# The Disc Center Finder

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I work with mixed media and sometimes I need to find the center of a disc.  If I made the disc myself, it was laid out from the center, but if it's say a round baking pan or a lid, I need to find it.

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## Step 1: Methods and Parts

One method of finding the center is with two squares, but that takes alignment, coordination, and the aforementioned two squares. Another method involves a compass to make several smaller circles and draw lines through their intersections. Yet another method involes drawing tangents, which are square to each other and then connecting to corners diagonally. Worst of all ... equations ... yuck.

Anyway, my solution uses plywood and hardboard (of course).

## Step 2: Cutting the Parts

I made mine from 3/4" plywood, ripped to 1 1/2" widths. Two true miters were made with a miter sled on a table saw. I then cut these to length (7 3/4") at the same time using a miter saw.

## Step 3:

These two pieces got glued to the hardboard as seen above. I squared to one side and then trimmed off the excess on the table saw after the glue dried. I cut the hardboard out of the V notch using a band saw. I also nipped off the points a little (you'll see) so I wouldn't stab myself in the future.

## Step 4:

Lastly, attach another 1 1/2" strip to one side. I used screws and no glue just in case I needed to replace it after time, because I could drop it or something. For me, this piece was 18" long, but can be whatever you want.

## Step 5: Mark the Center

Just slide the disc into the V notch, mark a line, rotate the disc, mark a second line.

It can also quickly find the center of squares, as long as the extension can reach the middle of the stock, which is much quicker than using a ruler to connect both corners and making more sanding work for yourself.

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## 27 Discussions

Nice project, thanks. Your method is better (in some ways) than using a machinist's V head square, which is small for large items.

But I initially stopped at the first method, with carpenter's square and speed square. I think I understand the method, do you just measure out 1/2 way, draw a line, and repeat?

2 replies

I use it to make a 2-3 intersecting lines in the center.

Thanks - I should have figured that out, obviously did not.

Excellent! and a simple tool all wood workers need in the shop! Thanx...I made mine using a Home Depot wood rules that slides out for some reach. Plus I made a small version for my drawing table.

I years of procrastinating I finally purchased a combination square set that has the v head to find the center of a round object. Making this makes more sense so it could easily be hungg ont the wall ready to go, and if it where to fall there would be no concern of warping a metal blade like the combo square has.

Nice, I just came across this problem myself. I was going to buy the proper tool but they are all a bit expensive for a one off job and even making the jig looked too complicated.

In the end all I did was get a piece of paper, put the circles on it and drew around them. Cut the circle out. Folded it edge to edge in three different places and where they intersected in the middle I stuck a hole.

Fortunately I am working with flat shapes so all I have to do is put the cut-out on top of the shape and mark through the hole in the middle.

Probably not much use to you if you are using curved round objects but might be of some use to someone.

Cool Tool. If you do not have the tool handy, you can always use the 2 chord method to find the center of any circle. Here are the steps:
Step 1.Draw 2 chords across the circle. (a chord is any straight line that goes from one point on the side of a circle to another point on the circle) Make sure that they are about 90 degrees or morefrom one another and each splits the circle into two parts (approx. 1/3, and 2/3). Remember, do this twice.
Step 2. measure and find the center of each of the chords. (bisect with a use a ruler)
Step 3. Using a protractor, find a point 90 degrees from one of the center points on one of the chords. connect the 90 dergee point with the center point of the chord. Exten this line across the circle. Repeat with the other chord's center point.
Step 4. Where the two 90 degree lines cross is the center of the circle.

http://www.onlinemathlearning.com/chords-circle.html

Instead of using Pointy plywood, you can use Perfectly Equal Diameter Discs as the Base.. and from their Contact points, you can use the vertical marking ruler..
That way, you can use it to mark the center of any circular object of any diameter, correctly anytime.. and if it gets slightly damaged on one side, you can just rotate the discs, and recalibrate..
Nice Instructable and tool by the way..

Adding in some more detail.. Take a two plywood pieces, and drill a hole thru them (approximate center..)
using a bolt and nut, clamp these two plywood pieces face to face into a lathe/drill
Start the Drill, and carve equal Diameter circular plates.. (Saves the time in measuring. and any diameter will do, the edges only have to be perfectly perpendicular..
Perpendicular edges can be checked by removing the plates, and just turning one plate around and checking if the circular faces match perfectly.. if the edges are not perfectly perpendicular, (it will turn out to be a cone), and the face diameters wont match..
Just repeat the lathe step, and get it trued..
Then use your ruler setup, on a cross. the discs go on the shorter arm, while the center marking ruler is the longer arm...

Very nicely done!
Q: Why did you add the hardboard? Seems, at first glance, to add unnecessary work (to cut it out on the band saw).

Also, it should be emphasized that the sharp points have to be trimmed equally and with precision so that the centerline straightedge remains in line with the center. What method did you use?

My reasoning on the hardboard was to keep the two pieces of plywood coplanar during glue up, because I knew they would slide around a bit if I didn't. So gluing to the hardboard as a backer with the help of a few pin nails was my solution and the band saw cut was just a few seconds. Honestly, I make most of it up as I go and fail a lot.

If your miter saw is dialed in, you could use that to nip the points. Mine wasn't, so I used a small parts cross cut sled on the table saw. I'm sure it's not machined precision specs, but it's close enough for my needs.

Well, high quality ones can range i the hundreds, but even Harbor Freight sells them for very cheap. The cheapest way would be to buy just the center head and a rule separately. Generally you would buy them in a 4-piece set including a rule, square head, center head and protractor head. eBay is another great place to find used stuff for cheap.

I'm glad you've found a method that woks for you, and might help others too. But, a little tip about circles: If you bisect any line that crosses through a circle with a perpendicular line (using the circles edges for all endpoints), that new line extends through the center of the circle, and the bisection point of that line is the center of the circle. If you know this property of a circle, you can find a circle's center with almost no math and with very simple tools (though an accurate ruler and a square would help).

Great jig.
In looking at it you could also just use a chop saw and cut duplicate angles on same width, length pieces. The angles don't even necessarily need to be 45, just identical. Also making the boards wider would make for a more accurate jig, so instead of the circle resting on the tips, they're resting inside.