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Coolant
Maintenance for Machining Operations
Machining coolants
are an important component of metal working operations.
Coolants improve machinability, increase productivity
and extend tool life by cooling and lubricating the
part and cutting tool. When performing these functions
coolants can quickly become contaminated with foreign
materials, causing coolants to lose effectiveness and
develop foul odors. As a result, many coolants are used
only for a short time then discarded.
The practice of discarding and replenishing
coolants can be costly and wasteful. Developing and
implementing a coolant maintenance program can help
minimize contamination, prolong coolant life and reduce
cost.
Coolant Selection
Coolants affect the overall performance of machining
operations, so proper selection is important. Using
an unsuitable coolant may lead to premature tool failure
or produce an undesirable finish on parts and may shorten
the useful life of the coolant. When selecting a coolant
determine if it is suitable for the type of metal and
machining operation being used and if it will produce
the desired finish quality.
Biodegradable fluids—subject
to breakdown by bacteria and living organisms—should
be considered if coolant spills are likely, or if spent
coolant will be discharged to a wastewater treatment
system or landspread. The most stable, non-biodegradable
coolants give most machine shops the best performance
over the longest period. Disposal of these coolants
generally includes separating the oil for fuel blending.
Facilities should use the minimal
number of coolant types. Many metalworking facilities
require no more than two types of coolants for their
operations, one for machining and one for grinding.
Using a large number of coolant types requires extra
storage space; adds to inventory and maintenance needs;
increases the chance for mix ups and cross-contamination;
and requires more staff expertise.
Contaminant
Removal and Prevention
Coolants are most frequently contaminated by metal chips
and fines, tramp oil, bacteria and dissolved salts.
Chips and fines are removed from coolants to eliminate
surfaces for bacteria to grow on. Free oils are removed
to eliminate a food source for bacteria and surface
films that prevent oxygen from dissolving into the coolant.
They also can deplete oil soluble components of the
coolant and oils reduce the cooling capability of the
metalworking fluid. Dissolved salts are removed because
they affect tool and coolant life, foaming characteristics,
filtering efficiency and emulsion stability.
By removing contaminants, machining
facilities can reduce costs by prolonging the effective
life of coolants. The table at the end of this fact
sheet describes the equipment and techniques used to
control and remove the four most frequent contaminants.
A good first step is teaching machine operators not
to use coolant sumps as trash receptacles. Waste food,
cigarettes and other trash cause bacterial contamination
and feed bacterial growth.
Coolant Monitoring
and Control
When a coolant is in use, heat and contaminants can
change its composition and performance. Routine monitoring
of coolants for quality is necessary to optimize performance
and prevent problems. Include monitoring pH and maintaining
concentration in a coolant maintenance program.
pH control. A change in pH
can quickly deteriorate coolant quality, corrode metals
and irritate skin. Proper coolant pH operating ranges
are specified by the manufacturer. For most coolants
this range is 8.5 to 9.0. Within the proper pH range,
corrosion and bacterial growth are minimized. Drops
below the recommended limit may be a symptom of a bigger
problem and are most likely caused by excessive bacteria
or excessive diluting. Correct these problems first.
If needed, pH levels can be increased to the recommended
range by carefully adding an alkaline chemical like
dilute caustic soda. Adding chemicals to coolants on
a frequent, ongoing basis can cause problems, such as
the buildup of dissolved solids, and indicates that
other problems have not been solved. Using low-cost
tools for monitoring pH levels such as portable pH meters
and pH paper, can help determine when pH needs adjusting.
For specific recommendations about maintaining coolant
pH, contact chemical or coolant suppliers.
Coolant concentration. Maintaining
the proper coolant-to-water ratio is critical because
coolants are designed to perform best at specific concentrations.
Overly diluted coolant may reduce tool life, and concentrated
coolant results in using more coolant than necessary.
The same is true for coolant additives. Coolant suppliers
can provide information on proper coolant concentrations
for specific applications.
Refractometers can provide accurate
measurements to determine the coolant-to-water ratio
and are fairly inexpensive and simple to operate. Titration
kits, supplied by some coolant manufacturers, are another
way to maintain proper concentration. Coolant metering
or feed pumps can also improve the rate at which coolants
are added.
In addition to monitoring pH and coolant
concentration, a comprehensive coolant maintenance program
should include monitoring tramp oil concentration, suspended
solids, alkalinity, spot corrosion of parts and bacteria
count.
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