There has been a lot of discussion over the last few years about the benefits of negative-rake scrapers. NRS’s usually are defined as the result of grounding down the end of a conventional scraper at a slight angle from the original top surface then grinding the end to produce an included angle (usually) somewhat less than 90 degrees. The latter grinding operation (or a subsequent burnishing operation) supposedly produces a delicate burr that leaves a very smooth finish when properly used.
I have long viewed the NRS with more than a little bit of skepticism. With a little thought, it is obvious that, with an appropriate grind at the end of a conventional scraper, exactly the same wood contact geometry as a NRS can be obtained by raising the handle of the conventional scraper slightly. The only difference is a trivial difference in the direction of force on the tool rest.
Okay, so why do some folks claim to be able to tell a difference? Some possible reasons are:
Magic or Divine Providence. Pick your favorite brand
Ease of tool control. Perhaps holding the handle high leads to less controlled movements.
Psychological. The big guns say it works so it must be true.
Frankly, I have always leaned toward the last reason, because I have seen it in other aspects of woodturning technology. However, a fourth possibility occurred to me recently that I believe bears considering. It is
Anisotropy of the metal of the scraper. I believe that the metal stock from which steel scrapers are made is hot and/or cold rolled. Steel rolling is known to affect the grain structure of the metal in such a way that it is not the same in all three directions. Possibly, therefore, the grinding/burnishing burr raised on the rolled surface of a conventional scraper differs significantly from one raised on a face that has been ground at a slight angle to the rolled surface. It would be interesting to see comparative pictures of the burrs obtained by the two types of scrapers using identical materials and techniques. Why might this be important? Possibly even better results could be obtained by using other orientations of the steel or other steels that have been processed by other methods.