How do you determine blade angle ?

H

HotSteel

Joined
May 28, 2010
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I've got a DMT Aligner, and I have scavenged info from the Internet regarding what angles are nominally appropriate for different types of knives; for example,

12 degrees: razor
15 degrees: fillet knife, scissors
20 degrees: kitchen knife
22 degrees: pocket knife
25 degrees: camp knife
30 degrees: survival knife
35 degrees: axe, machete, cleaver

But I would prefer to sharpen with the same angle the knife came with, otherwise I'm doing a LOT of sharpening the first time with a knife to force it to whatever angle I've decided upon if it's not the same angle already on the knife.

What approach do you use to read the existing angle ?

thanks
 
You can measure the width of the bevels and the edge thickness with a caliper/micrometer, and use trigonometry to find the included angle.

I'd say it's better just to paint the bevel with sharpie and set the Aligner as close as you can by eye, take a test stroke and see where sharpie was removed. Keep doing this until the test stroke removes sharpie evenly across the whole bevel. When that happens, you have matched the angle. Assuming you can make fine adjustments to the Aligner (I've never used one), that should be more accurate than trying to measure.
 
I do scissors at 11 degrees, but the measurement is taken from horizontal rather than vertical. So in your "system", the scissor angle would 79 degrees. Using the 15 degree standard, the angle would be 75 degrees off vertical. Digital clinometers will read from either the horizontal or the vertical plane. They are available at woodworking supply stores. They are often used to set the blade angle on power saws, but they work well on sharpening jigs. An internal magnet holds them on saw blades. On a sharpening jig, a small clamp is often needed to hold them while adjusting the working angle.
 
Thanks for all the suggestions.

I've already got a caliper and a knowledge of trigonometry, so I might start with the thickness vs edge width approach. The main issue I see there is measuring the width of an edge accurately.

I forgot about the tried-and-true sharpie approach. If plan A doesn't work that will be my plan B.

If I were sharpening a flat edge or scandi edge, there would be no problem measuring the edge width with a caliper or clinometer. But, when I sharpen a knife with a microbevel, such as a compound bevel or a hollow grind, I'm not measuring the width of the entire blade, just the final bevel. For example, on my Buck Vantage just now I measured about 2.5/64, or approximately 5/128. I'm not sure that's accurate enough to give me the angle from its trigonometric relation to the blade thickness at that point, but maybe that's a starting point.

Or maybe I'll just use the sharpie. :)
 
That is pretty much what I was getting at when I said that the sharpie would be more accurate than trying to measure. It would be impossible to measure a convex edge, or really any kind of microbevel.

Another approach would be to see what angle the knife starts to bite into a leather strop. You could measure the angle several different ways, accounting for spine thickness and/or primary grind angle. Also, I think the "bite" angle will be slightly greater than the actual edge angle.

Maybe the best way to go is once you've established an edge angle experimentally, via the sharpie method or whatever, and you have the aligner set like you want, use an angle finder of some sort to record the angle. Keep that in a notebook or something and the next time you use the Aligner on that knife you can see that it was last sharpened with, say, a 13 degree back bevel and a 18 degree microbevel, and quickly replicate those angles using the clinometer. If you also recorded the edge thickness at each sharpening, you could see over time if your edge is getting thicker and if you need to work down the primary grind.
 
You can also use some trig based on the distance between the spine of your blade and the surface of the hone, and I find this distance much easier to measure than the width of bevels and edges and all that, and it's pretty accurate.

Angle = asin(D/W) * 180 / pi

Where "D" is the distance between spine and hone, and "W" is the width of the blade. Just use the depth gauge on your calipers and set the frame on top of the spine, then measure the depth to the hone. Subtract half of the blades thickness since you're setting the caliper frame on top ( this isn't exact, but close enough ) and just make sure you're measuring the width of the blade at the same section of spine you're measuring for the distance--so for example, don't measure your width near the choil, but then the width in the middle of the blade.

Then the "/ 180 * pi" thing is just for calculators that are set to radians to convert it into angle. If your calculator is already set to "DEG" instead of "RAD", you don't need that part.

That's how I do it, but even if you do get an exact angle measurement, the aligner probably won't be able to be set to that specific angle. The best thing I've found to do with the aligner is to use a marker and just add a little micro-bevel to a knife. Otherwise, as you said, I wind up working on it a lot more than needed.
 
To find out the blade angle I look it up online or call the knife manufacturer and ask them. Even when they give a specific angle there is always a minor degree of varience as some manufacturers still hand sharpen their knives. I would say that most of your decent knives like spydercos are at 40° inclusive, that is 20°/side, while Benchmades are 30° inclusive, 15° per side.

With the quided clamp system your angles are set but not completely. Depending on where you clamp it can change the angle anyway. If the blade is clamped deep into the clamp the angle becomes bigger if clamped as far out in the clamp it becomes smaller so just because the slot says its a certain degree does not mean that is what the actual angle is out at the edge. The sharpie method works well as stated before to get it aligned right to it.
 
Mr. Pink said:
To find out the blade angle I look it up online or call the knife manufacturer and ask them. Even when they give a specific angle there is always a minor degree of varience as some manufacturers still hand sharpen their knives. I would say that most of your decent knives like spydercos are at 40° inclusive, that is 20°/side, while Benchmades are 30° inclusive, 15° per side.
Click to expand...

No offense, but that might be the least reliable method for production knives. At least if you are trying to get within a few degrees of exact. Factory grinds can vary a lot (to me, 5 degrees per side is a lot). But, it would be nice to know what the manufacturer intended the knife to have, since that ought to be a good starting point for a compromise between performance and durability.

Really though, knowing the exact degrees is not that useful to the knife sharpener. Unless you are a professionial sharpening to specs or something, or have some other reason for using an arbitrary angle. It is also useful to know for prospective buyers or to compare performance between blades. But for sharpening personal knives, the goal (IMO) should be simply to use the most acute angle that will withstand your intended uses. You only need to know if the angle you're currently sharpening at is equal to, slightly less than, or slightly greater than what it was previously sharpened at. What it measures in degrees is not particularly important, compared to performance and durability.
 
I did a little more work. I remeasured my Vantage edge width = 1.41 mm. The thickness at the point where the edge starts was 0.69 mm, or 0.345 for half of the thickness. From that I calculate an angle of 14.16 degrees, which is less than I would expect.

But, let's consider if my edge width is off by 10%. My results then range from 12.85 degrees to 15.78 degrees. If my edge width is off by 20%, my results range from 11.77 degrees to 17.81 degrees.

My DMT aligner angles options for a blade one inch wide are 16, 20, 24, 28, 32, 36, 40 degrees. If I have faith in my measurements, then I'm probably going to choose position 7, 16 degrees.

I was expecting something closer to the 22 degrees, my nominal value for a "pocket knife".

In this case, I think the chances of measurement error are sufficient that I will probably fall back to plan B, the Sharpie.

I do keep a spreadsheet showing what Aligner angles I use for any given knife, so that on repeated sharpenings I use the same angle as much as possible, although as Mr. Pink said that also depends on where you position the clamp on the blade.

I don't recall seeing blade angles specified on manufacturer's sites showing specs, but I would gladly use that, even if the finished product doesn't quite meet the spec. I would take their angle as the ideal for their product unless I determine otherwise for my own particular usage.
 
It is a pretty difficult thing to measure. To be useful you would need to have it within a couple of degrees. Sharpie is far more accurate but you don't know what the actual angle is.

When I was talking with recording angles was actually taking an angle finder or clinometer and resting it on the flat of your blade while the blade is clamped in the Aligner. That will give you an absolute angle that you can reproduce regardless of where the blade gets clamped next time. You would disregard the aligner's readings and rely on the clinometer. Of course you need to always sharpen on the same surface, or make sure the stone is perfectly level each time. This would allow you to make slight changes to your angle by adjusting the position of the blade in the clamps.
 
Robs92XJ said:
No offense, but that might be the least reliable method for production knives. At least if you are trying to get within a few degrees of exact. Factory grinds can vary a lot (to me, 5 degrees per side is a lot). But, it would be nice to know what the manufacturer intended the knife to have, since that ought to be a good starting point for a compromise between performance and durability.
Click to expand...

I like to know what angle was intended for the knife in the first place. Why completely change the angle unless you really have a good reason to do so. If the knife company is pretty decent they probaby chose that edge angle for some sort of reason. It will definitly give you a good starting point having this information. If it is off a few degrees it will take just a little bit of reprofiling to get it to where you want it. I use a sharpmaker and depending on how vertical you hold it you could be off a few degrees also. The key is consistency not what exact degree angle you have. I just like to get close to what the factory intent was for certain knives. Plus its nice to know what companies like to use what angles.
 
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I said it was useful to know the "factory" angle because that was their design intention.

But if you simply take that angle, set it on your device, and start sharpening, you may actually be doing a big reprofiling job (since the factory told you 30 but it's actually 40 on your knife).

I was suggesting using something like the sharpie method to match the existing angle on your knife, whatever it might be, instead of aiming for an arbitrary number (even if said number comes from the manufacturer). That is, unless you know from experience that the angle is too low or high.
 
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