Cutting Fluids and Maintenance
Working with machine tools requires several different types of fluids, oils, and greases. The most common of these are cutting fluids, hydraulic and slideway oils, as well as spindle and bevel gear greases.
The purpose of cutting fluid is to cool and lubricate the machining process. Additionally, cutting fluid cleans the tool and the machine, transports chips away, and provides corrosion protection (rust prevention) for the workpieces. A properly functioning cutting fluid allows for higher cutting speeds, extends tool life, and improves the surface quality and dimensional accuracy of workpieces.
When choosing a cutting fluid, it’s important to remember that the right cutting fluid is just as important as the right tool. By selecting the appropriate cutting fluid, you can extend the lifespan of both the tools and the cutting fluid itself, thereby reducing downtime caused by tool or fluid changes. Even the best cutting fluid requires monitoring in order to maintain consistent quality. Without regular monitoring, issues like corrosion (rust) or bacterial growth may occur.
The most common cutting fluid is an emulsion, also known as a water-soluble cutting fluid, with a typical concentration of 4–10%. Emulsion is more economical compared to straight cutting oil, which is used as is and is not mixed with water. Straight cutting fluids are mostly used for deep hole drilling, automatic lathes, and gear hobbing.
When choosing the right cutting fluid, consider the materials being machined, the machining method, and the machine tool's coolant system and its pressure. Cast iron and copper alloys are the easiest materials to machine. Highly alloyed steel and aluminum alloys are more challenging and require cutting fluids with excellent lubricating properties. It’s also worth noting that machining cast iron and copper alloys generates tiny metal particles that can contaminate the machine tool.
Changing Cutting Fluids
If the system contains cutting fluid that is still in good condition, replacement can be done by adding new cutting fluid diluted into the old mix. Before adding new cutting fluid, it’s recommended to conduct a compatibility test to ensure that mixing new and old products won’t cause foaming or separation issues. The mixture ratios to be tested are 10/90, 50/50, and 90/10. If the entire cutting fluid in the system is to be replaced, it’s best to use a system cleaner first before emptying the system. Read more..
The cutting fluid system should be cleaned regularly
Pump out the cutting fluid and remove chips and any impurities from the bottom of the tank before a summer or longer break. 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 mixed incorrectly, the emulsion may invert, causing water droplets in oil instead of oil droplets in water, and in such cases, the emulsion won’t function as intended. Read more..
Checking Emulsion Concentration
The concentration of the cutting fluid emulsion must remain at the recommended percentage level to function optimally. If the concentration is too high, the emulsion will be stickier, the risk of skin problems increases, and you’ll use more cutting fluid. If the concentration is too low, problems such as bacteria, fungi, corrosion, poor lubricating properties, and shortened tool and system life can occur. It’s important to measure the concentration regularly—at least twice a week. Read more..
How is the concentration measured?
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 reading given by the refractometer is multiplied by a coefficient (refractometer factor), which is specific to the water-soluble cutting fluid being used. The factor is listed in the product information sheet or on the product label.
Check the pH value
It’s important to monitor the pH value of the system. If the pH drops, the concentration is too low or there is bacterial growth in the system. Most cutting fluids must maintain a pH of at least 9.0. The pH can be checked with a pH meter or pH test strips. The pH value is product-specific; in a new emulsion, it’s usually 9.0–9.6. A drop in pH (which should be measured at least once a week) indicates that the emulsion is breaking down and it’s time for maintenance. Consequences of a low pH include bacterial growth and rust on materials and tools. Read more..
Measure the bacterial content of the emulsion
Bacteria cause the pH value to drop, resulting in corrosion problems and bad odors. Bacteria multiply quickly if growth forms in the system, so it’s important to measure bacterial content weekly. The easiest way to measure bacteria is with contact test strips.
