Notes on practical mechanical drawing, written for the use of the students in general engineering drawing in the University of Illinois

Front matter

otes on Practical Mechanical Drawing

COPYRIGHT OFFICE,

No registration of title of this book as a preliminary to copyright protection has been found.

Preface

The following book is a collection of notes upon practical mechanical drawing, originally the subject matter of a course of lectures, and was prepared to meet the needs of the students in the course in General Engineering Drawing in the University of Illinois.

It contains material not hitherto discussed but casually in existing books on drawing, notably, the treatment of sections, the arrangement and development of a set of working drawings, and machine sketching.

Attention is called to the fact that the book makes a long stride in advance of previous methods of teaching the subject. Formerly, and even to-day, in many colleges the elementary training is accomplished through line exercises and geometrical dravving. These are here reduced to a minimum, and practical problems take their place.

It is the author's belief, substantiated by experience, that the students' faithful adherence to minute directions as to methods of work, insures a ready grasp of the subject. And this is why methods are given such expanded treatment. It is also counting a great deal upon the student to expect him to grasp and apply all the practical points given, but in the past, this confidence has not proven vain, and by using practical problems from the beginning it has been vi Preface

possible to help him to do this and to keep him interested, as he never is in uninteresting geometrical figures. The departure is not altogether new, however, for the same progress is being paralleled in all manual training work.

The book is not yet complete. It still lacks a set of graded exercises upon working drawings and sketching, and a few illustrative examples. The elementary principles of orthographic projection are also left out, as they form the subject of a series of lectures with home exercises, intended to parallel the working drawings.

The author will be glad of any suggestions leading to a perfection of his idea. Quotations have been here and there been made from standard works upon drawing, and acknowledgement has been made where such has been done. The author wishes, in closing, to express his appreciation of the pains-taking work of Mr. C. L. McMaster, as shown in the illustrations, most of which were made by him. And not a little help was obtained from him, also, in the subject matter and in its arrangement.

Urbana, III., Sept. 1, 1907.

Chapter I. Straight Line Drawing

Sec. Page.

1. Definition of drawing 1

3. General directions with regard to preliminary steps 3

4. Detailed directions for Exercise No. 1 7

5. General directions in regard to inking 7

6. Detailed directions for inking Exercise No. 1 9

1 1 . General directions 14

Chapter II. Curves And Straight Lines Combined And Sections

12. Handling of the compass, dividers and bows 17

13. General directions for the treatment of Exercises No. 6 to No.

9, inclusive 22

14. Special detailed directions for drawing and inking Exercise

15. Special detailed directions for drawing and inking Exercise

19. Further points about drawing materials and instruments. ... 34

20. Sectioning 41

21. Some practical points about sectioning 49

22. Some practical points about and the care and handling of

drawing instruments 55 viii Index

Chapter III

Sec. IRREGULAR CURVES AND GEOMETRICAL DRAWING. Page.

27. Irregular curves 70

28. System in penciling and inking drawings 74

29. Geometrical drawing 79-

30. The conies 87

31. To draw an ellipse by the focii method 88

32. To construct an ellipse by the method of the trammel. ...'... 90

33. To draw an ellipse by the aid of the major and minor auxiliary

circles 92

34. To draw an ellipse approximately with the compass. 92

35. To draw a parabola by means of the focus 95

36. To draw a parabola by means of its envelope 95

37. To draw a hyperbola by means of its focii 97

38. To draw a hyperbola, given the two asymptotes and any point

on the curve . 99

39. To draw a hyperbola by the rectangle method 99

40. The cycloid * 100

41. To construct the cycloid 101

42. To draw the epicycloid ! 103

43. To draw the hypoc^'-cloid 103

44. Geometrical definitions, terms, etc 104

Chapter IV. Working Drawings

45. The difference between 1st and 3rd angle projection 106

46. The helical curve and the screw thread 107

47. The drawing of hexagonal and square-headed bolts 118

48. Orthographic projection and working drawing 124

49. Of what a set of working drawings is composed 125

50. The development and arrangement of working drawings 128

51. Dimensioning 137

52. Working drawings may violate the rules of orthographic pro-

jection 149

53. Relative value of tentative and exact processes 152

54. Checking drawings 153

55. Conventions in common use in working drawings 154

56. Tracings 158

Chapter V. Machine Sketching

58. Blue print process and reproduction 173

Chapter I. Straight Line Drawing

1. Definition of drawing.

Drawing is the art or science of recording a person's impressions about things by more or less accurate suggestion of form. All drawings may be divided into two general classes. (1). The drawings of objects as viewed at a finite distance. (2). The drawings of objects as viewed at an infinite distance.

The first of these is called perspective. The point of view at a finite distance is called the center of projection. It is as if the eye were at the point and the drawing of the object was made upon a transparent plane placed between the latter and the center of projection, that is, projected upon it from this center by lines from the center passing through all points of the object. It is a kind of drawing that is found in pictures.

In the second class the center of projection or the eye is theoretically moved to an infinite distance, that is, the projecting lines from the object to the plane become parallel. This, in a certain form of drawing, is what is called orthographic projection.

Now things are constructed and manufactured through the aid of drawings that are made according to the principles of the second kind mentioned, or orthographic projection. They may be made free-hand, that is by sketches, or they may be made by careful mechanical drawings; in the more careful and exacting work, they are ahvays made in this way.

A mechanical drawing used for the purpose of construction then, consists of one or more views made according to the principles of orthographic projection; in addition to which the sizes of parts are clearly set forth by dimensions with accompanying dimension lines and arrow heads to show their limits.

No matter how simple is the subject to be constructed, an accurate and comprehensive and unmistakable drawing should be made of it. The test of a good working drawing lies in the fact that the workman can make nothing out of the facts contained thereon than what was intended by the draftsman. The entire meaning should be clear beyond the shadow of a doubt.

To choose the number of views that this may be attained, to put on the dimensions w^hich the workman will need in making the subject is the problem of the draftsman. The needs of the Avorkman should be constantly in his mind.

This a perspective sketch of a form to be made by a workman in wood or metal. From this sketch construct a plan, side view and two end views, and put on them the dimensions shown. The sketch shows just the dimensions required and their place, and also the relative distance from the views at which the dimensions should be placed in the working drawing.

3. General directions with regard to preliminary steps.

Tack the paper by the upper left hand corner, then with the T sq. head against the left hand edge of the board, swing the paper into line with its upper edge. " Next, drawing it tight, put in the tack at the upper right hand corner, and then, stretching from the center out, put in the lower two tacks. Sometimes it is best just to clip the corners of the paper under the thumb tack head, in this case a very small amount of stretching can be effected by heading the point of the tack, as it is pushed in, slightly away from the corner to be tacked.

Drawings have frequently to be taken up from the board and reset again. When this is needed short horizontal lines drawn with the T sq. on each side of the sheet and extending onto the board are convenient for guiding the sheet into the same place it held on the board at first.

The border line for all plates, unless specially arranged for, is to be ^ inch from the edge of sheet.

Hold the T sq. with the hand over the head or by the blade very close to the head. The advantage in the latter method is that by means of the fingers held at the upper and lower edges of the blade, it can be made to creep byshort distances up and down the board.

Keep the T sq. always against the left hand edge of the board and always use, for working, only the upper edge of the blade. Sometimes large T sqs. are made tapering away from the head. When so made, the taper is always upon the under side.

Keep the triangles convenient to the T sq. always, that is, when not using, move to the right along the blade or away from the blade upward, but so that when needed again they can be drawn into place against the T sq. blade with the minimum of effort.

In using the triangle against the T sq. blade, observe the following proceeding. Adjust the T sq. first, with the right hand bring the triangle into place, and hold with the fingers of the left hand while the ball of the same hand holds the T sq. blade in place. The triangle can be thus moved, if desired, along the blade in easy stages _as for section lining, etc.

Rule vertical lines upward against the left hand edge of the triangle placed as just described. Do not draw to the extreme point of a triangle or to the point it touches an adjacent ruling edge against which it may be resting, for the sharpness of the angle cannot be depended upon.

Rule all lines in pencil as shown in Fig. 1. This direction is given now in order that the habit may be formed preparatory to the use of the ruling pen, which requires this treatment.

Remember, as a final direction, that points for locating lines should be as far apart as possible, while lines used in

Ouf-//he-.

locating points should cut each other as nearly as possible at right angles.

The lead pencil may be sharpened in one of two ways, by a long, tapering, round point or by a double-edged chisel. Cut the wood back for at least f of an inch from the end and leave from \ to f of an inch of lead exposed. Taper both down continuously to, if possible, a slightly concave form. The advantage of a tapering point is that it holds its sharpness for a longer time, and again, the point is not thick enough to cover up the work in hand or to mislead as to where the lead is marking.

The double-edged chisel should not be quite as wide across as the lead is thick, but reduced somewhat, say to | the diameter. A penknife for the wood and emery paper or a file for the lead will give the desired results most rapidly.

If the double-edged chisel is used it should only be for straight away lines, not for laying off measurements from the rule or scale. The round pointed form may be used either for picking off measurements from the rule or scale or to draw lines. If a chisel form is used for the lines, a pencil sharpened to a round point should be kept handy for picking off measurements.

In holding the pencil, keep it nearly perpendicular to the paper; if drawing lines with the round point, acquire the habit of slowly twirling the pencil during motion, so that the point will be worn down in a conical shape, and not irregularly.

Clean, firm lines, uniform in thickness and blackness both in penciling and in inking, are the kind which should be cultivated. The draftsman is known quite as well by the character of his penciling and construction as by his final inked drawing, carelessness and irregularity in the former is quite as surely to be repeated in the latter.

Avoid drawing superfluous lines, lines overrunning their proper limits or lines that are not to be inked. If accidental errors are made, correct them with the eraser at once. The neat draftsman is he who does his work with the least use of the pencil and eraser. Lines also which are to be dotted, in final drawing, should be dotted in pencil so that no mistake is made when inking. Wherever possible, locate the extremities of a line by the scale before drawing it, which is facilitated by placing the scale against the ruling edge.

After making an erasure, clean off the particles of dirt that are loose on the paper, for they interfere with the smooth and proper action of the other tools. This is a particularly important direction to observe preparatory to inking and when any alterations are made during inking. Too much care cannot be observed in freeing the paper and all tools from the dirt particles, for they are quite apt to get into the pen and give trouble.

4. Detailed directions for Exercise No. 1.

Draw the plan view first, laying out the center line, scaling the length over all, the width of each end, and then enclosing by lines. Draw the elevation next and then either end. After the views have been drawn put on the dimension lines, limiting lines, and arrow^ heads, lastly the dimension figures. The drawing is then ready to be inked.

5. General directions in regard to inking.

To put ink into a right line pen, hold the pen approximately horizontal with the kind of holding usually given to a wTiting pen. Hold the bottle down with two fingers and with two others lift the cork out and touch the quill end between the nibs of the pen, to let the ink run in, and do this over the bottle, for obvious reasons, corking it securely when through. If the nibs of the pen get ink on the outside, wipe oft' with a rag.

To hold the pen for ruling lines, raise the handle from the previously described position until it rests at or slightly in front of the second joint of the first finger. Have the end of the first finger just above the adjusting screw on the flat part of the nib, the thumb just opposite this, and the second finger touching the pen between the first finger and the thumb and just below them to steady it from any tendency to turn. Hold the pen always perpendicularly to the paper, steadying the hand against the ruling edge with the last finger or the last two.

The weight o'f the pen alone should be sufticient to make the desired line. If it does not, after trying a few times on a piece of trial paper, clean out and refill.

' Do not exert any pressure of the pen against the ruling edge, as it will tend to close the nibs. Make all lines by a continuous motion of the pen. Do not stop on a line to see where the rest of the line is to go. If it is absolutely necessary to stop in any case, even for a moment, lift the pen from the paper. Also, when stopping in this way, take the further precaution to move the ruling edge away from the wet line.

When requiring refilling, clean the pen out thoroughly so that no ink remains either upon the inside or outside of it. It is well to cultivate the habit of doing this early, making it invariable, for a pen clogged with ink is likely to give trouble. Devices for readily opening a pen to facilitate cleaning have been much advertised, and some are quite meritorious. However, they have not yet become indispensible to the experienced draftsman, for the cleaning can, with a little practice, be done very effectively without their aid. Use the nails of the thumb and first fingers successively, covered by one thickness of rag, and it will be found easy to clean one half of a blade with one finger and the other half with the other, and the remaining blade by turning the pen over and repeating the operation. Four wipes generally clean sufficiently and with perhaps no more expenditure of time than by the aid of the opening devices mentioned.

If the pen gives any trouble in marking at any time it is safest to empty, clean and refill.

The weight of an ink line on a drawing should be such that it will show clearly the form within the maze of dimension lines, etc., that it will blue print readily, giving an equally clear impression throughout, and that all annoyance is removed, due to the likelihood that the line will break through any obstructed flow of the pen. In Figure 2 are shown suitable conventions to use on a drawing, as well as the proper weight of line.*

It is highly important in the very beginning to cultivate a uniform line throughout. It takes experience and a trained eye to do this accurately and at the same time unconsciously.

6. Detailed directions for inking Exercise No. 1.

Rule the horizontal outlines of all the views first, beginning at the top and going down. Rule the vertical outlines next, including dotted lines or invisible edges, then the oblique lines. Rule the limiting and dimension lines next, and lastly put in arrow heads and dimension figures in the order named.

A drawing should be cleaned after all inking is finished. A very soft rubber will clean dirt very well, but it is hardly sufficient for superfluous pencil construction lines. Care should be exercised in erasing the latter with a harder eraser on account of injuring the ink lines, for much erasing will spoil the quality of any ink line.

In erasing, hold the paper firmly with one hand while the stroking of the eraser is in the direction away from it, or else hold with the thumb and forefingers of one hand opposed to each other and move the eraser between them, the whole object being to take the strain off the thumb tacks.

*NoTE. — Sometimes in fine careful drawings dotted lines can be made to appear equal in thickness to the solid lines, but to do this they have to be made actually thinner.

Draw the plan view first, laying out the center line^ scaling the length over all, and the width of the view. Next,. lay off the shorter horizontal distances, preferably upon the center line, marking continuously from the scale, then the vertical measurements, either side of the center line at the points marking the horizontal distances. Line in the view^ horizontal lines first and then verticle lines.

Lay out the elevation in the same manner as the plan. Put on the limiting lines, dimension lines, arrow heads and figures in the order named.

In inking, rule the horizontal outlines of all the views, first, beginning at the top and going down. Rule the vertical outlines next, including dotted lines or invisible edges.. Rule the limiting and dimension lines next and lastly put in arrow heads and dimension figures in the order named.

Draw the plan view first, laying out center line, scaling the length over all and the width of each end, then enclosing by lines. Draw the elevation next, and then either end. After this put on the dimension lines, limiting lines, and arrow heads, lastly the dimension figures.

In inking, rule the horizontal lines of all the views first,, beginning at the top and going down. Rule the vertical outlines next, finally the oblique lines. Next, rule the limiting lines and dimension lines, then put in arrow heads and dimension figures.

if f

Notes on Practical Mechanical Drawing Exercise No. 3

Draw the plan view first, laying out the center line, scaling the length over all, and the width of the view. Next, lay off the shorter horizontal distances, preferably upon the center line, marking continuously from the scale, then the vertical measurements, either side of the center line at the point marking the horizontal distances. Line in the view, horizontal lines first and then vertical lines.

Lay out the elevation in the same manner as the plan. Put on limiting lines, arrow heads and figures in the order named.

In inking, rule the horizontal outlines of all the views first, beginning at the top and going down. Rule the vertical outlines next, including dotted lines or invisible edges. Rule the limiting and dimension lines next, and lastly put in arrow heads and dimension figures in the order named.

10. Detailed directions for pencilling and inking Exercise

Draw the plan view first, laying out center line, scaling the length over all and the width of each end, then enclosing by lines. Draw the elevation next, and then either end. After this put on the dimension lines, limiting lines and arrow heads, lastly the dimension figures.

In inking, rule the horizontal lines of all the views first, beginning at the top and going down. Rule the vertical outlines next, finally the oblique lines. Next, rule the dimension and limiting lines, then put in arrow heads and dimension figures.

11. General directions.

Detailed directions were given for laying out and for inking exercises Nos. 1 to 5 inclusive. This was done because -method of execution is important, a certain procedure being economical of time and effort in the drawing and more likely to result in accuracy. It is now time to. discuss this matter a little more minutely.

The preceding exercises show a symmetry upon a center line. It is best to regard this symmetry in the construction of the drawing, that is, to lay out the main center lines of forms first and then measure either side of these center lines, saving a duplication of measurements.

Again, to avoid drawing superfluous lines, it is best to mark first the sizes of areas that are to be enclosed with lines, these sizes will aid in stopping the lines at the right places. Again, always size and enclose the larger areas first, the smaller features are then most likely to be Tight, and they are, moreover, better seen in their proper o

W^i :-l<o

\i places, when the larger things upon which they depend are there with which to compare them. It is a part of the training of the draftsman to acquire the judgment necessary to execute a drawing economically of time and effort. Such details as have been given will hereafter be omitted and only those processes outlined which differ from the preceding.