What is a convex edge and how do maintain it?
Below is an article on convex edges and how to maintain them. This article was written by Reid Hyken and I have been given permission to reproduce it here. Thank you Reid.
CONVEX SHARPENING THE BRKCA
WAY
By Reid Hyken aka: Sharpshooter
We’ve had a lot of conversations about how to sharpen a convex
edge. I’ve found that a first hand demonstration usually is the
best way to show how simple it really is. Perhaps a case of a
picture being worth a thousand words, so let’s try with pictures.
A convex edge isn’t what we have become accustomed to on
cutting tools. What we usually see is a bevel, a flats ground on
both sides of the blade creating a sharp edge
.
The convex edge is like a Bullet Point,
gently curving to the edge.
Most sharpening systems are made for the bevels that most
manufacturers currently use, the idea being to hold a consistent
predetermined angle along the length of the blade on both sides.
That won’t work on a convex edge since there really isn’t a
consistent predetermined angle on a convex edge, but rather a
gentle curve. To do this with a stone, one would need to
carefully “roll” the blade against the stone to follow the curve of
the blade.
There is an easier way, make the abrasive follow the natural
curve of the blade. To accomplish this we need a flexible
backing behind the abrasive.
Sandpaper held tight over a hard mousepad or piece of leather
glued to a hard backing such as a wood block.
To refresh a slightly dulled edge one starts with 1500 grit paper
and can work up to 2500 or finer. To resharpen a nicked or truly
dull edge I start with 320 grit and work my way up, the same
technique can be used to change a beveled edge to convex.
Whatever grit you are starting with, the technique is the same.
CONVEX SHARPENING THE BRKCA
WAY
By Reid Hyken aka: Sharpshooter
We’ve had a lot of conversations about how to sharpen a convex
edge. I’ve found that a first hand demonstration usually is the
best way to show how simple it really is. Perhaps a case of a
picture being worth a thousand words, so let’s try with pictures.
What is a Convex Edge?
A convex edge isn’t what we have become accustomed to on
cutting tools. What we usually see is a bevel, a flats ground on
both sides of the blade creating a sharp edge
The convex edge is like a Bullet Point,
gently curving to the edge.How Do I Sharpen the Convex Edge?
Most sharpening systems are made for the bevels that most
manufacturers currently use, the idea being to hold a consistent
predetermined angle along the length of the blade on both sides.
That won’t work on a convex edge since there really isn’t a
consistent predetermined angle on a convex edge, but rather a
gentle curve. To do this with a stone, one would need to
carefully “roll” the blade against the stone to follow the curve of
the blade.
There is an easier way, make the abrasive follow the natural
curve of the blade. To accomplish this we need a flexible
backing behind the abrasive.
Sandpaper held tight over a hard mousepad or piece of leather
glued to a hard backing such as a wood block.
To refresh a slightly dulled edge one starts with 1500 grit paper
and can work up to 2500 or finer. To resharpen a nicked or truly
dull edge I start with 320 grit and work my way up, the same
technique can be used to change a beveled edge to convex.
Whatever grit you are starting with, the technique is the same.
Converting A Bevel Grind to Convex.
Converting a bevel ground knife to convex is very simple and much
akin to the instructions that follow.
Bevel ground knives that are sharpened very often end up with a convex
edge as a matter of course. If you're acquainted with sharpening a bevel
grind you know the importance of maintaining a consistent angle between the
blade and abrasive. What "they" don't tell you is that if you aren't consistent
with the angle you end up with a convex edge.
Grandpa's great old whittler is probably convexed in this manner, truth be told
Grandpa didn't care about the what or the how, he only cared about the knife
being sharp and he knew how to get it very sharp. He was rolling the edge against
the stone creating a convex edge.
Using our method of sharpening to convert a bevel grind is simply a matter of
starting with a coarse paper like 320 and working your way up with equal strokes on
each side. Five at a time or ten or even 27, it doesn't matter so long as it the same on
each side.
Until you get the hang of it, I would concentrate more on technique than anything else.
Read the instructions and have at it.
READ THESE INSTRUCTIONS FULLY BEFORE STARTING…
The key to this is to pay attention to the contact between theedge and the abrasive
Lay the blade flat on the paper
The spine of the knife is lifted slightly from the paper until the
edge is contacting the paper; usually this is about a 13° angle.
The angle isn’t as important as the contact on the edge;
remember we have a flexible backing to follow the edge profile.
Once we have this angle established, the blade is drawn across
the abrasive, SPINE FIRST. We’re not trying to cut the paper, but
to drag the edge across it, so we’re removing the material behind
the edge not from the edge
DO NOT apply much down force on the blade; the weight of the
blade is sufficient to do the job. Press too hard and the abrasive
will actually be coming back up in the wake of the blade and
remove the edge, dulling the knife rather than sharpening it.
As you draw the blade across the abrasive, you can feel and hear
the abrasive doing the job. You’ll know it’s time to move to the
next finer grit when the drag stops and the blade seems to move
effortlessly across. The finer grits are removing the scratches
left by the more coarse grit. We’re really polishing more than
removing material, forming a wire on the edge and then removing
it when we work the opposite side. This is a complicated way of
saying that you need to equally work both sides of the blade.
Sharpening the Tip...
This would be the complete procedure were all blades to be
straight across; it’s the curve or belly of the blade that trips a lot
of people up.
At first glance, one would think that they need to lift the handle
as they follow the curve of the edge. That’s not correct…
remember, we have the blade almost lying down on the abrasive
and since the blade is flat the upsweep of the edge is not higher
but at a different angle to the sharpening motion.
The handle must be pivoted while maintaining the same plane to
maintain the edge contact with the abrasive. In this picture, I
have drawn a “Pivot Point” on the blade to illustrate this concept.
In reality, I put my off hand index finger on the blade at this point
and swing at that point. DON’T use the finger to apply down
force it’s only there to provide a point of reference.
The goal is to draw the edge across the abrasive at a
perpendicular angle.
Here is where a lot of people run into problems getting the tip sharp.
Usually this is because they don’t continue to move the blade perpendicular
to the abrasive, but rather draw the knife off the paper following the shape
of the curve.
blade is supposed to stay on the abrasive all the way across, the
stroke ending at the far right or far left of the sheet, not the top
or bottom.
Once you have run the course of abrasive papers you have a
sharp convex edge.
Heel to Toe or Toe to Heel?
I prefer starting at the tang and working the blade toward the tip of the blade.
Other people find it easier to sharpen from the tip to the tang.
There's no difference so long as you keep a smooth stroke and don't press
the blade into the paper.
Try both and go with your preference. They don't give points for style only
for sharpness.
Is it REALLY SHARP?
To check that there are no more flat spots on the edge, I use a
piece of hard plastic. They sell special rods of plastic to do this
test, but markers work just as well and are plentiful/cheap. I put
the edge against the plastic at about the same angle as I
sharpened at and with no pressure run the entire edge along.
Any flat spots will cause the edge to not bite into the plastic and
skip off.
Look closely at the picture and you can see how the edge is
actually slicing the plastic, a flat spot will skip across.
When you are satisfied that there are no more flat spots, you're
ready to move on.
Refining the Edge...
You can further refine the edge by honing it.
We often refer to this as stropping but that really isn't the correct term.
Barbers STROP razors on a hanging strop which leads people to thinking
that is the next step.
Stropping on a loose strap will DULL a knife.
Straight razors are designed to be stropped in that manner. A straight razor has a
thick spine and a very deep hollow grind that forces the edge to contact the strop which is held
flat because of the spine.
When we take a Convex Ground Knife and put it on the same strop, something happens...
The blade rides on the strop which causes the strap to continue to follow the curve when
it leaves the blade and wipes out the edge.
What we want to do is to essentially strop the edge. This is accomplished by "Stropping"
on a hard backing...
To avoid confusing the issue, I'm going to call this a Stropping Hone.
We now have a hard backing on the stropping leather so we can work the edge of the knife to
refine and perfect the edge.
You can buy a ready made Stropping Hone or simply make your own. There are many different
names for the Stropping Hone, what you're looking for is a hard backing on a leather face
that you can load up with compound.
To make a hone
2 pieces of leather can be glued down to the wood block, finish
side up. You need to prep the leather for before applying compound by
sanding the smooth surface until resembles a fine suede. This is the optimal
surface to hold compound and hone your knives to a perfect edge.
Bark River Knife and Tool sells bars of compound at a very reasonable price
through their dealers that have the correct degree of abrasiveness. The black
compound is more coarse that the green.
Applying compound is simply a matter of rubbing the bar of compound against the leather surface, The best way to explain how much is to think of how that prissy little girl in fourth grade colored, just a slight hint of color on the leather is all you need.
Compound is a fine abrasive that in conjunction with the leather refines the edge. Too much compound and the abrasive component of the compound simply flakes off the leather, this is a case of a little bit being a bunch better than a lot. The pads in the pictures have too much compound on them for honing but just enough for demonstrative photos.
Using the same technique as on the abrasive paper, hone the blade, several strokes on the
black followed by the green.
Once you have the edge good and sharp, you really only need to
hone it to keep it keen, saving the abrasive paper for when the
edge is nicked or if you get it really dull.
There are many ways of seeing how sharp and even an edge is,
one I use is slicing paper. The thinner the paper, the sharper the
knife must be. Here's a couple of fine curls made by the
NorthStar I sharpened for this article on some this white paper,
after passing it around to ten or so people over a weekend to try
it out, the edge was pretty rough.
Now you can see that it makes some very fine curls in the paper.
It only takes a few seconds to get the edge back to hair popping if
you get in the habit of stropping after a day’s work.
Feel free to link to this article so long as you credit the Bark River Knife Collector’s Association.
Reprinting for distribution or publication requires permission, please contact the President of BRKCA
for permission.
© Reid T. Hyken 2008
Comment