Picture of 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.
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Step 1: Materials

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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

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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

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

Step 4: Flatten the steel

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

Step 5: Pound it flat

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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

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

Step 7: Clamp while drying

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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

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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

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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.

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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 (author)  cannonball17022 days ago
Thank you for trying it and for commenting,
ant0ny1 year ago
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 (author)  ant0ny1 year ago
Exactly. And, making a diagonal slice across a straight piece automatically make the angle on the cut pieces equal.
ynze2 years ago
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!


Phil B (author)  ynze2 years ago
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.
Wow. Weird that this landed in my comments, but glad it did lol. I could use such a thing :)
grbennet2 years ago
Thank you very much for this simple method.
Phil B (author)  grbennet2 years ago
I hope it helps you. Thank you for your comment.
Learndy6 years ago
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 (author)  Learndy6 years ago
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 Phil B2 years ago
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 (author)  Col_uk2 years ago
Very clever. Thanks for your reply.
Phil B (author)  Phil B6 years ago
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."
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.
streetrod53 years ago
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 (author)  streetrod53 years ago
Thank you for looking and for your comment. I am glad to have solved a problem for you.
bettertimes4 years ago
If you would enjoy reading "The Boy Mechanic: Volume 1 by Popular Mechanics Co. " here is the down load link
this is a copy from the Deridder, Louisiana Library. enjoy
Phil B (author)  bettertimes3 years ago
I did download it and have looked at some of it. When I was growing up our public library had a couple of books by Alfred P. Morgan. One was on electricity. One was on all sorts of mechanical devices. I remember a small steam engine with bell cranks to operate sliding valves. The projects were marvelous, but always required tools and materials I did not have available. Thanks.
DoDo7293 years ago
Wow, so simple but works incredible. Solved one of my problems. Thanks for your post. Love this site.
Phil B (author)  DoDo7293 years ago
I am very pleased you found it and can use it. Instructables is a great place to share useful things. Thank you for looking.
nmh0105 Phil B3 years ago
Simple but yet great use... i could not follow you please help
Phil B (author)  nmh01053 years ago
Can you say what exactly you did not understand?
Creativeman4 years ago
I just found this, will definitely make one! Will post picture when done. Thanks for the idea/concept.
Phil B (author)  Creativeman4 years ago
Thank you. You may even find a way to improve on it.
wmarin Phil B4 years ago
You are still my hero !. Thanks
Phil B (author)  wmarin4 years ago
Thank you.
pgcagdol4 years ago
thanks great idea and just when i needed it
Phil B (author)  pgcagdol4 years ago
Thank you. I try to post useful things.
nice job..thanks..
Phil B (author)  zulhanifarifin4 years ago
Thanks. I hope it is useful to you.
Chadworkz5 years ago
You know, it's always the simplest things that are the best, and this just proved that point...excellent job, Phil! ;)
Phil B (author)  Chadworkz5 years ago
Thank you.  Unfortunately, it usually takes me a few attempts before I come to a really simple version of an idea.
una mente creativa ... Lo seplice muove il mondo benfatto!!!
Phil B (author)  roberto sirigu5 years ago
Grazie mille!
ignilc5 years ago

The angle doesn't need to be 45 degrees. It can be anything. As long as the angles on both pieces of wood are the same it will work well.

Phil B (author)  ignilc5 years ago
You are quite correct.  Thank you for your comment and thank you for looking at this.
This is very helpful :) thanks
Phil B (author)  mcanderson766 years ago
This is one of those things that makes me think, why didn't I thinkof that?! A straight forward solution to an annoying problem. Nice guide.
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