Coolant monitoring on CNC milling centres: metalworking fluid and spindle cooling

When people talk about coolant monitoring on CNC milling centres, they often mean two different things. One is the coolant (metalworking fluid) that cools and lubricates at the cutting edge and carries the chips away. The other is the cooling of the spindle and machine components, usually a separate, closed circuit with a chiller unit. Both affect tool life, surface finish and dimensional accuracy, but in different ways. This article deals with them separately: what wears or changes there, what you can measure and which care routines have proven themselves.

Two circuits, two tasks

Coolant: what changes during operation

A water-miscible coolant is not a stable medium. It changes with every day of operation:

• Concentration: water evaporates, coolant is carried out with the chips, and topping up is often mixed by feel. The concentration therefore drifts up or down. Too low means less corrosion protection, less lubrication and better conditions for bacteria. Too high puts a strain on skin and airways and wastes concentrate.
• pH value: water-miscible coolants are usually alkaline. A falling pH value is often a sign of microbial activity, because bacteria break down components of the coolant.
• Tramp oil: leakage oil from hydraulics, slideway and spindle lubrication gets into the coolant. It floats on top, reduces the oxygen supply in the tank and promotes bacterial growth.
• Chips and fines: the finest particles pass through the filters, settle in the tank and reach the workpiece and machine via the nozzles.
• Bacteria: bacteria and fungi multiply especially during standstill, in warm coolant and at low concentration. Typical signs are a foul smell after the weekend, slime formation and discolouration.
• Temperature: the coolant heats up through cutting and pump work. Warm coolant accelerates bacterial growth and can affect the temperature of the workpiece and fixtures.

Coolant monitoring in practice: what you can measure on the metalworking fluid

The most important quantities can be checked directly at the machine with simple means.

Concentration with a refractometer

The hand-held refractometer is the standard tool. A few drops of coolant on the prism, read off, convert. The reading is not directly the concentration, but has to be converted using the factor of the respective product:

Always measure at a representative point, for example at the nozzle outlet with the pump running, not at the surface of the tank. The target concentration for the respective machining operation is specified by the coolant manufacturer.

pH value, nitrite, bacteria and temperature

• pH value: with test strips or a pH meter. The trend is more important than the individual value. A steady fall is a warning sign.
• Nitrite: with test strips. Nitrite can be introduced via contaminated water, workpiece residues or microbial processes and is relevant under occupational safety law (see below).
• Bacterial load: with dip slides, which allow a rough classification after an incubation period.
• Temperature: with a simple thermometer in the tank or a permanently installed sensor.
• Visual and smell check: tramp oil layer, foam, discolouration, deposits, odour. This check costs nothing and belongs in every inspection.

For continuous monitoring there are inline sensors for concentration, conductivity, pH and temperature. They save time, but need care themselves and regular comparison with a manual measurement. A wrongly calibrated sensor is worse than a careful weekly measurement.

Care routines that have proven themselves

1. Measure and document regularly: concentration, pH value and nitrite at a fixed interval, with date and machine. Only the records make trends visible.
2. Mix correctly: add concentrate to water, not the other way round, and ideally with a dosing unit. To compensate for evaporation, top up with a lower-concentration mix, otherwise the concentration rises.
3. Remove tramp oil: with a skimmer or oil separator, and look for the source. A leaking hydraulic line often shows up in the coolant first.
4. Maintain filters and the chip conveyor system: change filters, and empty and clean the tank at intervals.
5. Consider standstill: during long standstills the coolant does not move. Circulating or aerating it as recommended by the supplier can prevent odour formation.
6. System cleaning when changing: before refilling, treat lines and tank with a system cleaner, otherwise residues quickly recolonise the new coolant.

Spindle cooling and thermal management

The second circuit is often more important for accuracy. Spindle bearings and the spindle motor generate heat, especially at high speeds. A chiller unit removes this heat via a cooling jacket. Many machines also cool the motors of feed axes, the control cabinet or the hydraulics.

The aim is not the lowest possible temperature, but the most constant one. Some units control to a fixed value, others maintain an offset from a reference temperature, such as that of the machine bed or the shop. Which principle your machine uses and which setting is correct is stated in the machine builder's documentation. Do not change these values without consulting them.

Typical problems in the cooling circuit develop gradually:

• Dirty fins or filters on the chiller unit reduce the cooling capacity, especially in summer.
• A falling fill level, air in the circuit or a weakening pump reduce the flow rate.
• Deposits in the cooling jacket impair heat transfer.
• An unintended medium or the wrong additives promote corrosion and deposits.

Thermal drift and dimensional accuracy

Heat changes geometry. The spindle grows along its axis, frame parts deform, and a heated screw shaft gets longer. The order of magnitude can be estimated with linear thermal expansion:

The example shows why even a few kelvin of difference matter in precision machining. Real machines behave in a more complicated way, because different components heat up at different rates. Many CNC controls therefore offer temperature compensation, and many plants use a warm-up program before the first precision machining.

You can measure how much your machine drifts yourself: a reference sphere or a fixed test piece in the working area is probed at intervals with the touch probe while the spindle runs under defined conditions. The displacement over time shows the thermal behaviour. The standard ISO 230-3 describes test methods for thermal effects on machine tools. Such a measurement is worthwhile as a reference and after interventions on the cooling system.

In the cooling circuit itself, these quantities are worth observing:

• Supply and return temperature: together with the flow rate, their difference shows how much heat is being removed.
• Flow rate: many machines have a flow switch that only reports a failure. A flow meter also shows a slow decline.
• Ambient temperature: it explains many fluctuations, especially in shops without air conditioning.
• Spindle bearing temperature: provided a sensor is fitted and the CNC control supplies the value. A rise with unchanged cooling can also point to a bearing problem.

The last point links cooling and bearing condition. An increased bearing temperature can come from the cooling or from a bearing with lubrication or raceway problems. Incipient bearing damage usually shows up in the vibration pattern earlier than in the temperature. More on this in the article Detecting rolling bearing damage early and on the question of where spindle noises come from.

What we offer today

Coolant and cooling circuits are on our roadmap. They are not part of our offer today. We will only offer them once we have shown on real machines that an assessment is reliable. The principle does not change: we only read data from the CNC control and do not change any settings.

Today we monitor the spindle and its bearings with a dedicated vibration sensor and an assessment against the learned baseline, i.e. the normal state of that spindle. How this works is explained in the guide to spindle monitoring. The current scope is described on the page Spindle monitoring for CNC milling centres.

Frequently asked questions

About the author

Sascha P. Forbach is the founder of Forbach & Partners and the software architect of its spindle monitoring system. For twelve years he has advised manufacturers on process quality and machine availability. About us