Convex bevel width; primary bevel scratching
- This topic has 24 replies, 5 voices, and was last updated 11/07/2018 at 12:03 am by
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Hey, y’all.
Made it to Hawaii. Helping folks teraform the lava. One more month, and I’ll be back in the forum more.
I’m practicing convexing. Love it deep, and smaller bevel sizes are easier to blend. Ground a drawknife into my machete, 1/2 inch convex bevel. Ended up doing 5 bevels: 18/14/12 as large bevels, 16/13 small to knock down the larger bevel edges. It’s beautiful, but I wasn’t able to blend them to a perfect curve… Still facetted.
Anyone experimented with blending bevels over a larger area? I’ll probably just need to do bevels each °. I’ll test it, but it may be a mm, or 1.5 mm bevel width max for seamless blending.
Another thing: I’m leaning more towards the method of doing the lowest angle bevel first, then the highest for apex, then bevels between. Problem is, on doing even 2° higher than apex bevel, the apex bevel gets scratched… Happened twice, and chapped my hide. Any idea what could be going on, or how to avoid it? I’m wondering if the swarf/grinding powder isn’t getting built up and getting dragged over the polished edge…
Sorry about not replying to all y’alls previous replies. I will, once I get back on the road for sweet, sweet R&R. Chomping at the bit to listen to everybody’s suggested playlists.
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I’ve only done convex on very thin knives, so my experience might be a bit off on this one. I think the facets you are getting are because of the large angle change between steps. I’ve been stepping 1/2 degree at a time, with a total of 4 degrees. Works great for thin knives. I also keep the number of strokes with each step to a minimum, After reaching an apex polished to 800 grit, I then step down 4 degrees in 1/2 degree steps and take maybe 3 or 4 strokes with the 800 at each step. After finishing the 4 degree facets with the 800, I start all over with the 1000 stones, first polishing the apex, then stepping down to the 4 degrees with 3 or 4 strokes at each step. I continue this with each of the following grits: 1500, 6 micron and 3 micron. You can see the facets by the scratch patterns (I alternate directions with each grit change), but the diamond film has just enough compliance to round the edges of the facets.
With a machete, I would still go with the 1/2 degree steps, but you’d probably start the convex protocol with a lower grit – maybe as low as 400-grit.
With the smaller steps, I think you can take fewer strokes at each step and this should make the facets less obvious.Good luck Drew.
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Maybe using the strops with emulsion (which burnishes instead of abraids) would smooth over the facets. It would take some experiments to determine the effectiveness against 2, 1, or 0.5 deg facet changes but with a USB scope you should be able to tell pretty fast. I love what the burnishing does to my bevel. I follow the “back off 2 deg from the apex” rule of thumb to avoid rolling the edge and it works great.
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I should point out that I do the 1/2 degrees steps on my Gen 3 Pro, where that’s really easy. If I were trying to decide how to manage it with a thicker blade, I’d finish the apex with the 800 grit and then I’d make the first 1/2-degree step down and then watch to see how many strokes it takes to get the new scratch pattern close to but not touching the apex. This would give you and idea of how much steel you’re removing and how many strokes it’ll take at each step. After 800-grit, not much additional steel is removed. I’ve developed a habit where every grit has a stroke direction change from the one before it. Also needs to be said that I don’t try to achieve mirror finishes and that you may require additional strokes if you’re OCD.
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tc: How do you determine the lowest grit to start each bevel below the primary? I want to avoid scratching the main bevel each time I drop down in degrees, like the first few times. You ever apex to 800, drop down 1/2°, use a 200, and find a few deep, weird scratches into the 800 grit pattern? If these stones are flat, and the cube is spot on, I don’t know how to explain that. .
Which grit to start the convex facets on depends on how much steel is to be removed. I’ve never gone backwards in grit progression, so producing scratches on previously polished bevels isn’t a problem. As I’ve said, I haven’t tried putting a convex edge on a thick blade, but each step removes very little steel, especially when taking 1/2-degree steps. When I start the secondary bevels, I watch to make sure I’m not erasing the primary bevel all the way to the apex. For thin knives, it’s been a few as three strokes.
If I haven’t been clear on it, let me explain further, with a typical, thin blade…
Regardless of which grit I’ve started the sharpening process with, I take each grit to the apex, erasing most, if not all the scratches from the previous grit. If I’ve decided to start the convex facets with my 800 grit stones, when I’ve finished the main bevel at the apex, I’ll then step down thru the convex facets with that 800-grit stone: -0.5 degrees, -1.0 degrees, -1.5 degrees, etc. Having created the series of convex facets with the 800-grit, I then go to the 1000-grit and follow the same sequence: apex, then -0.5 degrees, -1.0 degrees, -1.5 degrees, etc. Having finished with the 1000-grit, I then switch to the 1500-grit and so on and so on, all the way to my ultimate polish. When I’m done, the convex profile should be very nearly the same degree of polish as at the apex.
You should not require as many strokes for each convex facet as you would at the apex. This is because you are removing a relatively small amount of steel. Theoretically, you shouldn’t need as many strokes at the apex with grits following the initial creation of the convex facets. This is because the width of the apex bevel has been reduced.
Drew, just to add this, even though the tolerance of the stone paddle handles on the guide rods are pretty tight, by changing your finger placement and applied hand pressure along the paddle handle length, from holding it by the lower end to holding it by the upper end, while sharpening in each 1/2 º angle setting, this too will help blend your facets.
Marc
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I place the cube in the same position relative to the stones, and hold the stones in the same position relative to the knife edge, on both knife sides, each time I measure and set any guide rod angles, for consistency. For me the cube is usually even to the top of the stone. Remember our angles we measure are relative angles of the knife’s bevel relative to the imagined vertical knife center line. These are based on the digital angle cube zeroed to a flat horizontal position on the base. (I use this same position each time I check the cube for zero, again, for consistency). So each time, if you use a consistent method and technique to make your measurements and adjustments you should get consistent results.
The same thing goes for my sharpening strokes. I use consistent finger placement on the stones. Find what is comfortable for you and use this grip consistently. If your technique is repeatably consistent you’ll see predictably consistent results every time you re-sharpen the same knives. If it’s clamped the same each time.
Only when I choose to steer the stones action on the bevel do I move my finger placement to direct the stones point of contact more precisely.
All of these results are observable using a lighted magnified visual aide to see the scratch pattern your creating with your stone work. I use a USB microscope used with a laptop computer.
Marc
(MarcH's Rack-Its)MarcH: Gotcha. That’s a really good suggestion. I’ll pay attention to the finger placement on the stones, and find a position that’s comfortable and repeatable.
The neighbors must think I’m either a tweaker or on the spectrum. In my hut on the pitch black lava, headlamp on, scratching arcs over and over for hours at night… Maybe I do have a good touch of autism. Your talk of consistency and repeatable patterns give me a soothing reaction.
All these suggestions are pure gold. I consciously appreciate them.
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I also try to do things as consistently as possible. Holding the stones at the base of each paddle is my preference because it is consistent and it helps keep my hands below the edge of my knife for an added layer of safety. My fingers can’t get cut if they never go above the blade.
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