SECTION 3.5
Penetration of the coolant to the cutting edge
We should now consider penetration of the cutting fluid to the cutting edge of the tool. It is clear that the coolant has only two main directions in which it can go: (1) between the tool face and the chip, and (2) between the workpiece (part) and the tool land. Studies have shown that pressurized or forced coolant into either one or both of these directions is superior to normal flooding practices.
Even with pressurization, not much coolant flow will run down either of these two paths to the cutting edge, since both the chips and the work piece are moving in directions opposite to the fluid flow that is necessary. Furthermore, the extreme thinness of the labyrinth formed by the surface irregularities discourages copious fluid flow. But, we can do something that is extremely beneficial. (Note that the actual space the coolant must work in is about 0.000050" in diameter or smaller.)
The chart (Figure 3-1) below shows a comparison of the droplet sizes of various types of metalworking fluids.
Decreasing Particle Size |
Normal Emulsion |
Semi-Synthetic Emulsion |
Synthetic Emulsion |
Particle size
greater than 0.000,004" |
Particle size between
0.000,004" and 0.000,000,400" |
Particle size
less than 0.000,000,040" |
Typical of most cutting fluids, usually ranging from 0.0002" to 0.000,080" |
Described as colloidal solution particles |
A true chemical solution, with molecular and ionic dispersions |
Figure 3-1.
Coolant droplet size comparisons.
Picture a large and a small sphere, or globule, of oil surrounded by water, but separated by a thin layer of emulsifier. The oil phase of the emulsion carries the additives for film strength, oiliness, lubricity, anti-weld, etc. The water phase carries or transfers the heat. Now picture the action at the cutting edge at a single moment in time. The water is the cooling phase and the oil is the lubrication phase.
With smaller oil globule diameters, the cooling and lubricating will take place more simultaneously and evenly across the cutting edge than with the larger diameter globules. In addition, a proportionately greater amount of the coolant's lubrication can physically be moved closer to the place where it is needed. A fluid containing large oil spheres will have lower heat transferring capabilities. The size of the sphere depends on the amount and type of tramp oil present in the fluid, as well as the chemical design of the metalworking fluid. |
