It is now generally recognised that the old-fashioned method of teaching
machine drawing is very unsatisfactory. In teaching by this method an
undimensioned scale drawing, often of a very elaborate description, is
placed before the student, who is required to _copy_ it. Very often the
student succeeds in making a good copy of the drawing placed before him
without learning very much about the object represented by it, and this
state of matters is sometimes not much improved by the presence of the
teacher, who is often simply an art master, knowing nothing about
machine design. It is related of one school that a pupil, after making a
copy of a particular drawing, had a discussion with his teacher as to
whether the object represented was a sewing machine or an electrical
machine. Evidently the publisher of the drawing example in this case did
not adopt the precaution which a backward student used at an examination
in machine design: he put on a full title above his drawing, for the
information of his examiner.
Now, if machine drawing is to be of practical use to any one, he must be
able to understand the form and arrangement of the parts of a machine
from an inspection of suitable drawings of them without seeing the parts
themselves. Also he ought to be able to make suitable drawings of a
machine or parts of a machine from the machine or the parts themselves.
In producing this work the author has aimed at placing before young
engineers and others, who wish to acquire the skill and knowledge
necessary for making the simpler _working drawings_ such as are produced
in engineers' drawing offices, a number of good exercises in drawing,
sufficient for one session's work, and at the same time a corresponding
amount of information on the design of machine details generally.
The exercises set are of various kinds. In the first and simplest
certain views of some machine detail are given, generally drawn to a
small scale, which the student is asked to reproduce _to dimensions
marked on these views_, and he is expected to keep to these dimensions,
and not to measure anything from the given illustrations. In the second
kind of exercise the student is asked to reproduce certain views shown
_to dimensions given in words or in tabular form_. In the third kind of
exercise the student is required to make, in addition to certain views
shown to given dimensions, others which he can only draw correctly if he
thoroughly understands the design before him. In the fourth kind of
exercise the student is asked to make the necessary working drawings for
some part of a machine which has been previously described and
illustrated, _the dimensions to be calculated by rules given in the
text_.
The illustrations for this work are all new, and have been specially
prepared by the author from _working drawings_, and he believes that
they will be found to represent the best modern practice.
As exercises in drawing, those given in this book are not numbered
exactly in their order of difficulty, but unless on the recommendation
of a teacher, the student should take them up in the order given,
omitting the following:--26, 27, 28, 35, 40, 42, 43, 45, 49, 50, 54, 60,
61, as he comes to them, until he has been right through the book;
afterwards he should work out those which he omitted on first going over
the book.
In addition to the exercises given in this work the student should
practise making freehand sketches of machine details from actual
machines or good models of them. Upon these sketches he should put the
proper dimensions, got by direct measurement from the machine or model
by himself. These sketches should be made in a note-book kept for the
purpose, and no opportunity should be lost of inserting a sketch of any
design which may be new to the student, always putting on the dimensions
if possible. These sketches form excellent examples from which to make
working drawings. The student should also note any rules which he may
meet with for proportioning machines, taking care, however, in each case
to state the source of such information for his future guidance and
reference.
As machine drawing is simply the application of the principles of
descriptive geometry to the representation of machines, the student of
the former subject, if he is not already acquainted with the latter,
should commence to study it at once.
D. A. L.
GLASGOW: _March_ 1887.
_PREFACE TO THE THIRD EDITION._
To this edition another chapter has been added, containing a number of
miscellaneous exercises, which it is hoped will add to the usefulness of
the work as a text-book in science classes. The latest examination paper
in machine drawing by the Science and Art Department has also been added
to the Appendix.
D. A. L.
LONDON: _August_ 1888.