The Machine is based on a paper entitled "Some Medicolegal Aspects of Stab Wounds" by Dr. Bernard Knight, a British forensic pathologist. In his paper he describes how he tested the resistance of human skin to punctures from knives. What he found is that when the blade is set against the skin and pressed, it takes from ½ to 3 kilos (approximately 1 to 6 lbs.) of pressure to penetrate. The sharper the blade is, the easier it penetrates (which makes sense). Dr. Knight also found that the faster the blade was moving, the easier it penetrated as well.
Don Iolo FitzOwen and Don Christian Richard Dupré of Ansteorra did some experimentation based upon these findings. They discovered that a 2-inch bend in a Nr. 5 French foil blade (the "minimum kill" in Ansteorra at that time) required about 4 lbs. of pressure; well within the range of Dr. Knight’s data. They also discovered that thin rawhide which had been soaked thoroughly in water gave a reasonable approximation of the penetration resistance of human skin.
The Machine is designed to allow measurement of the force required to penetrate wet rawhide with a sharp blade, in a consistent and easily demonstrated manner.
Building the Machine
I made the base from a piece of 1 x 6 board about 16 inches (40 cm) long. The support bars that form the triangle are flat bar stock, about 1/8 x 5/8 inches (4 mm x 2 cm), and about 12 inches (30 cm) long. They are hinged at the base so they will fold down for ease of storage and transport. I twisted the bar stock 90 degrees and drilled holes in the upper ends of all four pieces to hold the pivot rod. I made the pivot rod from 1/4 inch (5 mm) round bar, about 5 inches (13 cm) long. See Figure 1.
The rawhide frame is made from more of the flat bar stock, bent into a diamond shape about 6-1/2 inches (16 cm) on a side, and bolted to the top of the center upright. The center upright is made from 1 inch (15 mm) aquare rod. There is a hinge attached to the bottom, which hangs down and catches the ratchet, to lock the frame so you can read the scale.
Note that the distance from the center of the frame to the pivot (A to B) and from the pivot to the eye-screw (B to C) must be the same (in mine it is 8 inches (18 cm) all the other dimensions can be changed to suit your needs. You may also use whatever materials are most convenient. I built my Machine mostly from scrap material I had in my workshop.
Figure 1: Side view and front view diagrams of The Machine
I used a small fish-weighing scale bought at a sporting goods store. It has a hook on the end, which hooks to the eye-screw in the back of the upright. The scale is mounted firmly on the base, so it won’t move while measuring the pressure. For the ratchet, I used a saber-saw blade, set into the base so it catches the hinge as it swings forward. See Figure 2.
Figure 2 Enlarged detail of hinge on bottom of the dowel, and ratchet on base
You will need to use the thinnest rawhide you can find. What I've found works best is goatskin rawhide, which my local Tandy Leather carries. You will need at least two pieces of rawhide for each demonstration, three or four if doing draw cuts as well.
Using the Machine
This is how the Machine works: cut a piece of the rawhide big enough to reach around the edges of the rawhide frame. Soak the rawhide in water for at least one hour. Clamp the wet rawhide to the frame (I use four of the black metal paper clamps from an office supply store.) Mount the upright to the base by sliding the pivot bar into the holes in the support bars, and hook the scale to the eye-screw. At this point your Machine should be self-supporting. Clamp the front end of the base to the table or whatever it is resting on, with a C-clamp.
Figure 3: The Machine set up and ready to use at KWAR 2008
When pressure is applied to the rawhide frame, the upright will pivot and pull the scale hook forward. This is why the distance from frame to pivot and pivot to scale must be the same; otherwise the unequal leverage would give a false reading on the scale. The hinge should catch in the ratchet to allow you to read the scale, once pressure is released.
I usually start my demonstration with a fencing blade, first pressing on the rawhide with the tip until the scale reads 4 lbs. (about 2 kilos) to show the audience how little the blade must bend to reach that; then hitting the rawhide a few times with "minimum kill" blows, and reading the scale each time. (Having an assistant to read the scale and reset it for you is useful—just make sure she stays well clear of the sharp blade. See Figures 4 and 5.).
Figure 4 Four pounds of pressure, using a foil, schlager and Del Tin. Note the lack of curvature with the heavier blades.
Figure 5 Back view of The Machine. Note the draw-cut frame lying on the table.
I then pick up my sharp rapier, and press it against the rawhide until it penetrates (see Figure 4). If the blade has been properly sharpened (I usually give it a final touch-up just before the class starts) and the rawhide is of the right thinness, the blade should penetrate at around 4 lbs. Note that once the blade has penetrated, the scale will not register any higher, because there is no more pressure on the rawhide. (Practice with this a few times, to make sure your rawhide is thin enough, and everything works properly.)
I then give the rawhide a few "minimum kill" thrusts with the sharp rapier. Because the blade is moving at combat speed, it should penetrate even easier. I also demonstrate that once the blade has penetrated the rawhide, it can continue on without any further resistance. I then hit the rawhide at an angle, to demonstrate that most "glancing blows" with a fencing blade would still penetrate with a real one.
At this point, I mount up a fresh piece of wet rawhide, and let the audience members each take a thrust at it, telling them to hit it with their usual killing blow. I will take a position where I can read the scale, and tell them how hard they hit with their thrust.
There are also two variations you can add to the presentation, if you wish. First, mount a ruler on the base of the Machine, so you can measure how deep the penetration is (bear in mind that anything more than a few inches or half a dozen centimeters into the torso is probably going to hit something vital).
Second, build a second upright-and-pivot unit. For the rawhide frame on this one, make it square, with several curved "ribs" across the front (see Figure 5). Attach a piece of wet rawhide, and use it to demonstrate draw cuts and "glancing" blows. (Draw cuts will use up rawhide very quickly; make sure you have enough.) The glancing blows should penetrate the rawhide at one side, travel beneath it for a little way, and then emerge from the other side. This is a particularly graphic demonstration of how a "glancing blow" with a sharp blade would behave, especially if you add a bit of draw cut pressure as you are withdrawing the blade.
Figure 5 Optional draw-cut frame