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  #1  
Old 08-13-2009, 06:51 AM
prkaye prkaye is offline
 
Join Date: Jun 2006
Location: Ottawa, Ontario, Canada
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Default finding the center

A number of times I have had a circle, and had to locate it's exact center (for drilling a pilot hole to center a hole-saw, for example). There are a number of ways I've thought of to do this (cut out a paper template and fold it in half two ways to find the center, or use a compass). But surely somebody must have invented a tool for doing this on any size circle. Does anybody know of such a tool?
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  #2  
Old 08-13-2009, 07:06 AM
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Phil Phil is offline
 
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Location: Waco, Texas
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Default

If you've got a compass, you can follow these steps. I keep a couple of compass' in the garage. One with lead and one with a sharpie.

http://www.makeitsolar.com/science-f...cle-center.htm

Phil
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  #3  
Old 08-13-2009, 07:33 AM
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flion flion is offline
 
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Location: Flagstaff, AZ
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Default

Of course, with a ruler you can simplify it a bit:

1)Mark a point on the circumference of the circle.
2)Using a compass (or even a ruler) mark the circumference of the circle at a straight line distance of about 3/4 the diameter of the circle. (Using a compass is easier).
3) Draw a line between the two points you just made and use a ruler to mark the midpoint of the line.
4) Draw a line across the midpoint you just marked to the first mark you made (in step 1). Extend this line to the other side of the circle.
5) Use the ruler to find the midpoint on the line you drew in step 4; the midpoint will be the center of the circle.

Another method is to use a drafting triangle (the 90/45/45 one, not the 90/60/30 one). Use the edges adjacent the 90 degree angle to 'corner' the circle (put both edges tangent to the circumference and then draw the 90 degree corner). Now place the hypotenuse on one of the lines you just drew with a 45 degree angle at the corner and draw the line across the circle, which will bisect it. Either use a ruler to find the midpoint or draw another corner and 45 to bisect the circle again (where the second 45 crosses the first is, of course, the midpoint).

There was a center-finding tool that used that last method; it was a combination of a square and a rule at 45 degrees where you could just lay the squared edges adjacent to the circle and use the rule to mark the bisecting line, but I haven't seen one in a while. Can't remember if it was a drafting tool or something model-airplane related.
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  #4  
Old 08-13-2009, 07:37 AM
mdoyle mdoyle is offline
 
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Location: Xenia, OH
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Default

Thale's Theorem, all you need is a right angle (a piece of paper, drafting triangle etc.)

http://www.mathopenref.com/constcirclecenter2.html
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  #5  
Old 08-13-2009, 07:50 AM
prkaye prkaye is offline
 
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Default Thale's Theorem

Thale's Theorem... that's awesome!!!! That's my new method! Thanks!!!
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  #6  
Old 08-13-2009, 10:11 AM
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az_gila az_gila is offline
 
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Smile It was a hardware store...

Quote:
Originally Posted by flion View Post
.....
There was a center-finding tool that used that last method; it was a combination of a square and a rule at 45 degrees where you could just lay the squared edges adjacent to the circle and use the rule to mark the bisecting line, but I haven't seen one in a while. Can't remember if it was a drafting tool or something model-airplane related.

...item made by Starrett...



http://www.mcfeelys.com/product/MS-2...r-Finding-Head
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  #7  
Old 08-13-2009, 01:34 PM
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Mike S Mike S is offline
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Default

Harbor Fright has a couple of them, link

Not the quality of the Starrett, but for occasional use, should be fine.
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  #8  
Old 08-14-2009, 03:41 AM
RV10Man RV10Man is offline
 
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Location: Oklahoma City, OK
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Default re: free

I like Thales the best. The price is certainly right.

Marshall Alexander
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  #9  
Old 08-14-2009, 06:05 AM
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jonbakerok jonbakerok is offline
 
Join Date: Jan 2005
Location: Houston
Posts: 361
Default Stencil for drawing circles

For small circles (less than 3 inches) I use a plastic circle stencil, purchased from a drafting or art supply store, for a couple of bucks. I think Hobby Lobby carries a few. It's much easier to use and usually more accurate than a compass.

Each circle size has index markers drawn at the edges to help line it up. So you start by drawing two intersecting lines with the center at the intersection. Then you place the stencil so that the lines line up with the index markers, and then just trace the circle. To find the center of an existing circle, you just do the reverse. Place the stencil over the circle and draw two intersecting lines using the index marks as guides.

I use this tool all the time for laying out fabricated parts. I even use it to draw the radiuses on corners so they all come out the same.
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  #10  
Old 08-14-2009, 09:51 AM
RFazio RFazio is offline
 
Join Date: Jan 2007
Location: LI, NY
Posts: 415
Default You want one of these

Check this one out.


http://littlemachineshop.com/product...=center+finder
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