SECTION 3.3
The Advantages (And Drawbacks) of Water

As was mentioned earlier, the cutting fluid must provide both a cooling and lubricating action. For a liquid to be most effective in dissipating heat, it must have a high specific heat and a high thermal conductivity. Oil is inferior to water in these respects. Water has a specific heat of 1 compared to 0.45 for a hydrocarbon oil. (Lower specific heat means that, when comparing equal weights of oil to water, for a given amount of heat input, water will have a resultant lower temperature.) This is significant because of the small volume involved. But what is even more important, water will transfer heat two to three times faster than oil.

Water sounds so much superior, but there are some real drawbacks. Water promotes rust, has no lubricity at all, and is not very wet. Therefore, water must have things added to make it slippery and wetter. In other words, surface tension is reduced by adding wetting agents, or surfactants. This enables the heat-transferring water to come in closer contact with the metal. Understanding that the heat transferring requirement for a coolant is quite obvious, as it is the most important function of the coolant.