========================================第1页========================================
附录
附录1
英文原文
Basic Machining Operations and Cutting Technology
Basic Machining Operations
Machine tools have evolved from the early foot-powered lathes of the Egyptians and John
Wilkinson's boring mill. They are designed to provide rigid support for both the workpiece and
the cutting tool and can precily control their relative positions and the velocity of the tool with
respect to the workpiece. Basically, in metal cutting, a sharpened wedge-shaped tool removes a
rather narrow strip of metal from the surface of a ductile workpiece in the form of a verely
deformed chip. The chip is a waste product that is considerably shorter than the workpiece from
which it came but with a corresponding increa in thickness of the uncut chip. The geometrical
服装行业前景分析shape of workpiece depends on the shape of the tool and its path during the machining operation.
Most machining operations produce parts of differing geometry. If a rough cylindrical
workpiece revolves about a central axis and the tool penetrates beneath its surface and travels
parallel to the center of rotation, a surface of revolution is produced, and the operation is called
turning. If a hollow tube is machined on the inside in a similar manner, the operation is called
boring. Producing an external conical surface uniformly varying diameter is called taper turning,
if the tool point travels in a path of varying radius, a contoured surface like that of a bowling pin
can be produced; or, if the piece is short enough and the support is sufficiently rigid, a contoured
surface could be produced by feeding a shaped tool normal to the axis of rotation. Short tapered
or cylindrical surfaces could also be contour formed.
Flat or plane surfaces are frequently required. They can be generated by radial turning or
facing, in which the tool point moves normal to the axis of rotation. In other cas, it is more
convenient to hold the workpiece steady and reciprocate the tool across it in a ries of
straight-line cuts with a crosswi feed increment before each cutting stroke. This operation is
called planning and is carried out on a shaper. For larger pieces it is easier to keep the tool
stationary and draw the workpiece under it as in planning. The tool is fed at each reciprocation.
Contoured surfaces can be produced by using shaped tools.
Multiple-edged tools can also be ud. Drilling us a twin-edged fluted tool for holes with
depths up to 5 to 10 times the drill diameter. Whether the
drill turns or the workpiece rotates, relative motion between the cutting edge and the
workpiece is the important factor. In milling operations a rotary cutter with a number of cutting
========================================第2页========================================
edges engages the workpiece. Which moves slowly with respect to the cutt
er. Plane or contoured
surfaces may be produced, depending on the geometry of the cutter and the type of feed.
Horizontal or vertical axes of rotation may be ud, and the feed of the workpiece may be in any
of the three coordinate directions.
Basic Machine Tools
Machine tools are ud to produce a part of a specified geometrical shape and preci I size
by removing metal from a ductile material in the form of chips. The latter are a waste product
and vary from long continuous ribbons of a ductile material such as steel, which are undesirable
from a disposal point of view, to easily handled well-broken chips resulting from cast iron.
Machine tools perform five basic metal-removal process: I turning, planning, drilling, milling,
and grinding. All other metal-removal process are modifications of the five basic process.
For example, boring is internal turning; reaming, tapping, and counter boring modify drilled
holes and are related to drilling; bobbing and gear cutting are fundamentally milling operations;
hack sawing and broaching are a form of planning and honing; lapping, super finishing.
Polishing and buffing are variants of grinding or abrasive removal operations. Therefore, there
are only four types of basic machine tools, which u cutting tools of specific controllable
geometry: 1. lathes, 2. planers, 3. drilling machines, and 4. milling machines. The grinding
process forms chips, but the geometry of the abrasive grain is uncontrollable.
The amount and rate of material removed by the various machining process may be I
large, as in heavy turning operations, or extremely small, as in lapping or super finishing
operations where only the high spots of a surface are removed.
A machine tool performs three major functions: 1. it rigidly supports the workpiece or its
holder and the cutting tool; 2. it provides relative motion between the workpiece and the cutting
tool; 3. it provides a range of feeds and speeds usually ranging from 4 to 32 choices in each ca.
Speed and Feeds in Machining
Speeds, feeds, and depth of cut are the three major variables for economical machining.
Other variables are the work and tool materials, coolant and geometry of the cutting tool. The
rate of metal removal and power required for machining depend upon the variables.
The depth of cut, feed, and cutting speed are machine ttings that must be established in
any metal-cutting operation. They all affect the forces, the power, and the rate of metal removal.
nap是什么意思
They can be defined by comparing them to the needle and record of a phonograph. The cutting
speed (V) is reprented by the velocity of- the record surface relative to the needle in the tone
arm at any instant. Feed is reprented by the advance of the needle radially inward per
revolution, or is the difference in position between two adjacent grooves. The depth of cut is the
penetration of the needle into the record or the depth o
f the grooves.
========================================第3页========================================
Turning on Lathe Centers
The basic operations performed on an engine lathe are illustrated. Tho operations
performed on external surfaces with a single point cutting tool are called turning. Except for
drilling, reaming, and lapping, the operations on internal surfaces are also performed by a single
point cutting tool.
All machining operations, including turning and boring, can be classified as roughing,
finishing, or mi-finishing. The objective of a roughing operation is to remove the bulk of the
material as rapidly and as efficiently as possible, while leaving a small amount of material on the
work-piece for the finishing operation. Finishing operations are performed to obtain the final size,
航空学校分数线shape, and surface finish on the workpiece. Sometimes a mi-finishing operation will precede
the finishing operation to leave a small predetermined and uniform amount of stock on the
work-piece to be removed by the finishing operation.
sreng
Generally, longer workpieces are turned while supported on one or two lathe centers. Cone
shaped holes, called center holes, which fit the lathe centers are drilled in the ends of the
workpiece-usually along the axis of the cylindrical part. The end of the workpiece adjacent to the
tailstock is always supported by a tailstock center, while the end near the headstock may be
supported by a headstock center or held in a chuck. The headstock end of the workpiece may be
held in a four-jaw chuck, or in a type chuck. This method holds the workpiece firmly and
transfers the power to the workpiece smoothly; the additional support to the workpiece provided
by the chuck lesns the tendency for chatter to occur when cutting. Preci results can be
obtained with this method if care is taken to hold the workpiece accurately in the chuck.
Very preci results can be obtained by supporting the workpiece between two centers. A
lathe dog is clamped to the workpiece; together they are driven by a driver plate mounted on the
spindle no. One end of the Workpiece is mecained;then the workpiece can be turned around in
the lathe to machine the other end. The center holes in the workpiece rve as preci locating
surfaces as well as bearing surfaces to carry the weight of the workpiece and to resist the
cutting forces. After the workpiece has been removed from the lathe for any reason, the center
holes will accurately align the workpiece back in the lathe or in another lathe, or in a cylindrical
grinding machine. The workpiece must never be held at the headstock end by both a chuck and a
lathe center. While at first thought this ems like a quick method of aligning the workpiece in
personal interest
the chuck, this must not be done becau it is not possible to press evenly with the jaws against
the workpiece while it is also supported by the center. The alignment provided by the center
will
not be maintained and the pressure of the jaws may damage the center hole, the lathe center, and
perhaps even the lathe spindle. Compensating or floating jaw chucks ud almost exclusively on
high production work provide an exception to the statements made above. The chucks are
========================================第4页========================================
save your heart
really work drivers and cannot be ud for the same purpo as ordinary three or four-jaw
chucks.
While very large diameter workpieces are sometimes mounted on two centers, they are
preferably held at the headstock end by faceplate jaws to obtain the smooth power transmission;
moreover, large lathe dogs that are adequate to transmit the power not generally available,
although they can be made as a special. Faceplate jaws are like chuck jaws except that they are
mounted on a faceplate, which has less overhang from the spindle bearings than a large chuck
would have.
Introduction of Machining
Machining as a shape-producing method is the most universally ud and the most
important of all manufacturing process. Machining is a shape-producing process in which a
power-driven device caus material to be removed in chip form. Most machining is done with
equipment that supports both the work piece and cutting tool although in some cas portable
equipment is ud with unsupported workpiece.
Low tup cost for small Quantities. Machining has two applications in manufacturing. For
casting, forging, and press working, each specific shape to be produced, even one part, nearly
always has a high tooling cost. The shapes that may he produced by welding depend to a large
degree on the shapes of raw material that are available. By making u of generally high cost
equipment but without special tooling, it is possible, by machining; to start with nearly any form
of raw material, so tong as the exterior dimensions are great enough, and produce any desired
shape from any material. Therefore .machining is usually the preferred method for producing one
or a few parts, even when the design of the part would logically lead to casting, forging or press
working if a high quantity were to be produced.
proviewClo accuracies, good finishes. The cond application for machining is bad on the high
accuracies and surface finishes possible. Many of the parts machined in low quantities would be
produced with lower but acceptable tolerances if produced in high quantities by some other
process. On the other hand, many parts are given their general shapes by some high quantity
deformation process and machined only on lected surfaces where high accuracies are needed.
Internal threads, for example, are ldom produced by any means other than machining and
small holes in press worked parts may be machined following the press working operations.
Primary Cutting Parameters
The basic tool-work relationship in cutting is adequately described by
means of four factors:
tool geometry, cutting speed, feed, and depth of cut.
The cutting tool must be made of an appropriate material; it must be strong, tough, hard,
五年级英语
and wear resistant. The tool s geometry characterized by planes and angles, must be correct for
========================================第5页========================================
each cutting operation. Cutting speed is the rate at which the work surface pass by the cutting
edge. It may be expresd in feet per minute.
For efficient machining the cutting speed must be of a magnitude appropriate to the
particular work-tool combination. In general, the harder the work material, the slower the speed.
Feed is the rate at which the cutting tool advances into the workpiece. "Where the
workpiece or the tool rotates, feed is measured in inches per revolution. When the tool or the
work reciprocates, feed is measured in inches per stroke, Generally, feed varies inverly with
cutting speed for otherwi similar conditions.
The depth of cut, measured inches is the distance the tool is t into the work. It is the width
of the chip in turning or the thickness of the chip in a rectilinear cut. In roughing operations, the
depth of cut can be larger than for finishing operations.
The Effect of Changes in Cutting Parameters on Cutting Temperatures
In metal cutting operations heat is generated in the primary and condary deformation
zones and the results in a complex temperature distribution throughout the tool, workpiece and
chip. A typical t of isotherms is shown in figure where it can be en that, as could be expected,
there is a very large temperature gradient throughout the width of the chip as the workpiece
material is sheared in primary deformation and there is a further large temperature in the chip
adjacent to the face as the chip is sheared in condary deformation. This leads to a maximum
cutting temperature a short distance up the face from the cutting edge and a small distance into
the chip.
Since virtually all the work done in metal cutting is converted into heat, it could be expected
that factors which increa the power consumed per unit volume of metal removed will increa
the cutting temperature. Thus an increa in the rake angle, all other parameters remaining
constant, will reduce the power per unit volume of metal removed and the cutting temperatures
will reduce. When considering increa in unreformed chip thickness and cutting speed the
situation is more complex. An increa in undeformed chip thickness tends to be a scale effect
where the amounts of heat which pass to the workpiece, the tool and chip remain in fixed
proportions and the changes in cutting temperature tend to be small. Increa in cutting speed;
however, reduce the amount of heat which pass into the workpiece and this increa the
temperature ri of the chip m primary deformation. Further, the condary deformation zone
tends to be smaller and this has the effect of increasing the temperatures in this zone. Other
changes in cutting parameters have virtually no effect on the power consumed per unit volume of
代价的意思metal removed and conquently have virtually no effect on the cutting temperatures. Since it
has been shown that even small changes in cutting temperature have a significant effect on tool
wear rate it is appropriate to indicate how cutting temperatures can be assd from cutting data.
========================================第6页========================================
The most direct and accurate method for measuring temperatures in high -speed-steel
cutting tools is that of Wright &. Trent which also yields detailed information on temperature
distributions in high-speed-steel cutting tools. The technique is bad on the metallographic
examination of ctioned high-speed-steel tools which relates microstructure changes to thermal
history.
Trent has described measurements of cutting temperatures and temperature distributions
for high-speed-steel tools when machining a wide range of workpiece materials. This technique
bridge
has been further developed by using scanning electron microscopy to study fine-scale
microstructure changes arising from over tempering of the tempered martens tic matrix of
various high-speed-steels. This technique has also been ud to study temperature distributions in
both high-speed -steel single point turning tools and twist drills.
Wears of Cutting Tool
Discounting brittle fracture and edge chipping, which have already been dealt with, tool
wear is basically of three types. Flank wear, crater wear, and notch wear. Flank wear occurs on
both the major and the minor cutting edges. On the major cutting edge, which is responsible for
bulk metal removal, the results in incread cutting forces and higher temperatures which if
left unchecked can lead to vibration of the tool and workpiece and a condition where efficient
cutting can no longer take place. On the minor cutting edge, which determines workpiece size
and surface finish, flank wear can result in an oversized product which has poor surface finish.
Under most practical cutting conditions, the tool will fail due to major flank wear before the
minor flank wear is sufficiently large to result in the manufacture of an unacceptable component.
Becau of the stress distribution on the tool face, the frictional stress in the region of
sliding contact between the chip and the face is at a maximum at the start of the sliding contact
region and is zero at the end. Thus abrasive wear takes place in this region with more wear
taking place adjacent to the izure region than adjacent to the point at which the chip los
contact with the face. This result in localized pitting of the tool face some distance up the face
which is usually referred to as catering and which normally has a ction in the form of a circular
arc.
