Knife sharpening - sharpening your knife dull
Has that happened to you?
You sharpen and sharpen but the darn thing won’t cut?
Well, it happened to me before I started to understand the most important principal in sharpening a knife……. Keep the edge angle as given by the maker!
Certainly steel is the most important factor in having a sharp, standing edge which will last a long time. Hardness (HRC) is needed but also appropriate toughness and wear resistance which are given by the steel and its proper HT.
But the best steel will fail in its cutting edge properties if the edge angle is changed. In case you make the edge angle to narrow the knife is razor sharp after your sharpening but the edge will not stand, dull fast, break out (chipping) etc.
Chipping can also occur in case the steel is too brittle after HT, just to mention.
The most commonis that the edge angle is changed into a thicker/blunter edge, creating the non-cutting edge. Now let’s face it: keeping always the same angle when sharpening by hand is kind of difficult. That’s why a lot of sharpening kits include an angle guide to eliminate as much as possible the human factor of being inconsistent when moving the knife over the stone, steel, diamond, ceramic etc.
The 2 most common mistakes in increasing the edge angle are
a) beveling the edge into a convex shape
If you are within the 2nd sketch you still get kind of a cutting edge, it is the edge you will find in chopping tools like a machete style knife or an axe. But this edge will not give you the razor edge you like in a slicer/carver for cutting meat and it will work only decent in a chef knife for cutting veggies.
If you are in he 3rd sketch your knife will feel dull in push cut applications and most pull cutting services.
b) increasing the edge angle till the edge starts to be dull
The red outline in the sketches below represents the reference to the original angle to demonstrate the effect of modifying the edge angle in let’s say 3x sharpening (it can happen from original cutting edge to non cutting edge in just 1 sharpening!!!!!).
the angle remains, it is just moved back.
On a straight blade this means the cutting edge will increase slightly in size, with a hollow ground blade this will start later.
This should help you to understand an important part in sharpening your knife in the field and also when at home in your kitchen.
Nevertheless after you have sharpened your knife a few times you will have changed the angle; there is now way around totally avoiding the human factor of being inconsistent (even so that some sharpening fanatics will argue this). It is a good practice to go once a while to a professional sharpening service or your knife maker to have the knife re-sharpened on a mechanical unit.
Most professionals offering sharpening services will use special grinders with a jig where the blade is fixed in its position to the grinding stone/wheel. They will restore the original edge angle and you have again the proper reference to sharpen your knife.
Just for illustration:
If your stone looks like this
you most likely will sharpen like shown in a)
And ‘touching up’ an edge on a steel or ceramic rod like the butchers do is not re-sharpen; it is just - as it says - ‘touching up’ and will most likely start the base for the edge angle demonstrated in a). Most meat cutters have a professional sharpening machine or use the service of a professional to maintain a proper edge on their knives.