" Caltrops. 605. A caltrop is a metal object having a number of spikes,
one of which will always land uppermost when the caltrop is thrown onto
the ground. There is no set design for caltrops, but three types can be easily
made in a unit workshop, and which will puncture the tires of the heaviest trucks.
If caltrops are linked together like a chain, a simple and effective roadblock
can be quickly set up." - Australian Army Manual of Land Warfare
The simplest method I have found for making caltrops is by using expanded metal.
This is used for industrial grating, ect. and has approximately 1 inch long wires between
the joints.
Using cutters (or a hacksaw, although this is much slower), cut an "X"
out of the grating. If you are do not plan on grinding points on the
tips, simply cut the wires at an angle.
Using a bench grinder, sharpen the tips.
With two pairs of pliers, bend the four prongs so one is sticking up and
the other three make a pyramid under the top one. This takes some practice,
but once you get the hang of it, mass-production becomes very easy.