Just figured out the obvious

[awestib]

Hi guys, this is in regards to recent shapening videos, advices etc. I have been stuggeling with scandi grind for quite some time. I am not talking about touching up an awesome made flat scandi on a fine diamond or waterstone etc., I am talking about serious "regrinding" of chipped scandis that may have never been flat from the very start.

Today I figured out the obvious, therefore for most of you it is probably not even something new at all.
HeavyHanded and me have been talking about this for some time and I absolutely believe, that the best approach to a scandi is to emphasise pressure differentially near the shoulder and near the apex. I took this approach from Martin. You will end up with a slight "convex" but this way you maintain the overall grind angle longterm.

Anyway, I did not want to give up and bought a large 120 grit !! DMT diamond stone just for the purpose of flattening that scandi grind (real flatt ?!). It felt extremely coarse at first and it really removes a lot of metal. I got the scandi flat all along quite fast with using lots of marker, therfore I got a burr also quite quick. Guess however what happened next?

I switched to my 600 grit DMT dotted stone and started doing the very same motion, lots of markers, felt very comfortable and smooth. Those stone are flat, I mean flat compared to waterstones in use. What I observed is that now I only got a new grinding pattern near the shoulder of the scandi? I tried all this in different movements, one was only edge leading, one was circular motions - but no difference.

I think I understand why. It must be the drag that occurs, the coarser the stone the higher the drag. On the coarse diamond, the tendency is towards the apex if I go edge leading, towards the shoulder if I go edge trailing. There is no easy solution to this as far as I can think. Even with the circular motion! So I guess if one wants to regrind a scandi properly, the best approach is to start out with not a too coarse surface. Rather use a 1000 grit JWS or so and take more time (days maybe ...). Stick with the same grinding medium also I guess. This is by the way not different if you use sandpaper on glass etc. Same dragging problem on coarse paper.

My next project is cutting a piece of the black/blue DMT diafold that is only slightly larger than the scandi bevel and will get into longitudinal scandi sharpening again. That makes completely more sense!

Would love to hear you guys suggestions, thoughts!

 

[Jason B.]

I find that you must "break through" the coarser scratch pattern and re-flatten the surface every time the abrasive becomes smaller. With each reduction in grit you also have a smaller contact area so it becomes increasingly difficult grind a flat surface.

Now I think of it like sharpening a yanagi and the way the pressure is used for grinding the soft and hard layers of metal. Overall the "flat" becomes slightly convex but its hardly extreme enough to notice.

A 1k waterstone works quite well for scandi blades and if finer is needed I typically used my 6k and focused the pressure near the edge. Like a microbevel but not. As per the coarse stone... Try a coarse waterstone and you will probably have better results. Personally I do not like diamond hones for large flat bevels.

 

[HeavyHanded]

The way I see it is this - you have a visual and tactile margin of error that is equal to the size of your abrasive. Some will be deeper at the shoulder, some deeper at the apex, most will have the same depth at the midpoint. Visually the bevel will appear completely flat. As you start working with a smaller grit size you will 'uncover' the imperfections as it grinds down - this is why the bevel will almost always appear somewhat convex when you first hit it on a finer stone - freehand anyway. On a Scandi it tends to be more obvious than on a V bevel. You overgrind at some point - shoulder, middle, ( on all my bevels I shoot for the shoulder first) and then the grind pattern expands as you overprint the remainder. Or you follow the curve and the bevel looks more convex at higher grit values than it did at lower ones, determined by how flat you held the grind angle initially. I do not believe someone (myself anyway) can freehand tighter than a degree or so deviation from shoulder to apex on an average Scandi (less on a V bevel), so why fight it. This is also much flatter than most come from the factory. The shift in emphasis from shoulder to apex should be something felt more than seen and result in no more than a degree or so/side. My Scandi edges also seem to hold up better this was too.

The above is why I always recommend newbies learn on cheap knives with a coarse edge - the margin of error and ability to hold an angle well becomes far more challenging as one goes up in grit values.