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Full text of "Modern Mechanical Engineering Vol-I"




particular cases, so that the use of tracing-paper may save considerable time.
Consider for a moment or two the case of, say, the lubricating pipe arrange-
ment round a large engine or the arrangement of platforms. If it be done
on drawing-paper the engines have to be drawn down to scale afresh each
time, and it is likely some apparently small details will be overlooked which
may seriously affect the arrangement. In a case like this a sheet of tracing-
paper can be laid over, say, the carefully made up drawing of the engines,
and the leads of lubricating pipes or platforms drawn in on tracing-paper,
without a single line of the engines proper. Of course, when finished, it
will be advisable to show the main outlines of the engine to convey a quick
picture of what is required to the men on the job. Against the use of tracing-
paper may be urged the fact that it tears more readily, gets dirty, does not
lend itself to erasure, and consequently leads to a good deal of annoyance
and irritation, and consequent inefficiency; but if properly and discreetly
used, great economy, financial and otherwise, ensues.
Fluctuating Nature of Work.—It is inevitable that there will be
periods of extreme pressure. Certain shops will demand work greedily to
keep them going, and to get the contract forward at the greatest possible
rate. Never may it be hoped that a whole contract can be designed and
detailed and modified to the draughtsman's satisfaction before he is called
upon to pass drawings and orders into the shop. Skill, judgment, and
experience are very necessary to know what things must have precedence.
For instance, it is obvious that where large castings form part of the product,
time will be needed in the production of the patterns, and a further period
must elapse before the foundry can deliver the castings.' In the machine
shop, moreover, many different operations may have to be performed on
one piece alone, and all of them at different times, so that the preparation
of such a drawing is generally a first call. But there are obvious risks. The
facings have to be fixed definitely when one would gladly do it tentatively,
in view of what may crop up at a later stage of the work. But this may not
be, and it is seldom indeed that a whole job is finished, and can be looked
back upon without a wish that it had been possible to alter many things.
It follows that often, in a squad, one or two of the men are trying the most
likely small arrangements to ensure that a reasonably suitable arrangement
of the major pieces can be made thus early. It is here that experience and
judgment are so necessary. But, whilst this is often the case, there are
periods of red slackness, when not much current work is on hand. The
staff is generally kept up, as it is usually bad policy to deplete a staff which
knows the run of the work and the office. During such periods the men
are generally turned on to the task of working up the data which may be
neglected during times of pressure; of preparing and altering, where found
necessary, standard drawings and sketches. This work, whilst it has no
visible return at the moment, proves useful in the long run.
Drawings of Standard Parts.—In making standard drawings it is
important not only to make the standards for different sizes of the same
group of articles show differences which shall be definite and progressive,