SG-250 - Oil Better Suited Than Water?

[tgbto]

The work being done can be measured by the material being removed (bonds being broken within the steel) and the generation of heat. You need to maximise the first and minimise the second. We want as little heat as possible and as much material removal as possible for maximum efficiency.

Agreed 100%.

To complement my response to OP's objection, the Higher MOH of the abrasive relative to the material being ground allows it to scratch said material. In doing so, there is resistance to the relative motion of the abrasive and the material. This resistance is precisely friction, and overcoming it generates heat (due to resistance to gliding at the molecular level AND to the the energy released by the breaking bonds within the material AND to the wear within the abrasive).

Lubricants work by separating surfaces. A lubricant that would work so well that there would be no resistance to the relative movement of the abrasive with respect to the ground material would also prevent the abrasive from scratching the material. For grinding to be effective, surfaces have to be allowed to come into contact. Where they come into contact, the kinetic friction coefficient depends only on the surfaces, not the lubricant.

Machinists know that a dry bit removes material much faster than a lubricated one. Unfortunately, it also breaks down so much faster that it is impractical to use it dry.

Water for the Tormek is a good compromise : it properly transfers heat away from the point of contact, lubricates a bit but still allows grinding to happen quickly enough.

TL/DR : the better the lubricant, the less abrasive and steel come into contact, the slower steel is being ground.