Flood, dry, mist or high-pressure through-tool: the right choice is set by the material and the operation, not by habit. Coolant does three jobs, cool, lubricate and clear chips, and on some materials one of those jobs flips the whole decision. Here is the brand-neutral guide by ISO group.
Before you decide wet or dry, know which job you need. The material decides which one dominates, and sometimes the job you do not need is the one that hurts you.
| Job | When it dominates | What it buys you |
|---|---|---|
| Cooling | High-speed steel turning, anything heat-limited | Controls crater wear and plastic deformation by pulling heat from the edge |
| Lubricity | Gummy, ductile materials at lower speed | Stops material cold-welding to the edge, the cure for built-up edge |
| Chip evacuation | Drilling, deep pockets, blind holes | Flushes chips out so they do not re-cut, pack flutes or scratch the bore |
On cast iron and on interrupted milling cuts, coolant can do more harm than good: it turns abrasive iron dust into slurry, and intermittent wetting of a hot insert causes thermal-fatigue cracks. Sometimes the right move is no coolant at all. See thermal cracking in the wear guide.
The default that gets you started, why it is the default, and the thing to watch. Always defer to your machine, fluid and tooling makers' specifics.
| Group | Default | Why | Watch |
|---|---|---|---|
| P Steel | Flood, or dry with coated carbide | Versatile. Modern coated grades turn steel dry well; coolant still helps finish and chip control | Crater wear climbs with speed; coolant or a CVD alumina grade tames it |
| M Stainless | Flood, high lubricity; high-pressure if deep | Work-hardens and forms built-up edge; coolant kills adhesion and clears chips | Never let the tool dwell or rub; that hardens the surface wet or dry |
| K Cast iron | Usually dry | Abrasive graphite dust plus interrupted cuts; coolant makes slurry and risks cracks | Use dust extraction; if wet, flood consistently, never intermittent |
| N Aluminium | Flood or MQL, high lubricity | Gummy and sticky; needs lubricity against build-up and flushing of soft chips | Built-up edge if too slow or dry; keep speed up, geometry sharp |
| S Titanium / superalloy | Copious flood, high-pressure through-tool | Very low thermal conductivity keeps heat at the edge; HP coolant cools and breaks chips | Titanium fines are flammable; do not dry-grind. Heat is the enemy |
| H Hardened steel | Often dry or light air | Hard turning uses controlled edge heat to shear; coolant can thermally shock CBN | Thermal cracking on interrupted hard cuts; stability matters more than fluid |
Defaults are starting points per common practice. Material subgroup, operation and setup can shift the call.
| Fluid | Strength | Best for |
|---|---|---|
| Soluble / emulsion oil | Balanced cool + lube, economical | General machining. Typical mix around 6 to 10 percent, set by refractometer |
| Semi-synthetic | Cleaner, good cooling | Mixed shops wanting less residue and better sump life |
| Synthetic | Best cooling, cleanest | Grinding and high-heat work where lubricity matters less |
| Straight (neat) oil | Maximum lubricity, low cooling | Threading, broaching, gun-drilling, gummy low-speed work |
| MQL (mist) | Minimal fluid, near-dry | Aluminium and dry-leaning ops; cuts waste and cleanup |
| High-pressure through-tool | Chip breaking + targeted cooling | Stainless and superalloy drilling, deep cavities, stringy-chip materials |
Most coolant problems are mix problems. Too lean invites rust, bacteria and poor lubricity; too rich brings residue, foam and waste. Check emulsion with a refractometer against your fluid maker's target, do not eyeball it, and top up with pre-mixed fluid rather than neat water.
Coolant rarely fixes a problem alone. If the edge is failing, read the wear first; if the grade is wrong for the group, no fluid will save it. That is the brand-neutral problem this tool solves: the right grade might be a Sandvik number while your shop stocks Kennametal.
Free, no strings: 8-brand grade cross-reference (PDF) · ISO material-group cheat-sheet (PDF)