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附：外文翻译
外文原文：
Fundamentals of Mechanical Design
Mechanical design means the design of
things and systems of a mechanical
nature
—
machines, products,
structures, devices,
and
instruments.
For
the
most
part
mechanical
design
utilizes
mathematics,
the
materials
sciences,
and
the
engineering-mechanics sciences.
The total design process is of interest
to us. How does it begin? Does the engineer simply
sit down at his desk with a blank
sheet
of paper? And, as he jots down some ideas, what
happens next? What factors influence or control
the decisions which have
to be made?
Finally, then, how does this design process end?
Sometimes,
but
not
always,
design
begins
when
an
engineer
recognizes
a
need
and
decides
to
do
something
about
it.
Recognition of the need and phrasing it
in so many words often constitute a highly
creative act because the need may be only a
vague discontent, a feeling of
uneasiness, of a sensing that something is not
right.
The
need
is
usually
not
evident
at
all.
For
example,
the
need
to
do
something
about
a
food-packaging
machine
may
be
indicated
by the noise level, by the variations in package
weight, and by slight but perceptible variations
in the quality of the
packaging or
wrap.
There is a distinct difference
between the statement of the need and
the identification of the problem.
Which follows this
statement? The
problem is more specific. If the need is for
cleaner air, the problem might be that of reducing
the dust discharge
from power-plant
stacks, or reducing the quantity of irritants from
automotive exhausts.
Definition of the
problem must include all the specifications for
the thing that is to be designed. The
specifications are the
input and output
quantities, the characteristics of the space the
thing must occupy and all the limitations on
t
hese quantities. We
can
regard the thing to be designed as something in a
black box. In this case we must specify the inputs
and outputs of the box
together with
their characteristics and limitations. The
specifications define the cost, the number to be
manufactured, the expected
life, the
range, the operating temperature, and the
reliability.
There are many implied
specifications which result either from the
designer
'
s particular
environment or from the nature of
the
problem
itself.
The
manufacturing
processes
which
are
available,
together
with
the
facilities
of
a
certain
plant,
constitute restrictions on a
designer
'
s freedom, and
hence are a part of the implied specifications. A
small plant, for instance, may
not own
cold-working machinery. Knowing this, the designer
selects other metal-processing methods which can
be performed in
the plant. The labor
skills available and the competitive situation
also constitute implied specifications.
After the problem has been defined and
a set of written and implied specifications has
been obtained, the next step in design
is the synthesis of an optimum
solution. Now synthesis cannot take place without
both analysis and optimization because the
system under design must be analyzed to
determine whether the performance complies with
the specifications.
The design is
an
iterative process in
which we proceed through several steps, evaluate
the results, and then return to an
earlier phase of the procedure. Thus we
may synthesize several components of a system,
analyze and optimize them, and return
to
synthesis to
see
what
effect
this
has
on the
remaining
parts
of the
system.
Both
analysis
and
optimization
require
that
we
construct
or devise abstract models of the system which will
admit some form of mathematical analysis. We call
these models
mathematical models. In
creating them it is our hope that we can find one
which will simulate the real physical system very
well.
Evaluation
is
a
significant
phase
of the total
design
process.
Evaluation
is the
final
proof
of
a
successful
design,
which
usually involves the testing of a
prototype in the laboratory. Here we wish to
discover if the design really satisfies the need
or
needs. Is it reliable? Will it
compete successfully with similar products? Is it
economical to manufacture and to use? Is it easily
maintained and adjusted? Can a profit
be made from its sale or use?
Communicating
the
design
to
others
is
the
final,
vital
step
in
the
design
process.
Undoubtedly
many
great
designs,
inventions, and
creative works have been
lost to
mankind simply because the originators were unable
or unwilling to explain
their
accomplishments to others. Presentation is a
selling job. The engineer, when presenting
a new solution to administrative,
management, or supervisory persons, is
attempting to sell or to prove to them that this
solution is a better one. Unless this can be
done successfully, the time and effort
spent on obtaining the solution have been largely
wasted.
Basically, there are only three
means of communication available to us. There are
the written, the oral, and the
graphical
forms.
Therefore
the
successful
engineer
will
be
technically
competent
and
versatile
in
all
three
forms
of
communication.
A
technically competent
person who lacks ability in any one of these forms
is severely handicapped. If ability in all three
forms is
lacking, no one will ever know
how competent that person is!
The
competent engineer should not be afraid of the
possibility of not succeeding in a presentation.
In fact, occasional failure
should be
expected because failure or criticism seems to
accompany every really creative idea. There is a
great to be learned from
a failure,
and the
greatest gains are
obtained by those willing to risk defeat. In the
find
analysis, the real failure would
lie in
deciding not to make
the presentation at all.
Introduction
to Machine Design
Machine
design
is
the
application
of
science
and
technology
to
devise
new
or
improved
products
for
the
purpose
of
satisfying human needs.
It is a vast field of engineering technology which
not only concerns itself with the original
conception of
the product in terms of
its size, shape and construction details, but also
considers the various factors involved in the
manufacture,
marketing and use of the
product.
People who perform the various
functions of machine design
are
typically called designers, or design engineers.
Machine
design
is
basically
a
creative
activity.
However,
in
addition
to
being
innovative,
a
design
engineer
must
also
have
a
solid
background
in
the
areas
of
mechanical
drawing,
kinematics,
dynamics,
materials
engineering,
strength
of
materials
and
manufacturing processes.
As
stated previously, the purpose of machine design
is to produce a product which will serve a need
for man. Inventions,
discoveries
and
scientific
knowledge
by
themselves
do
not
necessarily
benefit
people;
only
if
they
are
incorporated
into
a
designed product will
a benefit be derived. It should be
recognized, therefore, that a human need
must be identified before a
particular product is designed.
Machine design should be considered to
be an opportunity to use innovative talents to
envision a design of a product is to be
manufactured.
It
is
important to
understand
the
fundamentals
of
engineering
rather
than
memorize
mere
facts
and
equations.
There are no
facts or equations which alone
can be
used to provide all the correct decisions to
produce a good design. On the
other
hand, any calculations made must be done with the
utmost care and precision. For example, if a
decimal point is misplaced,
an
otherwise acceptable design may not function.
Good designs require trying new ideas
and being willing to take a certain amount of
risk, knowing that is the new idea does
not work the existing method can be
reinstated. Thus a designer must have patience,
since there is no assurance of success for the
time and effort expended. Creating
a completely new design
generally requires that many old and
well-established methods be
thrust
aside.
This
is
not
easy
since
many
people
cling
to
familiar
ideas,
techniques
and
attitudes.
A
design
engineer
should
constantly search for
ways to improve an existing product and must
decide what old, proven concepts should be used
and what
new, untried ideas should be
incorporated.
New designs generally
have “bugs” or unforeseen problems which must be
worked out before the superior
characteristics of
the new designs can
be enjoyed. Thus there is a chance for a superior
product, but only at higher risk. It should be
emphasiz
ed
that if a design
does not warrant radical new methods, such methods
should not be applied merely for the sake of
change.
During the beginning stages of
design, creativity should be allowed to flourish
without a great number of constraints. Even
though many impractical ideas may
arise, it is usually easy to eliminate them in the
early stages of design before firm details are
required by manufacturing. In this way,
innovative ideas are not inhibited. Quite often,
more than one design is developed, up to
the point where they can be compared
against each other. It is entirely possible that
the design which ultimately accepted will use
ideas existing in one of the rejected
designs that did not show as much overall
promise.
Psychologists
frequently talk about trying to fit people to the
machines they operate. It is essentially the
responsibility of the
design engineer
to strive to fit machines to people. This is not
an easy task, since there is really no average
person for which
certain operating
dimensions and procedures are optimum.
Another important point which should be
recognized is that a design engineer must be able
to communicate ideas to other
people if
they are to be incorporated. Initially the
designer must communicate a preliminary design to
get management approval.
This is
usually done by verbal discussions in conjunction
with drawing layouts and written material. To
communicate effectively,
the following
questions must be answered:
(1)
Does the
design really serve a human need?
(2)
Will it be
competitive with existing products of rival
companies?
(3)
Is it
economical to produce?
(4)
Can it be readily maintained?
(5)
Will it sell
and make a profit?
Only time will
provide the true answers to the preceding
questions, but the product should be designed,
manufactured and
marketed
only
with
initial
affirmative
answers.
The
design
engineer
also
must
communicate
the
finalized
design
to
manufacturing through the use of detail
and assembly drawings.
Quite
often, a problem well occur during the
manufacturing cycle. It may be that a change is
required in the dimensioning or
telegramming of a part so that it can
be more readily produced. This falls in the
category of engineering changes which must be
approved by the design engineer so that
the product function will not be adversely
affected. In other cases, a deficiency in
the
design may appear during
assembly or testing just prior to shipping. These
realities simply bear out the fact that design is
a living
process. There is always a
better way to do it and the designer should
constantly strive towards finding that better way.
Machining
Turning
The engine lathe,
one of the oldest metal removal machines, has a
number of useful and highly desirable attributes.
Today these lathes are used primarily
in small shops where smaller quantities rather
than large production runs are encountered.
The engine
lathe has been
replaced in today's production shops by a wide
variety of automatic lathes such as automatic of
single-point tooling for
maximum
metal removal, and
the use of form tools for finish and
accuracy, are now
at the
designer's
fingertips with production
speeds on a par with the fastest processing
equipment on the scene today.
Tolerances
for
the
engine
lathe
depend
primarily
on
the
skill
of
the
operator.
The
design
engineer
must be careful
in
using
tolerances of an experimental part that
has been produced on
the
engine
lathe
by
a
skilled
operator.
In
redesigning
an
experimental
part
for
production,
economical tolerances should be used.
Turret
Lathes
Production
machining
equipment
must
be
evaluated
now,
more
than
ever
before,
in
terms
of
ability
to
repeat
accurately
and
rapidly.
Applying
this
criterion
for
establishing
the
production
qualification
of
a
specific
method,
the
turret
lathe
merits
a
high
rating.
In
designing
for
low
quantities
such
as
100
or
200 parts,
it
is
most
economical
to
use the
turret
lathe.
In
achieving
the
optimum tolerances possible on the
turret lathe, the designer should strive for a
minimum of operations.
Automatic Screw
Machines
Generally, automatic screw machines
fall into several categories; single-spindle
automatics,
multiple-spindle automatics
and automatic chucking machines. Originally
designed for rapid, automatic production of screws
and
similar threaded parts, the
automatic screw machine has long since exceeded
the confines of this narrow field, and today
play
s a
vital role
in the mass production of a variety of
precision parts. Quantities play an important part
in the economy of the parts
machined on
the automatic to set up on the turret lathe than
on the automatic screw machine. Quantities less
than 1000 parts may
be more
economical to set up on the turret
lathe than on the automatic screw machine. The
cost of the parts machined can be
reduced if the minimum economical lot
size is calculated and the proper machine is
selected for these quantities.
Automatic Tracer Lathes
Since surface
roughness depends greatly upon material turned,
tooling, and fees and speeds
employed,
minimum tolerances that can be held on automatic
tracer lathes are not necessarily the most
economical tolerances.
Is
some
case,
tolerances
of
±
0.05mm
are
held
in
continuous production
using
but
one
cut.
Groove width
can
be
held
to
±
0.125mm on
some parts. Bores and single-point finishes can be
held to ±
0.0125mm. On high-production
runs where maximum
output is desirable,
a minimum tolerance of ±
0.125mm is
economical on both diameter and length of turn.
Milling
With the exceptions of
turning and drilling, milling is undoubtedly the
most widely used method of removing
metal. Well suited and readily adapted
to the economical production of any quantity of
parts, the almost unlimited versatility of
the milling process merits the
attention and consideration of designers seriously
concerned with the manufacture of their product.
As in any other process, parts that
have to be milled should be designed with
economical tolerances that can be achieved in
production milling. If the part is
designed with tolerances finer than necessary,
additional operations will have to be
added
to
achieve these
tolerances
——
and this will
increase the cost of the part.
Grinding
is
one
of
the
most
widely
used
methods
of
finishing
parts
to
extremely
close
tolerances
and
low
surface
roughness. Currently, there are
grinders for almost for almost every type of
grinding operation. Particular design features of
a part
dictate to a large degree the
type of grinding machine required. Where
processing costs are excessive, parts redesigned
to utilize a
less expensive, higher
output grinding method may be well worthwhile. For
example, wherever possible the production economy
of center less grinding should be taken
advantage of by proper design consideration.
Although
grinding
is
usually
considered
a
finishing
operation,
it
is
often
employed
as
a
complete
machining
process
on
work
which can be ground down from rough condition
without being turned or otherwise machined. Thus
many types of forgings
and other parts
are finished completely with the grinding wheel at
appreciable savings of time and
expense.
Classes
of
grinding
machines
include
the
following:
cylindrical
grinders,
center
less
grinders,
internal
grinders,
surface
grinders, and tool and cutter grinders.
The cylindrical and center less
grinders are for straight cylindrical or taper
work; thus splices, shafts, and similar parts
are
ground on cylindrical
machines either of the common-center type or the
center less machine.
Thread
grinders
are
used
for
grinding
precision
threads
for
thread
gages,
and
threads
on
precision
parts
where
the
concentricity between the diameter of
the shaft and the pitch diameter of the thread
must be held to close tolerances.
The internal grinders are used for
grinding of precision holes, cylinder bores, and
similar operations where bores of all kinds
are to be finished.
The
surface grinders are for finishing all kinds of
flat work, or work with plain surfaces which may
be operated upon either
by the edge of
a wheel or by the face of a grinding wheel. These
machines may have reciprocating or rotating
tables.