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Introduction
Increases
in productivity, improvement in quality and decreases in cost are the goals of
every manufacturing process.
In all cases there are many ways in which this can be accomplished.
At corporate level purchasing is a powerful tool (whether by buying
components directly or purchasing materials) and the management of the supply
chain has recently formed a strong focus.
In house at the design level concurrent engineering and the
philosophies of design for manufacture has lead to increased efficiencies in
this part of the manufacturing cycle.
When we look at the manufacturing operations cellular manufacture has
concentrated resource on developing similar products to gain efficiency in
tooling and the skills of the personnel.
These
trends have made companies concentrate more and more on “core competencies? which in turn has forced the supply chain to become more and more the source
of expert information in many fields of manufacture.
Often then when problems arise, it is a matter of many suppliers being
involved with the corrective actions and this has the potential to cause
conflicts. Many
times the problem may “cure itself?without the actions taken being
responsible and hence a repeat problem will start at the beginning to find the
solution again.
From this it follows that there is a need to restore some of the
understanding back to the shop floor where it can be used to help improve
operations in a timely manner.
This is particularly true of the metal cutting operations as these not
only turn expensive materials into waste they also are some of the more
expensive operations (even with modern machine tools and methods).
This
paper considers the three material systems appropriate to metal cutting and
their interactions in the machining operations in light of present day
pressures. It
also considers the financial implications in the real world.
The
Workpiece
Perhaps
the most important material to consider in the metal cutting system.
This is the one where the “shop floor?has the least opportunities
to improve matters.
This is because in many cases the properties of the workpiece have to
satisfy many product safety and environmental aspects.
Thus the design process has to consider these primarily to ensure
performance and consider aspects of machining to be rather less important.
Early
attempts at improving the workpiece involved the introduction of “soft? inclusions, which had two effects.
The first is to act as an internal lubricant lowering cutting forces
and hence temperatures and wear rates.
The second was to act as fracture sources to help chip breaking.
This second effect has a little appreciated affect on surfaces that are
produced. It
can be clearly shown that these secondary phases are associated with
discontinuities in the surfaces that may act as corrosion or fatigue fracture
surfaces. Modern
day stylus instruments that measure surface finish do not usually record this
as this measurement often reflects the feedrate of the cutting tool which in
many cases has a dominant effect on the measurement but may not have on the
performance.
In depth knowledge of the surface and its behaviour is still required
before these effects can be quantified.
Soft inclusions in this sense include manganese sulphide, lead,
tellurium and selenium.
Recently
less deformable inclusions have been shown to have benefit in the machining
operation in recent years, provided that the stress and temperature conditions
can activate them.
These inclusions do not deform in manufacture of the material and as
such do not confer the same directional properties that may have inhibited
designers in specifying these free machining materials previously.
These “harder?inclusions have been reported in the literature for
over 50 years but it is only in the last 20 that commercially they have been
available and improving.
The early reports looked at silicate systems whereas focus today has
shifted to the calcium-based inclusions often formed in deoxidation processes.
Less
used but maybe, as effective in controlling the machinability of workpieces is
the heat treatment condition.
It was shown some time ago that small inclusions of the size required
for precipitation hardening, could have considerable effect in machining but
as far as I am aware this has not been exploited.
Costs of adding inclusions are normally in the range of 5 to 15% of the
cost of the material whereas a controlled cooling process would be (on a
subcontract basis) be much more than this.
The add-on cost at the time of manufacture is less easily costed.
The
Tool
Changes
in tool materials are the most easily affected ones on the shop floor.
The cost of tools is relatively low allowing some experiment.
Today with the plethora of tool materials about the challenge is to
establish a much more logical approach to selection from different
manufacturers.
High speed steel metallurgy and its links to performance is relatively
well understood even for the newer grades.
The original uncoated carbides composition and structure was for many
years held confidential and as such users were at a considerable disadvantage
with these materials.
Today much of the newer coated carbides fall into this category.
This limits the transferability of the knowledge acquired from in house
testing. To
some extent this also is true of many of the modern ceramic and ultrahard
cutting tools.
While the structures and generic compositions are known details of the
commercial products are difficult for researchers let alone shop floor
personnel to obtain.
The
coolants
Coolants
follow the same trend as above with much of the focus of development aiming
today at complying with legislation.
The in depth chemical knowledge required to understand these substances
is rarely observed in manufacturing industry.
This is particularly true of the water based solvents while many oil
based ones are finding tougher legislation on disposal.
However there is evidence around that the amount of lubricant required
may be considerably less than would originally be thought.
In many cases there is a move to dry machining.
This is aided by modern tooling that will allow much higher temperature
to be tolerated.
However the coolant not only cools the tool but also is responsible for
ensuring dimensional accuracy by minimising temperature rise in the workpiece
and removing the swarf generated.
The
objectives of the paper
This
paper will show how some of the basic understanding of the process can be
applied in decision making on the shop floor and look at some
of the economic issues involved when deciding on the appropriate materials to
choose.
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