Stropping Theory
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I don’t know how many of you follow Curry Custom Cutlery on YouTube, but Cliff has been a long time WE user and has recently (the last year or two) gotten into powered belt sharpening as well. He makes superbly informative videos on sharpening and I thought I’d share one of his more recent examples here. The video contains an interesting discussion on the theory of stropping and how to overcome the rounding of the apex. Enjoy!
That’s a good video Organic, I have watched several videos by C.C.C. and they’ve been informative.
The one thing I would like to add to the information is the effect of the shoulder of the bevel and the effect it has on the cutting action of an edge. Since we back off the angle so as not to roll the cutting edge during stropping the highest pressure on the strop is at the shoulder of the bevel. The further away from the shoulder of the bevel the less pressure on the bevel at that point and thus the micro convex bevel(pressure lessening the further from the contact point you get). The rounding and polish of the bevel shoulder is very advantageous on thin knifes especially. On FFG knives that tend to be thicker steel not rounding the shoulder off to much helps material that would normally adhere to the blade after/while cutting less of an issue. By leaving the shoulder intact the sharp angle of the bevel shoulder in relationship to the blade grind will help the blade repel the material that would normally adhere to the it. I accomplish this by putting a micro bevel on at 2 or 3 deg. more than the sharpened angle (in this scenario 3 deg was easier) and then stropping lightly and without to many passes at the same angle as sharpened. example… sharpened at 17 then micro at 20 then strop at 17 deg. This puts even pressure on the 17 deg. angle and spreads the pressure evenly across that bevel. The trick is not to use so much pressure stropping that you wind up coming over the cutting edge which is now at 20 deg. or rounding the shoulder more than desired. I think if you try this method you will be surprised that you have to put quite a bit of pressure on it in that scenario to effect either adversely. Because the pressure is spread out over a greater area the bevel will not sink into the leather as much thus does not rebound as much or as quickly as it would when all the pressure is put at the shoulder. By using lighter pressure, it helps me keep more of the integrity of the shoulder while still cleaning up the edge.
I’ve been trying to understand why some of what I have been doing with the strops works and why some things I’ve tried didn’t. When trying to help someone with stropping the difficulty comes in the fact that I can’t tell what heavy and light means to a specific person, how consistent are their strokes, and conveying all the variables that come with stropping and how it effects the entire bevel and not just the cutting edge. Stropping is more sensitive to different steels, the speed you use to strop, the pressure, the sharpened angle, the stropping material so on and so on.
I use different angles, mediums, compounds and pressure depending on what I’m trying to accomplish. This makes it very difficult to help someone with stropping in general and makes the above video valuable to start to understand what is happening with a said material against a said metal at a said angle.
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That’s a good video Organic, I have watched several videos by C.C.C. and they’ve been informative. The one thing I would like to add to the information is the effect of the shoulder of the bevel and the effect it has on the cutting action of an edge. Since we back off the angle so as not to roll the cutting edge during stropping the highest pressure on the strop is at the shoulder of the bevel. The further away from the shoulder of the bevel the less pressure on the bevel at that point and thus the micro convex bevel(pressure lessening the further from the contact point you get). The rounding and polish of the bevel shoulder is very advantageous on thin knifes especially. On FFG knives that tend to be thicker steel not rounding the shoulder off to much helps material that would normally adhere to the blade after/while cutting less of an issue. By leaving the shoulder intact the sharp angle of the bevel shoulder in relationship to the blade grind will help the blade repel the material that would normally adhere to the it. I accomplish this by putting a micro bevel on at 2 or 3 deg. more than the sharpened angle (in this scenario 3 deg was easier) and then stropping lightly and without to many passes at the same angle as sharpened. example… sharpened at 17 then micro at 20 then strop at 17 deg. This puts even pressure on the 17 deg. angle and spreads the pressure evenly across that bevel. The trick is not to use so much pressure stropping that you wind up coming over the cutting edge which is now at 20 deg. or rounding the shoulder more than desired. I think if you try this method you will be surprised that you have to put quite a bit of pressure on it in that scenario to effect either adversely. Because the pressure is spread out over a greater area the bevel will not sink into the leather as much thus does not rebound as much or as quickly as it would when all the pressure is put at the shoulder. By using lighter pressure, it helps me keep more of the integrity of the shoulder while still cleaning up the edge. I’ve been trying to understand why some of what I have been doing with the strops works and why some things I’ve tried didn’t. When trying to help someone with stropping the difficulty comes in the fact that I can’t tell what heavy and light means to a specific person, how consistent are their strokes, and conveying all the variables that come with stropping and how it effects the entire bevel and not just the cutting edge. Stropping is more sensitive to different steels, the speed you use to strop, the pressure, the sharpened angle, the stropping material so on and so on. I use different angles, mediums, compounds and pressure depending on what I’m trying to accomplish. This makes it very difficult to help someone with stropping in general and makes the above video valuable to start to understand what is happening with a said material against a said metal at a said angle.
The sharpness of the bevel with relation to the primary grind of the knife as it affects product release is not something I had considered before. That’s pretty interesting. Getting a blade with a thin, high performance bevel and good product release is a highly sought after combination for chef’s knives in particular.
I have found that heavy pressure when stropping usually improves cutting performance more effectively than light pressure but I couldn’t explain why that is. When I first tried stropping with the WE I wasn’t getting much of an improvement (if any) compared to the edge directly off the stones. I had been using too little pressure because I had read so much about stropping causing the apex to get rounded off. I did some further research and Clay had posted about how he uses enough pressure while stropping that sometimes the whole setup lifts off the table so I decided to give it a much firmer hand and that has worked for me.
I hear you Organic, for me the pressure on the strop has to match not only how much you back off the angle but the material of the strop and the surface area of the contact point as well. When you back off the sharpened angle 2 deg. like is recommended when you start with the WE you need a lot of pressure on the strop to reach the cutting edge. I’ve found 2 deg. back and heavy pressure is approximately correct for cow hide, but with kangaroo I found I could not get to the cutting edge at -2 deg. That’s my problem in explaining stropping as I see it because there are so many variables to be considered it makes explanations confusing and extensive. Add that to the fact that I’m not the sharpest knife in the drawer and…there you go!
My general rules for strops on a V edge with no micro bevels are
Kangaroo minus 1 deg. medium/heavy pressure
Balsa minus 1.5 deg. medium/heavy pressure
Leather minus 2 deg. heavy pressure
Micro beveled edges(dependent on the size of the micro bevel)
Kangaroo minus zero to .5 deg. medium pressure
Balsa minus .5 to 1 deg. medium pressure
Leather minus .5 to 1.5 medium/heavy pressure
These are starting points for me and then I take other factors into consideration.
Thanks for the detailed breakdown of your stropping starting points. I only have experience with the cow leather and I have been stropping at -1.5 degrees from the sharpened angle. If I sharpened at 20 degrees per side, I change the angle to 18.5 degrees per side before stropping with heavy pressure using the cow leather strops and that gives really good results in my experience.
I believe I referred to it at the beginning as a micro convex if I’m not mistaken. A strop will not create a pronounced convex bevel like on a slack belt sander, thus my term of micro convex.
A strop rounds the bevel starting at the shoulder of the bevel (point of greatest pressure) and diminishes from that point all the way to the cutting edge where hopefully the strop has very little pressure so not to roll over the edge. The more the angle is backed off the more pressure necessary to get the strop back to the cutting edge and the quicker the metal is removed from the shoulder.The more strokes used the more the convex as the metal is removed faster at the shoulder and lessens all the way to the edge.
A hard strop will not convex much and a soft strop will convex more as you have to back the angle off more to prevent rolling the edge. A belt sander with a tight belt will not convex as much as a sander with a slack belt will. The more give that there is in the strop the more it will convex the bevel.
So you see it’s not just pressure that produces a convex bevel, it’s the combination of pressure, angle, number of reps. and the amount of give in the strops. (soft or hard)
Gotcha, I read what your saying. I know it applies to a slack belt sander with a stropping belt. I’d love to see a USB Photo of this done with a Wicked Edge and a strop showing this convexing. It seems like it would have to be a very very good balancing act between strop hardness or softness, stropping pressure and the angle backed off and a lot of experience with all three.
Marc
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Another great video, Cliff. Since learning of your angle guide for the HF 1X30, I’ve been playing with mine with similar (but not as refined) results. I hadn’t thought of re-orienting the rig to put it in the horizontal plane. Gonna have to switch that around, I see.
I have tried sharpening only on my buffing wheels with pretty fair results, too. I have the left wheel at 600-grit and the right wheel at 4 micron diamond compound. Certainly, if there is significant damage, one would have to go to the belt sander to start with.
I see that you started with 120 grit and wondered why you went so coarse. Everything I’ve seen on belt sharpening shows them starting at 220. It seemed to me by the size of the burr that you were removing quite a bit of steel. I would be interested if you’ve ever checked the “before and after” blade width, just to get an idea if it (starting with 120 versus 220) makes a difference.
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