What Grit Sandpaper for Making a Knife Convex

I use 220 (120 if it really needs some meat taken off). I usually finish at 400 grit or maybe 800. If its a light use knife I might go as high as 2000 grit or even .3 micron lapping compound. Its just like a V bevel in that respect, what type of cutting are you doing, what type of edge do you prefer? Only caveat is if it's a carbon steel blade and you're doing a full convex edge, you might want to polish the blade face to 600-800 minimum to help w/ rusting. This can be done fairly easily even if you don't want to sharpen the cutting edge that fine, just something to consider.

Not going into the coarse vs polished debate (what IS a polished or "fine" edge anyway?) as far as which one cuts better, but have a few observations - why would the "teeth" at 120 grit be more fragile than the "teeth" at 1200 grit? Properly finished edges should all look pretty much the same except for scale.
Also, have been getting into neck knives lately, converting them to belt wear for EDU. Using a Kydex sheath, my edges dull relatively quickly (compared to a folder or fixed w/ a leather sheath) just being taken many times in and out of the sheath daily. The higher the finishing grit, the faster they seem to dull - 12c27, 1095, and 1095CroVan all seem to follow this pattern. Was over on the Becker forum and there are a few threads where some owners have noticed the same trend on their BK11 and BK14s.

HH
 
xtestifyx said:
...Saw teeth are much larger than micro serrations. We're talking in the thousands here, Teeth don't mean teeth. Teeth are jagged edges, aren't they?

Saw and knives are different tools in that one locks onto the material and rips it away and the other splits the material in two
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Saw teeth are a form of "macro"-serration. Both are teeth. However, the design of the teeth and position in relation to one another determines how they will function during a cut.
The reason the teeth are advantageous at all on a cutting edge is that they reduced the surface area over which the perpendicular force of the cut is exerted, thereby GREATLY increasing pressure on that surface (assuming a decent design). Try pushing the wider belly of a blade through tough material, then try stabbing the material with a narrow tip using the same amount of force - the tip attacks a smaller surface area with that same force, and since P=F/A...
What a toothy edge accomplishes is this same reduction in surface area, which gives it its "bite" compared to a smooth edge that might only "skate" over the surface unless greater force is applied.

Whether or not the serrations remove material during the cut depends upon design of the teeth. Saw teeth can be designed to cut on either one or both the pull and the push stroke depending on the edge angles of each tooth in 3 dimensions. A tooth which, after penetrating into the medium being cut, is then drawn "horizontally" through the medium, will split the material if the edge facing the direction of motion is designed such that it applies sufficient force to cause such separation. However, if that side of the tooth is angled horizontally perpendicular to the direction of draw (e.g. it's flat), then the force of the draw may result in the material fracturing rather than splitting, and being removed as the tooth's flat edge plows it along.

Saw teeth can also be designed into a single or multiple staggered rows along the main edge. The off-set of such teeth results in maceration of the material being cut as the teeth first penetrate vertically and then draw horizontally along. This makes for a very effective saw.

The effects you see with the various designs of saw teeth is a magnified demonstration of what occurs with a coarse finish or micro-serrations.

An effect you are also likely to see is damage to or loss of teeth, since a tooth is inherently more fragile the narrower it is made (at a given length), having less metal to support it. However, if there are many teeth then the loss of only one or a few along the entire edge will not significantly effect cutting performance.

Is this making sense? Should a new thread be started? Applied physics can be a pain without proper illustrations... I hope this helps clarify the stance, at any rate.
 
What method do you guys use to find the angle for a convex edge? I've seen most tutorials say around 13˚, is that correct?

Erdbeereis
 
Good question. It all depends on how conformable the backing is. I find I need to drop between 5 and 10 degrees from a V grind. That is if you went down to 13 degrees, it should produce about an 18 degrees cutting angle/side - approx 36 degrees inclusive. A lot depends on the backing and very small differences in pressure. For a full convex on a thick blade, I might drop down to 8 degrees on a fairly soft backing. If the backing is harder, you can polish/convex the entire blade face using multiple passes and change your angle slightly per pass.

HH
 
Do you need to remove the burr in between grits, or just a the end?
How do you recommend removing it?:confused:

Erdbeereis

P.S. Sorry for all the newb questions.:o
 
That's sort of a personal preference, but I'd recommend doing so. At the lower grits - under 1000 - I'd say use black emery compound on a strop. You could also run the edge over a scrap piece of hardwood and that actually works OK most of the time. Just cut across the wood grain with moderate to light pressure, and then lightly back-drag the edge at a steep angle along a corner of your scrap to see if you got it all - if you didn't, it'll stand up on the same side and you'll be able to see it easier. Repeat and change sides as needed. I'm too much of a stickler to rely on that for my final burr removal, but practically speaking its about all you need to do.
HH
 
If you're changing an entire blade over to convex, its (IME) very important to use a sharpie on the the entire blade face - make a grid or a bunch of dots. Then you'll know exactly where you're grinding metal from and if you need to change your angle, pressure, backing etc. If you're working along the actual cutting edge with your sandpaper instead of grinding the shoulder off the old V grind, or somewhere further back up the face of the blade, you'll raise a burr very quickly using 80 or 120 grit - maybe 20 passes and for sure by 30-40 passes. That said, it can take a lot of passes (100s using light pressure) to get to the point where you've made it to the apex, so it's important to mark up the entire area w/ a sharpie. If you're just convexing the edge you'll still be able to tell from checking your sharpie, but again it can take many passes to remove the old shoulder after which you'll raise a burr very quickly. Hope this is intelligible.

HH
 
Thanks, the knife looks like it's convex but it just doesn't get any sharper. I've tried the sharpie, and it looks like it's grinding it away.
I think I just need to spend a little more time on it.

Thanks a lot!

Erdbeereis
 
Convex edges are deceptively simple. Once they've been successfully converted they're pretty easy to maintain, but it's all too easy to convex unintentionally at rather obtuse edge bevels. If your edge is turning out at 40+ degrees, it'll get plenty sharp, but still not slice well. Lay it flat on a piece of leather and slowly push the edge as you elevate the spine, like you're trying to lightly shave a slice off the top. Wherever it catches is what your edge angle wound up at per side. If this is too large that could be your problem. You also have to pay very close attention to getting rid of that burr. All that edge-trailing grinding you've been doing could easily leave a bunch of played-out metal at the edge. Make sure your angle isn't too broad, and then I'd say you either need to further refine the edge, or you've got some issues w/ rounding it over or burr removal. Very light pressure. What grit paper are you working with at this point?

HH
 
That 150 grit will give you a pretty aggressive toothy edge, but its at the low end of what will push cut newspaper or shave arm hair without really knowing what you're doing to finish it off. Move on up to 400 grit and you'll start to see a change in how she cuts. What do you have for a strop? 400 grit stropped w/ the black emery from Sears will produce a very nice all-purpose edge, and I've also gotten arm hair-shaving edges from 400 grit carefully stropped on cardboard. From there you could step up again to 800 grit and so on. I cannot stress enough how easy it is to convex an edge that is very thick without trying, or accidentally round the edge over. Very light pressure and go a little steeper than you think is necessary.

HH
 
I'm trying to do an old dull SAK, I'vs done over around 100-200 passes but I don't think I forming a burr.
I think it's getting a little sharper, but I'm not sure.

I don't really have anything to strop with, but some people seem to have success with a phonebook cover, but I'm not going to worry about that until I can form a burr on one side of the blade.

Erdbeereis
 
Definitely time to mark up the entire side of the blade and find out exactly where you're grinding. Repeat with the sharpie as needed till you get it under control. 200 passes w/ 150 grit should have something to show for it.
 
Erdbeereis said:
I'm trying to do an old dull SAK, I'vs done over around 100-200 passes but I don't think I forming a burr.
I think it's getting a little sharper, but I'm not sure.

I don't really have anything to strop with, but some people seem to have success with a phonebook cover, but I'm not going to worry about that until I can form a burr on one side of the blade.

Erdbeereis
Click to expand...

I'll offer this suggestion. A lot of people, myself included, advise to 'form a burr' before progressing beyond the initial coarse stage. To someone who's fairly accustomed to looking for a burr when sharpening, it's a fairly straightforward objective. But, I do remember when I was still trying to learn the process, sometimes it's not quite so obvious (by the naked eye, or by 'feel') to detect when or if the burr's been formed. The turning point came for me, when I began to use some decent magnification to closely inspect the edge. With at least 10x or better, it's much easier to see if the edge has been completely apexed (formed to a nice, sharp 'V'). A burr, if present, will also become obvious under magnification. I really do believe it's better to actually SEE the burr, than to 'feel' for it by other means. The 'feel' aspect will come with repetition and experience, and you'll begin to recognize other 'signs' that the burr has been formed. Another variable is, some steels make more of a burr than others. I've had at least a few knives that never produced much of a burr at all, in spite of the fact that the edge had long since been apexed. But I couldn't see that until I actually inspected the edge under magnification. It will also reveal other issues, such as if the edge is rounding off (which means the sharpening angle is not being consistently maintained).

I really do think a magnifier would help you (and everyone) greatly. I've been in the habit of recommending this a LOT lately, and probably sound like a broken record to some on this point. But I keep bringing it up because it made a HUGE difference for me. Give it a try.
 
Erdbeereis said:
Does it matter if the knife is really dull before convexing it?

Erdbeereis
Click to expand...

Essentially, no. BUT, I have found that convexing is a LOT easier when I've used something like a diamond hone to initially put a sharp edge on it. I view it as sort of a 'short-cut' for getting the job done. Once that's done, you DO need to be extra careful to not undo that effort. In a sense, that's a very good thing, because it'll force you to focus on keeping the angle low and pressure light, so you don't round off the edge. Most of 'convex' sharpening involves shaping the bevel itself, and rounding & smoothing the 'shoulders' of the bevel. The very edge itself, is still just the 'edge'. It's either sharp, or it isn't.
 
What exactly does a burr look like.

Erdbeereis

P.S. how do you post a pic?
 
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Erdbeereis said:
Here is a link of where the sharpie is gone:http://www.flickr.com/photos/erdbeereis/5598832281/
What exactly does a burr look like.

Erdbeereis

P.S. how do you post a pic?
Click to expand...

The link you posted is requiring a login to Flickr. Can't see the pic. You need to copy the entire link location (should be an URL, like your link, but ending in the name of the picture file itself (usually with a '.jpg' or '.jpeg' file extension). When you've copied the file location, use the 'Insert Image' icon in the editor here to paste the link into your post.

The burr is just a thin, ragged sliver of metal, left at the edge when it's become apexed. In essence, the edge has become thin enough, that the metal there will 'fold' over, one way or the other. Sometimes, it'll appear in isolated locations, looking like ragged 'teeth' hanging loosely from the edge. Other times, it'll be a longer sliver, running along the length of the edge. That's usually referred to as a 'wire edge'.
 
5598832281_538a54325d_z.jpg

There that should work.

Erdbeereis
 
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