It just seems to make sense to me to run a bur forward instead of backward. In 1979, I read an article in 'Scientific American Magazine' entitled 'The Mechanisms of Abrasive Machining' and it got me started understanding why some files cut so much better than others. Up to this point it never occurred to me that endo instruments had much in common with tools used in machining, carpentry and stone-cutting. A pretty dry definition until you begin to dig into it.
Rake angles come in two varieties, positive and negative .
If the leading edge of the blade is ahead of the perpendicular, the angle is, by definition, negative. If the leading edge of the blade is behind the perpendicular, the angle is by definition, positive. Examples of positive rake instruments are my Fine-Cut files, Hedstrom files, Dynatrak files and Most dental burs. There are several characteristics of cutting that are important to us in dentistry and particularly in endo. When a negative rake angle is used, you cut by applying downward pressure which creates a compression wave ahead of the blade.