An Introduction to Machine Drawing and Design Part 10

The ill.u.s.trations in this chapter are in most cases not drawn to scale; they are also in some parts incomplete, and in others some of the lines are purposely drawn wrong. The student must keep to the dimensions marked on the drawings, and where no sizes are given he must use his own judgment in proportioning the parts. All errors must be corrected, and any details required, but not shown completely in the ill.u.s.trations, must be filled in.

EXERCISE 62: _Single Riveted b.u.t.t Joint with Tee-iron Cover Strap._--Two views, one a side elevation and the other a sectional elevation, of a riveted joint are shown in fig. 60. Draw these views, and also a plan projected from one of them. Show the rivets completely in all the views. Scale 4 inches to a foot.

[Ill.u.s.tration: FIG. 60.]

[Ill.u.s.tration: FIG. 61.]

EXERCISE 63: _Girder Stay for Steam Boiler._--The flat crown of the fire-box of locomotive and marine boilers is generally supported or stayed by means of girder stays, an example of which is shown in fig. 61. A B is the side elevation of a portion of one of these girders. Each girder is supported at its ends by the plates forming the vertical sides of the fire-box. The flat crown is bolted to the girders as shown. Observe that the girders are in contact with the crown only in the neighbourhood of the bolts. Consider carefully this part of the design, and then answer the following questions: (1) What objections are there to supporting the girders at the ends only without the contact pieces at the bolts? (2) What objections are there to having the girders in contact with the crown plate of the fire-box throughout their whole length?

Draw the views shown in fig. 61, and from the right-hand one project a plan. Scale 4 inches to a foot.

[Ill.u.s.tration: FIG. 62.]

EXERCISE 64: _End of Bar Stay for Steam Boiler._--On page 12 one form of stay for supporting the flat end of a steam boiler is described. Another form of stay for the same purpose is shown in fig. 62. A B is a portion of the end of a steam boiler. C D is one end of a bar which extends from one end of the boiler to the other. The ends of this bar are screwed, and when the bar is of wrought iron the screwed parts are generally larger in diameter than the rest of the bar. When made of steel the bar is generally of uniform diameter throughout. In the case of wrought-iron bar stays the enlarged ends are welded on to the smaller parts.

Welding is not so reliable with steel as with wrought iron. Write out answers to the following questions: (1) What is the advantage of having the screwed part of the bar larger in diameter than the rest? (2) Why are steel bar stays not generally enlarged at their screwed ends?

Draw the views shown in fig. 62, and project from one of them a third view. Scale 4 inches to a foot.

EXERCISE 65: _Knuckle Joint._--Draw the plan and elevation of this joint shown in fig. 63, and also draw an end elevation looking in the direction of the arrow. The parts at A and B are octagonal in cross section. Scale 4 inches to a foot.

[Ill.u.s.tration: FIG. 63.]

EXERCISE 66: _Locomotive Coupling Rod Ends._--A form of knuckle joint used on locomotive coupling rods is shown in fig. 64.

In this case two rods meet and work on the same pin, as shown at (a) fig. 64. Draw, in addition to the views shown in fig. 64, a plan and a vertical section through the axis of the pin. Scale 6 inches to a foot.

Would it be practicable to replace the two rods A B and B C by a single rod working on the crank pins at A, B, and C? Give reasons for your answer.

[Ill.u.s.tration: FIG. 64.]

EXERCISE 67: _Bell Crank Lever._--Draw the plan and elevation of the lever shown in fig. 65. Scale 6 inches to a foot.

[Ill.u.s.tration: FIG. 65.]

EXERCISE 68: _Back Stay for Lathe._--Draw a plan and two elevations of the stay shown in fig. 66. Make all necessary corrections and show all the details in each view. Scale full size.

[Ill.u.s.tration: FIG. 66.]

[Ill.u.s.tration: FIG. 67.]

EXERCISE 69: _Conical Disc Valve and Casing._--Draw, half size, the views shown in fig. 67 of the conical disc valve and casing, and also add an elevation looking in the direction of the arrow.

EXERCISE 70: _Connecting Rod End._--The student should carefully compare this connecting rod end (fig. 68) with those ill.u.s.trated on pages 50 and 52. The lower part of fig. 68 is a half plan and half horizontal section, and the upper part is a half side elevation and half vertical section. Draw these views and also an end elevation.

Scale 6 inches to a foot.

[Ill.u.s.tration: FIG. 68.]

[Ill.u.s.tration: FIG. 69.]

[Ill.u.s.tration: FIG. 70.]

[Ill.u.s.tration: FIG. 71.]

[Ill.u.s.tration: FIG. 72.]

[Ill.u.s.tration: FIG. 73.]

[Ill.u.s.tration: FIG. 74.]

EXERCISE 71: _Engine Cross-head._--The cross-head shown in fig. 69 is for an inverted cylinder marine engine. A is the piston rod, and B B are pins, forged in one piece with C, to which the forked end of the connecting rod is attached. Draw the upper view with the central part in section as shown. Make the right-hand half of the lower view a plan without any section, and make the left-hand half a horizontal section through the axis of the pins B B. Scale 4 inches to a foot.

EXERCISE 72: _Ratchet Lever._--The lever shown in fig. 70 is used for turning the horizontal screw of a traversing screw jack. Draw the two views shown, and from one of them project a plan. Scale full size.

EXERCISE 73: _Steam Whistle._--Draw, full size, the elevation and section of the steam whistle shown in fig. 71. Draw also horizontal sections at A B, C D, and E F.

[Ill.u.s.tration: FIG. 75.]

EXERCISE 74: _Screw Coupling for Railway Carriages._--Draw the three views of the screw coupling shown in fig. 72. Scale 6 inches to a foot.

If the link A is fixed, through what distance will the link B move for two turns of the lever?

[Ill.u.s.tration: FIG. 76.]

EXERCISE 75: _Loose Headstock for a 6-inch Lathe._--Two views of this headstock are shown in fig. 73. On one of these views a few of the chief dimensions are marked. The details, fully dimensioned, are shown separately in figs. 74, 75, and 76.

Explain clearly how the centre is moved backwards and forwards, and also how the spindle containing it is locked when it is not required to move.

Draw, half-size, the views shown in fig. 73, and from the left-hand view project a plan. Draw also the detail of the locking arrangement shown in fig. 74.

APPENDIX A.

_SCIENCE AND ART DEPARTMENT, SOUTH KENSINGTON._

SYLLABUS.

SUBJECT II.--MACHINE CONSTRUCTION AND DRAWING.

It is a.s.sumed that the student has already learnt to draw to scale, and that he can draw two or more views of the same object in simple or orthographic projection. To pa.s.s in machine construction and drawing, he must be able to apply this knowledge to the representation of machinery.

He must be acquainted with the form and purpose of the simpler parts of which machines are built up and must have had some practice in drawing them. To test his knowledge, rough dimensioned sketches, more or less incomplete, of simple machine details will be given him, and he will be required to produce a complete drawing in pencil to a given scale. Two or more views of at least one subject will be required, and these must be so drawn as to be properly projected one from the other, _in order to show that the student appreciates that he is producing a representation of a solid piece of machinery, and not merely copying a sketch. No credit will be given unless some knowledge of projection is shown._ The centre lines of the drawings should be shown, and parts cut by planes of section should be indicated by diagonal shading. Bolts and other fastenings should be carefully shown where required. Any indication that a candidate has merely copied the sketches given, without understanding the part represented, will invalidate his examination.

FIRST STAGE OR ELEMENTARY COURSE.

In the elementary stage, a knowledge is required of the simple parts only of _machines in common use_. _Some_ of these are enumerated in the following list. The student should be practised in drawing them till he recognises their forms, and the object of the arrangement should be explained to him. He should also know the simple technical terms used in describing them.