How would one measure radii?

[Monkeyvoodoo]

I was wondering if anyone knew how to measure the radius of a curved edge. I know I can replicate to another piece of wood but I'm looking to create a blueprint for my zback so others can make their own.

[shponglefan]

When you say "measure" what exactly are you referring to?  Are you talking about which point to measure from?

For example, I cut a curved edge at the bottom of my CP template which was made using a 7 foot jig.  So if the curve was part of a full circle, the radius would be 7.5 feet.  I'm sure if that's exactly how one would measure it, but in terms of using jig, that would give the point of rotation for which to use the jig.

[PL1]

This post assumes that the curves being measured have a fixed length radius.

Small curves -- Find a coin, jar lid, paint can, etc. that matches the curve, measure the diameter, divide by 2.

Large curves -- There is a well diagrammed instructable here showing how to do this.

Draw two or three decent sized chords on the curve about 90-120 degrees apart if possible. (90 degrees separation gives the best accuracy for two chords, 120 degrees for three chords)

Find the center point of each chord and draw a line through that point that is perpindicular to the chord.

Where the lines meet is the center of the circle.

You can now measure the radius from the center to the curve.

To describe curves for build plans, show the X/Y measurements for the center point and the radius of the curve.

Some plans only list the radius of the curve which requires the builder to reverse engineer the center point using the last straight/first curved point at the beginning of the curve and the last curved/first straight point at the end of the curve. (See attached)

Draw two circles (red, green) of the given radius centered on these two points.

The circles overlap at the center point for the desired curve. (blue)


Scott