Mechanical Drafting

Mechanical Drafting

REVISED IN 1915

By

THE DEPARTMENT OF GENERAL ENGINEERING DRAWING

H. W. MILLER, M.E.

R. K. STEWARD, C.E.

F. M. PORTER, M.S.

H. H. JORDAN, B.S.

H. O. RUGG, C.E.

R. CRANE, S.B.

C. A. ATWELL, B.S.

In the University of Illinois Urbana, Illinois

Copyright © 2013 Read Books Ltd.

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British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library

Technical Drawing and Drafting

Technical drawing, also known as 'drafting' or 'draughting', is the act and discipline of composing plans that visually communicate how something functions or is to be constructed.

It is essential for communicating ideas in industry, architecture and engineering. The need for precise communication in the preparation of a functional document distinguishes technical drawing from the expressive drawing of the visual arts. Whereas artistic drawings are subjectively interpreted, with multiply determined meanings, technical drawings generally have only one intended meaning. To make the drawings easier to understand, practitioners use familiar symbols, perspectives, units of measurement, notation systems, visual styles, and page layout. Together, such conventions constitute a visual language, and help to ensure that the drawing is unambiguous and relatively easy to understand.

There are many methods of constructing a technical drawing, and most simple among them is a sketch. A sketch is a quickly executed, freehand drawing that is not intended as a finished work. In general, sketching is a quick way to record an idea for later use, and architects sketches in particular (in a very similar manner to fine artists) serve as a way to try out different ideas and establish a composition before undertaking more finished work. Architects drawings can also be used to convince clients of the merits of a design, to enable a building constructer to use them, and as a record of completed work. In a similar manner to engineering (and all other technical drawings), there is a set of conventions (i.e particular views, measurements, scales, and cross-referencing) that are utilised.

As opposed to free-sketching, technical drawings usually utilise various manuals and instruments. The basic drafting procedure is to place a piece of paper (or other material) on a smooth surface with right-angle corners and straight sides – typically a drawing board. A sliding straightedge known as a 'T-square' is then placed on one of the sides, allowing it to be slid across the side of the table, and over the surface of the paper. Parallel lines can be drawn simply by moving the T-square and running a pencil along the edge, as well as holding devices such as set squares or triangles. Other tools can be used to draw curves and circles, and primary among these are the compasses, used for drawing simple arcs and circles. Drafting templates are also utilised in cases where the drafter has to create recurring objects in a drawing – a massive time-saving development.

This basic drafting system requires an accurate table and constant attention to the positioning of the tools. A common error is to allow the triangles to push the top of the T-square down slightly, thereby throwing off all the angles. Even tasks as simple as drawing two angled lines meeting at a point require a number of moves of the T-square and triangles, and in general drafting this can be a time consuming process. In addition to the mastery of the mechanics of drawing lines, arcs, circles (and text) onto a piece of paper – the drafting effort requires a thorough understanding of geometry, trigonometry and spatial comprehension. In all cases, it demands precision and accuracy, and attention to detail.

Conventionally, drawings were made in ink on paper or a similar material, and any copies required had to be laboriously made by hand. The twentieth century saw a shift to drawing on tracing paper, so that mechanical copies could be run off efficiently. This was a substantial development in the drafting process – only eclipsed in the twenty-first century with 'computer-aided-drawing' systems (CAD). Although classical draftsmen and women are still in high demand, the mechanics of the drafting task have largely been automated and accelerated through the use of such systems. The development of the computer had a major impact on the methods used to design and create technical drawings, making manual drawing almost obsolete, and opening up new possibilities of form using organic shapes and complex geometry.

Today, there are two types of computer-aided design systems used for the production of technical drawings; two dimensions ('2D') and three dimensions ('3D'). 2D CAD systems such as AutoCAD or MicroStation have largely replaced the paper drawing discipline. Lines, circles, arcs and curves are all created within the software. It is down to the technical drawing skill of the user to produce the drawing – though this method does allow for the making of numerous revisions, and modifications of original designs. 3D CAD systems such as Autodesk Inventor or SolidWorks first produce the geometry of the part, and the technical drawing comes from user defined views of the part. This means there is little scope for error once the parameters have been set. Buildings, Aircraft, ships and cars are now all modelled, assembled and checked in 3D before technical drawings are released for manufacture.

Technical drawing is a skill that is essential for so many industries and endeavours, allowing complex ideas and designs to become reality. It is hoped that the current reader enjoys this book on the subject.

PREFACE

In writing the original edition of this text it seemed wise to the author to base its arrangement and content upon two principles which considerable experience proved sound. These principles are: first, that the student can just as well and perhaps better, be taught the use of instruments on work that will at the same time have educational value; second, that for greatest efficiency in teaching drawing the text should be made so complete and follow the class room work so closely that lecturing is unnecessary.

The above principles were followed by first designing a very flexible course in drafting, substituting drawings of machine parts for the conventional geometrical figures. The work was arranged into definite groups, according to subject, each group being scheduled for a definite amount of time. Second, the text was so arranged that section, lesson or chapter one, gave all information necessary for the work included in group one, etc.

After three years’ very satisfactory trial of the text, the department of drawing has undertaken a complete revision with the desire that the work shall be not only a text, more complete than the first, but also a book of reference that will be of service after the student has completed the course.

H. W. MILLER

October, 1915.

CONTENTS

Chapter 1. Lettering, Freehand and Mechanical

Chapter 2. Use of Instruments

Chapter 3. Orthographic Projection

Chapter 4. Working Drawings

Chapter 5. Fasteners, Threads, Bolts and Nuts, etc.

Chapter 6. Shop Terms, Tools, Machines, etc.

Chapter 7. Isometric and Oblique Projection

Chapter 8. Machine Sketching

Chapter 9. Perspective

Appendix

MECHANICAL DRAFTING

CHAPTER 1

LETTERING

FREEHAND

(1) Freehand or offhand lettering is so much a part of every engineer’s daily routine that to be unable to letter with speed and grace is considered an inexcusable discredit. The results of practice show that no one need be embarrassed long because of the lack of this skill, for anyone can learn to letter. However, the acquisition of proficiency demands what skill in any manual performance requires,—more or less experience and careful study of principles.

It is fortunate for the beginner in lettering that there are very few elements that must be mastered. Most engineers use extremely simplified styles of freehand letters. The Reinhardt alphabet (slant or vertical) is especially noted for its simplicity as it has been stripped of all superfluous appendages that made formed styles both complicated and time-consuming in their use. In the practice of either type the beginner will find that all of the letters are made up of but two or three characteristic elements or strokes, each of which is easily constructed.

The first style or type presented is the Reinhardt slant. It should be mastered thoroly because it is in use in most drafting rooms and colleges. It is probable that its use in over eighty per cent of the large drafting rooms of the country is due to its legibility and ease and rapidity of construction.

EQUIPMENT

(2) Selection of Lettering Pens. For a pure type of either Reinhardt or vertical letter, such a pen should be used as will give a stroke of uniform width, weight, or heaviness, when it is moved up, down, right, left, or diagonally on the paper; otherwise the letter will have a shading, which does not belong to the types mentioned. Some pens which have proven satisfactory for letters of a uniform weight are the Sheppard lettering pen, Paysant pen, Moore’s Non-Leaking fountain pen, and any of the steel points known as ball pointed pens.

Fig. 1

Inking of Pens. Apparently a great part of the trouble experienced in the use of the above pens is due to improper inking. No lettering pen should ever be dipped into the ink bottle. The proper method is to transfer by means of the quill attached to the cork a small amount of ink to the inside of the ball pointed pen or between the nibs of the Sheppard or Paysant pens. It is well to hold the pen point over the bottle so that any superfluous ink may not be spilled on the drawing or desk. The word of warning