Page 630 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 22 12 Cuttnng Flunds

Mist, fumes, smoke, and odors from cutting fluids can cause severe skin reactions
and respiratory problems, especially in using fluids with chemical constituents such
as sulfur, chlorine, phosphorus, hydrocarbons, biocides, and various additives.
Much progress has been made in ensuring the safe use of cutting fluids in manufac-
turing plants, including reducing or eliminating their use by adopting the more
recent trends in dry or near-dry machining techniques (see Section 21.12.1).
Cutting fluids (as well as other metalworking fluids used in manufacturing
operations) may undergo chemical changes as they are used repeatedly over time.
These changes may be due to environmental effects or to contamination from vari-
ous sources, including metal chips, fine particles produced during machining, and
tramp oil (from leaks in hydraulic systems, oils on sliding members of machines, and
lubricating systems for the machine tools). The changes involve the growth of mi-
crobes (bacteria, molds, and yeast), particularly in the presence of water, becoming
an environmental hazard and also adversely affecting the characteristics and effec-
tiveness of the cutting fluids.
Several techniques (such as settling, skimming, centrifuging, and filtering) are
available for clarifying used cutting fluids. Recycling involves treatment of the fluids
with various additives, agents, biocides, and deodorizers, as well as water treatment
(for water-based fluids). Disposal practices for these fluids must comply with federal,
state, and local laws and regulations.

22.I2.l Near-dry and Dry Machining
For economic and environmental reasons, there has been a continuing worldwide
trend since the mid-1990s to minimize or eliminate the use of metalworking fluids.
This trend has led to the practice of near-dry machining (NDM), with major benefits
such as the following:

° Alleviating the environmental impact of using cutting fluids, improving air
quality in manufacturing plants, and reducing health hazards.
° Reducing the cost of machining operations, including the cost of maintenance,
recycling, and disposal of cutting fluids.
° Further improving surface quality.

The significance of this approach becomes apparent when one notes that, in
the United States alone, millions of gallons of metalworking fluids are consumed
each year. Furthermore, it has been estimated that metalworking fluids constitute
about 7 to 17% of the total machining costs.
The principle behind near-dry cutting is the application of a fine mist of an
air-fluid mixture containing a very small amount of cutting fluid, which may be refor-
mulated to contain vegetable oil. The mixture is delivered to the cutting zone through
the spindle of the machine tool, typically through a 1-mm-diameter nozzle and under
a pressure of 600 kPa. lt is used at rates on the order of 1 to 100 cc/hr, which is esti-
mated to be (at most) one ten-thousandth of that used in flood cooling. Consequently,
the process is also known as minimum-quantity lubrication (MQL).
Dry machining also is a viable alternative. With major advances in cutting
tools, dry machining has been shown to be effective in various machining operations
(especially turning, milling, and gear cutting) on steels, steel alloys, and cast irons,
but generally not for aluminum alloys.
One of the functions of a metal-cutting fluid is to flush chips from the cutting
zone. This function seems to be problematic with dry machining. However, tool
designs have been developed that allow the application of pressurized air, often
through the tool shank. The compressed air doesn’t serve a lubrication purpose and