Wednesday, November 4, 2015

The cutting edge

In our recent discussions between Ralph and I about sharpening, I often mentioned the importance of tweaking the edge with a micro bevel if the edge seems to crumble too fast for a given wood. He mentioned on numerous occasions that he never saw any references to that in older text, so what gives?

So here is my take on this.
There always was, it just wasn't called micro bevels like we do today...

Let me try to explain this.


Every text I ever read about sharpening always makes mention of two angles;
The grinding angle and the whetting angle. 
Top the primary or grinding angle, need to be shallow.
Bottom, angle A is the primary, B is the secondary.
You want to add a third one? Be my guess, but its not necessary...

So what are they and why bother with two angles??

You need a shallow angle to penetrate easily into the wood. That shallow angle is referred to as the primary bevel or the grinding angle. It is usually between 20 and 30 degrees, as seen in older text. Today we seems to favored 25 degrees (smack in the middle).
If you are using a chisel for paring, 20 would be fine, but it would crumble pretty quick if you were chopping mortise.

Nobody really likes to spend too much time sharpening, we all rather spend our time actually working wood.

The next angle is the secondary angle or referred to as the whetting angle in older text. That is the angle that we use on our stones after the grinding operation. Which grinding is usually done on a mechanical or electrical grinder, but I do mine on stones as well, as I am not a big fan of a hollow edge, produced by a round wheel. A bigger wheel will give you a shallower hollow, but it is still there.

Why that second angle? Because if will significantly reduced the time required to sharpen, since we are not doing the whole bevel again, AND perhaps more importantly, it will beefed up the edge (more metal behind the edge)

That small land of steel at a higher angle makes a lot of difference for the edge retention, but it is too small to make much of an effect on how the tool feels into the wood. If you were not adding that small secondary bevel and instead were to sharpen two chisels, one at 25 the other at 30, you will find a  difference in how it penetrate the wood. Now touch up that 25 degrees by adding a 30 degrees micro bevel and compare it to the other still at 30. The one with the whole edge at 30 will be harder to push still than the one with the micro bevel of 30.

I called that secondary bevel MICRO, because really you do not need to have a wide land of steel at that secondary bevel to affect its performance.

I know some would argue that what we call micro bevel today are really a tertiary bevel (3rd one) but to me that is semantic.
I do my edge usually at 25 then tweak the edge by whatever work for the wood I'm working with. The exact angle?  Don't know don't care!

What does that mean? If my edge is crumbling too fast, I'll simply touch up the edge with a steeper secondary bevel.
I do strop my edges often as I work, an habit I picked up from my carving days.
It keep my edge sharp longer and delay the inevitable trip back to the stones...

Other than that, that is all I do.
Oh, I do the back first real good the first time around (past pitting at the edge), then...
i'll just continue to polish it to remove the small burr introduced by whetting.

I do not care to take my edge to a blinding shine level, I stop at my finest stone, a 6000 grit waterstone, and I most certainly do not test my edges on any of my body parts. Nail, hair on my arms etc, paper or whatever (those test proves nothing, except that you are about to cut yourself and probably damaged that edge :-)

I'll have you know on good authority that blood cause rust on tools, so clean up immediately, save the tool, don't worry about your body parts, i'll grow back right?


 I simply look at the edge under a strong lite trying to catch reflections and I test the actual edge on a scrap piece of pine.
 
My all times favourite knife.
Bought that knife in 1998, has been sharpened a bazillions times by now...

Can you tell if its sharp or not?

Same picture, blown up.
See the white bright white spots on the edge??
These means that there is sufficient thickness to reflect light, 
it is overdue for sharpening :-)


Why pine? Being a softwood, the end grain crumble easily if the edge is not properly sharpen.
Once I got a shiny burnished surface left on the end grain cuts, I am done!

First cut with nicked edge shown above.
Cut easily still, but look at the scratches left 

After a couple strokes on my 6000 stone
Better but I can still see lite spots on the edge

A few more strokes followed by stropping.
Pretty good, it is leaving a burnished surface


BUT... I will tweak that edge depending on how it react to the wood I am working on, by raising the edge a tad. Whetting angle, secondary bevel, micro bevel, whatever you want to called it...

Brazillian hardwood, forgot the name.

Look at the clean pore holes, cuts are shiny, burnished

AND I do not put any back bevels what so ever on my edges thank you very much... Not on my plane irons, and never on my chisels.
I may be lazy but never that lazy (ruler trick) ... :-)

Bob, sharpening his keyboard with help from Rudy.

That is his little stool to go up on the couch.
Its starting to shows some signs of puppy chew...

Must really puppy proof my shop... :-)

6 comments:

  1. You make a good argument with some facts to back it up. You almost have me swayed into this camp. Your way makes more sense to me than the disagreeable ruler trick. Maybe I have seen/read it but it didn't register on me. I need to read it for myself as have never heard of what you posted before.

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  2. Almost, uhh :-) OK go read what Stanley wrote on that same subject in 1929. Picture on my Google page.

    Bob, sipping a cold one eh!

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  3. Bob,

    A new "boss", glad to see he is well cared for.

    Good over view. When using oil stones it helps if you sharpen small surfaces e.g. your secondary bevel.

    I expect few can freehand a perfect flat bevel even with hollow grinding. The final edge, how ever you get there, will have some rounding, which I think is a good thing. From my experience a "perfect" edge is a fragile edge, at the first touch of wood it will break down, fracture or whatever term you use. A stropped edge will not be perfect because no matter how careful there is some dubbing, if small enough there will be no real difference in sharpness but a great difference in strength.

    I guess you could say I use a method that is much like the one Paul Sellers or Richard Maguire use, a slight increase in angle near the end of the sharping stroke to raise a good wire edge. Once I have a wire edge on the set up stone (currently a Lilly White Washite) I pull it off on a Hard Black Arkansas then raise a finer wire edge on the Arkansas I will then lightly back and forth until I can no longer feel a wire on either side. Then a light strop and pull through the end grain of some Oak to make sure there is no wire edge left. Takes longer to write than to do :-).

    Once the resulting bevel is too steep, I will grind the bevel back to some where close to 25 degrees. If I understand your post, we end up in the same place by maybe slightly different technique.

    One last thought, shinny is not necessary nor is it necessarily sharp, though it sure looks impressive and pretty.

    As always with anything wood.....YMMV.

    Ken full of it, but loves me some puppies.

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  4. Hi Ken
    Yes the puppy pretty well rule the hacienda right now :-)
    Agreed however you do it the results are the same, two angles at the edge.
    The one good thing that shiny does, is it help the shavings glide and not stick.
    How shiny before you get no more benefit? I don't know, but stopping At 6000 plus dropping, work for me.
    No matter how sharp, the edge start to degrade on contact with the enemy... Oops talking military, I meant on contact with the wood :-)

    Rudy, with inputs from Bob :-)


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  5. &@$()!? Auto correct, meant stropping, not dropping
    Dropping does not help the edge whatsoever :-)

    Bob

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