Coolant, Lubrication and Chip Evacuation
Where the heat goes, flood, mist and air, minimum-quantity lubrication, chip control and clearing chips from a pocket.
Where the heat goes
Nearly all the energy of cutting turns into heat in a thin zone at the edge. A cutting fluid has two jobs: to cool (carry heat away) and to lubricate (reduce friction on the rake face and the flank). It also washes away chips. At higher cutting speeds most of the heat leaves with the chip, which is why high-speed machining can be run dry.
Worked example: how hot do the chips get?
- The aluminum slotting cut needs 3.4 kW at the cutter (see Cutting Force, Power and Torque). If 85 % goes into the chips, that is 2.9 kW.
- The chip flow is 138 cm³/min × 2.7 g/cm³ = 6.2 g/s. With a specific heat of 0.9 J/(g·K) the chips rise by ΔT = 2 890 ÷ (6.2 × 0.9) = 517 K: very hot chips that are carrying the heat away from the tool.
- This is why a thin, slow cut heats the workpiece (no chip to carry the heat) while a faster, thicker cut leaves the part cool.
Coolant or not?
| Material and cut | Recommended | Why |
|---|---|---|
| Aluminum | Mist, air blast or flood; lubricant to stop welding | Chips weld and pack; heat is low |
| Mild and alloy steel | Flood or through-tool; or dry with coated carbide and air | Cooling helps life; milling with carbide: avoid thermal shock |
| Stainless steel | Flood, rich emulsion, high pressure if possible | Heat stays in the tool; keeps it from work-hardening |
| Titanium | Flood or high-pressure through-tool | Very poor heat conduction |
| Cast iron | Dry with air or extraction | Fine abrasive dust mixes with coolant into a grinding paste |
| Brass and bronze | Dry or light oil | Free-cutting; little heat |
| Plastics | Air blast (no oil on some plastics) | Coolant can cause stress cracking; chips melt without air |
| Wood and MDF | Dry with dust extraction | No coolant; keep dust out of the air |
| Drilling and tapping | Flood or cutting oil; through-tool for deep holes | Heat and chips are trapped in the hole |
Worked example: how much heat can the coolant carry?
- A flow of 20 L/min that warms by 10 K carries 20 ÷ 60 × 4.18 × 10 = 13.9 kW (an emulsion is a little less, about 13.2 kW): roughly 4.1 times the cutting power in the example.
- So the coolant can carry the heat; the real problem is getting it to the edge, which is why through-tool coolant and a well-aimed nozzle matter more than the flow.
- Interrupted milling cuts with carbide heat and cool the edge each revolution; flood coolant can cause thermal cracks, so run dry with an air blast, or use a high-speed strategy.
Types and concentration
| Type | Use | Notes |
|---|---|---|
| Neat (straight) oil | Heavy cuts, low speed, difficult materials (tapping, broaching, gear cutting) | Best lubrication; poor cooling; smoke and fire risk at high speed |
| Soluble-oil emulsion (milky) | General machining, 5 to 10 % in water | Good cooling and lubrication; the usual shop coolant |
| Semi-synthetic | High-speed machining, mixed materials | Cleaner, longer life, less oil mist |
| Synthetic | Grinding, aluminum, high cooling | Best cooling; little lubrication; can attack seals and paint |
| Mist and MQL oil | Aluminum, plastics, light cuts | Tiny quantities of ester oil; no waste fluid |
Worked example: mixing and topping up
- A 20 L tank at 8 % needs 20 × 8 ÷ 100 = 1.6 L of concentrate and 18.4 L of water. Pour the concentrate into the water (not the reverse) so that it emulsifies.
- Water evaporates faster than oil, so the mix gets stronger: after 1 L evaporates the concentration is 1.6 ÷ 19 = 8.4 %. Top up with water, not mix, and check the concentration with a refractometer.
- A refractometer reading of 4.0 Brix with a factor of 1.8 (the maker gives it) is 7.2 %. Too weak causes rust and poor lubrication; too strong causes foam, residue and skin irritation.
- Keep it healthy: a pH of about 8.5 to 9.5, skim tramp (leaked) oil, keep the sump clean and covered; a coolant that smells has bacteria and should be replaced.
Mist, minimum-quantity lubrication and air
Minimum-quantity lubrication (MQL) delivers a few milliliters of oil per hour as a fine mist in an air stream, which lubricates the edge without cooling; it is common in aluminum and in dry machining of steel with coated tools. Air blast (compressed air through a nozzle) clears chips and cools slightly, with no fluid to dispose of.
Worked example: MQL against flood
- MQL uses about 30 mL of oil an hour. A flood system pumping 20 L/min circulates 1 200 L an hour (about 40 000 times the volume), and even a 2 % loss to chips, mist and evaporation is 24 L an hour to replace and dispose of.
- A 3 mm air nozzle at 6 bar flows about 527 L/min (19 CFM) of free air: enough to clear chips from a small pocket, but a small compressor must keep up (see Power, Lighting and Compressed Air).
- Aim the air at the edge, not at the work in general; two nozzles on opposite sides clear a pocket.
Chip control and evacuation
- Short, curled or comma-shaped chips are the goal: easy to clear and a sign of the right feed and depth.
- Long, stringy chips (turning) wrap the tool and workpiece: raise the feed or depth, change the chip breaker, or peck.
- Powdery dust means the feed is too low (rubbing) or the material is brittle (cast iron).
- Packed chips in a slot or pocket are re-cut, which destroys the edge: reduce the depth, use fewer flutes, add air or coolant, or machine with a helical entry and a trochoidal path.
Worked example: a pocket of chips
- A pocket 50 × 50 mm and 10 mm deep removes 25 cm³ of solid metal. The chips bulk up to about 4 times that, 100 cm³ of loose chips, which must be cleared as they form.
- In a closed pocket there is no way out except up the flutes: slow the plunge, take shallower layers and blow the chips out as you go.
- In wood and plastics the same chips become dust and static: use a vacuum shoe on the router (see Dust, Fumes and Ventilation).
Thermal effects on size and finish
Worked example: a warm part measures too large
- A 100 mm steel part 8 K warmer than when it will be measured is longer by 11.7 × 10⁻⁶ × 100 × 8 × 1 000 = 9.4 µm; as it cools it shrinks by that much (see Measuring Tools and Accuracy).
- For a tight tolerance, finish the part with coolant on and measure it at 20 °C, or leave it to reach room temperature.
- Dry cutting leaves the heat in the work: a few hundredths of a millimeter of growth is common on a long aluminum part.
Health, fire and disposal
- Skin contact with coolant causes dermatitis; wear nitrile gloves when handling the fluid and barrier cream, and wash hands (but not near rotating tools).
- Oil mist is a lung hazard: the typical limit is about 5 mg/m³; a mist system that releases 20 mg/min into a 50 m³ room with 6 air changes an hour reaches about 4.0 mg/m³ without extraction: use an enclosure and extraction.
- Fire: neat oil and mist ignite on hot chips and sparks (especially with titanium and magnesium chips); never use water on a metal fire (see Fire, Fuels and Explosion).
- Disposal: used coolant is hazardous waste: store it in a closed container and send it to a licensed collector; never pour it down a drain.
Troubleshooting
| Symptom | Likely cause | Try |
|---|---|---|
| Coolant smells or the sump is slimy | Bacteria, tramp oil, too weak | Skim the oil, check pH and concentration, clean the sump, replace |
| Rust on the machine and parts | Concentration too low | Check with a refractometer; correct the mix |
| Foaming | Too strong, too much air, a leak in the pump suction | Reduce the concentration, fix the leak, use a low-foam coolant |
| Chips weld to the tool (aluminum) | No lubricant or a dull edge | Add mist or a lubricating coolant; use a polished sharp tool |
| Thermal cracks on carbide inserts | Flood coolant on an interrupted milling cut | Run dry with air, or use a continuous high flow |
| Chips jam in a pocket | No evacuation | Air blast or coolant; shallower layers; fewer flutes |
| Parts measure large then shrink | Heat in the workpiece | Use coolant, let the part cool, measure at 20 °C |
Related
- Speeds and Feeds: the overview of this guide.
- Speeds and Feeds by Material: the material notes on coolant.
- Tool Materials, Coatings and Geometry: coatings that run dry.
- Machining Metals and Chemicals, Gases and Welding.
FAQ
Do I need coolant?
For steels, stainless and titanium, yes (or an air blast with coated carbide); for aluminum, mist or air with a lubricant; for cast iron, brass, plastics and wood, usually none.
How strong should the coolant be?
Typically 5 to 10 % for a soluble oil, as the maker advises; check it with a refractometer, and top up with water, not mix.
Why do chips pack in a slot?
There is nowhere for them to go but up the flutes: reduce the depth, use fewer flutes, blow them out and use a trochoidal path.
Is MQL as good as flood coolant?
It lubricates but does not cool: excellent for aluminum and for coated tools in dry steel machining; flood is still better for stainless and titanium.
References
- Oberg, E., et al., Machinery’s Handbook (cutting fluids).
- Byers, J. P. (ed.), Metalworking Fluids, CRC Press; Sandvik Coromant, Coolant in Metal Cutting.
- NIOSH, Criteria for a Recommended Standard: Occupational Exposure to Metalworking Fluids (the 0.5 mg/m³ thoracic mass limit) and OSHA oil-mist limits.
Last reviewed: October 2026. Concentrations and flows are typical: follow your coolant maker’s instructions.