Kershaw Outcast

[Cliff Stamp]

...I should put the full flat ground side on the platen, and regrind it so my new primary bevel runs from the spine (leaving that at its current thickness) to the shoulder of the edge, and the thickness at that shoulder should be around 0.025"...

This is the optimal solution, you then follow this by adjusting the edge angle to a suitable profile. Both the exact thickness of the edge and the edge profile depend on your strength/experience and the local wood type. If you grind down to 0.025" then add a relief grind to the edge of about 8/10 degrees which arcs to about 12/14 degrees in the last 0.015" or so of steel you should have a very durable edge (spruce, oak, birch, etc.) which cuts very well.

Applying the relief to the edge will widen it significantly as the NIB profile is far more obtuse so it will widen back to 0.030"+ at the shoulder. This will prevent it from rippling in extreme work such as heavy batoning. Assuming the steel is solid it should be able to just push cut off small limbs requiring almost no actual chopping motion from the shoulder, and still be easily durable enough to cut through thick knots while batoning.

If you skip the primary grind modification and just apply a relief to the edge it will end up severely wide and be horrible to sharpen due to the low grindability of D2. Most users can't actually work on primary grinds hence the popularity of edge relief grinds but they are in fact the worst solution, but still a large improvement over most initial edges. You can combat the drawback of having to sharpen a wide bevel with micro-beveling but it is still vastly less efficient than thinning the primary grind.

Specifically for example I recently took a heavily used Rucksack which had a very acute edge, less than ten degrees per side, but which was very wide and thus inefficient to sharpen, and ground the primary grind down so the edge was reduced to under 0.005" thick. The stainless steel they used is easy to grind so this was only a few minutes work. It now cuts much better and is *way* easier to sharpen as the edge is a small fraction of a millimeter wide.

-Cliff

 

[Nick Hyle]

OK, I have a better idea of exactly what you both mean. I'll hold off for a little while so I can try to learn from Ken's grinding mistakes instead of having to learn from my own.

 

[Ken C.]

Nick,

My knife was reground by Mike Stewart of Bark River Knives. The man knows a thing or two about convex grinding! I would never take on a task such as that without some help. I make sheaths, not knives. I truly suck at grinding!

 

[Cliff Stamp]

You really can't make a mistake aside from aesthetics and overheating. The biggest concern is going to far, in the above I was somewhat cautious in that respect. On a personal knife I was using I would go significantly thinner in the primary grind. Probably take it down to 0.015 before sharpening if the stock angle was 20+ degrees. Start off with something like a $10 machete to give you an idea of your personal limits on geometry on local woods because any decent blade should be able to exceed that (go thinner).

-Cliff

 

[Nick Hyle]

Ken, you bum, I was figuring you were toughing it out like I was going to.

Anyway, start smacking that reground edge around and give us some feedback. I'm definitely interested.

Cliff, my temptation is going to be to dust it down a bit at a time. Feel free to critique my thinking, which goes like this.

I am a big, clumsy, accident prone bastid. I'd like to get my Outcast "as thick as it can be and still do good work", with a safety factor for idiocy, rather than "Thin as it can be and still be tough enough". Call me a weenie.

I'm thinking of the Outcast as a general-use badass big blade - I'd rather leave it tough enough to hit the odd nail, rock, etc, than optimize it as a woodworking tool.

Since for heavy sharpening I use the same 2x72 Burr King I'll be using to grind it down, touching up even a really wide bevel just isn't that much work. It may be a pain in the ass to get it right, but it won't be work in the sense it would be if I were doing it by hand or even on a baby 1x42 grinder.

So right now my plan goes:

Wait for Ken's feedback;

Decide whether I want to play flat or convexed to start;

Dust it down to ~0.035 to ~0.030 or so;

See how big a performance jump I get, and what kind of edge toughness;

then decide whether or not to take it down to the 0.015" to 0.025 range.

I do like your "screw up something cheap first" idea - this weekend I think I'll go digging in the shop for a machete or similar to thin down as a test piece.

I have an old stainless Blackjack Panga, and the edge as ground is thin enough that I've rippled it in a couple places. I'd like to avoid that with the Outcast.

 

[Cliff Stamp]

...dust it down a bit at a time.

That was what I did many times on different blades for the reason you noted, to gauge the extent of the performance increase as the geometry changed and its nature.

I am a big, clumsy, accident prone bastid. I'd like to get my Outcast "as thick as it can be and still do good work", with a safety factor for idiocy ...

The origional edge thickness I listed assumed that, it was far heavier than a pure wood working blade. I could not ripple a Heafner bowie for example (also D2) with an edge just under 0.030" thick and I was pounding it through cross knots in rounds so twisted that it was making fractional progress. The impacts were heavier than full strength hits from a 28 oz framing hammer. You can't approach this type of impact when chopping, even abusive cutting like inverted clipping of deadwoods.

If you want to be able to cut nails without significant damage, modifing the primary grind stands out because it allows you to increase the cutting ability without losing shallow edge durability as the edge angle stays the same and edge angle has a critical influence on preventing fracture in such an impact. Most applications of convex edges are actually just relief grinds. The edge doesn't cut better because it went from flat to convex, it cuts better because the cross section was reduced. It isn't like if the edge thickness stayed the same and the edge curvature increased it would cut better.

With a significant relief to the edge which cuts heavy into the shoulder and reduces the edge angle. The edge will react far worse when you crack it into something hard such as rocks hidden by bush. For cutting hard materials like metals with minimal damage on a large knife of that size you want an edge about 20 degrees per side. The edge thickness isn't really an issue, I have never ripped an edge on a nail or rock impact, the damage is usually really light in terms of depth because the edge will just take localized damage immediately. It is difficult to get damage to be above 0.015" and even that tends to need multiple impacts on the same spot.

Since for heavy sharpening I use the same 2x72 Burr King I'll be using to grind it down, touching up even a really wide bevel just isn't that much work. It may be a pain in the ass to get it right, but it won't be work in the sense it would be if I were doing it by hand or even on a baby 1x42 grinder.

With power equipment it doesn't matter then the width of the bevel in regards to sharpening. Even a tiny belt sander rips off steel so fast that sharpening any steel is trivial in terms of time. Just consider how fast the belt goes around and how many passes on a stone that equates to in just a few seconds. There are still other issues though as noted. Consider in general why knives have primary grinds and how they effect performance. Why don't you just take flat stock and grind an edge on it and have an efficient cutting knife?

...then decide whether or not to take it down to the 0.015" to 0.025 range.

We all have to make out decisions on cutting ability vs durability because at some point the knife will either get limited in scope of work or the lifetime reduced because of increased sharpening due to grinding out edge damage. Personally, with working knives I am concerned more about me than the knife which is why all of mine are at the point where if I do something really abusive like full strength inverted hardwood clipping the edges will turn. Because if I had to leave them durable enough to take that without any effect they would be far too heavy to be optimal for the majority of work. I do that type of cutting still, I just reduce power. The knife will see occasional damage from a rock contact, and this will be more extensive than with stock profiles but I'll take that damage when it happens to be able to turn this :

into this :

by removing this :

in just minutes due to the more optimal cutting profile. With a much higher level of cutting ability methods also change because less force is required and you can work much longer for much safer. As noted though when I mentioned 0.015", that is the edge before sharpening and I assume the edge angle on the Outcast is 20+ per side. After sharpening you want it at 0.025"+ to prevent rippling, adjusted as noted by user strength/experience and wood type. For general emergency/survival heavy chopping blades I leave the edges at a higher angle than I normally run for working blades. Working parang class knives are 8/10 degrees at the shoulder to 12/14 at the edge. The general survival class ones get increased by about 2-4 degrees, the edge thickness stays the same at about 0.030" at the shoulder after sharpening, 0.035-0.040 on really soft blades. Machetes for example are usually fairly soft and and need more steel, they also need more angle so they get increased 1-2 degrees as well.

-Cliff

 

[Nick Hyle]

Cliff, I think I get 80% of your points, but I'd like you to expand on

If you want to be able to cut nails without significant damage, modifing the primary grind stands out because it allows you to increase the cutting ability without losing shallow edge durability as the edge angle stays the same and edge angle has a critical influence on preventing fracture in such an impact. Most applications of convex edges are actually just relief grinds. The edge doesn't cut better because it went from flat to convex, it cuts better because the cross section was reduced. It isn't like if the edge thickness stayed the same and the edge curvature increased it would cut better.-Cliff

I'll say I don't really care whether we're talking about nails, rocks or knots. What I care about is I have an old full ground Blackjack Panga, which I just mic'd at over 0.025" for the entire edge, which has significantly rippled (not ripped out) in a couple spots. I haven't done anything I'd call deliberate abuse to it, just heavy chopping.

Now I think the D2 is better steel than whatever the BJ Pangas were made of (AUS-8? 440B?), but I'm guessing this is one of those things where geometry is more important than metallurgy.

I do not want my Outcast to look like my Panga.

That's my reluctance to go under 0.030 - 0.035.

If I'm missing something please try again with smaller words.

 

[Cliff Stamp]

What I care about is I have an old full ground Blackjack Panga, which I just mic'd at over 0.025" for the entire edge, which has significantly rippled (not ripped out) in a couple spots. I haven't done anything I'd call deliberate abuse to it, just heavy chopping.

That doesn't sound unreasonable to be given that thickness and that steel on that size of blade. When cutting heavy ingrowth it isn't uncommon to have less than perfect hits due to branches crossing and overlapping and this can produce a lot of lateral strain on the edge when cutting small diameter hardwoods.

That's my reluctance to go under 0.030 - 0.035.

After sharpening yes that is about where I would want the edge. Since the edge on the Outcast is also much more obtuse than it needs to be aside from being thicker, once you reduce the edge angle it will thicken the edge significantly. Thus if you cut the edge back to 0.025 on the primary grind and the resharpen it to a optimal wood cutting profile the edge will climb back up to 0.030-0.035" or greater.

Probably the most efficient solution is to apply a proper edge relief until the blade cuts as well as desired and then thin the primary grind back down to get the edge at a sensible width. This will raise the cutting ability again and allow for a greater sweep in the very edge which will make it more durable at the edge in case of accidental impacts.

As a side note, there is one case where you will want to leave edges really thick and where reducing the primary grind tends to be a hinderance and this is cutting soft sapwoods. Here you want some thickness to the edge to try to clear chips. However the Outcast isn't a blade designed to cut thick softwoods, it is more of a style to cut small woods in direct cuts.

-Cliff

 

[Nick Hyle]

So in small words for my small brain , your position is:

Grind the primary bevel down until the point where the sharpened bevel WILL meet the main bevel (when it is reground and resharpened) is about ~0.035 thick.

0.025 is your estimate of what this would give in thickness of a fully unsharpened primary bevel taken out to the edge.

Yes?

 

[Cliff Stamp]

Grind the primary bevel down until the point where the sharpened bevel WILL meet the main bevel (when it is reground and resharpened) is about ~0.035 thick.

Yes, after sharpening. It would taken an insane amount of impact energy to ripple that edge that thick on a piece of properly hardened tool steel. If I had access to a heavy grinder nothing I carried would be thicker than that and there would be a number of blades I would readily recut.

0.025 is your estimate of what this would give in thickness of a fully unsharpened primary bevel taken out to the edge.

Yes assuming you are aiming for a wood cutting profile which is typically a lot more acute than most stock edges. Felling axes for example are typically 15 degrees per side in the very edge, most tactical knives are 20-25. As a specific example of such a modification to the edge and the change it induces; my Ratweiler came with an edge 0.035" thick at the shoulder and ground at 22 degrees per side.

When I reduced it to a pure wood working profile by sweeping the shoulder back to 8 degrees and the edge to 12 tapering to 14 in the final mm or so, the edge climbed in thickness at the shoulder to 0.060 and is now about 3/16" wide. Ideally this would be taken to a large grinder and the primary taken in to reduce the edge thickness back down to 0.030". I'd go lighter on a pure chopper, but want about 0.030" for heavy batoning.

-Cliff