finding your blade angles.

B

bubblewhip

Joined
Dec 18, 2009
Messages
248
I've thought about how to find out the angles of your blades. Since a lot of companies do not put their blade angles on their info, I've developed a trigonomic system of measuring blade angles.

Materials:
-Calipers (precision digital)
-Fine ruler
-Calculator (scientific or higher)

Measure the thickness of the edge from the relief cut, then measure the height of the relief cut from the edge.

(-)=Theta
The equation should be Tangent(-)= (1/2thickness)/relief cut height)

So what you should punch in is...
Tan^-1 (inverse of tan)*((1/2thickness)/relief cut height))=(-)

(-)*2 should give you the edge angle.

So for example...

My cold steel kukri machete (because it's easy) has a 3mm thickness and a relief cut that is 5mm high.

Therefore my equation should be...

Tan(-)= 1.5mm/5mm

Therefore what I should punch in is
Tan^-1*(1.5/5mm)=(-)
(-)=16.6 degrees
2(-)=33 degrees

Therefore I should sharpen on my sharpmaker or lanskey (in theory) on the 30 degree setting for this knife.


If you have a true full flat ground knife with no bevel it becomes much easier as you just measure the full thickness of the knife and the full width of it and input it into the same equation. This equation will not work with convex grinds with no bevel in them. It does not matter which units you use (mm, meters, centimeters, inches, ect.) as the units will cancel each other out.

Hope this helps.

P.S: This will also help in re-profiling if your sending it out to a heavy duty sharpener with stone wheels or a belt sander or even if your doing it yourself. Just halve the value of what angle you want and punch it back into the equation solving for the height of how high you want your relief cut to be.

So if I wanted a 22 degree angle I would go...

Tan(11)=(1.5mm/x)
x=1.5mm/Tan(11)
x=7.7mm
So I would need to move that relief cut about 2.5mm up to get a 22 degree edge.
 
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I've used the height of the blade from the stone and the height from spine to edge but I'm not that good at math so for me it was just a close guess.

Because I freehand I don't really bother with knowing my angle. From experience I've gained what I call a understanding of angles required to make a given steel and blade geometry perform with a selected sharpening angle.
 
knifenut1013 said:
Because I freehand I don't really bother with knowing my angle. From experience I've gained what I call a understanding of angles required to make a given steel and blade geometry perform with a selected sharpening angle.
Click to expand...

This. Once I learned how to actually get a knife sharp, the number didn't matter to me anymore. It would be cool to know what angle I've actually got, but it wouldn't make too much a difference (I wouldn't go and try to shave off a degree if it were 21°, for example.
 
knifenut1013 said:
I've used the height of the blade from the stone and the height from spine to edge but I'm not that good at math so for me it was just a close guess.

Because I freehand I don't really bother with knowing my angle. From experience I've gained what I call a understanding of angles required to make a given steel and blade geometry perform with a selected sharpening angle.
Click to expand...

Well for people that re profile and sharpen their edges with belt sanders and even diamond files, I don't think it's a big issue, but there are a lot of people with stories going like "I DID 1000 PASSES WITH MY SHARPMAKER AND IT'S STILL DULL!" Which maybe just an outgrowth that they decided to take a 40 Degree knife and use the 30 Degree angle setting which makes them question to hell why their knife in D2 or S30V can't get sharp on the wrong angles. Though I do know what your talking about the "understanding of angles" you can generally tell by the amount of grinds and what type of grind that an edge will contain what angle. Even though I'm a beginner I think it's possible to also "feel" the angle on a stone especially if the relief cut is pretty high. You can feel the blade stop at a point on the stone when your angling the blade, and that's where I feel to sharpen.
 
When you sharpen its A LOT about feel so keep doing just that.
 
Nothing wrong with the math, but it's easier just to mark the edge with a Sharpie, and/or look under magnification (or even just a good light), to see where you're hitting the edge, and adjust accordingly.

cbw
 
cbwx34 said:
Nothing wrong with the math, but it's easier just to mark the edge with a Sharpie, and/or look under magnification (or even just a good light), to see where you're hitting the edge, and adjust accordingly.

cbw
Click to expand...

That's how to get a knife sharp, but it won't give you the numbers. So we see here that the math is good and all, but in the end, math doesn't give you a finished product (I'm an engineer here taking a stab at mathematicians :p)
 
I sharpen 2 x 20 degrees
I made a little 20 degree wooden block to get the feeling
I find myself using it less and less, getting the feeling naturaly
 
Hengelo_77 said:
I sharpen 2 x 20 degrees
I made a little 20 degree wooden block to get the feeling
I find myself using it less and less, getting the feeling naturaly
Click to expand...

I just made a wooden block also, as a rough guide. Cut some in 5 degrees (ex. 20,25,30,40, etc.) on a mitre saw. gives a good idea where you're at. Cool math, but not my forte.
 
If you have a triangle with one 90degree angle and it's legs are 3:1, the smallest of the two other angles is 20 degrees
A handy way to find it
 
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