Introduction to Machining Processes
MACHINING is a term that covers a large collection of manufacturing processes designed to remove unwanted material,
usually in the form of chips, from a workpiece. Machining is used to convert castings, forgings, or preformed blocks of
metal into desired shapes, with size and finish specified to fulfill design requirements. Almost every manufactured
product has components that require machining, often to great precision. Therefore, this collection of processes is one of
the most important of the basic manufacturing processes because of the value added to the final product. By the same
token, machining processes are often the most expensive.
The majority of industrial applications of machining are in metals. Although the metal cutting process has resisted
theoretical analysis because of its complexity, the application of these processes in the industrial world is widespread.
Machining processes are performed on a wide variety of machine tools.
dual-turret numerically controlled (NC) lathe. Workpieces are held in workholding devices, such as a three-jaw chuck.
The tools used to cut metal are in the turrets. Other examples of basic machine tools are milling machines, drill presses,
grinders, shapers, broaching machines, and saws.
Each of the basic machine tool types has many different configurations. Lathes, for example, may be engine lathes, turret
lathes, tracer lathes, or automatic-screw machines. Lathes have followed the trend of other machine tools, and NC lathes
can now be routinely purchased.
The primary chip formation processes are listed below, with alternative versions in parentheses. Each process is
performed on one or more of the basic machine tools. For example, drilling can be performed on drill presses, milling
machines, lathes, and some boring machines:
· Turning (boring, facing, cutoff, taper turning, form cutting, chamfering, recessing, thread cutting).
· Shaping (planing, vertical shaping)
· Milling (hobbing, generating, thread milling)
· Drilling (reaming, tapping, spot facing, counterboring, countersinking)
· Sawing (filing)
· Abrasive machining (grinding, honing, lapping)
For each of the basic machine tool types, there are many different kinds of workholders, cutting tools, and cutting tool
holders, resulting in a rather formidable list of equipment and processes. In this Volume, a Section entitled "Fundamentals
of the Machining Process" is presented first, with the intent of putting these processes into perspective and helping the
reader to understand the problems associated with using machining processes in the manufacture of products.
Overview of Machining Process Variables
Metal cutting processes can be viewed as consisting of independent (input) variables, dependent variables, and
independent-dependent interactions or relationships. The engineer or machine tool operator has direct control over the
input variables and can specify or select them when setting up the machining process. Several input variables are
described below. Figure 3 summarizes the input/output relationships associated with metal cutting.
Workpiece Material. The metallurgy and chemistry of the workpiece can either be specified or is already known.
Quite often, a material is selected for a particular application chiefly because it machines well. Cast iron and aluminum,
for example, are known to machine easily. Other metals, such as stainless steel or titanium, are difficult to machine. They
often have large cutting forces or poor surface finishes, which can result in short cutting tool life, yet these metals are
selected to meet other functional design criteria. Machining practice for specific workpiece materials are reviewed in the
Section "Machining of Specific Metals and Alloys" in this Volume.
Starting Geometry. The size and shape of the workpiece may be dictated by preceding processes (casting, forging,
forming, and so forth) or may be selected from standard machining stock (for example, bar stock for screw machines).
Usually this variable directly influences the machining process or processes that are selected, as well as the depths of cut.
Specific Machining Processes. The selection of machining processes required to convert the raw material into a
finished product must be based on the geometry of the part (size and shape, rotational or non-rotational), the required
finishes and tolerances, and the quantity of the product to be made. Machining processes can be grouped into three broad
categories. These include traditional chip formation processes, abrasive machining processes, and nontraditional
machining processes.
Chip Formation Processes. As described earlier, there are seven basic chip formation processes: turning, shaping,
milling, drilling, sawing, broaching, and abrasive machining. The equipment and principles of operation associated with
each of these processes (with the exception of abrasive machining, which is treated separately) are described in the
Section titled "Traditional Machining Processes" in this Volume.
Abrasive machining is the basic process by which chips are formed by very small cutting edges that are integral parts
of abrasive particles. The principles of abrasive machining, the fundamental differences between metal cutting and
grinding, and the abrasives and equipment used for abrasive machining operations are described in the Section "Grinding,
Honing, and Lapping" in this Volume.
Nontraditional Machining Processes. Machining processes that involve compression/shear chip formation have a
number of inherent disadvantages. These include:
· High costs incurred with chip formation (high energy output and chip removal, disposal, and/or
recycling)
· Heat buildup that often results in workpiece distortion
· High forces that create problems in holding the workpiece and which can also cause distortion
· Undesirable cold working and residual stresses in the workpiece that often necessitate further processing
to remove the harmful effects
· Limitations as to the size and delicacy of the workpiece
Tool Materials.
The three most common cutting tool materials currently in use for production machining operations are
high-speed steel (HSS), both in wrought and powder metallurgy (P/M) form; carbides; and coated tools. Cubic boron
nitride (CBN), ceramics, and diamonds are also being widely employed. Generally speaking, HSS is used for general-purpose
tools, for tools of complex design or for tools used when cutting speeds are more modest. Carbide and ceramic
tool materials, which can operate at faster cutting speeds, come in a wide variety of grades and geometries. Titanium
nitride and titanium carbide coatings for HSS and carbides are now commonplace. Selection of a tool material that
provides reliable service while fulfilling the functional requirements is still an art. The harder the tool material, the better
it can resist wear at faster cutting speeds. The faster the cutting speed, the higher the cutting temperature and the shorter
the tool life. Retention of hardness at elevated temperatures as well as long tool life are desirable characteristics in cutting
tools. See the Section "Cutting Tool Materials" in this Volume for descriptions of the processing, properties, and
applications associated with the aforementioned materials.
Cutting Parameters.
For every machining operation, it is necessary to select a cutting speed, a feed, and a depth of
cut. Many factors impinge on these decisions because all of the dependent variables are influenced by them. Proper
selection of variables also depends on the other input variables that have been selected; that is, the total amount of
material to be removed, the workpiece and tool materials, and the machining process or processes. These need to be
selected before preliminary choices for speed, feed, and depth of cut can be made.
Tool Geometry.
Cutting tools are usually designed to accomplish specific operations, and thus the tool geometry
(angles) is selected to accomplish specific machining functions. Generally speaking, large rake and clearance angles are
preferred, but they are possible only on HSS tools. Tools made from carbides, ceramics, and other very hard materials
must be given small tool angles, which keep the tool material in compression during machining and thereby avoid tensile
failure and brittle fractures of the tool. The greater the precision required of the process, the better the geometry of the
cutting edge itself must be.
Dependent Variables
Dependent variables are determined by the process based on the prior selection of the input or independent variables.
Thus, the manufacturing engineer's control over these is usually indirect. The important dependent variables are cutting
force and power, size and properties of the finished product, surface finish, and tool wear and tool failure.
Cutting Force and Power. To machine metal at a specified speed, feed, and depth of cut, with a specified lubricant,
cutting tool material, and geometry, generates cutting forces and consumes power. A change in any of the variables alters
the forces, but the change is indirect in that the engineer does not specify the forces, only the parameters that generate
those forces. Forces are important in that they influence the deflections in the tools, the workpieces, and the workholders,
which in turn affect the final part size. Forces also play a roll in chatter and vibration phenomena common in machining.
Obviously, the manufacturing engineer would like to be able to predict forces (and power) so that he can safely specify
the equipment for a manufacturing operation, including the machine tool, cutting tool, and workholding devices. The
basic concepts associated with the modeling and understanding of cutting forces and power are explained in the article
"Forces, Power, and Stresses in Machining" in this Volume.
Size and Properties of the Finished Product. Ultimately, the objective of machining is to obtain a machined
surface of desired size and geometry with the desired mechanical properties. Because machining is a localized, plastic
deformation process, every machined surface will have some residual deformation (stresses) left in it. These residual
stresses are usually tensile in nature and can interact with surface flaws to produce part failure from fatigue or to cause
corrosion. In addition, every process has some inherent process variability (variations about average size) that changes
with almost all of the input variables. Thus, the manufacturing engineer must try to select the proper levels of input
variables to produce a product that is within the tolerance specified by the designer and has satisfactory surface properties.
Surface Finish. The final finish on a machined surface is a function of tool geometry, tool material, workpiece material,
machining process, speed, feed, depth of cut, and cutting fluid. Surface finish is also related to the process variability.
Rough surfaces have more variability than smooth surfaces. Often it is necessary to specify multiple cuts, that is, roughing
and finish cuts, to achieve the desired surface finish, or it may be necessary to specify multiple processes, such as
following turning with cylindrical grinding, in order to obtain the desired finish. The effect of various machining
processes on surface finish and on the properties of the final products are described in the article "Surface Finish and
Surface Integrity" in this Volume.
Tool Wear and Tool Failure.
The plastic deformation and friction inherent in machining generate considerable heat,
which raises the temperature of the tool and lowers its wear resistance. The problem is subtle, but significant. As the tool
wears, it changes in both geometry and size. A dull cutting edge and change in geometry can result in increased cutting
forces that in turn increase deflections in the workpiece and may create a chatter condition. The increased power
consumption causes increased heat generation in the operation, which accelerates the wear rate. The change in the size of
the tool changes the size of the workpiece. Again, the engineer has only indirect control over these variables. He can
select slow speeds, which produce less heat and lower wear rates, but which decrease the production rates because the
metal removal rate is decreased. Alternatively, the feed or depth of cut can be increased to maintain the metal removal
rate while reducing the speed. Increasing either the feed or depth of cut directly increases the cutting forces. Therefore,
while tool life may be gained, some precision may be lost due to increased deflection and chatter. Wear mechanisms,
determination of modes of tool failure, and tool life testing are examined in the article "Tool Wear and Tool Life" in this
Volume.
Shear Front-Lamella Structure.
The shear process itself is a nonhomogeneous (discontinuous) series of shear fronts
(or narrow bands) that produce a lamellar structure in the chips. This fundamental structure occurs on the microscale in all
metals when they are machined and accounts for the unique behavior of the machining process.
Individual shear fronts (Fig. 2) coalesce into narrow shear bands. The shear bands are very narrow (20 to 200 nm)
compared to the thickness of a lamella (2 to 4 m) and account for the large strain and high strain rates that typify this
process.
These fundamental structures are difficult to observe in normal metal cutting, but can be readily observed in a scanning
electron microscope with specially prepared workpieces. Figure 3 shows micrographs from an orthogonal machining
experiment performed inside a scanning electron microscope. The fundamental shear front-lamella structure is readily
observed. The side of the workpiece has been given a mirror polish so that the shear fronts can be observed. The shear
fronts are produced by the activation of many dislocations traveling in waves from the tool tip to the free surface. The
lamella represents heavily deformed material that has been segmented by the shear fronts. When machined, all metals
deform by this basic mechanism. The shear fronts relieve the applied stress.
Mechanics of Chip Formation
Introduction
THE BASIC MECHANISM involved in metal cutting is that of a localized shear deformation on the work material
immediately ahead of the cutting edge of the tool. The relative motion between the tool and the workpiece during cutting
compresses the work material near the tool and induces a shear deformation (called the primary deformation), which
forms the chip. The chip passes over the rake face of the cutting tool and receives additional deformation (called the
secondary deformation) because of the shearing and sliding of the chip against the tool.
These two plastic deformation processes have a mutual dependence. The material element that rubs the rake face has been
heated and plastically deformed during its passage through the primary shear process; therefore, the secondary process is
influenced by the phenomena on the shear plane. At the same time, the shear direction is directly influenced by the rake
face deformation and friction processes. The shear direction influences the heating and straining of the chip in the primary
process. In terms of metal cutting theory, this means that shear stress and shear direction must be determined
simultaneously. Such theoretical analyses are usually based on the mechanics of the process.
This article will review the following:
· The fundamental nature of the deformation process associated with machining
· The principles of the orthogonal cutting model
· The effect of workpiece properties on chip formation
· The mechanics of the machining process
Additional information on the modeling and analysis of chip formation can be found in the article "Forces, Power, and
Stresses in Machining," which immediately follows in this Section.
Forces, Power, and Stresses in Machining
THE MODELING AND ANALYSIS of chip formation has been a continuing exercise over the past century. The metal
cutting process is a unique and complex production process distinguished by:
· Large shear strains, usually of the order of 2 to 5 (Ref 1)
· Exceptionally high shear strain rates, typically from 103 to 105 s-1 with local variations as high as 107 s-1
(Ref 2, 3)
· The rubbing of the tool flank over a freshly cut surface that is chemically clean and active
· Many process and tooling parameters with a wide range of settings that can drastically alter the cutting
process
· A large number of metallurgical parameters in the workpiece that can influence its response to the
cutting tool
These factors and others make the modeling of metal cutting a difficult task that continues to evolve over time. The
models and the discussion presented in this article will attempt to explain the basic concepts of the many complex factors
that influence the forces, power, and stresses in machining.
Forces and Energy in Orthogonal Machining
Although most production machining processes are oblique (that is, having three component forces), models of the
orthogonal (that is, two force) machining of metals are useful for understanding the basic mechanics of machining and
can be extended for modeling of the production processes.
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