Laser device for edge angle determination

[Jan]

Some time ago Wootz posted reference to the study "Experiments on Knife Sharpening" by John Verhoeven. In the appendix 1 there is a sketch of a laser device for edge angle determination.  https://www.wickededgeusa.com/wp-content/uploads/2012/10/knifeshexps.pdf)

I was wondering if it will work with the laser line module which I am utilizing for LOC visualization. Using children mechanical construction set I prepared simple optical bench shown below.



As I expected it works fine for knives with wide bevels which reflect the laser beam well (e.g. the TORMEK/MORA knife shown). The laser line is splitted and reflected on the knife edge. The reflected rays draw two lines on the cross board. From the separation of those lines and the distance of the cross board from the edge the edge angle can be easily calculated.



Much less satisfying results I have obtained for my quite new Victorinox kitchen knife where the bevel is narrow (less than 1 mm) and does not have mirror quality finish. With some difficulties I was able to determine the angle of the blade tapering. The lines reflected from the tapered sides were heavily blurred. In my thinking the reason may be that Victorinox side grind is not exactly flat but slightly convex. The estimated angle of the approximately "V" grind was circa 3^o.
The reflections from the own bevel were very weak, even in a darkened room, and even more indicating an edge angle of 20^o instead of 30^o.

So those are my provisional results, I still have to work on that.

Jan

[stevebot]

May I suggest a circular target area centered at the knife edge. Mark in degrees divided by two - a 45 will deflect the beam 90 deg, etc.
CATRA makes a handheld unit for about $100.

[Jan]

Steve, thank you for your suggestion! 

This will completely eliminate any calculations by preparation of the semi-circular scale which will directly provide the bevel edge angle in degrees. (By the way the planar scale can also be calibrated in degrees for a fixed distance from the knife edge.)

You are correct, the scale in degrees should be divided by two, because the laser beam is deflected at an angle which is two times greater than the bevel angle.

At this moment I am experimenting and trying to find optimal distances. You can see that the laser module is movable in both directions, longitudinal and transversal while the knife clamp is movable only longitudinally but can be pivoted around vertical axis.

Jan

[RichColvin]

Jan, where do you get your lasers ?

Kind regards,
Rich

[Jan]

Rich, I use the same 5 mW focusable laser line modules which are offered e.g. by Amazon. I power it from 3V battery without any resistor.

https://www.amazon.com/Focusable-650nm-Module-driver-Plastic/dp/B012V3U3KK/ref=pd_sim_200_2?ie=UTF8&dpID=41mmSR02trL&dpSrc=sims&preST=_AC_UL160_SR160%2C160_&psc=1&refRID=DYR5W9EK8H60TGMZDRQ7https://www.amazon.com/Focusable-650nm-Module-driver-Plastic/dp/B012V3U3KK/ref=pd_sim_200_2?ie=UTF8&dpID=41mmSR02trL&dpSrc=sims&preST=_AC_UL160_SR160%2C160_&psc=1&refRID=DYR5W9EK8H60TGMZDRQ7

Jan

[Jan]

CATRA makes a handheld unit for about $100.

Yes Steve, you are correct, the CATRA Hobby laser goniometer "HOBBIGONI" costs US$ 136 + shipping + VAT. 

The Portable Model laser goniometer Mk1 with aluminium body and focussed laser  costs US$ 2200 and the Bench Model laser goniometer is even more expensive. 

I hope my prototype is in the highest category.  I'll have to tell my spouse how much money I had saved. 

I am wondering if CATRA uses dot laser modules because the image of the beam reflected at the edge has circular symmetry. I use line laser module which delivers line image of a planar bevel. Both approaches have their pros.

Jan

[wootz]

Jan, that's a very nice setup of yours.
I always wanted to do something alike, but never dared, and finally bought CATRA's hobby goniometer.
Yes, CATRA uses dot laser pointer.
And as you noticed with that other knife, not all edges angle can be measured using the laser method. Now and then I get a knife that CATRA goniometer cannot read, this is sort of expected limitation of the method.

If you wish, I could upload scan of the CATRA manual if their description might help you in any way...
BTW your description of Victorinox blurred reflections is how CATRA describes convex reflections in their manual.

[Jan]

Wootz, thanks for your kind response, appreciated! 

My setup was inspired by John Verhoeven sketch and only than Steve pointed me to Catra products. I will appreciate if you could upload the Catra manual. I would love to learn how they describe reflections from hollow grind bevels which are our daily bread. 

Yesterday night I was experimenting with my bench goniometer and can confirm that in full darkness I was able to measure weak reflections from very narrow secondary bevels also. 

The Catra dot lasers may be better for curved edges while my line laser shape is good for straight edges. I am using 5 mW laser modules while Catra uses 1 or 2 mW modules. For very weak reflections it is good to have reserve in laser beam intensity. To properly adjust and focus the line laser is more difficult.

Jan

[wootz]

Sent you the link to the uploaded manual via PM, Jan.

[Jan]

Wootz, thanks so much for the manual upload. 

It is very important for me to know that Catra interprets the screen images of the hollow ground bevels in the same way as I did independently in my thoughts. It was a great pleasure for me to read the relevant section. 

Jan

[RichColvin]

Ok, ya'll gotta stop talking like I'm smart or something.  What is a "goniometer" ?

Kind regards,
Rich

[Jan]

Rich, goniometer is an (optical) instrument that measures angles. Originally used in surveying. The correct term for my device is "laser reflecting goniometer". 

Catra uses the term "laser goniometer" as synonym for "laser knife edge protractor".

Jan

[Herman Trivilino]

Much less satisfying results I have obtained for my quite new Victorinox kitchen knife where the bevel is narrow (less than 1 mm) and does not have mirror quality finish.

Jan, have you tried polishing the edge with the SJ grindstone?

The reason I ask is that when I use my SG grindstone prepared in the fine state, followed by polishing with Tormek compound on the leather honing wheel, what I see with my dissecting microscope are smooth, mirror surfaces separated by scratches. I conclude tentatively that the 1000 grit grindstone and the leather wheel are too far apart in grit. Something in between is needed to get all the scratches out. Either that, or apply something like the SJ grindstone.

Of course, none of this affects the performance of the knives and tools that I use, so it's just an intellectual curiosity for me.

[Jan]

Herman, I did not polish the Victorinox knife until now, because it is still with the factory grind, untouched by Tormek wheels and my primary interest now is to measure the edge angle of the cutting edge.

I have tuned my laser goniometer setup and now I am able to measure the angles of my Victorinox knife in low ambient light also. Sorry for lower photo quality, they were not enough photons available.   



Image interpretation:
A and A' are reflections from the cutting edge
C and C' are bands of reflections from the sides of the blade. The sides are probably convex and such a grind is called cannel or rolled. The width of the band tells about the curvature of the grind.
X centralised blade position.

Based on my measurements the edge angle of the cutting edge is circa 20 degrees which is not in compliance with the generally declared 30 degrees. 

Jan

[Ken S]

Herman

Valve grinding compound applied to a leather honing wheel might be the answer. I have found that it is noticeably faster cutting than the Tormek honing compound. It does not leave as smooth a polish. I use it with a separate leather honing wheel. The honing wheels are easily interchanged in only a few seconds. With two Tormeks, adding the fourth step would add almost no time.

Ken