Finding the middle of a circle

Jakko

[ˈjɑ.ko]
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I made a Centre finder as an apprentice 50 years ago,still using it now.
That’s a device I had never even heard of, and didn’t know how it works until somebody posted a picture of it somewhere on here. I suppose it’s one of those things where if you’ve had certain types of education and/or are in certain lines of business, you know all about it — but if you’re not, then you probably don’t even know such a thing exists.

as an ex art director I'm used to eyeballing the center. ;)
Eyeballing it works fine if you’re trying to, say, put something in the middle of a diorama base. If you want to put something exactly in the middle of an item like a turret floor, though, you’ll probably be quite a bit far out of the real centre :smiling3:
 

Martin64

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Important Note: The method I explain below only works because I happened to get lucky drawing the triangle. See further down this thread for a way that does actually work

I needed to saw a disc (actually a wargames base) in two equal halves, but it didn’t have a clear centre point visible on it.

View attachment 298589

Here’s how to do it. First, draw a triangle whose points are on the disc’s circumference:

View attachment 298590

Make this as large as you can, meaning it’ll be approximately equilateral — though there’s no need to make the corners exactly 60°. Just eyeballing it will be accurate enough. (You could draw the triangle much narrower, but you’ll make things easier for yourself if you don’t :smiling3:)

Next, from each of the triangle’s corners, draw a line that’s at a 90° angle to the opposite side:

View attachment 298591

These three lines should all cross the same point: the so-called orthocenter of the triangle. Because the triangle’s points are on its circumference, this coincides with the centre of the disc.

In my case I just had to saw straight through that to get two halves:

View attachment 298592

but you could of course also use it to drill a hole for an axle, glue something dead centre, or for anything else that requires you to find the middle of a circle.

(With the added note that for cutting a disc in halves, you don’t even need to draw six lines: three will do. Draw two sides of the triangle and then the third line from one of the “open” corners to the line opposite. This will go through the centre of the disc if you do it right. It’s just that it’s easier to spot errors if you do draw all six lines, because you can see whether the ones in the middle all go through the same point.)
[/QUOT
 

John Race

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This was interesting to re read and the methods we use. From Dave's mathematics to 570s method, which as another thick ex joiner I would and have used .
 

adt70hk

I know its a bit sad but I like quickbuild kits!!!
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Just found this, John thanks for giving it a nudge. Very helpful. Always wondered how to do this.

Now I know!

Thanks Jakko.

Andrew
 

John Race

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Just found this, John thanks for giving it a nudge. Very helpful. Always wondered how to do this.

Now I know!

Thanks Jakko.

Andrew
Andrew, yes it's one of those things you never think about until the need arises. Thanks to Jakko as you rightly say .
 
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