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I just had a phone conversation with Dr. Ed Ashby to address “Why 25 degree angle of cut”. He has a health issue which prohibits him from using his arm, and with his permission, I am thus forwarding a summation of our conversation for the benefit of all.
To start, whether the BH was 25 or 35 degrees, assuming a similarly sharp angle, it is suspected to have no discernable difference in regards to PENETRATION. No scientific study was done to verify this hypothesis: a study of this type is suspected to require 1,000 shots under a controlled study to reach any meaningful conclusions.
Next, 25 1/2 degrees was confirmed to be Dr. Ashby’s preferred angle of cut. I will address why Dr. Ashby came to this conclusion, and how 25 1/2 is preferred to an angle such as 35 degrees for example.
WHY – The thinner the cutting edge, the greater the mechanical advantage a BH gains with regard to severing the blood vessels: less pressure is required to sever the blood vessel with a thinner edge. Unless pneumo thorax occurs(my insert), an archer/arrow kills with hemorrhaging. [A cell when cut, wants to coagulate through a series of reactions by blood plasma enzymes and proteins…prothrombin…thrombin….fibrinogen….fibrin which attaches to the tissue tags at the edge of a cut, causing coagulation].
HOW – Simply, through much independent trial and error, any thinner of an angle of cut than 25 1/2 degrees was causing an edge roll to the BH because it lacked durability and thus defeated the purpose of WHY. It is emphasized that a minimum Rockwell hardness of 52 is required to shoot an angle as small as 25 1/2 degrees. Assuming a softer metal is chosen, this requires a greater angle to BH, so integrity is not compromised(Dr. Ashby did not recommend a softer metal- 52-57 is recommended hardness range for the compromise between durability and brittleness).
In confirmation to Dr. Ashby’s conclusions 1).Larry Hanify, an engineer with Abowyer Broadhead Co. came to the same conclusion from his studies. 2). Bacon skinners all use 25 degrees single bevel for the perfect balance between sharpness and durability. 3). Tire cutters all use 25 degrees also.
One last note about methodology of obtaining the angle of cut. Dr. Ashby said it is critical to both penetration and hemorrhaging for the edge to be refined. A file is not good enough, even a smooth edge file. File burrs cause extra tissue tags to form, and these increase coagulation.
It is his educated opinion a file and a subsequent refinement using a window edge,coffee cup or similar is not refined enough. They may cut paper and rubber bands well, but on live cells, nothing bleeds as well as a refined edge. Dr. Ashby is quoted as preferring to prepare for the worst case scernerio, to put all odds in his favor, and I am in complete agreement with this philosophy.
Thank you Dr. Ashby for sharing this knowledge to better the Bowhunting community. Saying a prayer for a speedy and complete recovery to your health issues.
Good hunting to you all.