Metalworking Fluids and Maintenance

Machine tools require a variety of different fluids, oils, and greases. The most common include cutting fluids, hydraulic and way oils, and spindle and bevel gear greases.

The purpose of cutting fluid is to cool and lubricate the machining process. In addition, cutting fluid cleans the tool and machine, carries chips away, and protects workpieces against corrosion (rust). A properly functioning cutting fluid enables higher cutting speeds, extends tool life, and improves the surface finish and dimensional accuracy of workpieces.

When selecting a cutting fluid, it is important to remember that choosing the right cutting fluid is just as important as choosing the right tool. Selecting the correct cutting fluid can extend the service life of both the tools and the cutting fluid itself, thereby reducing downtime caused by tool or fluid changes. Even a high-quality cutting fluid requires monitoring to ensure that it remains consistent at all times. If it is not monitored, corrosion or rust problems and bacterial growth may occur.

The most common cutting fluid is an emulsion, also known as a water-soluble cutting fluid, which is typically used at a concentration of 4–10%. An emulsion is less expensive than straight cutting oil, which is used as supplied and is not mixed with water. Straight cutting fluids are generally used for deep-hole drilling, automatic bar machines, and gear milling.

When selecting the right cutting fluid, the materials being machined, the machining method, and the machine tool’s coolant system and pressure must be taken into account. Cast iron and copper alloys are among the easiest materials to machine. High-alloy steel and aluminum alloys are more difficult to machine and require highly lubricating cutting fluids. However, it should be noted that machining cast iron and copper alloys releases small metal particles that can contaminate the machine tool.

Changing the cutting fluid

If the system contains cutting fluid and it is in good condition, the fluid can be changed by adding new cutting fluid and diluting the existing fluid. Before adding new cutting fluid, it is recommended to perform a compatibility test to ensure that mixing the new and old products does not cause foaming or separation problems. The products being tested should be mixed at ratios of 10/90, 50/50, and 90/10. If all the fluid in the cutting-fluid system is being replaced, it is best to use a system cleaner first, before draining the system. Read more..

The cutting-fluid system must be cleaned regularly

Pump the cutting fluid out and remove chips and any contaminants from the bottom of the tank before a summer shutdown or other extended stoppage. Clean the conveyor, tank, and machine, then filter the cutting fluid back into the system. Read more..

Mix the cutting-fluid emulsion using a mixer

If the emulsion is mixed incorrectly, it may become inverted, with water droplets dispersed in the oil instead of oil droplets dispersed in the water. In this case, the emulsion will not function as intended. Read more..

Checking the emulsion concentration

The concentration of the cutting-fluid emulsion must remain within the recommended percentage range for the emulsion to function optimally. If the concentration is too high, the emulsion becomes stickier, the risk of skin problems increases, and more cutting fluid is consumed. If the concentration is too low, bacteria, fungal growth, corrosion, poor lubricating properties, and reduced tool and system service life may result. It is important to measure the concentration regularly, at least twice a week. Read more..

How is the concentration measured?

Measure the concentration of cutting fluid with a refractometer. Before using the refractometer, calibrate it with the same water used to dilute the cutting fluid. Multiply the reading shown by the refractometer by the factor (refractometer factor) specified for the water-soluble cutting fluid being used. The factor is provided in the cutting fluid’s product data sheet or on the package label.

Check the pH value

It is important to monitor the system’s pH value. If the pH value drops, the concentration is too low or there is bacterial growth in the system. The pH value of most cutting fluids must be at least 9.0. The pH value can be checked with a pH meter or pH test strip.  The pH value is product-specific, and in a new emulsion it is usually 9.0–9.6. A decrease in pH (the value must be measured at least once a week) indicates that the emulsion is beginning to break down and that maintenance is required. Consequences of a decreasing pH may include bacterial growth and rust on the surfaces of the material and tools. Read more..

Measure the bacterial content of the emulsion

Bacteria cause the pH value to drop, which can lead to corrosion problems and unpleasant odors. Bacteria multiply rapidly if growth develops in the system, so it is important to measure the bacterial content weekly. The easiest way to measure bacteria is with contact test strips.