Physical strength, bone and muscle, must be elements in successful engineering experience; and if these things are not acquired at the same time with a mechanical education, it will be found, when ready to enter upon a course of practice, that an important element, the "propelling power," has been omitted.
(1.) What is the difference between geometric and artistic drawing?—(2.) What is the most important operation in making a good drawing?—(3.) Into what three classes can working drawings be divided?—(4.) Explain the difference between elevations and plans.—(5.) To what extent in general practice is the proportion of parts and their arrangement in machines determined mathematically?
CHAPTER XXII.
PATTERN-MAKING AND CASTING.
Patterns and castings are so intimately connected that it would be difficult to treat of them separately without continually confounding them together; it is therefore proposed to speak of pattern-making and moulding under one head.
Every operation in a pattern-shop has reference to some operation in the foundry, and patterns considered separately from moulding operations would be incomprehensible to any but the skilled. Next to designing and draughting, pattern-making is the most intellectual of what may be termed engineering processes—the department that must include an exercise of the greatest amount of personal judgment on the part of the workman, and at the same time demands a high grade of hand skill.
For other kinds of work there are drawings furnished, and the plans are dictated by the engineering department of machinery-building establishments, but pattern-makers make their own plans for constructing their work, and have even to reproduce the drawings of the fitting-shop to work from. Nearly everything pertaining to patterns is left to be decided by the pattern-maker, who, from the same drawings, and through the exercise of his judgment alone, may make patterns that are durable and expensive, or temporary and cheap, as the probable extent of their use may determine.
The expense of patterns should be divided among and charged to the machines for which the patterns are employed, but there can be no constant rules for assessing or dividing this cost. A pattern may be employed but once, or it may be used for years; it is continually liable to be superseded by changes and improvements that cannot be predicted beforehand; and in preparing patterns, the question continually arises of how much ought to be expended on them—a matter that should be determined between the engineer and the pattern-maker, but is generally left to the pattern-maker alone, for the reason that but few mechanical engineers understand pattern-making so well as to dictate plans of construction.