Metalworking fluids

A metalworking fluid is a process parameter

A cutting fluid does four things at once: it cools the cutting zone, lubricates the tool-workpiece contact, flushes the chip away, and protects part and machine from corrosion. The balance between them decides the type: at high speed and light cuts, cooling dominates, so a low-oil emulsion; in broaching, threading or gear cutting, lubrication dominates, so a neat oil. What is most often overlooked is that the fluid does not degrade through wear but through management. Concentration allowed to drift, pH unmonitored and machine oil leaking into the sump turn a good fluid into a growth medium — bacteria, smell, corrosion on parts and eventually a sump that has to be dumped and disinfected.

Fluid types and where each is used

The usual classification starts from mineral oil content. Conventional oil-rich emulsions give good lubrication and solid corrosion protection; semi-synthetics, with less oil, balance cooling against lubrication and are the default choice for general turning and milling; synthetic solutions, with no mineral oil, cool best and stay transparent, which matters in grinding where the operator has to see the part. Neat oils are used on severe operations — broaching, threading, gear cutting, automatic turning — and on difficult alloys. Separately there are quenching oils, chosen on the cooling rate the heat treatment requires, and forming fluids for drawing and pressing. On aluminium and copper alloys the formulation has to be checked explicitly: active sulphur additives stain non-ferrous materials.

Sump management, where it is won or lost

A mediocre fluid well maintained beats an excellent one left alone:

Measure concentration weekly with a refractometer, but correct for accumulated tramp oil — otherwise the reading looks better than reality.
Watch the pH. A drop below roughly 8.5 signals bacterial growth several days before the smell does.
Get machine oil out of the sump. A cheap skimmer extends emulsion life more than any biocide.
Mix by pouring concentrate into water, not the reverse, and use water of the recommended hardness — too soft foams, too hard splits the emulsion.
Do not top up with plain water to "bring the concentration down". Evaporation leaves concentrate behind, so topping up with pure water unbalances the sump progressively.