Trent E.M., Wright P.K. Metal Cutting
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A SHORT HISTORY OF MACHINING 3
By 1860 the basic problem of how to produce the necessary shapes in the existing materials
had been solved. There had been little change in the materials which had to be machined - cast
iron, wrought iron and a few copper based alloys. High carbon tool steel, hardened and tempered
by the blacksmith, still had to answer all the tooling requirements. The quality and the consis-
tency of tool steels had been greatly improved by a century of experience with the crucible steel
process. Yet even the best carbon steel tools, pushed to their functional limits, were increasingly
insufficient for manufacturers’ needs, constraining production speed and hampering efficiency.
From the mid-1880s on, innovative energies in manufacturing shifted from developing basic
machine tools and producing highly-accurate parts to reducing machining costs and cutting new
types of metals and alloys. With the Bessemer and Open Hearth steel making processes, steel rap-
idly replaced wrought iron as the workhorse of construction materials. Industry required ever
greater tonnages of steel (steel production soon vastly exceeded the earlier output of wrought
iron), and required it machined to particular specifications.
Alloy steels proved much more difficult than wrought iron to machine, and cutting speeds had
to be lowered even further to maintain reasonable tool life. Towards the end of the 19th century,
both the labor and capital costs of machining were becoming very great. The incentive to reduce
costs by accelerating and automating the cutting process became more intense, and, up to the
present time, still acts as the major driving force behind technological developments in the metal
cutting field.
The discovery and manipulation of new cutting tool materials has been perhaps the most
important theme in the last century of metal cutting. Productivity could not have significantly
increased without the higher cutting speeds achievable using high-speed steel and cemented car-
bide tools, both important advances over traditional carbon steel technology. The next major step
occurred with the development of ceramic and ultra-hard tool materials. Recently, a group of new
techniques, including electrical discharge and water-jet machining, have joined ceramic and
ultra-hard materials at the forefront of metal cutting technology.
Machine tool manufacturers have created machines capable of maximizing the utility of each
generation of cutting tool materials. Designers and machinists have optimized the shapes of tools
to lengthen tool life at high cutting speeds, while lubricant manufacturers have developed new
coolants and lubricants to improve surface finish and permit increased rates of metal removal.
Tool control has also advanced considerably since the days of manually operated machines.
Automatic machines, computer numerically controlled (CNC) machines and transfer machines
produce better tool efficiency, greatly increasing output per employee.
Increasingly, the process of metal cutting is integrated with computer software and hardware
that control machine tools. The age of “mechatronics” accompanies a trend toward integrated
manufacturing systems composed of cells and modules of machines rather than individual, stand-
alone units. Machining today requires a wider range of skills than it did a century ago: computer
programming and knowledge of electronic equipment, among others. Nevertheless, knowledge
of the physical realities of the tool-work interface is as important as ever.
One last note should be added to our understanding of the evolution of machine tool technol-
ogy concerning the double role of basic metal producers. Many new alloys have been developed
to meet the increasingly severe conditions of stress, temperature and corrosion imposed by the
needs of our industrial civilization. Some of these materials, like aluminum and magnesium, are
easy to machine, but others, such as high-alloy steels and nickel-based alloys, become more diffi-
cult to cut as their useful properties (i.e. strength, durability, etc.) improve. The machine tool and
4 INTRODUCTION: HISTORICAL AND ECONOMIC CONTEXT
cutting tool industries have had to develop new strategies to cope with these new metals. At the
same time, basic metal producers have responded to the demands of production engineers for
metals which can be cut faster. New heat treatments have been devised, and the introduction of
alloys like free-machining steel and brass has made great savings in production costs.
1.3 MACHINING AND THE GLOBAL ECONOMY
Today, metal cutting is a significant industry in most economically developed countries,
though small in comparison to the customer industries it serves. The automobile, railway, ship-
building, aircraft manufacture, home appliance, consumer electronics and construction indus-
tries - all these have large machine shops with many thousands of employees engaged in
machining. Worldwide annual consumption of machine tools (metal cutting + metal forming
units) over the last several years has been on the order of $35 - $40 billion per year.
5
As shown
on the left of Figure 1.2, the U.S. is currently the world’s largest consumer, purchasing over $7
billion in new machine tools in 1996.
6
FIGURE 1.2 Top five machine tool consumers (Gardner Publications)
The U.S., once the world leader in machine tool production, suffered from foreign competi-
tion in the 1970s and 1980s and has struggled to regain market position ever since. While
domestic demand fell during the recession of 1982 - 1983, Asian and European demand, espe-
cially for cheap, reliable CNC technology, exploded. Although this type of machine control was
originally developed in the United States, Japanese firms successfully commercialized and
exported the technology, taking a pronounced lead in the machine tool industry by the late
1980s.
As U.S. consumption grew after the 1982 - 1983 recession, so did imports from Japan and
Germany (Figure 1.3). Despite this competition from Japan and Germany, the U.S. remains a
major machine tool producer (Figure 1.4), with shipments of more than $4.5 billion and exports
of $1.2 billion in 1996.
6
MACHINING AND THE GLOBAL ECONOMY 5
FIGURE 1.3 U.S. machine tool consumption and imports (U.S. Bureau of the Census)
The U.S. machine tool industry is also an important job provider for skilled, technical workers,
employing between 50,000 and 100,000 workers during the last decade in firms of all sizes.
6
The
recent resurgence of the U.S. machine tool industry can be seen in Figure 1.5 by the growth in
average revenue and number of employees in machine tool companies. This resurgence during
the mid-1990s can be attributed to several factors, including:
• Improved business practices in the larger, better-established machine tool companies
• The entry into the market of smaller, newer U.S.-based machine tool companies supplying
high quality, “easy-to-buy-and-maintain” machine tools in the $50,000-$100,000 price range.
• More favorable exchange rates in the period of the mid-1990s.
Imports
1,000
1,500
2,000
2,500
3,000
3,500
4,000
millions of U.S. dollars
84 85 86 87 88 89 90 91 92 93 94 95 96
500
0
Consumption
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
millions of U.S. dollars
81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96
6 INTRODUCTION: HISTORICAL AND ECONOMIC CONTEXT
FIGURE 1.4 U.S. machine tool exports (U.S. Bureau of the Census)
Top Export Markets
50
100
150
200
250
300
350
millions of U.S. dollars
0
1995
1996
Canada
Mexico
China
Brazil
South
Korea
United
Kingdom
Japan
Germany
Thailand
Singapore
Exports as a % of Production
84 85 86 87 88 89 90 91 92 93 94 95 96
15%
0%
5%
10%
20%
25%
30%
35%
40%
45%
Exports
800
1,000
1,200
1,400
1,600
millions of U.S. dollars
84 85 86 87 88 89 90 91 92 93 94 95 96
600
0
200
400
SUMMARY AND CONCLUSION 7
FIGURE 1.5 U.S. machine tool companies, 1995-96 statistics (U.S. Bureau of the Census)
1.4 SUMMARY AND CONCLUSION
1.4.1 Economics
To summarize the economic importance, the cost of machining amounts to more than 15% of
the value of all manufactured products in all industrialized countries.
7
Metcut Research Associ-
ates in Cincinnati, Ohio, estimates that, in the U.S., the annual labor and overhead costs of
machining are about $300 billion dollars per year (this excludes work materials and tools). U.S.
consumption of new machine tools (CNC lathes, milling machines, etc.) is about $7.5 billion dol-
lars per year. Consumable cutting tool materials have U.S. sales of about $2 to 2.5 billion dollars
per year.
8,9
For comparison purposes, it is of interest to note a ratio of {300 → 7.5 → 2.5} billion
dollars for {labor costs
→ fixed machinery investments → disposable cutting tools}.
1.4.2 Sociology
Progress in machining is achieved by the ingenuity, logical thought and dogged worrying of
many thousands of practitioners engaged in the many-sided arts of metal cutting. The machinist
operating the machine, the tool designer, the lubrication engineer, and the metallurgist are all
constantly probing for solutions to the challenges presented by novel materials, high costs, and
the needs for faster metal removal, greater precision and smoother surface finish. However com-
petent they may be, there can be few craftspeople, engineers or scientists engaged in this field
who do not feel that they would be better able to solve their problems if they had a deeper knowl-
edge of what was happening at the cutting edge of the tool.
1.4.3 Technology
It is what happens in a very small volume of metal around the cutting edge that determines the
performance of tools, the machinability of metals and alloys, and the final qualities of the
machined surface. During cutting, the interface between tool and work material is largely inac-
cessible to observation, but indirect evidence concerning stresses, temperatures, metal flow and
other interactions has been contributed by many researchers. This book will endeavor to summa-
8 INTRODUCTION: HISTORICAL AND ECONOMIC CONTEXT
rize the available knowledge contained in published work, the authors’ own research, and that of
many of their colleagues to provide a thorough understanding of metal cutting.
1.5 REFERENCES
1. Doyle, E.D. and Aghan, R.L., Metall. Trans. B, 6B, 143 (1975)
2. Rabinowicz, E., Wear,
18,169 (1971)
3. Rolt, L.T.C., Tools for the Job, Batsford (1965)
4. Cookson, J.O. and Sweeney, G., ISI Publication No 138, p. 83 (1971).
5. A Technology Roadmap for the Machine Tool Industry, The Association for Manufacturing
Technology, McLean, VA (1996)
6. 1997-98 Economic Handbook of the Machine Tool Industry, The Association for Manufactur-
ing Technology, McLean, VA (1997).
7. Merchant, M.E., Machining Science and Technology, 2, 157 (1998).
8. Komanduri, R., Tool Materials, Encyclopedia of Chemical Technology, Fourth Edition, John
Wiley and Sons Inc., Volume 24, 390 (1997)
9. Huston, M.F., and Knobeloch, G.W., Cutting Materials, Tools and Market Trends, in the Con-
ference on High-Performance Tools, Dusseldorf, Germany, 21 (1998)
CHAPTER 2 METAL CUTTING
OPERATIONS AND
TERMINOLOGY
2.1 INTRODUCTION
Of all the processes used to shape metals, it is in machining that the conditions of operation are
most varied. Almost all metals and alloys are machined - hard or soft, cast or wrought, ductile or
brittle, with high or low melting point. Most shapes used in the engineering world are produced by
machining. As regards size, components from watch parts to aircraft wing spars, over 30 meters
long, are machined. Many different machining operations are used; cutting speeds may be as high
as 3,500 m min
-1
(11,500 ft/min) for aluminum wing panels, or as low as a few centimeters per
minute; cutting time may be continuous for several hours or interrupted in fractions of a second.
It is important that this great variability be appreciated. Some of the major variables are, there-
fore, discussed and the more significant terms defined by describing briefly some of the more
important metal cutting operations with their distinctive features.
2.2 TURNING
The basic operation of turning, (also called semi-orthogonal cutting in the research laboratory),
is also the one most commonly employed in experimental work on metal cutting. The work mate-
rial is held in the chuck of a lathe and rotated. The tool is held rigidly in a tool post and moved at a
constant rate along the axis of the bar, cutting away a layer of metal to form a cylinder or a surface
of more complex profile. This is shown diagrammatically in Figure 2.1.
The cutting speed (V) is the rate at which the uncut surface of the work passes the cutting edge of
the tool, usually expressed in units of ft/min or m min
-1
. The feed (f) is the distance moved by the
tool in an axial direction at each revolution of the work.
10 METAL CUTTING OPERATIONS AND TERMINOLOGY
FIGURE 2.1 Lathe turning showing a vertical cross-section at top right and a detail of the insert geometry
at bottom right. The dynamometer platform and the remote thermocouple on the bottom of the insert are not
used during today’s production machining. However they are useful in the research laboratory for routine
measurement of cutting forces and overall temperature.
TURNING 11
The depth of cut (w) is the thickness of metal removed from the bar, measured in a radial direc-
tion.
†
The product of these three gives the rate of metal removal, a parameter often used in mea-
suring the efficiency of a cutting operation.
V f w = rate of removal (2.1)
The cutting speed and the feed are the two most important parameters which can be adjusted
by the operator to achieve optimum cutting conditions. The depth of cut is often fixed by the ini-
tial size of the bar and the required size of the product.
Cutting speed is usually between 3 and 200 m min
-1
(10 and 600 ft/min). However, in modern
high speed machining, speeds may be as high as 3,500 m min
-1
when machining aluminum
alloys. The rotational speed (RPM) of the spindle is usually constant during a single operation,
so that when cutting a complex form the cutting speed varies with the diameter being cut at any
instant. At the nose of the tool the speed is always lower than at the outer surface of the bar, but
the difference is usually small and the cutting speed is considered as constant along the tool edge
in turning. Recent computer-controlled machine tools have the capacity to maintain a constant
cutting speed, V, by varying the rotational speed as the work-piece diameter changes.
Feed may be as low as 0.0125 mm (0.0005 in) per rev. and with very heavy cutting up to 2.5
mm (0.1 in) per rev. Depth of cut may vary from zero over part of the cycle to over 25 mm (1 in).
It is possible to remove metal at a rate of more than 1600 cm
3
(100 in
3
) per minute, but such a
rate would be very uncommon and 80-160 cm
3
(5-10 in
3
) per minute would normally be consid-
ered rapid. Figure 2.2 shows some of the main features of a turning tool. The surface of the tool
over which the chip flows is known as the rake face. The cutting edge is formed by the intersec-
tion of the rake face with the clearance face or flank of the tool. The tool is so designed and held
in such a position that the clearance face does not rub against the freshly cut metal surface. The
clearance angle is variable but is often on the order of 6-10°. The rake face is inclined at an
angle to the axis of the bar of work material and this angle can be adjusted to achieve optimum
cutting performance for particular tool materials, work materials and cutting conditions. The
rake angle is measured from a line parallel to the axis of rotation of the work-piece (Figures 2.1
and 2.2). A positive rake angle is one where the rake face dips below the line. Early metal cut-
ting tools had large positive rake angles to give a cutting edge which was keen, but easily dam-
aged. Positive rake angles may be up to 30°, but the greater robustness of tools with smaller rake
angle leads in many cases to the use of zero or negative rake angle. With a negative rake angle of
5° or 6°, the included angle between the rake and clearance faces may be 90°, and this has
advantages. The tool terminates in an end clearance face, which is also inclined at such an angle
as to avoid rubbing against the freshly cut surface. The nose of the tool is at the intersection of
all three faces and may be sharp, but more frequently there is a nose radius between the two
clearance faces.
_________________________________
†
.The feed rate (f) during turning is also called the undeformed chip thickness, (t
1
) in Figures 2.1 and 3.1. The depth
of cut, w, in turning is also referred to as the undeformed chip width. Since machining has developed from a practitio-
ner’s viewpoint, the terminology is not really consistent from one operation to another. For example, in Figure 2.5, for
end milling, the term depth of cut is used in a different way. This is unfortunately confusing for a new student of the
field. Perhaps the best way to accommodate these inconsistencies is to always view the “slice” of material being
removed as the “undeformed chip thickness” (t
1
) and the direction normal to this (into the plane of the paper in Figure
2.1) as the “undeformed chip width”.
12 METAL CUTTING OPERATIONS AND TERMINOLOGY
FIGURE 2.2 Cutting tool terminology
This very simplified description of the geometry of one form of turning tool is intended to help
the reader without practical experience of cutting, follow the terms used later in the book. The
design of tools involves an immense variety of shapes and the full nomenclature and specifica-
tions are very complex.
1,2
It is difficult to appreciate the action of many types of tools without actually observing or,
preferably, using them. The performance of cutting tools is very dependent on their precise
shape. In most cases there are critical features or dimensions which must be accurately formed
for efficient cutting. These may be, for example, the clearance angles, the nose radius and its
blending into the faces, or the sharpness of the cutting edge. The importance of precision in tool
making, whether in the tool room of the user, or in the factory of the tool maker, cannot be over
estimated. This is an area where excellence in hand-skills is still of great value despite the well-
controlled automated aspects of sintered tool material manufacturing. A number of other
machining operations are discussed briefly, with the objective of demonstrating some character-
istic differences and similarities in the cutting parameters.
2.3 BORING OPERATIONS
Essentially the conditions of boring of internal surfaces differ little from those of turning, but
this operation illustrates the importance of rigidity in machining. Particularly when a long cylin-
der with a small internal diameter is bored, the bar holding the tool must be long and slender and
cannot be as rigid as the thick, stocky tools and tool post used for most turning. The tool tends to
be deflected to a greater extent by the cutting forces and to vibrate. Vibration may affect not only
the dimensions of the machined surface, but its roughness and the life of the cutting tools.