In conclusion, I will say on the subject of patterns and castings, that a learner must depend mainly upon what he can see and what is explained to him in the pattern-shop and foundry. He need never fear an uncivil answer to a proper question, applied at the right time and place. Mechanics who have enough knowledge to give useful information of their business, have invariably the courtesy and good sense to impart such information to those who require it.
An apprentice should never ask questions about simple and obvious matters, or about such things as he can easily learn by his own efforts. The more difficult a question is, the more pleasure a skilled man will take in answering it. In short, a learner should carefully consider questions before asking them. A good plan is to write them down, and when information is wanted about casting, never go to a foundry to interrupt a manager or moulder at melting time, nor in the morning, when no one wants to be annoyed with questions.
I will, in connection with this subject of patterns and castings, suggest a plan of learning especially applicable in such cases, that of adopting a habit of imagining the manner of moulding, and the kind of pattern used in producing each casting that comes under notice. Such a habit becomes easy and natural in a short time, and is a sure means of acquiring an extended knowledge of patterns and moulding.
A pattern-maker no sooner sees a casting than he imagines the kind of pattern employed in moulding it; a moulder will imagine the plan of moulding and casting a piece; while an engineer will criticise the arrangement, proportions, adaptation, and general design, and if skilled, as he ought to be, will also detect at a glance any useless expense in patterns or moulding.
(1.) Why cannot the regular working drawings of a machine be employed to construct patterns by?—(2.) What should determine the quality or durability of patterns?—(3.) How can the arrangement of patterns affect certain parts of a casting?—(4.) What means can be employed to avoid inherent strain in castings?—(5.) Why is the top of a casting less sound than the bottom or drag side?—(6.) What are cores employed for?—(7.) What is meant by venting a mould?—(8.) Explain the difference between green and dry sand mouldings.—(9.) Why is sand employed for moulds?—(10.) What generally causes the disarrangement of cores in casting?—(11.) Why are castings often sprung or crooked?—(12.) What should determine the amount of draught given to patterns?—(13.) What are the means generally adopted to avoid cooling strains in castings?
CHAPTER XXIII.
FORGING.
Workshop processes which are capable of being systematised are the most easy to learn. When a process is reduced to a system it is no longer a subject of special knowledge, but comes within general rules and principles, which enable a learner to use his reasoning powers to a greater extent in mastering it.
To this proposition another may be added, that shop processes may be systematised or not, as they consist in duplication, or the performance of certain operations repeatedly in the same manner. It has been shown in the case of patterns that there could be no fixed rules as to their quality or the mode of constructing them, and that how to construct patterns is a matter of special knowledge and skill.