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I bought these a while back and have been very satisfied with them so far.
http://www.biggatortools.com/products.html

I have always found that a dull edge needs more force sowhen it slips it usually cuts deeper and not near as clean. I do seem to get alot more very small nicks with a sharp edge. I think that it is...

No expert but I would think that the purplehart is shrinking after the cutter head and rubbing on the side of the forster bit causing drag.