedge piece. Countersink for the screw heads so they are flush.

Step 6: Clamp a New Straightedge Piece

Remove the center finder blank. Place a second straightedge piece against the first. Clamp it to the drilling guide. Loosen the C clamps on the first straightedge piece and remove it.

Step 7: Drilling the Second Hole

Place the center finder blank against the screw stop and against the straightedge piece in place now. Drill the second hole. Its location will mirror the location of the first hole.

Step 8: Insert Aluminum Rod Pieces

I cut two pieces of 1/4 inch aluminum rod about 3/4 inch long and chamfered the ends of the rods. Then I pressed the rods into the holes in the center finder blank with a vise.

Step 9: Trim

I marked and trimmed the profile of the center finder with a bandsaw. The second photo shows the profile of the center finder when finished.

Step 10: Use It

Press the aluminum rods firmly against the round object whose center you wish to find. Do this from three positions to be certain. Notice that all three intersect at the same point.

In April 2009 I did another Instructable on making a center finder. That center finder required fewer workshop operations to make and will work with relatively small round objects through fairly large round objects. This center finder will not work with objects less than a certain minimum diameter. But, that diameter can be selected by you through deciding how far apart the aluminum rods should be.