One more trick

At some point during your woodworking career, you’re going to have to determine the radius required to create a certain arc (or circular segment).  You may want to put arched windows or doors in that new castle you’re building or you may simply want to make a few arched topped panels for a cabinet or chest.  Well, as with many things in life, it turns out that there are two ways to get to the same place.  The first, and quicker of the two is by mathematical calculation:

The second method, although more time-consuming, works well when you have to lay out multiples of the same segment, in larger scale, i.e. laying out rough openings, building curved walls, etc.  A nail is simply driven at both ends of the chord.  A third nail or simply a mark is set to indicate the TDC of the arc (arc height).  A triangle is constructed from lathe, plywood strips, or straight furring strips.  Each “leg” of the triangle should be somewhat longer than the chord length.  A pencil or nail is affixed at the apex of the triangle.  When the triangle is held against the nails at the ends of the chord and moved right and left, the segment is described.  This technique is especially useful when there is insufficient space to lay out the arc using a line or beam to swing the radius.

QED

BTW, if there’s wood lathe laying around on the job you’re working on, you’re taking way too long to complete it!

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