How sharp is that? Goniometer testing.

L

Longden

Joined
Jun 22, 1999
Messages
579
On another sharpening related thread: http://www.bladeforums.com/ubb/Forum32/HTML/000728-2.html

I talked about the possibility of using a laser pointer to measure the edge angles of a knife by testing the deflection of a beam head-on (normal) to the edge.

I bought a cheapo laser pointer to try this out, but while in class yesterday I noticed the article on Buck Knives in Combat Knives magazine (latest issue, article is titled "High-Tech Edging", p. 84) and one photo shows a Buck knife's edge being tested via a laser "goniometer", which is like the setup I envisioned.

If Buck uses this goniometer, does any other knifemaker? Has anyone else tried to make a homegrown setup like this with any success?
 
So have you tried it for yourself yet and if so how have you determined the results(good v. bad)?

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I can see how it might be possible for a man to look down upon the earth and be and atheist, but I cannot conceive how a man could look up into the heavens and say there is no God.
*Abraham Lincoln

Romans 1:20-22

 
Scarman, it's inconclusive. I can get a defined beam deflection on both sides, but the beam is distorted. Don't know yet if this due to the low quality of the light, edge aberrations, or because my setup so far has yet to be lined up with any precision.

Got a bunch of house and plumbing problems when I get home, and that's obviously the priority in a house with 3 females.
 
I have access to a goniometer because others in my research group use it to measure contact angles of water on thin silane films. I guess I could use it do determine the angle of the edge. That is what you are looking for right?
 
Try it on a new single-edge razor blade to see if that gives you a distorted beam -- after you get the plumbing fixed, of course. We can wait....
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-Cougar Allen :{)


[This message has been edited by Cougar Allen (edited 09 November 1999).]
 
Critical Mass, yes the edge angle's what I'm after.

Drew, sounds like this is an industry standard method of sorts for QA on production knives.

Cougar Allen, good idea on the razor. I was going to try calibrating first using mirrors at fixed known angles and then using a clean blade to test ... a razor's fine. And yes, it'll have to wait for the plumbing (sewer line, leaking pipe and a possible water heater leak).

The main objective is to see if something relatively simple (using readily available supplies) and cheap (say, under $50) could be cobbled together so anyone could do their own edge testing. Sharpness could be measured in specific angles rather than just "shaving" or "scary" sharp. I think this was something suggested by member Gollnick (Chuck).

Main problem is precise alignment of light and blade, and calibration. I figured I could gain precision by using a large setup so the measured distances are large (maybe 6ft).

The problem now is time and finding 6 sq ft of uncluttered space in my house
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If anyone is curious enough to get this to work before I do, I'd appreciate hearing the details.

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Longden Loo - Ventura, CA
Technology's the answer, what's the question?

 
Goniometers come in many levels of precision. A protractor with a pointer rod pivoting at 0 degrees or an adjustable angle drafting triangle can make gross angle measurements. Short of engineering/laboratory grade devices you'll find that the error associated with anything economical will be greater than the typical 20 degree bevel angle on most knives.

Regardless, angle is not the only measure of "sharpness".

Take care,

Mike Crenshaw

Crit. Mass, I assume you're at Vandy. What department.
 
hso, I don't dispute that anything I do at home with a $12 laser pointer will come close to what the lab gadgets can do.

I also agree that this is not a sole measure of sharpness, only a factor, and aims only to give us a quick and easy rough measure.

Humor me with this test, if you have a laser pointer and a cardboard box (and of course, you always have a knife
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)

1) With the box upside down and on the table, plunge the knife into the box about midway down the blade, with the edge facing you.

2) Lay the laser pointer so the beam points at the edge, and angle the beam slightly down at the intersection of the box and the edge. You may have to move the beam slightly to center it (shadow of the blade should be directly behind the spine).

3) You should be getting a beam deflection on both sides of the blade, angling away from the edge. Possibly several lines if you have a double-beveled blade.

4) Use your free hand and a pen to mark the deflected beam lines on both sides (you don't have to mark the whole line). If there are several lines on each side, mark the lines farthest from the blade (this is the line caused by the edge bevel).

5) Pull out the knife and use a ruler to connect the marks with the cut made by the front edge of the blade.

6) If you measure the resulting angle, this should be double the included angle of the bevel.

My Spyderco Calypso Jr shows about 60 deg (I don't have a protractor on me), which means about 30 deg for the edge, which jibes with Spyderco's claims of a 30 degree factory edge.

My BM 330 shows a much wider angle, and is in fact, not as sharp as my Spydercos.

My new Buck 501, which has astonished me with its sharpness, was angled measurably narrower than the Calypso.

Useful kitchen science.

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Longden Loo - Ventura, CA
Technology's the answer, what's the question?

 
To my understanding, the purpose of this test as it is used by Spyderco is simply to ensure uniform edge angles, not to test sharpness. Spyderco knives are sharpened by hand at their US and several Japanese factories. they are supposed to have a 30-degree included angle on the edge, 15 degrees per side (this allows easy sharpening with the 40-degree sharpmaker). To make sure that they are, in fact, being ground to these angles, examples are randomly pulled from each batch and subjected to this test (and others).

-Drew
 
Maybe "sharpness test" is a misnomer. Lots of people are always asking what the factory bevel for their knives are, or whether the angle they're grinding at is at such and such an angle.

The intent here is just to give us the means to quantify that measurement ourselves. The other way to find a factory angle is to use a marker on the edge and actually try grinding at a suspected angle and checking the edge for how the ink is removed... a rather destructive way to measure.
 
Hello!

I've been using my cheap laser pointer for a while now to determine the angle of the bevel. It's hard to do only because I don't have any good setup made. (Someday I will do a setup for it...) Anyways I take the knife and hold it against the side of the table (for instance). Then I point the laser to some flat part of the blade so that the light reflects perpendiculary from the blade. Actually I hold it so that the light hits a paper on the table. Then I will draw the reflected line (this is where I need some more hands
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) on the paper. Then I just move the knife so that the bevel I want to measure hits the laser and draw that reflection on to the paper also. Then just measure the angle between the lines. The quality of the reflected light beam isn't very good usually. It can be really distorted. The cheap laser pointers don't give very good quality light, but the main distortion comes from the edge bevel. Of course the beam is also wider than the bevel so you get reflections from the other parts of the blade at the same time, and the borders of the bevels - these all give you more distortions and funny reflections. But still it's quite easy to do and gives good results (they depend on the quality of the bevel and you setup). But it doesn't give any hints about the sharpness of the blade.

Hugo.
 
Hugo, your idea sounds good, but it appears to measure the angle between the edge bevel and the blade bevel (unless I'm misunderstanding). This should work with a partially ground blade where the side of the blade has an unground portion to use as a reference, but what about fully ground blades where the blade is angled from the spine all the way to the edge bevel?
 
Hi Longden,

If you aren't doing it already, I would lay a piece of white paper on your box before plunging in the knife. Otherwise it sounds a little like some optics demos I've done for school kids. Another trick might be to try some sort of nulling process where you rotate the blade until your reflected beam comes straight back to your laser.

Jeff
 
Jeff, I drew the angles directly on the surface of the box just fine, but the paper's a nice touch to show someone without toting a box around
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Your 2nd suggestion sounds something like what Hugo mentioned. I tried a variation which was to hold the knife tip down on a piece of paper and bounce the beam off the edge bevel where it meets the blade bevel. This results in two reflections (at least, ... depending on any secondary edge bevels) whose angle will indicate the angle of change from the blade bevel to the edge bevel. Unfortunately, this is not a good indicator of the true edge angle unless you can then determine the blade bevel angle (difficult for a full flat ground blade).

My cardboard box test seems to give a straight-forward indicator of the complete edge angle. I'd like to test it on a Moran or some other convex grind blade NIB to see how the lines are affected. My guess would be a uniform but fuzzy arc of some sort on each side due to the curved edge.

Overall, the test is NOT, as others have already pointed out, a definitive indicator of sharpness.

But it can throw some light
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on the edge characteristics (how sharp an angle, how rounded is the edge) and a blunt edge should even reflect light backwards, tho I hardly think anyone on this list needs a laser to tell them a knife is that dull.

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Longden Loo - Ventura, CA
Technology's the answer, what's the question?

 
Longden, you are correct in that when the laser hits the bevel, I do get 2 reflections. One from the blade bevel and one from the edge bevel. But what I do (and tried to explain) is that first I position the knife and the laser pointer so that the light from the laser is reflected back from some flat part of the blade, not the blade bevel. You can usually find some part of the blade that has no bevel (near the handle maybe). I try to hold the handle against the side of the table so that I can then slide the knife to another position and keep the the same angle towards the laser (which I don't move anymore).

Hugo

[This message has been edited by Hugo (edited 15 November 1999).]
 
Gotcha Hugo. I was wondering how you kept the plane of the knife constant while moving the knife's position so the edge hit the beam. So from that point it's just a ruler and some trig to find the angle...great!

I'll give it a try and see if the results match up with my method. What kind of distance from the knife to the reflected spot are you using?
 
What kind of distance... anything that seems to work
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. Like I said I don't have any good setup for doing this... also my laser pointer is on only when I press the button - so I have to keep on pressing it. If you make somekind of setup, even a crude one, it should be much easier to calibrate and maintain the angle between the blade and the laser. I have used this pretty much for just determining the angle of a knife before first sharpening. I have the DMT aligner that I use for sharpening pocket knives. I also calculate the angles for the different settings of the aligner for each knife (angles change with different blade sizes).
If you come up with a good setup, keep us posted. I think it's definitely worth trying.

Hugo.

[This message has been edited by Hugo (edited 15 November 1999).]
 
Hugo, I don't know how your laser pointer is constructed, but most of the cheap ones here in the US have similar on/off buttons that require pressure to maintain the beam.

I just wrap a bit of transparent tape around the pointer to hold the button down when I need "hands off" operation.

I like my box method for now, only because it's a bit simpler. I much prefer yours because it would seem more accurate, given that the blade plane can be kept constant as the knife is moved over the beam. There's a distinct change in deflection from the flat of the blade, to the blade bevel to the edge bevel, and with sufficient distance, should give a reasonably good angle measurement even given minor errors in measuring the lengths.

I'll let you know if I ever come up with a simple rig to test using your method (requiring less hands). Thanks for your input.
 
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