The most likely reason for your edge crush problem is that you are not abraiding the edge of your
piece of flint or chert that you are trying to knap, or you need to abraid them more heavily. As a
matter of physics, your sharp edge of your piece of flint or chert digs into your billet and expends a
significant amount of the energy that your billet is generating by your swing. Instead of the force
carrying through the biface and resulting in a long thinning flake, it only has part of the energy that
it should, resulting in a short flake and crushed edges. The edge can also crush due to it being too
brittle and thin to withstand the billet strike. Once you learn to abraid, your knapping results will
improve a great deal as you start to remove longer thinning flakes.
As a side note, if you use copper billets, in general you will have to abraid your biface edges more
than with antler billets. With antler billets, you will need to abraid enough to let the energy carry
through the piece (the billet hangs for a few miliseconds on your striking platform thus storing up
energy to carry a flake through), but abraid too much and an antler billet will fail to remove a flake
See the below picture to get an idea of what crushed edges look like. If your knapping results in
bifaces with a lot of step fractures and hinge fractures like the below piece, chances are you need to
abraid. Thanks Don for the teaching piece.