The most precise method I know for determining the sharpness of an edge is to measure the amount of force it takes to cut through a loop of thread. Repeated tests of this type give data that can be evaluated using statistical methods.

For each test a loop of thread is tied and suspended from a small scale. The plane blade is placed inside the loop and pressure is applied against the thread. The maximum force before the thread is cut indicates the sharpness of the blade, with smaller numbers reflecting a sharper blade. This test involves no slicing due to sideways movement of the thread along the blade edge. I believe this best reflects the cutting action of a plane blade through wood. At the bottom of this page is a diagram of how the scale and thread are used.

Extensive testing of premium knives has been done using this test. I developed my test based on work done by a knife tester named Cliff Stamp.

The cutting force needed depends on the type of thread being used. My first tests were done using basting thread, a 100% cotton thread designed to have low tensile strength. After the first round of testing I looked for ways to make the results more precise, and found that rayon thread was much more uniform in thickness. I’m now using 40 wt. rayon thread made by Coats & Clark. The force required to cut this thread is about 65% of the force required for basting thread.

For each sharpness test seven or more loops are cut and the average of the values is used to describe the blade’s sharpness.

To make the results of this test easier to compare to more common tests of sharpness, here is a chart that records my impressions of edges at various stages of use

Force to Cut 40 wt. Rayon Thread	Tests of Sharpness	Woodworking Uses
50 g	Will catch on a hair above the surface of the skin and cut through it. I’ve achieved this degree of sharpness only on blades with bevel angles of 31º or less.	Most work doesn’t require an edge this sharp
65 g	Pops hair off as it shaves, cuts through light paper without tearing, cuts soft pine end grain cleanly	Excellent for any use
100 g	Shaves hair like a dull razor, catches on fingernail	Sharp enough for all but the most demanding work
125 g	Will barely scrape hair off, still catches on fingernail	Sharp enough for many uses but requires more effort to push the plane
150 g	This can’t be described as sharp.	A plane blade at this point is no longer sharp enough for smoothing, but may work for stock removal.

 

 

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This is how I measure the force needed to cut through a loop of thread:

 

To make this test more precise, I move the blade using a traveling holder made from a desk flap stay with a friction sleeve. This allows me to lower the blade very slowly and evenly.