The Sean O’Rambo Knife – 5

When someone is carrying something very long, and very sharp, it’s important to keep from being the agent of their demise. You’ve got to think in terms of things like “what happens if this person is wearing it on their hip, and they slip in mud, land on the knife, the scabbard pops, and the blade winds up in their Femoral Artery?”

The short answer is, “I faint!” But let’s not have it happen. When I am making a scabbard for carry, I tend to go way overboard because of that scenario. So, we start off with 3 pieces of ABS plastic sheet:

The pieces get stacked and taped together, then Mr Happy Dancing Bandsaw cuts the outside shape, then they’re unstacked and the inside shape of the welt is cut out. Welted sheathes are a really good design if you’re worried about things coming apart. One of the things I have always disliked, somewhat, about classical Japanese scabbards is that typically they are assembled out of two pieces, and the edge of the blade sits right along the seam. There’s a failure more there.

Also, you’ll see in a bit, I cut the outer shape a bit differently so it’s easier for the carrier to tell which edge is the sharp one. These things instinctively add up.

For welding ABS plastic, you can use a variety of acrylic glues or a solvent glue. I prefer to solvent weld it using methylmethacrylate, which makes the material sticky and then you press it together and it becomes one piece. If you care, you can get the solvent in the form of lucite welding fluid, and you can get sheets of ABS plastic from Ridout Plastic, Inc. [rid] One of the great things about that business is they will cut materials to size for a small fee, so if you’re making a lucite box you can do all the calculation and have it pretty much shipped flat-pack to your door. At various times I have used large sheets of fiberglass or polypropylene from Ridout, and then it gets interesting: a truck shows up and drops off a pallet with 6×10-foot sheets strapped to it. Whee! Anyhow, put some thinking into your materials and I’d say favor big over fragile.

Another reason I really like ABS is because, on a sander, it works about like hardwood. So you can scuff it, round it, clean it up, etc. I am not exaggerating at all when I say that this is less than 5 minutes of work. (solvent welding takes less than a minute):

Next: a bit more safety, and some functionality, and decoration. All in one! This is a problem I have wrestled with for many years, and how I now handle it is I make the scabbard, and then a carrying frog that straps or attaches to it, which means I need attachment points. One attachment point that looks great with a wet-formed leather scabbard is wrapped wire bumps:

Since there is going to be leather wrapped around that, I don’t have to worry a whole lot about the wire unwrapping, so I drill a small hole (front left of the wrap closest to camera) bend the wire over with pliers, wrap it, and go around to the other side and latch the wire into the other side of the hole. You can also see what the texture of the ABS looks like after a bit of sanding and whacking it on things and filing and whatnot.

Now, we’ve moved into the sewing/leatherwork area and I didn’t get pictures of what this looks like all blocked up, but… You take 2 pieces of thick plywood and cut a hole in one that is pretty much the outline of the frame of the scabbard. Then you take some vegetable tanned leather, soak it in cool water for about an hour, wrap it tightly around the frame, then position the piece of plywood with the hole over the frame, and clamp the whole mess together. If you want the stitch-line to be centered on the scabbard, you use 2 pieces of plywood, each with a hole, and be a bit more careful how you position it. Since I want this to be carried flat to the body, the stitch-line should be on the rear edge. This is the basic technique for wet-forming pretty much anything, by the way. Once the leather has sat for a couple days it’s nice and hard and tight around the form. Since I’m using ABS plastic for the scabbard, I can just use the scabbard itself. That way everything comes out very dimensional.

If you want to make it really tight, you wet the leather, make sure the frame is pretty snug to you object, roughly fit everything in the frame, then grab the flap-edge with pliers and pull it tight then clamp down the frame.

Now’s an important step you can’t see. You gotta think about scabbard frogs and how they will attach, because if you’re going to do any sewing of anything onto the leather, you need to do that before you stitch the welt. In this case, I had decided that the frog was going to be supported by two straps that were going to be laced through the leather, riding between the ABS and the leather wrap. Which means that I gently removed the scabbard, used an oval hole punch to make 4 holes in a position that looks properly situated with the reinforcing wires, and then grabbed a brush, covered the outside of the frame and the inside of the leather with Barge’s rubber cement, and assembled the whole thing together. There’s a certain amount of fiddling and cussing at this stage and traditionally I get some rubber cement in my mustache around about here. Before everything sets up I slip the butt of a scalpel through the punched holes in the back and feed through the straps so the leather will have time to stick/form around them. Another thing to think about: now is the time to dye the leather. Because with things like punched holes that are going to go below a frog, you’re going to have a hell of a time getting a nice even dye job if you can’t reach the holes, and you’ll look like a sloppy worker if someone ever takes the frog off and there are big ugly un-dyed patches.

This is not how it assembles. I guess for photography reasons I put the frog on the wrong side so it’s visible, but the resulting image really confuses the construction:

You can see the straps are fed through the back. Those will feed through slots in the frog, then loop back around front where they clasp. So what you see there is the back of the frog that will be against the back of the scabbard. There’s also a tail that comes up a ways. It turns out with very sharp things, it’s not a bad idea to have a piece of leather there in case someone draws the blade and slides it against one of their love handles. Yeeow! The stitches on the frog are to hold the two halves in position against eachother, so one could loop a belt through one of three possible positions. Or, make another frog, which would be super easy, that hangs from a bandolier, or a leg belt, or whatever. Also, having multiple positions on the frog allows a leg strap or something to be added. I figure: 1) tactical, 2) options. Also, easy to fix! I could make another frog that rides higher, or hangs lower, or whatever. Usually when I do something like this I make a tracing and stick it in my Big Pile Of Patterns just in case.

Suddenly, it’s black:

Leather groovers are wonderful: you set them for the depth you want and it cuts a groove. Then, you put the stitches in the groove. Done! Of course you have to have everything trimmed up nicely, etc, first. I did not illustrate any of that. For that, I use a mixture of kiridashi and scalpels, blood, sweat, and tears.

Standard lock stitching, then touch up here and there with dye, neatsfoot oil and wax, and it’s done.


  1. kestrel says

    Using the ABS inside like that is clever, I remember making knife sheaths and only using harness leather. This is way better. Also your stitching is nice. The knife is really stunning. And deadly!

  2. says

    That certainly looks better and more functional than what Rambo had in the movies. It is a knife to strike fear in the hearts of many a lawmaker.

    I never used plastic for a liner. I do not doubt it is stronger than wood, especially since it won’t split and won’t be punctured either. ABS is a rigid material. However, given my experience from my previous job, I would test it with neatsfoot oil before using it on the leather over it. One of the most destructive tests for plastics was sunblock lotion and hand lotion resistance. Apparently, chemicals in these, including fatty acids, can be really harmful to some plastics. It could be that neatsfoot oil (or any other animal or plant-based fat) will embrittle it over time, especially in combination with heat. I can’t remember how susceptible ABS was, I do remember that for PC the sunblock lotion was like kryptonite.
    It is food for thought though, ABS is cheap and I could buy some easily.

    As far as splitting wood goes, when using modern woods it should be of no consequence that the seam is directly opposite the cutting edge – modern woods are usually stronger than the natural lignin bonds within the wood so when failure happens, it is the natural wood that usually fails before the glued joint. And that is when talking about PVA glue, epoxy is even stronger.

  3. Tethys says

    Can neatsfoot oil penetrate through the leather to the ABS? I would expect that the rubber cement would prevent the oil from contacting the plastic, but I can’t see any issue with using a little on the outside of the finished scabbard.

    It came out really well. Pointy, gleaming, and so sharp that it opens Husbands with a flick of the wrist. I’m a little confused how to wear it. On a thigh? On a waist belt? It looks very large. Perfect for invading Brittania.

  4. says

    @Tethys, I don’t know if the oil can penetrate through the leather and the rubber cement. One of the points of that test used to be to test exactly that – if the oils can penetrate the surface decorations (paint, lacquer, plastic foil, print, etc). If they could not, one of the other most frequent points of failure was edges, where the oils did not need to penetrate anything but could come directly into contact with the unprotected plastic. That has led to edge delaminations, swellings, embrittlement, etc. In a knife scabbard, such a point would be the throat.
    It might be that it is just fine, I’m just saying that I would not assume it is fine and would test it.

  5. says

    ABS is a styrene, it is proof against oil and gasoline, but not acrylic solvents. I’m not particularly worried about someone carrying one of my knives falling into a vat of trichlor. Also, the steel wires might help.

    It’s even flame resistant at temperatures that will ignite the carrier. Waterproof, too, though the leather isn’t. I have not tested against gravity from a black hole but I think the scabbard and knife would come apart.

  6. says

    @Marcus, you are probably right. But could you humor me, put a drop of neatsfoot oil on an offcut, put it somewhere warm and dry for a few days, maybe even in the sun, and after that look if it has changed (swellings, cracks, embrittlement)?

    I am not trying to be difficult, I have two reasons.

    The first is theoretical – “Resistant against oils” usually means against mineral oils and other petroleum derivatives. The chemistry of those is completely different from the chemistry of plant and animal-derived oils. If you google polycarbonate oil resistance it will tell you that it is resistant against oils but I do remember with 100% certainty that PC will fail in contact with some organic oils, especially in combination with heat.

    The second is practical – this spring the ABS housing on my electric chainsaw failed. It cracked along the in-mold flow lines, which is exactly the kind of embrittlement that happens with organic oils on some plastics that I remember. The oil receptacle on the saw was a bit leaking and when in storage, everything became all covered with oil. And I am using biodegradable oil for my chainsaw. Thus I do not know if it failed because it was cheap junk or if said biodegradable oil eats away ABS. I suspect it is the former because ABS is very resistant, but I am not entirely sure. Alas, I no longer have access to a database of tests for this kind of thing and my memory is not as good as it used to be.

  7. flex says

    @12, Charly,

    It sounds like an interesting test for Marcus to run, if he desires. But I don’t think there would be any issue.

    We use a lot of ABS in automotive and one of the corrosion tests we perform on interior components is a Suntan Lotion test.
    ABS seems to survive okay.

  8. says

    @flex, you have probably missed it in my comment #2, but those tests were exactly the reason why I started this conversation. I was working in automotive for 12 years and I was doing these tests on interior components regularly. But I could not remember how ABS performs. So thank you for the info.

  9. says

    So… oddly serendipitous, it turns out that the stock on my L1A1 is ABS plastic. Rifle stocks get randomly soaked with all sorts of solvents including various kinds of gun lubes with teflons and dog knows what all else in them. The stock, which is at least 20 years old (probably closer to 40) looks new. I’m going to just conclude right here that ABS plastic is tough stuff that does not dissolve in the presence of a wide range of oils.
    Your chainsaw may have had problems with some kind of additive in the bar oil. I know that they put all kinds of weird stuff in bar oil – ceramic powders, molybdenum powders, blah blah blah. But it’s all mostly carried in oil of some kind or another. I’d be inclined to look at the chainsaw manufacturing rather than the plastic – maybe the casting just wasn’t very good?

  10. says

    I never used plastic for a liner.

    A lot of knife makers are using kydex, now, which is fairly similar to ABS, I think it’s a thermo-forming variant that maintains flexibility after being heated.
    The main reason I like to avoid wood or leather as the sole material in a sheath is because I have spent many hours of my life polishing Japanese swords in the humidity of the Baltimore/Washington corridor. The standard wood for the scabbard (saya) is pawlonia (ho) or catalpa. Those are very tight grained woods which don’t absorb a lot of moisture, or release it. But, for long-term storage of carbon steel blades, wood is better than leather, and neither is remotely close to plastic. A blade in a wooden scabbard will eventually rust. A blade in a leather scabbard will quickly rust (“quickly” is 10-20 years) I have blades I laid up in college that are in ABS scabbards I made back in the 80s and they’re rust free because the whole assembly is practically dripping with WD-40. (Do NOT put WD-40 on a nihon-to if it’s made with bloomery steel/tamahagane, it will stain it sometimes and that’s a disaster)

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