How To Create an easy accurate angle guide for freehand sharpening

[maximus83]

I wanted to capture a method I've been using to...

• Accurately calculate the spine height of a blade to achieve a given angle in degrees per side (dps) during freehand sharpening.
• Easily create a blade-specific sharpening height guide based on the desired sharpening angle.

This information is not new, it's available in the knife community and bits and pieces of it have been posted in BF threads as well. But I wanted to pull all the useful information together into a single easy to follow guide.

Scenario: You want to sharpen a knife to a predetermined dps angle, you want that angle to be accurate not merely estimated, and you want a simple angle guide to keep you on track while sharpening on a flat bench stone.

1. Go to: https://www.triangle-calculator.com/?what=aas
   • Note: This is an "angle-angle-side" (AAS) calculator.
2. Enter the first angle (the α angle) as 90 degrees.
3. Enter the second angle (the β angle) as the desired sharpening angle in degrees per side.
4. Measure your blade height at the center point of the blade. Enter that in the calculator's "a" field as the length of the side.
5. Click Calculate.
6. Measure the thickness of your blade at the spine and divide by 2. This is your thickness adjustment value.
   • Note: you use this value to adjust the spine height value to take into account the thickness of the blade.
7. Take the calculated height value (shown in the calculator as the height of side C), and subtract from it the thickness adjustment value. The resulting value is the sharpening height at which you should hold the spine for the desired sharpening angle.
8. Divide the sharpening height value from the previous step by 0.068. Round the result to the nearest whole number. This number is your quarter stack height, the number of stacked quarters to use for your angle guide.
   • Note: 0.068 is the approx thickness of a US quarter. Also, you are of course rounding the result to the nearest full quarter. This is usually a difference of a few hundredths and is close enough for freehand sharpening.
9. Place the stack of quarters on your sharpening stone. To set your correct sharpening height for each stroke, place the BOTTOM edge of the spine on the TOP of the quarter stack.

Example:

• You have a blade with a blade height of 1.6", and a thickness at the spine of 0.1875", and you want to sharpen to 15 dps angle.
• You plug in these values to the triangle calculator:
  • α angle: 90
    
  • β angle: 15
    
  • "a" side length (blade height): 1.6"
• Click calculate
• Determine your thickness adjustment value: 0.1875 / 2 = 0.09375
• Calculate sharpening height: 0.414 - 0.09375 = 0.320
• Calculate quarter stack height: 0.320 / 0.068 = 4.71
• So: use 5 quarters for your target sharpening height of 0.320".

PS: Please let me know if some of you pro knife grinders see a better, more accurate rule-of-thumb way to compensate for the spine thickness, and to calculate blade height.

 

[HeavyHanded]

I used to have lines etched into both sides of my sharpening block, indicating 18, 15, 12 degrees. I would sight down the spine and along the desired line or somewhere between, depending. Every few passes stop and reset by sighting along the line and making sure that's your working angle.

Checking against high powered microscope and angle finder, this method proved to be surprisingly accurate with a bit of practice.

 

[Chris "Anagarika"]

Good formula!

I use center of spine at 0.25 of the blade width to get approximately slightly below 15° per side. Sin 15° is 0.2588. Most of the time I go lower though, free handing it