Scandi grind - mystery revealed

[Jan]

Last year on the margins of some microscopy thread we have discussed why some outdoor knives with scandi grind have unusually small edge (included) angles.    http://tormek.com/forum/index.php?topic=3190.15

Scandi grind is defined as a flat grind and is often intended for sharpening on a flat stone in an outdoor environment. The attached pictures show a blade of a new Swedish Morakniv made of stainless steel hardened to HRC 56-58. The length of the scandi grind is 6 mm and the edge (included) angle is 22 to 23°.

Using a microscope with magnification 50X I have revealed tiny micro-bevel of less than 0.1 mm length. In the second picture the micro-bevel is depicted as the upper horizontal white belt which is not intersected by the vertical scratches.  I was interested in the edge angle of this micro-bevel and so I have used my laser goniometer to measure it. But it was in vain because such a tiny micro-bevel has not reflected enough laser light. So I have thought to focus the laser beam using a watchmaker's loupe. And suddenly I have revealed a very weak reflection indicating that the edge angle of the micro-bevel is circa 36°.

As already mentioned by Grepper in the previous discussion the purist will say that it is not a really scandi grind but for me it is more important to know that the cutting edge angle is not 22° but realistic 36°. 

Jan

[WolfY]

Scandi grind (Long secondary bevel) can't be used for cutting as its end (if used as long primary bevel) will result in very thin and breakable edge.
Scandi grind is used to give lower profile to the knife so there will be lower cutting resistance and smoother transition the primary edge to the back of it.

Today you can see it in almost all kitchen knives. Long bevel from the back of the knife to the primary that is made with belt grinder and is so small sometimes that ppl don't notice it.

Remember Victorinox recommendation of thinning the knife? Same idea but with secondary bevel already established making giving the knife 3 bevels and no straight knife side.
This is why I add 1~2 dgrs to the VM-200 reading compared to the angled measured with chisel profile.

[grepper]

Wow, great sleuthing there Mr. Jan!

That makes much more sense.  I didn't understand why an "outdoor" knife, especially HRC 56-58, would be sharpened to 11°.   

Also...  Outstanding idea to focus your laser.  I'm surprised that worked, and that the magnifier didn't diffuse the initial focus of the laser.  From burning stuff as a kid, a magnifier never focused the light from the sun to more than a mm or two diameter hot spot.  I'm guessing that a thinner beam produces a tighter focus through the magnifier lens?

Anyway, you da man!  Thanks for clearing that up.  I know I'll finally sleep better tonight. 

[Jan]

Thank you Grepper!    It was an issue from my "Why is it so?" list. I am happy to understand it better now. It is interesting that even the expert review from the UK Blade forum considers Morakniv blades for pure scandi grinds.

The small inexpensive laser modules have diverging beams which are not fully focusable. The picture shows the watchmaker's loupe inserted between the laser module and the edge. It is only first aid solution, some compound lens may surely work better. 

Jan

[Jan]

Wolfy, there is a question what is the difference between a secondary bevel and a micro-bevel. For me the secondary bevel is visible by the necked eye while the micro-bevel is not.   

Jan

P.S.: Figure reference https://www.protoolreviews.com/tools/hand/cutting-chisels/knife-blade-grinds/14119/

[Jan]

I have prepared a drawing showing the situation of a knife blade with scandi grind and micro-bevel.

The knife manufacturer confirmed for me today that the micro-bevel edge angles are usually between 30-40° depending on the model. 

Jan

[WolfY]

For me the secondary bevel is visible by the necked eye while the micro-bevel is not.   

Jan

Depends on your eye condition Isn't it?
I would always assume there is a micro bevel when there is a scandi grind. Otherwise the edge would be to vulnerable. The visibility of it depends on how far up on the secondary it is placed and if it is straight or convex bevel. The convex is difficult to distinguish. Also the convex will not reflect the laser so you can measure it right. 

[WolfY]

Jan, do you use MEKANO parts for your building projects?

[Jan]

Wolfy, my devices are built from MERKUR which is a Czech toy metal construction set. http://www.merkurtoys.cz/en/

Jan

[Jan]

I would always assume there is a micro bevel when there is a scandi grind. Otherwise the edge would be to vulnerable. The visibility of it depends on how far up on the secondary it is placed and if it is straight or convex bevel. The convex is difficult to distinguish. Also the convex will not reflect the laser so you can measure it right.

Good thoughts Wolfy, you are correct! 

If the bevel is large enough the reflected laser beams tell me whether the grind is flat or not. Flat grind reflects laser beam as a line while convex and hollow grinds reflect a bunch of lines. The width of this bunch caries info about blade curvature. 

Recently I was sharpening an Opinel folding knife for an academician from the Charles University in Prague. He thought that the grind is flat. To persuade him that he is not correct I have send him the attached picture which clearly shows two bunches of laser reflections from a convex bevel grind.   

Opinel recommends to sharpen the cutting edge angle circa 40°. I think it is reasonable for a durable edge of an outdoor knife made of HRC 57-59 stainless steel.

Jan

[WolfY]

Wolfy, my devices are built from MERKUR which is a Czech toy metal construction set. http://www.merkurtoys.cz/en/

When I saw your construction I remembered the MEKANO factory in Sweden where I use to live near to. They moved from that location long ago. The factory was situated in a very nice wood outside of Stockholm.

MERKUR is nice too. Great parts for small projects when the fantasy and mechanical abilities develop. Today kids only think digital.

[Ken S]

Thanks for pointing out Mekano and Merkur. My grandson is very involved with Lego robotics. M and M seem more durable and can provide (as seen in Jan's work) a practical alternative to machinist work.

Ken

[Jan]

Ken, for kids Lego is much easier than Meccano or Merkur. My laser goniometer could be assembled from Lego Technic without any difficulties. A robotic hand could be designed and programed to measure the edge angle of simple grinded blades automatically. 

Jan

[Thomas at MORAKNIV]

I guess there is some cunfusion regarding the grindings and the edges and I might be able to spread som light in this issue.
First, dont confuse ordinary "Scandi Grind" (used on 90% of our construction workers and outdoor knives) with "Scandi Grind Zero" (used on carving and whittling knives).
Ordinary Scandi Grind ALWAYS has a smal secondary bevel. In our factory we use different methods on different blades but the result is similar - to make the edge stronger, less voulnerable and get a better edge retention. This is the way we have been making knives for 126 years now so I think we can claim the right to tell what is what when it comes to our knives
This is the purpose of the leather wheel on your TORMEK grinder, like stroping, to deburr and create a sharp, strong and durable edge.

Some European, American and Asian knife manufacturers has a 1-5mm high "secondary bevel" and that is NOT Scandi Grind, even if the rest of the blade looks like it would be.

One more thing to keep in mind that often causes missunderstandings: Primary grinding = secondary bevel and secondary grinding = primary edge.
This is depending on point of view; from a production angle or from a user view.
/Thomas at MORAKNIV

[Jan]

Thomas, welcome to the Tormek forum. 

Thank you very much for your terminological clarifications, especially the one concerning the Scandi grind. Sometimes it is for us not easy, because not all manufactures around the globe use the same terminology.

For us, knife sharpeners, it is important to know the factory angle of the secondary bevel. On your web pages you recommend: "You need to place the knife against the steel at the same angle that the very outer cutting edge has been ground to, and simultaneously pull the knife all the way at the same angle, from the hilt to the tip of the blade." But what is the angle of the very outer cutting edge? Are there some data sheets for specific models available? 

Because the height of secondary bevels on some Morakniv knives is only 0.1 mm, the sharpener has no chance to estimate this angle and keep it equal to the factory grind. Not every sharpener has a measuring equipment described in this thread – microscope and laser goniometer. 

As you know I have measured the height of the small secondary bevel on a new Morakniv Pro S knife and also estimated its bevel angle. I was very happy that you kindly assured me that my values were in principle in compliance with the factory grind parameters.

Morakniv knives I have chosen for testing because they are relatively widespread and popular in this market. The goal of this thread was to help other sharpeners to sharpen Morakniv blades with Scandi grind correctly.   

We would highly appreciate a small table with primary and secondary bevel angles for typical Morakniv knives.

Jan