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If you’ve ever struggled to locate the center of a circular plate or a piece of round stock using a ruler or a square, you know it can be a tricky proposition. My usual ham-fisted method involves marking the midpoint of several diameters and kind of visually averaging all those sloppy centers to get roughly in the middle of the thing. If I need more accuracy, I try to sandwich the circle between two framing squares and then draw lines between opposing corners—lines which, at least in theory, should bisect each corner at 45 degrees and intersect in the middle of the circumscribed circle. In theory.

But this cleverly-designed gauge from Robert Larson Woodworking Tools makes short work of the job, and does it more accurately, to boot. Basically, it’s just a small framing square with a built-in 45-degree rule: Fit the workpiece against the square, mark the bisector, rotate it about 90 degrees, and do it again. The lines will intersect right in the center, no matter the exact angle between them, whether you draw two or twenty.

Though marketed mainly to turners for centering stock on a lathe, I’ve already found plenty of “off-label” uses for mine on cans, jar lids, dowel rod, and so forth. The gauge is about 6″ square, and the bisector rule is 5.75″ long, which means it can be used on any round object up to 11.5″ in diameter.

The biggest thing I’ve used mine on so far is this paint can, which is 6.5″ across.

The gauge actually has two sides—one with 90-degree jaws for use on round, square, or octagonal stock, and one with 60-degree jaws for use on hexagons. At first, I was dubious about the value of using it on polygons. After all, if the shape has corners, I thought, one can just use a ruler to draw lines between them and find the center that way.

Which is true, of course, unless the corners are rounded, as in the case of this glass box. Because the center finder indexes off the edges of the profile, it works just as well where the corners themselves are indistinct or inaccessible.

Frankly, I love this thing. It’s dead simple, fun to use, works great, and saves lots of time. And for less than five bucks ($3.65 from M&M Tool and Machinery as of this writing) it pays for itself in just a couple of uses.

More:
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Sean Michael Ragan

I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I write for MAKE, serve as Technical Editor for MAKE magazine, and develop original DIY content for Make: Projects.


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Comments

  1. migpics says:

    I think this is a great tool and for the price, you can’t beat it. Looks like a good time saver (if only I could find it when I needed it!)
    Here’s how to find the center of a circle using a right triangle (at least untill you get your tool in the mail)
    http://www.mathopenref.com/constcirclecenter2.html

  2. br3ttb says:

    My first thought was “I should 3D print one of those.” But of course, thingiverse already beat me to it: http://www.thingiverse.com/thing:7465

  3. Eric says:

    I’ll have to dig mine out to verify this though I’m pretty sure you can use the same overall method that you might use a compass to find the center point of a triangle, etc., and I don’t mean by guessing the radius of the circle.

  4. Eric says:

    Here’s one method with a compass and a straight edge http://www.mathopenref.com/constcirclecenter.html

  5. dr says:

    Every once in a while I see a post here that makes me realize there are whole fields of tools you guys are unaware of.

    What you are looking at here is a pale shadow of a real combination square. Now granted, a genuine Starrett will set you back $100-$200 dollars. But it’s *precision*. Also, you can pick them up (much much) much cheaper used and at flea markets.

    1. Nah, I’ve got a combination square. Not a Starrett, mind you; in fact, quite the opposite. But I’ve never seen one with a 5.75″ arm on it. I’m sure they exist, somewhere, but I’m equally sure they cost more than $3.14.

      1. dr says:

        Absolutely they cost more than $3. They also do a lot more than just finding the center of a circular object. They true up 45 and 90 degree angles and also measure things at the very least and halfway decent ones have levels and protractors. Uni-taskers are usually a bad investment.

        A normal combination square, including the one behind my link, should have an arm of either 6″ or 12″.

  6. Rik Marshall says:

    or you can print one out if you have a makerbot!
    http://www.thingiverse.com/thing:7465

  7. Anonymous says:

    Next up: The Sine Bar.