Machining Coolant Selection and Maintenance

This presentation will primarily deal with Emulsified Oils (Soluble Oils) which are mixtures of oil, usually napthenic, and emulsifiers, and can be combined with bactericides, corrosion inhibitors, chelating and Extreme Pressure (EP) agents such as Chlorine. These products are offered as concentrates and mixed with water at rates varying from 3 to 20%, dependent on the application, typically referred to as a ratio, example 5%=20-1, 20 parts water to 1 part concentrate. The emulsified solutions create a milky, opaque appearance solution of oil in water droplets.

These products offer lubrication, they contain oil, and the water aids in dissipating heat. Speeds and feeds can be increased and better size control obtained. But soluble oils have several disadvantages. When mixed with hard water, some soluble oils (no chelates) can form a precipitate (hard water salt) which can deposit on the parts and machines. This can interfere with filtration and in extreme cases break the emulsion. At high concentrations the mist from soluble oil can leave machines and work areas in a messy, slippery condition. Depending on the presence and amount of rust preventatives built into the product, rust can be a problem. The water can support microbial growth, which leads to rancid odors and short sump life if sufficient bactericides are not present.

Two other water soluble products are Chemical Emulsions (Semi-Synthetics) and Chemical Solutions (Synthetic Fluids). Semi-Synthetics have lower oil content and a higher emulsifier package which results in a smaller droplet formation and a mixture that is translucent or transparent. These solutions usually have better settling and cleaning properties than Emulsified Oils. Synthetics are true solutions and are completely clear; they consist of organic and inorganic salts dissolved in water. Many are used as grinding fluids since they offer rust protection and good heat removal. Heavy-duty products are good general purpose coolants that provide good lubrication and fast heat removal. They keep the wheels open and free grinding, enable the operator to see the work and produce considerably less mist than other cutting fluids. Synthetic solutions are the less prone to microbial infestation and have superior settling and cleaning properties that help extend fluid life. Synthetic solutions are typically stable even in hard water.

Cutting fluids should not leave residue on the parts or the machine and should be non-toxic and non-irritating to eyes, nose and skin. Rancidity, accompanied by objectionable odors should be controlled by microbicides built into the formulation. Rancidity can be controlled by good sanitation, correct concentrations, aeration and the use of good water. Water hardness “uses up” the coolants concentrate and tends to force it out of solution and part of the concentrate does not contribute to cutting efficiency. It may appear as a gummy deposit residue on the machine or part. In addition the lost concentrate can cause parts and machines to rust. Cutting fluid should provide rust protection on parts for 72 hours provided the product is used at the recommended concentration and under favorable conditions.

Cutting Fluid Controls

Control of the system is as important as the cutting fluid selection in prolonging the life of the coolant. Basic steps to obtain long fluid life and to avoid problems;

  1. Assign responsibility of control to one person
  2. Clean the system thoroughly before recharging with fresh mix
  3. Maintain the cutting fluid concentration at recommended dilution
  4. Keep the fluid free of chips and grit
  5. Provide good chip flushing
  6. Employ good housekeeping
  7. Remove tramp oil
  8. Minimize aeration, contributes to build up of hard water salts

Microbial tests should be done on working coolants at periodic intervals. Companies that already use this technique have set different maximum levels ranging from 1 to 20 million organisms per milliliter. Keeping the level under 10 million organisms at all times can extend coolant life from a few months to one or more years.

It is impossible to entirely prevent the introduction of organisms into a working fluid; however it is possible to minimize contamination of the system. High bacterial counts can lead to corrosion. Bacteria consume metalworking fluid components, and the byproduct of this activity can lower the pH of the fluid. The metabolic products include mild organic acids, which lower the pH of the fluid, compromising the corrosion resistance of the fluid. If left unchecked the bacteria can split the emulsions.

Metalworking fluids are designed to be effective within a specified dilution range depending on the machining application and the machinability rating of the metal. Overcharging a system can interfere with the required cooling capability. The goal is to Maximize Wheel / Tool Life and Minimize Coolant Usage. If the fluid is to lean the product may not be able to perform their designed jobs. This applies to rust inhibitors, which may not be able to protect the newly ground or machined parts from corrosion. If corrosion protection beyond these limits is required, then the use of longer term rust preventive is recommended. Check fluid concentration frequently. Titration methods and hand held refractometers are the best methods of determining concentrations.

The pH is a measure of the acidity or alkalinity of a metalworking fluid mix, and it is a sound indicator of the condition of the coolant. A low pH can lead to rancidity, ferrous corrosion and mix instability. a pH range of 8.8 to 9.2 should be maintained in the system this will help protect ferrous metals.

When adding make-up coolant to the reservoir, always add the oil into the water to avoid an inverted emulsion. Remember the coolant droplet is oil in water, the water cools the piece and the oil lubricates. An inverted emulsion is water in oil, the end result is less cooling and inadequate lubrication. If the concentration range has been determined between 8-10%, make-up should be less than 8% to account for water evaporation. This assists in maintaining the correct range established for the machining center. Never just add water to a system unless it is absolutely necessary. This drops the pH and consumes the reserve alkalinity that is intended to keep the pH buffered to 8.8-9.2. This drastically reduces bath life.

Small metal particles in a metalworking fluid referred to as “swarf” or “dirt”. Swarf deposited on the part and not properly washed away can form a galvanic cell and rust will occur underneath the swarf. To help settle swarf , select the settling aid or clarifying agent compatible with the fluid and filter system. Check the coolant pressure at the coolant nozzle to determine if the lack of coolant pressure is causing the swarf to build up on fixtures or workpieces. Normal coolant pressure is 20 psi.

Free oil or referred to as tramp oil is oil or oil-like material that is not emulsified and floats on the surface of the metalworking fluid. Tramp oil is usually hydraulic fluids and way oils that have leaked into the coolant. Tramp oil should be less than 0.5% because a high percentage in the coolant can lead to microbial growth and residue.

If the emulsion appears watery and the sample shows stratification, the emulsion is unstable. Rust can happen if an emulsion splits or breaks down, in which case the rust inhibitors will be ineffective. Broken and unstable emulsions are often caused by hard excessive bacterial contamination. It is important to determine the cause of the split emulsion before taking corrective action. Many additives are available that can boost corrosion protection. The determination of which one to use depends on the fluid in use and chemical restrictions. Always consult your fluid supplier before making any additions.

Cutting Fluid Selection

Both the needs of the job and performance properties must be considered in selecting a cutting fluid. The type of operation combined with the most critical operation in a machining environment may dictate the cutting fluid selection. Threading, reaming, tapping and broaching are some of the most difficult machining operations, and require a cutting fluid that reduces friction. Turning, drilling boring, and similar less difficult operations can be machined with products having low to moderate friction-reducing properties. Some grinding applications can be machined with fluids that provide little friction reduction. Form grinding requires a heavy-duty fluid.

Severity of Operation, stock removal rates, feeds, speeds and finish requirements must be considered. Divided into four general categories:

  1. Metal removal in which adhesion between the chip and tool interface is minimal is considered light duty. Turning 1112 steel or surface grinding cast iron are examples
  2. Operations in which adhesion between the chip and tool is noticeable and causes poor finish and size control is considered moderate duty. Key seat milling and internal race grinding are examples.
  3. Severe adhesion between the chip and tool is considered heavy-duty. Sawing of large parts, deep slotting and centerless grinding of heavy parts are examples.
  4. Extremely severe adhesion between the chip and tool and small tool clearances which create severe rubbing conditions between the tool and work-piece are extremely heavy-duty. As severity increases it may be necessary to increase the lubricity of the fluid. Thread chasing, tapping 4140 steel, deep hole drilling, form or thread grinding are examples.

Machinability differentiates metals into six different classes basing the standard on SAE 1112 steel as 100% machinable. Variations in microstructure, amount of alloy, heat treatment and many other factors affect machinability. These are some examples.

  1. Ferrous >70%, primarily low carbon steels, 1016 – 1030, some alloy 1109- 1144, cast steel, malleable iron, stainless iron.
  2. Ferrous 50 – 70%, medium carbon steels, 1020 – 1070, alloy 4028 – 4047, cast iron
  3. Ferrous ≤40 – 50%, alloy 1320 – 1340, 4340, 6120, wrought iron, stainless
  4. Ferrous <40, 317, tool steel
  5. Non-Ferrous <100%, Aluminum
  6. Non-Ferrous ≤100%, Nickel, Brass, Copper

Using trend analysis can assist in tracking each machining centers daily checks for ph and concentration. Review the attachment included with this correspondence.