Make a Center Finder

Make a precision center finder from scrap materials.

Recently egbertfitzwilly posted an Instructable on Finding the Center of a Circle. WirelessMonk posted a link to a machined steel center finder. Rimar mentioned making one. I decided to try making one that would be very precise.

Step 1: Materials

I had a steel bracket left over from something. I also had a piece of wood with good straight edges. In addition I used a little wood glue and a couple of short screws.

Step 2: Cut the Wood at an Angle

I cut a 45 degree angle across the piece of wood. By cutting it as shown I needed to make only one angle cut for two pieces.

Step 3: The Second Wood Cut

Lay the free piece over the main piece of stock. Mark and cut for the second piece.

Step 4: Flatten the Steel

My scrap piece of steel needed to be flattened. I began by squeezing it with my vise.

Step 5: Pound It Flat

The vise did not do all that was necessary to flatten the steel. Use a ball peen hammer on a flat surface. Pound from both sides to make it as flat as possible.

Step 6: Glue the Wood Pieces

Smear wood glue on the facing edges of the wood pieces.

Step 7: Clamp While Drying

Clamp the wood pieces to a flat surface. A piece of paper from the newspaper or the phone book keeps the glue from sticking to the flat surface. I placed the flat piece of steel under one of the pieces of wood before clamping to lift it so there would be a raised edge at the joint that I could use to align the piece of steel when fastening it to the wood. The edge of the steel will need to bisect exactly the angle formed by the two pieces of wood. The glue line is on the line that bisects the angle. While the glue is not yet hardened scrape away the excess glue with a chisel so the raised edge is clean and sharp.

Step 8: Attach the Steel

When the glue is dry, press the piece of steel against the raised edge. Clamp the steel to the wood so it does not move while you drill and attach the steel to the wood with screws.

Step 9: Use

No allowance has been made for the thickness of a pencil line. I like to use a knife to score a very fine line. Make three or more score lines from different positions around the work piece to see if your center finder is truly accurate. If it is not completely accurate, you will get a small triangle, which also marks the center rather well. I chose steel so it would stand up to the wear imposed on it from the knife I use for marking.

If you want to try another version of a center finder made without laser cutters, etc.; try this link.

Comments

[leames1]

This is great. So simple, yet so accurate. And zero cost to boot.

[Phil B]

Thank you for looking and for commenting. I am glad you can use it. When I use mine, I make three scribe marks. I wish I could say all of them intersect at the same point, but they do leave a tiny triangle. The open area in it is a very good indication of the center. Also, be aware some wooden rods appear round, but are actually a little egg shaped. That can also lead to confusing indications of the center.

[ChandrakanthM]

Good work done.

But, to me center finding ruler is quite simple and is more precise.

[cannonball1702]

Hi Phil! I made this handy little tool last night to solve the conundrum of finding the exact center of a 1 1/4" iron cap that I need to drill a hole in to mount a toggle switch. The beauty of this thing is that it can handle circles that aren't flat on top and are dome-shaped, etc. I used a Kreg Jig and pocket screws to join the wood because I was too excited to use it to wait for glue to dry! Used a 12" Simpson strong tie strap for the straight edge part. Thanks for sharing your knowledge and for solving my problem!

[Phil B]

Thank you for trying it and for commenting,

[ant0ny]

Great idea! Does not even matter what angle of the cut - 30, 40 or 47,358 degree! :) Still glued line will divide the angle in half.
Thank you!

[Phil B]

Exactly. And, making a diagonal slice across a straight piece automatically make the angle on the cut pieces equal.

[ynze]

Very smart! I bumped into the problem of finding the center of round wood recently. I just noticed this I'ble (first picture), and.... Owwwwwww, of course, that's the way!

Thanks!

Y.

[Phil B]

Thank you for looking. I am glad it is helpful to you. It was fun to develop and I use mine more than I thought I would.

[barefootbohemian]

Wow. Weird that this landed in my comments, but glad it did lol. I could use such a thing :)

[grbennet]

Thank you very much for this simple method.

[Phil B]

I hope it helps you. Thank you for your comment.

[Learndy]

Can we extend this mechanism somehow to find the center axis of a sphere, of a ball?

Application: Find a center axis of a table tennis ball to make it a cheap wheel for a small robot.
--
Airspace V - international hangar flying!
http://www.airspace-v.com/ggadgets for tools & toys

[Phil B]

Here is an idea for finding the center of a sphere, like a table tennis ball. For the sake of illustration, your table tennis ball is red in color. Cut a collar of PVC that is just a little smaller in diameter than the table tennis ball. The PVC collar is shown in cutaway and is gray in color. Cut it to length so the top of the ball is roughly even with the top of the ball when the collar is resting on the ball. Use the center finder to mark the center from a couple of directions. You will need to turn the table tennis ball over to mark the extension of the axis. Do that by making a circle on a piece of paper, the diameter of which is the same as the PVC collar. Mark the center of the circle. Align the center you marked on the ball with the dot on the paper. Support the ball from several sides so the center do on the ball remains squarely on the center dot marked on the paper. Place the PVC collar on top of the ball so it is level. Without moving anything, use the center finder to locate the extension of the axis.

[Col_uk]

Hi Phil
Thanks for the instructable, it came in very useful.
For a sphere, i found it easier to make a center finder the same depth as the ball diameter and put steel on both ends. That allows you to mark both poles at the same time.

best regards

[Phil B]

Very clever. Thanks for your reply.

[Phil B]

Oops! "Cut it to length so the top of the ball is roughly even with the top of the ball when the collar is resting on the ball." should read, "Cut it to length so the top of the collar is roughly even with the top of the ball when the collar is resting on the ball."

[Datawolf]

Use the center finder to draw a first "equator" line. Turn the ball and use the center finder to draw a "meridian" line.
The two intersections (the "poles") give you the axe for an axle.

[streetrod5]

Phil, this is something I've needed for years, but didn't know it existed! I've been "finding" centers by... well, not the most accurate way. Thank you!

[Phil B]

Thank you for looking and for your comment. I am glad to have solved a problem for you.