If you look at the top of the fluted portion on twisted files you will always see three or four flat surfaces which catch the light as you turn them. Ground files simply have a groove that plays out at the top of the flutes.
What About Diamond Instruments
Diamond instruments are all negative rake. If you can imagine a chunk of diamond glued to the shaft, it would be next to impossible to orient each particle to create a positive rake.
Lasers for Canal Preparation
A quaint concept now that we realize that there are lots of cheaper, easier and more efficient ways to open, enlarge, shape and prepare a canal.