Measuring blade angles

This may have been posted before or may just be well known, but just in case you want to measure a blade bevel angle with a bit more certainty than those crappy high school grade protractors they sell (which are only accurate to +/- a couple of degrees if you are lucky), then try this.

It does rely on knowing some basic trigonometry but these days you can get calculators or EXCEL doing the hard work anyway. I have shown the EXCEL formula you need in the picture.

- You need a reasonable quality marked square and a straight edge of some kind.
- I usually clamp the blade or tool I want to measure in a vice. I use a precision engraved Japanese Shinwa square.
- Lay the square down alongside the blade so the bevel forms the start of a triangle on the inside of the right angle of the square.
- Align the straight edge along the face of the bevel such that the straight edge crosses the two arms of the square. See Picture.
- Read off the distances X and Y shown in the diagram.
- Calculate the Inverse tangent (ATAN) of the X/Y ratio which should be the bevel.

If you maximize the distances X and Y and the bevel is reasonably consistent across the blade face you can determine the blade angle to better than 0.25 of a degree.
If you repeat the measurement across a the face of bevel you can measure how consistent your bevel angle is.
If you are really careful and have enough "flat" you can also check micro-bevel angle as well.
If you have a rounded main bevel you can determine roughly what the degree of roundness is (max angle and minimum bevel).

It works very well for small blades like spokeshaves and shoulder plane blades

A pair of tape measures will also work reasonably well if you don't have a marked square.

You can also use this method in reverse to set angles like on sliding bevels etc.

I hope it works for you.

Cheers