Earnest T. Childs.

The amateur who has closely followed these talks on mechanical drawing, and who has applied himself sufficiently to have completed reproductions of the specimen drawings which have been shown, will have attained by this time a fair knowledge of what is required of the mechanical •draughtsman as a copyist. By this is meant that he will have learned the necessary details which must he acquired in order that he may draw lines correctly on tracing cloth or paper, at the same time working from some predetermined outline or dimensions. This is the first step in the development of the mechanical draughtsman, and is extremely important. A person capable of making accurate copies with reasonable celerity has the foundation upon which the future draughtsman may stand. Without doubt accuracy is most important, but rapidity follows a close second. The ability to make free-hand sketches in a clear, concise manner, as described in the last chapter, is also a valuable requisite.

All the above requirements are, however, those of the copyist only, and it is on this basis that the young draughtsman generally starts his work in the drawing-office. As his ability to comprehend in-•creases by practice in copying or tracing, and as he profits by his observation of work with which he comes in contact, just so fast will he advance. Once started in the work, it depends upon himself alone just how fast and how far he will be able to go.

There are other things required, however, for the production of the first-class mechanical draughtsman. He must be well posted on engineering subjects, must understand the principles of mechanics, must know about the strength of materials, and must have a working knowledge of steam and electricity, not to mention a dozen other kindred subjects.

These requirements are enumerated, not to discourage the student, but rather to direct the course of his study into the proper channels. The draughting board is not a thing to which every draughtsman expects to be tied throughout his natural life. It may be looked upon as a stepping-stone to something broader and higher. And it is an extremely essential stepping-stone. It would surprise many, if statistics were available, as to the number of superintendents and chief engineers who started their careers over the drawing-table.

But aside from the future possibilities of the man who actually works over the board in the drawing-office, a thorough knowledge of drawing is of vast importance to every artisan, no matter whether he be journeyman or apprentice. Let us consider for a moment a few of the trades in which a knowledge of and an ability to read plans will be absolutely necessary. Imagine, if possible, a machinist starting to build an engine without plans to work from, or a patternmaker making a pattern for a cylinder by guesswork. The result may be a thing which will run, but it can be obtained only by a cut and try process, and when the machine is complete there will be no record to show what it is like. The same will apply to a motor or any other piece of mechanism. Ask a carpenter to build a house, and the first thing he does is to make a plan, even though the house be only a beach cottage. A mason must be able to read plans to lay out foundations, walls, etc. A plumber or a steam-fitter always makes a diagram before piping a job.

In fact, there is hardly a trade where a knowledge of drawing will not be useful to the workman, and it is more particularly to help the apprentice and the young journeyman that these talks have been given. If a young man is anxious and ambitious to become something more than an ordinary workman, the first step will be to learn to read drawings, and this will be succeeded by courses of study along the lines of his chosen occupation.

This chapter will close the present series of articles on mechanical drawing, and the assembly drawing of the engine, together with the last details, are given herewith. It has been necessary to draw the assembly to a very small scale (11/2" = 1') on account of the size of the sheet; but in redrawing, this should be enlarged to half size in order to better show the various details. Particular attention should be paid to the choice of sections of the engine to full scale. There is little to be said regarding the sheet showing the cylinder head and steam chest. These are necessary to complete the final details of the engine. It will be seen that in this course we have drawn every detail of this 4x4 engine, and the complete work is attained in the assembly drawings.

Mechanical Drawing VII The Assembly 189Mechanical Drawing VII The Assembly 190

Fig. 22.

Mechanical Drawing VII The Assembly 191

Fig. 23.

Mechanical Drawing VII The Assembly 192

Fig. 24.

Incidental to the acquisition of a knowledge of mechanical drawing, it has been possible to obtain also a small idea of the drawing required for the parts which go to make up a small upright engine. This series will not be complete, however, without a little additional data. In Chapter III is given an outline of the various common types of screw threads. The United States standard screw thread is one with which all draughtsmen should be familiar. This thread is slightly flattened at the apex, and the angle is 60°. The standard dimensions of threads from 1/4" to 2" are given in the accompanying table (see Table No. 1) and the load which each will hold is given, based on a tensile strength of 5,000 lbs. per square inch of cross-section. It will be observed that the rough iron sizes vary slightly from the standard, the latter being smaller, due to the flattening of the V to prevent injury to the thread.

Supplementing the data on screw threads, it is essential that one should possess some knowledge of nuts and boltheads. With this in mind, a table has been made giving the most important dimensions for hexagonal nuts, in sizes from 1/4" to 2" in diameter. (See Table 2.)

In connection with these tables it will be found convenient to have one giving decimal equivalents of fractions of an inch. (Table 3.) These tables will do to start a reference notebook, which every draughtsman should have and should add to it the scraps of information secured from time to time. The best kind of a book for handy reference is one which is indexed, thus making it possible to classify the information the more readily. Later, when the notebook may become overcrowded, the information may be transferred to a card index, alphabetically arranged. This is, however, rather beyond the range of the present subject, and is given as a hint for future work. Part II, Projection, will be the subject of continued talks.

Decimal Equivalents

1/64.......015625

1/92....03125

3/64....046875

1/16.............0625

5/64....078125

3/32....09375

7/64....109375

1/6....125

9/64....140625

5/32....15625

11/64...171875

3/16....1875

13/64...203125

7/32....21875

15/64...234375

1/4....25

17/64....265625

9/32....28125

19/64....296875

5/16....3125

21/64...328125

44/32...34375

23/34...359375

3/8....375

25/64...390625

13/32...40625

27/64...421875

7/16....4375

29/64...45/3125

15/32...46875

31/64...484375

1/2....5

33/64......515625

17/32......53125

35/64...546875

9/16.........5625

37/64...578125

19/32......59375

39/64...609375

5/8....625

41/64...0625

21/32...65625

43/64...671875

11/16...6875

45/64...703125

23/32...71875

47/64...734375

3/4.................75

49/64.............765625

25/32............78125

51/64.............796875

13/16.............8125

53/64............828125

27/32.............84375

55/64.............859375

7/8.............875

57/64.............890625

29/32............90625

59/64.............921875

15/16.............9375

61/64.............953125

31/32.............96875

63/64.............984375

1...1