When is Machining Fluid No Longer Safe?

The Occupational Saftey and Health Administration has developed a document of best practices that it recommends following when dealing with metalworking fluids.


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In its Metalworking Fluids: Safety and Health Best Practices Manual, The Occupational Safety and Health Administration (OSHA) (Washington, D.C.) lays out some general guidelines on working with metalworking fluids (MWF). It notes that there are many signs that a fluid has undergone changes and is no longer safe to use because of emerging health hazards. If one or more of the following changes occur, the fluid should be evaluated to see if it is safe for continued use or if it should be replaced.

Low Sump Level

Check the sump level at the start of the shift. A low sump level (30 percent below the full mark) shows metalworking fluid loss or water evaporation (increasing the concentration of chemicals present in the MWF). Check the concentration. If it is too strong, add water to reach the proper concentration. If the concentration is correct, then fluid was lost due to dragout. You should add fluid at an appropriate dilution, or if pre-diluted fluid is not available, water and concentrate should be added. All systems should be monitored carefully and metalworking fluid additions should be made on a regular basis to maintain a constant working concentration.

Abnormal Fluid Appearance

Determine if the fluid color looks normal. When in good condition, many synthetic fluids are clear; semi-synthetics are often transparent to milky; and soluble oil usually looks milky white with no free oil layer. If the fluid turns gray or black, then bacteria are often present. If the fluid picks up a yellow or brown tint, then tramp oil may be present. Dye fading may indicate that a fluid is aging.

Foul Smell

When fluids smell bad, it usually means that there is uncontrolled microbial growth. Although it may be possible to cover up the odor, it's best to address the cause because microorganisms present in the fluid can be aerosolized into the air as part of the mist. Exposure to microorganisms in the air may cause adverse health effects to exposed employees. If the fluid has a strong and "locker room" odor, it likely has biological growth and should be treated with biocide and evaluated. If need be, the fluid should then be discarded, the sump properly cleaned and the fluid replaced.

Floating Matter on the Fluid

If the fluid has floating chips, swarf or mold growth, this is not normal. Try to remove as much as possible with a skimmer or have it pumped off. The level of dirt (total suspended solids) in the fluid is a measure of the efficiency of the filtering system. Periodic checks and maintenance of the filtration system and oil skimmer are necessary to assure that they are functioning as designed.

Tramp Oil Floating on the Surface

With water-diluted fluids, if the sump is completely covered with oil and the machinist cannot swish the oil out of the way for more than five to eight seconds before the sump is covered again, there is too much tramp oil present. Skim or pump the surface oil to remove it. Tramp oil is one of the main causes of dermatitis. These oils are not developed with repeated skin contact in mind, and some components of these machine lubricants are highly irritating to the skin. Unemulsifed (tramp) oils can be a significant carrier of metallic fines, which can be deposited on the skin and cause mechanical irritation.

Excessive Foam

A lot of foam may be caused by soft water with some products. The fluid also may be too highly concentrated, or may be contaminated by cleaners, or there may be an imbalance in the fluid surfactants. Another possibility is that you could have an undersized system, excessive flowrates or the fluid may not be at rest long enough to allow air to escape. In addition, the level of cutting fluid in the reservoir may be low, causing air to be drawn into the pump.

Dirty Machines or Trenches

This could mean that the emulsions are becoming unstable, the cleaners in the fluid have been depleted, the contaminants are being deposited from the fluid, there is a filter failure or there is poor housekeeping.

Skin Irritation

If employees have skin irritation, it could mean that the fluid has one or more of the following properties: too high a concentration, high alkalinity, metal contamination, an unstable emulsion or contamination from workpiece coatings. Of course, skin irritation also can be due to causes not directly related to metalworking fluids, such as changes in the weather, poor personal hygiene, poor work habits, the use of harsh hand soaps, wearing contaminated clothing or prolonged exposure to the fluid.

Respiratory Irritation

Exposure to MWF aerosols can lead to complaints of irritation and tightness in the chest. Factors that can contribute to irritation could be the improper delivery of fluid to the cutting zone; improper use of additives; a high coolant concentration; a heavy concentration of machines in a small area; inadequate or poorly designed enclosures and mist collectors; loss of microbial control; poor general ventilation of the shop; insufficient fresh air make-up rates; and high mist concentrations (even in the absence of machining operations) may be present in areas where coolant flumes make sharp turns.

Other problems that might be fluid related and that should be investigated include:

  • Rust or corrosion of the machine tool or of the part produced.
  • Staining of the metal machined or machine tool.
  • Tool failure due to the loss of performance additives.
  • Growth of fungi that block fluid flow.
  • Change of fluid viscosity (thinner or thicker).
  • Accumulation of water at the bottom of the oil sump drain, in straight oils.
  • Dirt and grit suspended in the fluid.
  • Failure at the workpiece-tool interface (for example, burning of a ground part due to excessive heat buildup).

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