An Introduction to Machine Drawing and Design.

by David Allan Low.

PREFACE.

It is now generally recognised that the old-fashioned method of teaching machine drawing is very unsatisfactory. In teaching by this method an undimensioned scale drawing, often of a very elaborate description, is placed before the student, who is required to _copy_ it. Very often the student succeeds in making a good copy of the drawing placed before him without learning very much about the object represented by it, and this state of matters is sometimes not much improved by the presence of the teacher, who is often simply an art master, knowing nothing about machine design. It is related of one school that a pupil, after making a copy of a particular drawing, had a discussion with his teacher as to whether the object represented was a sewing machine or an electrical machine. Evidently the publisher of the drawing example in this case did not adopt the precaution which a backward student used at an examination in machine design: he put on a full t.i.tle above his drawing, for the information of his examiner.

Now, if machine drawing is to be of practical use to any one, he must be able to understand the form and arrangement of the parts of a machine from an inspection of suitable drawings of them without seeing the parts themselves. Also he ought to be able to make suitable drawings of a machine or parts of a machine from the machine or the parts themselves.

In producing this work the author has aimed at placing before young engineers and others, who wish to acquire the skill and knowledge necessary for making the simpler _working drawings_ such as are produced in engineers' drawing offices, a number of good exercises in drawing, sufficient for one session's work, and at the same time a corresponding amount of information on the design of machine details generally.

The exercises set are of various kinds. In the first and simplest certain views of some machine detail are given, generally drawn to a small scale, which the student is asked to reproduce _to dimensions marked on these views_, and he is expected to keep to these dimensions, and not to measure anything from the given ill.u.s.trations. In the second kind of exercise the student is asked to reproduce certain views shown _to dimensions given in words or in tabular form_. In the third kind of exercise the student is required to make, in addition to certain views shown to given dimensions, others which he can only draw correctly if he thoroughly understands the design before him. In the fourth kind of exercise the student is asked to make the necessary working drawings for some part of a machine which has been previously described and ill.u.s.trated, _the dimensions to be calculated by rules given in the text_.

The ill.u.s.trations for this work are all new, and have been specially prepared by the author from _working drawings_, and he believes that they will be found to represent the best modern practice.

As exercises in drawing, those given in this book are not numbered exactly in their order of difficulty, but unless on the recommendation of a teacher, the student should take them up in the order given, omitting the following:--26, 27, 28, 35, 40, 42, 43, 45, 49, 50, 54, 60, 61, as he comes to them, until he has been right through the book; afterwards he should work out those which he omitted on first going over the book.

In addition to the exercises given in this work the student should practise making freehand sketches of machine details from actual machines or good models of them. Upon these sketches he should put the proper dimensions, got by direct measurement from the machine or model by himself. These sketches should be made in a note-book kept for the purpose, and no opportunity should be lost of inserting a sketch of any design which may be new to the student, always putting on the dimensions if possible. These sketches form excellent examples from which to make working drawings. The student should also note any rules which he may meet with for proportioning machines, taking care, however, in each case to state the source of such information for his future guidance and reference.

As machine drawing is simply the application of the principles of descriptive geometry to the representation of machines, the student of the former subject, if he is not already acquainted with the latter, should commence to study it at once.

D. A. L.

GLASGOW: _March_ 1887.

I. INTRODUCTION.

_Drawing Instruments._--For working the exercises in this book the student should be provided with the following:--A well-seasoned yellow pine _drawing-board_, 24 inches long, 17 inches wide, and 3/8 inch or 1/2 inch thick, provided with cross-bars on the back to give it strength and to prevent warping. A =T= _square_, with a blade 24 inches long attached permanently to the stock, _but not sunk into it_. One 45 and one 60 _set square_. The short edges of the former may be about 6 inches and the short edge of the latter about 5 inches long. A _pair of compa.s.ses_ with pen and pencil attachments, and having legs from 5 inches to 6 inches long. A _pair of dividers_, with screw adjustment if possible. A _pair of small steel spring pencil bows_ for drawing small circles, and a _pair of small steel spring pen bows_ for inking in the same. A _drawing pen_ for inking in straight lines. All compa.s.ses should have _round points_, and if possible _needle_ points. A piece of india-rubber will also be required, besides two pencils, one marked H or HH and one marked HB or F; the latter to be used for lining in a drawing which is not to be inked in, or for freehand work.

Pencils for mechanical drawing should be sharpened with a _chisel point_, and those for freehand work with a _round point_. _Do not wet the pencil_, as the lines afterwards made with it are very difficult to rub out.

Drawing-paper for working drawings may be secured to the board by _drawing-pins_, but the paper for finished drawings or drawings upon which there is to be a large amount of colouring should be _stretched_ upon the board.

The student should get the best instruments he can afford to buy, and he should rather have a few good instruments than a large box of inferior ones.

_Drawing-paper._--The names and sizes of the sheets of drawing paper are given in the following table:--

Inches Demy 20 15 Medium 22 17 Royal 24 19 Imperial 30 22 Atlas 34 26 Double Elephant 40 27 Antiquarian 52 31

The above sizes must not be taken as exact. In practice they will be found to vary in some cases as much as an inch.

Cartridge-paper is made in sheets of various sizes, and also in rolls.

Hand-made paper is the best, but it is expensive. Good cartridge-paper is quite suitable for ordinary drawings.

_Centre Lines._--Drawings of most parts of machines will be found to be symmetrical about certain lines called _centre lines_. These lines should be drawn first with great care. On a pencil drawing centre lines should be thin continuous lines; in this book they are shown thus -- - -- - --.

After drawing the centre line of any part the dimensions of that part must be marked off from the centre line, so as to insure that it really is the centre line of that part: thus in making a drawing of a rivet, such as is shown at (_a_) fig. 1, after drawing the centre line, half the diameter of the rivet would be marked off on each side of that line, in order to determine the lines for the sides of the rivet.

_Inking._--For inking in drawings the best Indian ink should be used, and not common writing ink. Common ink does not dry quick enough, and rapidly corrodes the drawing pens. The pen should be filled by means of a brush or a narrow strip of paper, and not by dipping the pen into the ink.

In cases where there are straight lines and arcs of circles touching one another _ink in the arcs first_, then the straight lines; in this way it is easier to hide the joints.

_Colouring._--Camel's-hair or sable brushes should be used; the latter are the best, but are much more expensive than the former. The colour should be rubbed down in a dish, and the tint should be light. The mistake which a beginner invariably makes is in having the colour of too dark a tint.

First go over the part to be coloured with the brush and _clean_ water for the purpose of damping it. Next dry with clean blotting-paper to take off any superfluous water. Then take another brush with the colour, and beginning at the top, work from left to right and downwards. If it is necessary to recolour any part let the first coating dry before beginning.

Engineers have adopted certain colours to represent particular materials; these are given in the following table:--

_Table showing Colours used to represent Different Materials._

MATERIAL COLOUR

Cast iron Payne's grey or neutral tint.

Wrought iron Prussian blue.

Steel Purple (mixture of Prussian blue and crimson lake).

Bra.s.s Gamboge with a little sienna or a very little red added.

Copper A mixture of crimson lake and gamboge, the former colour predominating.

Lead Light Indian ink with a very little indigo added.

Brickwork Crimson lake and burnt sienna.

Firebrick Yellow and Vand.y.k.e brown.

Greystones Light sepia or pale Indian ink, with a little Prussian blue added.

Brown freestone Mixture of pale Indian ink, burnt sienna, and carmine.

Soft woods For ground work, pale tint of sienna.

Hard woods For ground work, pale tint of sienna with a little red added.

For graining woods use darker tint with a greater proportion of red.

_Printing._--A good drawing should have its t.i.tle printed, a plain style of letter being used for this purpose, such as the following:--

[Ill.u.s.tration: ABCDEFGHIJKLMNOPQRST UVWXYZ 1234567890]

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The following letters look well _if they are well made_, but they are much more difficult to draw.

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For remarks on a drawing the following style is most suitable:--

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All printing should be done by freehand.