Ignoramus10071 wrote:

A metalworking fluid perform several functions.

1. It is a great conversation starter
2. It cools the cutter
3. It lubricates the contact area where the tip of the cutter removes
material, and therefore less heat is generated. (not the same as 2.)
4. It prevents microscopic welding of material to cutter, thus
improving surface finish and effective sharpness of the tip.
5. For high speed machining, high pressure stream of coolant removes
chips. (not something we encounter with manual machines)

I would also like to learn when straight oil is more appropriate than
soluble oil.,

I suppose its a tradeoff. A lubricant provides coolant and (as in your #3) a
coolant lubricates. The more heat you generate, the better the fluild's
cooling properties should be.

I'm also thinking about the use of a lubricant to reduce the load on the
workpiece/tool and resulting distortion. Cutting a thread by hand with oil,
for example, seems to produce a better result than without, as the tap (or
workpiece) doesn't twist as much. Its not a heat factor, as I can just go
slow enough to allow conduction to dissipate that. The ultimate in
distortion when not using oil is busting a tap off. But short of that,
anything that reduces the forces will result in a more accurate cut.

--
Paul Hovnanian
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Have gnu, will travel.