Improving the edge geometry on an Ambush Alpha

[timichango]

Hi hi!

I recently added an Ambush Alpha to my knife collection, primarily to serve as an all-around bush knife with a bit more reach than the 1st gen BKRT Gunny that I've been using for a number of years. I love the gunny, but it's a smidge short for batonning with some of the wood I typically find myself wanting to knock down for kindling, and I've been using a big BK9 for most of my batonning as a result. Rather than need to rely on a 2-knife system for camp work, I grabbed the Alpha when I saw it for sale at closeout pricing at KSF —*thinking that the in-between size of it might allow it to replace both of the other knives in general use.

Initial impressions after a couple of weekends up the Squamish valley camping with the knife are that it's a great size, and overall I'm in love with it in the hand. Really really comfortable, and the design makes it a great all-rounder.

The main complaint that I have about it, though (and this might be limited to my copy — dunno) is that the final edge bevel, which is convex, is fairly obtuse. It did come pretty sharp, out of the box, but overall it's not a terribly great knife for slicing, draw cuts, etc. and I initially found that it was challenging to ascertain the proper stropping angle to touch up the edge. Additionally, even after a considerable amount of stropping with CBN compounds (8 and 4 micron) on one of the Ambush/DLT strops, the edge just doesn't bite as much as I'd like.

The same routine on the gunny transformed that knife from a very sharp knife to a terrifyingly sharp knife, so I don't think that it's the gear or technique in sharpening the Alpha that's holding it back — I think it's the geometry. I'm surprised by this, given my experience with other BKRT knives, which have all seemed to emphasize cutting ability.

So I wound up attacking it with the coarsest sharpening medium I have around: a DMT fine whetstone (6" interrupted diamond plate), and worked on easing back the cheeks of the convex bevel in order to make the tip geometry more acute. This did help somewhat, and there's now a somewhat longer convex radius from the initial bevel to the edge, but I'm still not totally satisfied by how it cuts.

My question is this: what's the best approach (short of power tools, bench grinders, etc.) to more aggressively pull back and extend the edge bevel in order to arrive at a more acute convex edge? Should I repeat the approach I've tried above, but carefully using a coarser DMT stone to do the bulk of the reprofiling work? Or is there an easier and more efficient way to thin out the edge somewhat?

I should also mention that I'm not too concerned about sacrificing durability at the edge by going thinner: the A2 gunny is considerably thinner at the edge, and I've positively abused that knife with zero ill-effect. Considering the Alpha is made from 3v, I'm confident that it can take a much more aggressive edge than it currently has, and still be immune to damage given what I'll be using it for.

Anyways, any advice on how to take this blade from great to excellent would be sincerely appreciated!!

 

[Jason B.]

I would get something coarser and grind it down further, that or send it off to one of the professionals on this forum such as myself. Either way, there is some thinning that needs to happen to it to cut how you want.

Given that it's a flat grind to a convex edge I would think a full regrind is in order. Without reducing the primary grind there will be a limit to what you can do at the bevel.

 

[awestib]

Why don't you check this video out and follow through his further approach: https://m.youtube.com/watch?v=bj8PP8xJhIQ

 

[timichango]

I would get something coarser and grind it down further, that or send it off to one of the professionals on this forum such as myself. Either way, there is some thinning that needs to happen to it to cut how you want.

Given that it's a flat grind to a convex edge I would think a full regrind is in order. Without reducing the primary grind there will be a limit to what you can do at the bevel.

Hm, interesting —*hadn't considered sending it off for aftermarket modification (actually, didn't even really realize that anyone offered that kind of service!), but realistically, given that I'm assuming most folks are in the US, and I'm in Canada, return shipping plus the exchange rate probably makes that prohibitive. In any case, I'm a fan of developing new skills and tools, so reckon I might go coarser and grind further as you suggest.

I'm not sure at what point an edge modification becomes a full regrind, but in my mind, what I'm envisioning is an alteration something like this:

Grey is (roughly) what I think the current geometry is, and pink is the direction I imagine would leave plenty of strength in the edge, being 3v, and would be considerably 'slicier' while still being able to baton, chop, etc.

Would the coarse DMT stone be the best way to go for reprofiling 3v in this manner? Or are there other stones that I should look at that'd also work well on this steel?

Additionally, I'm assuming that once the new geometry's been ground in, I'm going to have lost a considerable amount of the nice stonewashed blade finish. Any way to restore that (ie. grind, refinish, final sharpening)? I'm fine with sacrificing the knife's looks in exchange for more utility, but if I can have both, then... great

Thoughts?

Thanks!!

 

[timichango]

Hah — thanks for that. Pretty much what I'm envisioning.

 

[Jason B.]

Re-stone washing is the obvious answer to get the proper finish but that's not likely to be an option. It would be considered a full overhaul at that point because the handle would need to be removed. Sandpaper is always a good alternative and with proper technique the finish can be very nice.

Do you have a caliper gauge? Can you measure the thickness behind the bevel?

For stones,

The DMT would work and I like my DMT stones but... for convex work I much prefer a waterstone, so much more control of the curve. Personally, I would probably start at my 150 grit stone then move to a 320 and later a 1000 before smoothing with sandpaper and adding a microbevel with a fine stone. I find a good thin convex supported by a microbevel allows for easy maintenance and even better cutting performance.

To clarify, when I say microbevel I don't mean it in the classic sense. After I finish smoothing the grind with sandpaper I hold the edge on the stone at the apex angle and just polish the existing geometry of the edge. It looks like a microbevel but it flows into the grind almost seamlessly. I adopted this method because it's not only much faster but way more consistent and faster to touch up.

There is also the option of just using stones but some don't like the appearance.

 

[timichango]

Re-stone washing is the obvious answer to get the proper finish but that's not likely to be an option. It would be considered a full overhaul at that point because the handle would need to be removed. Sandpaper is always a good alternative and with proper technique the finish can be very nice.

Do you have a caliper gauge? Can you measure the thickness behind the bevel?

For stones,

The DMT would work and I like my DMT stones but... for convex work I much prefer a waterstone, so much more control of the curve. Personally, I would probably start at my 150 grit stone then move to a 320 and later a 1000 before smoothing with sandpaper and adding a microbevel with a fine stone. I find a good thin convex supported by a microbevel allows for easy maintenance and even better cutting performance.

To clarify, when I say microbevel I don't mean it in the classic sense. After I finish smoothing the grind with sandpaper I hold the edge on the stone at the apex angle and just polish the existing geometry of the edge. It looks like a microbevel but it flows into the grind almost seamlessly. I adopted this method because it's not only much faster but way more consistent and faster to touch up.

There is also the option of just using stones but some don't like the appearance.

Awesome info — thanks, sincerely!

The edge, measured at the back of the polished convex bevel (the one I extended back and thinned out on the 'fine' DMT interrupted-diamond whetstone) is currently 0.04"ish along the majority of the edge (bit thicker at the very tip of the knife, but the bevel's a bit longer there too — ie. the avg. bevel angle is probably similar there).

I'm curious: how does a waterstone provide more control of the curve, as compared to an equivalently rated DMT? Just because of the slurry/feel of the stone? I'm assuming you're talking about a stone that's flat-lapped, not radiused, right? Realistically I'd be aiming to finish through the existing equipment I have (fine DMT + 8 and 4 micron CBN emulsion and a dual-sided strop), so I'm trying to figure out what's ideal for use at the coarser end of the spectrum... ie. will a conventional aluminum oxide based waterstone like a norton actually abrade the 3v steel properly? I can get a 220/1000 grit norton combination stone up here in Canada for around $50, so if that'll work, that's an attractive option, price-wise... that grit spread would out the fine DMT stone that I already have right in the middle of the grit range, I think (afaik, the DMT fine stones' grit is around 600 grit).

Regarding re-stonewashing, I'm not too concerned, really — realistically, I'll be smacking the knife through logs regularly, so before long I expect that it'll have a bit of patina to help blend things out My gunny looks like hell, and cuts like stink, and that's just fine with me. Shame it wasn't bigger. Here's hoping that the Alpha can give me as good service as that Gunny once I've got the edge set up a bit better...

 

[Jason B.]

0.04 is pretty thick, I consider .03 in need of a regrind, .02 is a "thicker edge", .015 ideal on large blade, .01 is ideal on folders and .005-.008 as ultra high performance.

Waterstones work better because the slurry and the softness of the stone. Can't say I would recommend the Nortons though, I lump them in the "average" waterstone class. They will likely work but it will be slow going.

3v can be sharpened by most any stone, some stones will do much better than others though. With very coarse stones it shouldn't matter much but as you go finer the problems will become apparent. Beyond about 500 grit I would want a stronger abrasive like ceramic or silicon carbide. That could also be another budget friendly option, the Norton combo silicon carbide stone. Kinda like a waterstone in the way it works and would be a good addition to what you have. Usually around $20

 

[timichango]

0.04 is pretty thick, I consider .03 in need of a regrind, .02 is a "thicker edge", .015 ideal on large blade, .01 is ideal on folders and .005-.008 as ultra high performance.

Waterstones work better because the slurry and the softness of the stone. Can't say I would recommend the Nortons though, I lump them in the "average" waterstone class. They will likely work but it will be slow going.

3v can be sharpened by most any stone, some stones will do much better than others though. With very coarse stones it shouldn't matter much but as you go finer the problems will become apparent. Beyond about 500 grit I would want a stronger abrasive like ceramic or silicon carbide. That could also be another budget friendly option, the Norton combo silicon carbide stone. Kinda like a waterstone in the way it works and would be a good addition to what you have. Usually around $20

How do you like the ceramic "shapton glass" stuff? I can get a set of the 220 and 320 grit stones for around $130 CAD up here, shipped free, and if those're a good investment in gear, I won't lament the cost too much.

I suspected that 0.04 might be 'pretty thick' Wonder why BRKT ground it that way. Now I'm wondering if that's the actual design spec, or if I just got a badly ground copy. Hmmm...

 

[timichango]

Update: I haven't pulled the trigger on any coarse stones yet (though I think I'm going to pick up a 120 grit shapton to start with, since I have intermediate grits in other mediums to polish out the 120 scratches), but I realized that I have a 2" wide roll of some very good cloth-backed sandpaper from some metal fabrication I was doing a couple of years ago. Originally I was using it for working unhardened cro-moly tubing, which is a much softer material, but I figured I'd give it a shot on the 3v and see how far it'd get me.

I wound up tightly clamping a strip of it across a 20" length of 90° aluminum angle stock to form a sanding surface, and clamped the whole thing in my shop vise and went to work.

After an initial 15 minute run at it (one strip's worth), I managed to ease back the starting point of the convex edge bevel so that it now starts radiusing in about 1cm behind the original start of the bevel, which both thins things out, and pulls back the shoulder. I need to do some more work on it to thin out the area near the apex, which is still a bit more obtuse than I'd like, but at present, after removing some stock, and then polishing it out a bit on some finer silicon carbide wet/dry sandpapers, followed by a quick run across the DMT fine stone, followed again by the strops and CBN, the material behind the original bevel is now a smidge less than 0.03, and radiusing out smoothly from there to blend with the original flat grind about 1cm back.

Going to continue refining this further once I get my hands on a stone (or two), but in the interim this has definitely improved things enough that it definitely takes a keener edge, and slices with a lot less resistance. Going to use it like this for a few weeks, and see how it holds up to abuse before taking off more material and/or figuring out how I want to refinish the blade now that the stonewash is pretty scuffed up.

Thanks again for all advice so far!

 

[Jason B.]

The SG320 is a nice stone, they are hard ceramic stones that wear slow and cut fast. A little difficult to lap flat without a diamond plate but for the coarse stones you really want to use another coarse stone or some SiC powder to gain the proper texture.

The 120 and 220 Pro stones would be a little better than the glass stones in the same grit. It's not until you get to the 320 grit in the Glass series that the cutting action really comes alive. It's almost like the binder is just too hard on the 120 and 220 for them to be stand out stones.

Clamping down abrasive strips is a good method, can work faster than stones at times.

 

[timichango]

The SG320 is a nice stone, they are hard ceramic stones that wear slow and cut fast. A little difficult to lap flat without a diamond plate but for the coarse stones you really want to use another coarse stone or some SiC powder to gain the proper texture.

The 120 and 220 Pro stones would be a little better than the glass stones in the same grit. It's not until you get to the 320 grit in the Glass series that the cutting action really comes alive. It's almost like the binder is just too hard on the 120 and 220 for them to be stand out stones.

Clamping down abrasive strips is a good method, can work faster than stones at times.

Thanks for that!

Weirdly, the shapton pros seem to be a pain to find domestically here in canada, but (of all places) fendrihan, a Canadian online straight-razor retailer, carries the shapton glass stones. Thanks for the tip on the 120/220 being less than ideal in the glass-stone line.

As an alternative at a lower grit, it's looking like I might pick up a Sigma power select II in 220 grit for my coarse stone. Lee Valley (woodworking tool retailer up here) sells'em, and from what I can find out there on the 'net, they seem to be well-liked for dealing with wear-resistant steels ...which describes more than one of the knives that I want to deal with. Weirdly, though, Lee Valley only sells the 220, and then the 1000 grit (and on up from there), but nothing between 220 and 1000. Might grab those two, and if I'm feeling like I need something intermediate, grab the 320 shapton glass at that point to bridge up.

Anyways... thanks again!

 

[Jason B.]

The Sigma stones are known to be fast cutters so you might not need a between stone.

 

[timichango]

Wound up finally getting my sigmas (the 240 and 1000), and — wow! So much easier to work the blade compared to the other sharpening mediums I've been using. Got the edge of the Ambush thinned out quickly on the 240, and reworked the rest of the primary bevel up to the sabre line (if that's what the top of the primary grind on a sabre-ground knife is called?) so that it's now a continuous convex bevel down to the edge, rather than flat, with a steep convex right at the edge.

I tested pounding it through some knotty scrap firewood and plywood around the garage, and it's still plenty tough in that application despite the thinning — and it now cuts much better than before.

I recently purchased a Winkler/Tad Combat axe as well, which also had both an extremely obtuse edge, and a rather lopsided edge grind. The 240 Sigma sorted that out pretty quickly, and the edge on the axe now has a more conventional convex axe grind — still with plenty of steel behind the edge for chopping strength, but much keener than it came originally. It's similar to the edge on my Gransfors Bruks 'Outdoor Axe' now (which, incidentally, might be the sharpest blade I own).

Anyways — thanks again for the advice.