SECTION 3.6
Why the Solution Does Not Always Have to be Synthetic

Based on the evidence presented this far, it would appear that a synthetic solution type would be the best overall. However, that category is often lacking the running lubrication that broaching requires. It is quite likely that semi-synthetic emulsions will penetrate closer to the cutting edge than the normal emulsion types (and some go all the way due to capillary action). It is also probable that synthetic solutions will penetrate even further due to their smaller particle size. But, because of the high temperatures at the chip-tool interface, the liquid vaporizes and may penetrate further as a gas.

This gas provides a pressure separation between the chip and the tool face and carries to the tool edge, or built-up edge, the additives which provide the adsorbed film. These are the contaminants that prevent clean metal-to-metal rubbing. The proper choice of additives will promote formation of adsorbed films on freshly cut metal. These films are more easily sheared than the parent metal, thereby protecting the tool face from pressure welding