Sunday, December 18, 2016

The Dreaded Bevel Angle Calculation

Online Knife Bevel Angle Calculator

It's been super cold the past two weeks and I haven't had a lot of quality time in the sub-zero garage working on knives. I have to keep busy, and warm, so I've been working inside on a tool that lets us set the bevel angle on a blade by the grind height and thickness.

To work this out we need to look at the thickness of the steel and the height of the grind and make a simple right triangle.

Here our steel is laying on the flat. Imagine we want bevels that look like 'A'.

Adding a horizontal center line will divide the steel in half.

Adding a vertical line at the grind stop will allow us to extract a right triangle. Voila! High School was finally useful!


In order to get from a right triangle to real world grinding angle, we need to assign the Adjacent (A) and opposite (O) values.


There is a mnemonic used to remember which trigonometric function to use when we need to find some missing part of a right triangle. This mnemonic is SOH CAH TOA. Okay... what the?

SOH CAH TOA helps us remember to use the Sine function when we know the Opposite and Hypotenuse, we use the Cosine when we know the Adjacent and the Hypotenuse and we use the Tangent when we know the Opposite and Adjacent values.

In our example for blade grinding we us TOA or Tangent for Opposite divide by Adjacent.

Let's put some real numbers in. Say our knife is going to be a hefty 0.25" thick. We take 1/2 of that to make the right triangle, so O is 0.125".  We'd like a grind height of 3/4" or 0.75".

Using the ATAN function on our calculator we can determine the angle in radians. But, radians is not degrees. We need to multiply radians by 180/pi to get the degrees.

Here's the link  for Imperial and this link is for the Metric Grind Angle Calculator.

I promise no more math for the rest of the year.

Merry Christmas,