Since he was not prepared to re-teach my course he devised this demonstration to illustrate what happens when a negative rake is used and again when a positive rake is used.
Remember, a negative rake angle is when the leading edge of a cutting tool is ahead of the perpendicular to the surface being cut.
A positive rake means that the blade is BEHIND the perpendicular.
Examples of negative rake instruments are K-files, K-Flex files, Flex-R files, Reamers, and most of the current crop of NiTi files. Examples of positive rake files are Hedstroms, Fine-Cut Files, and S-files.
He brought to the meeting a stick of butter, a butter knife and a pepper shaker.
The butter represents dentin, the knife an endo instrument and the pepper, the debris, bacteria and necrotic dentin on the surface of the dentin lining a canal.
He then covered the butter with pepper, applied the knife at a negative rake and attempted to remove the pepper.
It took several attempts because each stroke although it removed some pepper, burnished what was left further into the butter.
He then re-covered the butter with pepper and this time holding the knife at a positive rake, in one stroke removed all of the pepper.
When a negative rake is used, it burnishes as well as cuts. In the process it drives bacterial debris into the dentinal tubules, fins and lateral canals. This in turn necessitates removing more tooth structure to get at the bacteria, which in turn...well you get the idea.
When a positive rake is used, only light pressure need be applied and because the canal can be filled with water, the debris is scraped off, and immediately gets suspended in water and can be flushed out.
Notice again the amount of butter removed and how the remaining pepper is burnished into the butter.