Fluids and maintenance

The use of machining tools requires various fluids, oils, and greases. The most typical among these are cutting fluids, hydraulic and slideway oils, as well as spindle and angular head greases.

The function of cutting fluid is to cool and lubricate the machining process. In addition, the cutting fluid cleans the tool and the machine, transports chips away, and provides corrosion protection for workpieces (rust prevention). Properly functioning cutting fluid allows for higher cutting speeds, extends the life of tools, and improves the surface quality and dimensional accuracy of workpieces.

When selecting a cutting fluid, it's important to remember that the right cutting fluid is just as crucial as the right tool. By choosing the correct cutting fluid, the durability of both the tools and the cutting fluid itself can be enhanced, thus reducing downtime due to tool or fluid replacement. Even a good cutting fluid requires monitoring to ensure it maintains consistent quality. If not monitored, issues such as corrosion (rust problems) or bacterial growth may occur.

The most common cutting fluid is an emulsion, or water-soluble cutting fluid, typically used at a concentration of 4-10%. Emulsions are cost-effective compared to straight cutting oils, which are used in their pure form without being mixed with water. The applications for straight cutting fluids usually include deep hole drilling, bar automatics, and gear milling.

When selecting the right cutting fluid, consider the materials to be machined, the machining method, and the cooling system of the machine tool along with its pressure. The easiest materials to machine are cast iron and copper alloys. High-alloy steel and aluminum alloys are more challenging to machine and require well-lubricating cutting fluids. However, it should be noted that machining cast iron and copper alloys releases small metal particles, which contaminate the machine tool.

Changing the Cutting Fluid

If the system contains cutting fluid in good condition, replace it by adding new cutting fluid, diluting it into the old mix. Before adding new cutting fluid, it's advisable to conduct a compatibility test to ensure that mixing the new and old product does not result in foaming or separation issues. The mixing ratio of the products to be tested should be 10/90, 50/50, and 90/10. If the entire cutting fluid system's fluid is being replaced, it is best to initially use a system cleaner before draining the system.

The Cutting Fluid System Should Be Regularly Cleaned

Pump out the cutting fluid and remove chips and any impurities from the bottom of the tank when anticipating a summer or other long break. Clean the conveyor, tank, and machine, then filter the cutting fluid back into the system.

Mix Cutting Fluid Emulsion Using a Mixer

Improperly mixed emulsion can become inverted, resulting in water droplets in oil rather than oil droplets in water, causing the emulsion to not function as intended.

Checking Emulsion Concentration

The concentration of the cutting fluid emulsion should remain at the recommended percentage level for optimal operation. If the concentration is too high, the emulsion becomes stickier, increases the risk of skin problems, and consumes more cutting fluid. If the concentration is too low, the outcome is bacteria, mold growth, corrosion, poor lubrication properties, and shortened lifespan for tools and systems. It's important to measure concentration regularly, at least twice a week.

How to Measure Concentration?

The concentration of the cutting fluid is measured with a refractometer. Before using the refractometer, it should be calibrated with the same water used to dilute the cutting fluid. The refractometer reading is multiplied by a factor (refractometer factor) specific to the water-soluble cutting fluid used. This factor is stated on the cutting fluid's product information sheet or packaging label.

Check pH Value

Monitoring the system's pH value is important. If the pH value decreases, the concentration is too low or there is bacterial growth in the system. The pH value of most cutting fluids should be at least 9.0. The pH value can be checked with a pH meter or pH test strip. The pH level is product-specific, and in a new emulsion, it is usually 9.0–9.6. A decrease in pH (measured at least once a week) indicates that the emulsion is beginning to break down and requires maintenance. The consequences of a lower pH value may include bacterial growth and rust on material and tool surfaces.

Measure Emulsion Bacterial Content

Bacteria cause the pH value to lower, leading to corrosion problems and bad odors. Bacteria multiply quickly if growth develops within the system, so it is essential to measure the bacterial content weekly. Bacterial measurements are easiest to perform with contact strips.