Measuring Edge Angles

K

KennyB

Joined
Jan 19, 2010
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I'm just curious what my edge angles are and want a really accurate way of measuring them so that I can familiarize myself with how a knife performs at various angles, and be accurate. Right now the best way I have is to using my calipers and do some math stuff, but I wonder how accurate it is. I've tested it on some stock production knives by calculating how high I think I need to hold the spine to match the factory grind, and it's close, but I can't tell how much error is in my measurement, in how well I'm setting it on the guide I set up, or how well I hold the angle. It seems dead on though.

Anyway, I figure that a laser pointer would never be as accurate as the CARTA thing, but I wonder if it would be more accurate than using calipers. I figure that if I used something like a 5 gallon bucket and cut the bottom off so it's kind of a tray, I could mark out the degrees on the walls of it and use those little "divots" that are in the center of them to line the laser pointer and blade up, but who knows if that would be accurate enough. There's nothing saying that the light in the laser is mounted square.

Not that I need to be really accurate or precise, I just can't really see my edge very well because it's so thin, and it'd be nice to tell what the actual edge angle is . Using calipers is fairly accurate from what I can tell, but a laser goninometer would be better... I'm just not sure doing it with a laser pointer and what not is any more accurate than using calipers. Anyone know how much the one from CARTA is? May be a worthwhile investment since I want to get into making knives, and I'll need to test different edge angles.

How do people here measure edge angles? I saw one user using JB weld on the edges, and that looks like something that would be nice to use, and maybe possible to measure with a good machinist's protractor, but this is the most expensive option outside getting the CARTA machine.
 
Setting an edge angle on a knife is a cinch, finding out the factory angle is a bit tougher. If you are sharpening in a jig, use a bevel gauge or protractor on the blade and stone/hone while it's in the jig. Unless you have an minutely adjustable jig, you will be stuck sharpening at the angles the jig has slots for so you may not be able to precisely match a factory angle. If you are free-handing, or building your own jig, you could use trigonometry and base your calculations on the height of the center of the blade spine off the stone, and the width of the blade. Set the height with an appropriate gauge block. Or set the angle with an appropriate angle block and hold it while sharpening
 
Mahoney said:
Setting an edge angle on a knife is a cinch, finding out the factory angle is a bit tougher. If you are sharpening in a jig, use a bevel gauge or protractor on the blade and stone/hone while it's in the jig. Unless you have an minutely adjustable jig, you will be stuck sharpening at the angles the jig has slots for so you may not be able to precisely match a factory angle. If you are free-handing, or building your own jig, you could use trigonometry and base your calculations on the height of the center of the blade spine off the stone, and the width of the blade. Set the height with an appropriate gauge block. Or set the angle with an appropriate angle block and hold it while sharpening
Click to expand...

Why the center of the spine and not the underside of it? When I take the measurement with the depth gauge on my calipers, I set the calipers on the upper-side of the blade spine. I just assumed that since the equation is basically measuring the angle of the side of a triangle which the blade forms with the stone, I figured that measuring from the bottom of the spine would be the best way to measure how high the surface of the blade forming the side of the triangle would be.

For example, with one of my knives

Knife width at point of measurement: .990"
Spine distance measured from top side of spine: .310"
Spine distance measured from center of spine (based on subtracting half of blade thickness ): .252"
Spine distance measured from bottom side of spine ( based on subtracting full blade thickness): .195"

So what's the right measuring between 18 degrees per side, 14 per side and 11 per side

Then there's also the question of whether this angle measurement corresponds to the actual grind of the edge angle when taking into consideration the angle that the relief grinds are made at. For example, on the knife I measured, the relief grind is 6 degrees; so when measuring something like 14 per side based on the center of the spine, does this take into account the 6 degrees for the relief angle? How do you account for it? Just subtract the angle of the relief grind from the measurement you get with the spine?
 
With a sharpmaker, edgepro, etc you can find the angle that best fits the edge by trial and error. With a factory grind it usually varies a bit along the blade and from side to side.
 
Because you should not be sharpening for a angle in relation to each side but to the exact center of the knife. Making a even bevel does not always mean you created a centered cutting edge.

Finding your angle is useful for testing that will be shown but for personal knowledge sharpen to the angle that feels best and adjust as needed. Believe me, angles will just make it that much more confusing.
 
knifenut1013 said:
Because you should not be sharpening for a angle in relation to each side but to the exact center of the knife. Making a even bevel does not always mean you created a centered cutting edge.

Finding your angle is useful for testing that will be shown but for personal knowledge sharpen to the angle that feels best and adjust as needed. Believe me, angles will just make it that much more confusing.
Click to expand...

Yeah, that makes sense, and I guess that would eliminate the need to try to compensate for the relief grind too since it's measuring it from that axis rather than based on the surface. Plus the measurement I get for this specific knife makes a lot more sense measuring from the center.

Oh, and yeah, I'm not really doing it for personal use aside from making spacers to match factory bevels. Otherwise I just need a way to mark down the actual angle for when I start making knives and deciding which edge angle to put on them.

What do you think of this method?

1. Place blade on the surface as if I was taking another stroke, and making a mark on the spine to set the caliper's frame on, then take depth measurement to the stone's surface and keeping the depth gauge's flat bottom flush with the stone's to ensure the calipers are at 90 degrees to the stone on all sides
2. After taking the depth measurement, I measure the width of the blade from the mark I made to set the caliper's frame on to the very edge of the knife; I'm careful to not tilt the blade to help make contact with the calipers and keep it as square as I can so the point of view I'm taking the measurement from isn't at an angle itself.
3. Subtract half of the blade's thickness from the depth measurement so that it's reading as close to the center of the spine as I can get.

Then I take the distance (d) measurement and the width (w) measurement, and do this

angle = asin(d/w) * 180 / pi

Think there's any reason to need it more accurate than that? I just feel like there's lots of room for error doing it like this, with all the measurements that need to be taken, and some of which are invariably off a little bit. The CARTA machine says it's only accurate to +/- half a degree though, and when I play around with the measurements with a 1" wide blade, the "d" measurement varies the final result by half a degree with a .008" - .009" variance, so I wonder if the measurements I'm taking are even accurate to within +/- .010 or not--there's just not really a way to verify. It seems accurate enough though, we're still talking about half of a degree.

Do you think the angle being within +/- 1 degree is good enough? I suspect I'm at least within that kind of tolerance, and it's not like I'm doing CARTA level testing. However, like I mentioned, I want to try to match factory grinds; would being 1 degree off matter a great deal? I've never really measured to know, and don't recall anyone but myself ever caring. :P
 
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Sill kinda a waste, what steel will you be using? what type of grind? thickness behind bevel? intended use of knife?

All that and more will decide the proper bevel for your knife and why I say forget knowing a angle it will only confuse you. Now knowing angle of the main blade grind is something I would want to know.
 
knifenut1013 said:
Sill kinda a waste, what steel will you be using? what type of grind? thickness behind bevel? intended use of knife?

All that and more will decide the proper bevel for your knife and why I say forget knowing a angle it will only confuse you. Now knowing angle of the main blade grind is something I would want to know.
Click to expand...

Finding the primary grind angle is one of the reasons I want to find a reliable way of measuring edge angles, but I hope to do that with a protractor before the edge is ground. It's just right now I I'm dealing with wanting to know angles that might be a little too small to measure with a machinist's protractor, and of course I don't want to mess up my edges. So the laser or just doing it with measurements seemed the logical idea.

Anyway, I don't think it will confuse me to know the angles of the edges. It's not as if I'm going, "Well, everyone says that 30 degrees is awesome, so I'll be sure to set this at 30," I just want to know the actual angle of an edge so it's easier to replicate. Suppose I have a factory bevel or a bevel I've done ( or someone I sharpen for has done) that I just want to polish, then just by measuring the width and thickness of the blade I could determine what height to cut a spacer for so I could be sure to match the bevel as best as I could. I mean, having a set of spacers around marked for each particular knife may work, but it has its own set of complications (what if you lose one, you start sharpening with something different, etc.). Angles are just a more universal thing, so I might as well work with them instead of arbitrary things.

I mean, I suppose buying an EdgePro would be "easier", but then I'd have to learn how to grind on it, spend all that money, etc. To me it just seems that using the measurements and the trig is a cheaper way to do it, I just want to make sure I'm doing it right and that it's actually accurate enough. I guess by testing I kind of lead you to believe I'd be testing different angles, but it's more about replicating angles that I've found that work for a particular knife; after all if I make a knife I like, I'm going to want to make more.
 
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