Convex Edge Maintenance

David, work shap convex sharpening are made on a slack belt. The pressure you use decide how the belt grind away material from the edge.

If your knife have a convex edge in for example 4 degrees and your pressure against the belt if just as hard that the belt follow the 4 degree sphere it will be grinded perfectly in 4 degrees.
If your pressure are just a little harder the belt will grind hard on the cutting edge and the backbevel and the cutting edge will get a big change in angle very fast. The cutting edge will be sharp, you cannot see the change, you will probebly dont feel the change in edge angle.
Over time this will make your edge angle so high that the knife do not penetrate material - and you must regrind the edge back to its starting point.

My tool Chef can meassure the convex sphere, with the help of the built in protractor you can meassure how many degrees the convex sphere holds.

Thomas
 
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Bear71,

fvveya.jpg


The image above are from my homepage 2002 and shows my sharpening tool AxePal with my innovation the bended guide rod that sharpen convex edges.

AxePal have a strong magnetic foot and the pivot point can be adjusted in two ways, the bar can slide along the standing rod and the pivot screw can be very fine adjusted, half a turn on the pivot screw change the edge angle 15/100 parts of 1 degree. 3 full turns = 1 degree change of the angle.

The bended guide rod for convex edges, and AxePal, are innovations I made for more then 15 years ago and I own the Copyright on them both.

Thomas
 
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Havyhanded,
I have meet a lot of people that clime that they, by freehand, can grind/sharpen edges without changing the edge angle what so ever - but I have never seen it be made. A lot of people have show me how they do - but the edge angle what not the same after the sharpening.

The cutting edge are very thin, just some thousends part if 1 mm thick. The differance between 10 and 11 degree on the cutting edge are less then 1/1000 part of 1 mm. 0,05 mm behind the edge the change are 1/1000 part of 1 mm.

What happens on the edge in exact 10 degrees if a diamond sharpener on 25 mikron slide across the edge in 11 degrees 1 time.
Will the edge angle on the cutting edge change from 10 to 11 degrees on the part it slided on?

1 micron (1,000 001 meter) are 1/1000 mm thick (= 1,000 001 meter).

How deep go the 25 micron diamonds in the edge material in one stroke - and how many diamonds are passning the cutting edge side, and how many scratches will be made?

I agree in the principle that when a 3 degree convex sphere edge shall be sharpened by freehand it is a nice method to try to sharpen it flat and only in the middle of the convex sphere. That method will give the less change of the cutting edge angle - but it will not keep the angle there was on the edge, it will just delay the convexing process to the point where the edge is to steep for ghe knifes porpouse.

Sorry about my English. I know that I can "sound" aggressive - but I am not. :)

Thomas
 
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First off, your English is very good, no aggression whatsoever.

First off I would never claim I can manually hold a perfect angle or even guarantee one every time. I will assert that if I manually sharpen the same tool five times it will fall in a relatively narrow deviation around the target - it will not drift in one direction only. In general if it does develop a consistent trend away from the target angle, it will become more acute, not less.

When I first got into convex edges I ran into a bit of apex creep. Once I took a look at where I had to do a lot of work to correct this, I began to simply keep track of it every touch-up.

Being mindful that this sort of creep can effect every edge, esp chisels and axes, I made sure to thin every time and mark my progress. I began doing the same to all my convex edges, and on my V bevels I make a point to work from the shoulder to the edge every progression. In theory this should make the edge more acute at every step but due to freehand creep, it generally manages to simply maintain the target angle. Convex edges properly maintained should only need a handful of light passes at the apex after having the approach properly thinned.

Initially I estimated my angles by gauging where they caught when sliding forward on pine, then I began to use the focal depth on a high powered optical microscope. Once I built my own guided equipment I had a chance to verify my assumed angles.

My Bark River full convex I estimated at 12°/side at the apex was actually 11. Likewise my Puukkos were within a degree at the apex and most had a degree or two drift from shoulder to apex. Probably more consistent then when I bought them, and this after many resharpenings. Most of my V bevel edges were close enough that pressure variances on a guided system might account for nearly all of it - within a degree or two at the apex.

My freehand when I'm hot might hold to within a degree per side, but really I'm mostly interested not in the total deviation (though I constantly shoot to decrease this) as much as I am in holding the angle at the apex < the target angle.

This is not the easiest to learn, but not impossible. People just have to recognize this is the normal trend and guard against it.
 
Thanks about my English :)

I agree about what you write. I lived 6 month at the time outddors and my knifes was my only tools and I sharpen them daily, sometimes many times per day. I could se how the edge changes over time - and of cause i tryed to hold the same edge angle - in the target angle as you say. I freehand sharpen for 40 years - and I study my edges by loupe and microskope to understand what happens -and why.
This experiance make me construct a sharpening tool becouse my needs was to have a functional knife edge and I needed functional edges, both flat and convex edges. This demand force me to solve the problem how to sharpen convex edges with full control of the angle so that my knifes edges did not change during 6 month outdoors and daily sharpening. My tecnical solution was the bended guide rod.
Today the bended guide rod is well known and used all over the world - but when I innovate it there was a lot of resistance. People did not belive, or understood, that it really works and that it give control of the cutting edge angle - and also control of the convex sphere degrees.

if you have a Lansky, just bend the allready 90 degree bend about 10 degrees more on one of the guide rods - and now the sharpener makes a controled convexing move across the edge. The same curve every time. Use 2 stopnuts to decide how long surface you shall use on the sharpener so that you get a starting point and a stop point. Study how it works on the edge and meassure with a angle Cube. Sharpen a knife and evaluate the edge.

Thomas
 
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