Engineering drawing is a language which is understood throughout the world by engineers and fabricators. Other languages may fail to describe the size, shape, physical aspects, inner details, finish, etc., but the engineers’ language known as ‘Engineering Drawing’ never fails. The most intricate assemblies with their various complicated parts can be easily represented by engineering graphics. There are various types of engineering drawings and all have one simple purpose, i.e. the communication of ideas to others. The drawings used for communicating the ideas of the design engineer to the production engineer and technicians is known as ‘Workshop Drawing’.
Engineering Drawing – A drawing Prepared by an engineer, for an engineering purpose is known as an engineering drawing. It is the graphic representation of physical objects and their relationship. It is prepared, based on certain basic principles, symbolic representations, standard conventions, notations, etc. It is the only universal means of communication used by engineers and technicians.
Pictorial Drawing – Every person cannot understand the orthographic projection. Its execution requires a thorough understanding of the principles of projection and its reading requires a good practice of constructive imagination. We can describe the shape of a job by means of pictorial drawing also, which can be understood quite easily. Pictorial drawing is the drawing of a picture in graphic language of engineers, to represent a real thing by means of picture views. It shows the appearance of the object by one view only. Following three methods of pictorial projections are commonly used in engineering drawing:
(a) Isometric projection
(b) Oblique projection
(c) Perspective projection
A trained eye and good imagination will be able to understand the three dimensions of an object. Several orthographic views on different planes are to be drawn to understand fully an object. But in isometric projection, only one view on a plane is sufficient to represent an object in its realistic appearance. Anyone can understand by looking at a view what the job is by isometric projection.
Isometric projection is a type of pictorial projection. Isometric means equal measure. In this isometric projection, all the plane surfaces and the edges formed of these plane surfaces should be equally inclined to the metric plane. Metric plane is the same horizontal plane which is used in orthographic projection.
To represent the three dimensions (length, breadth and height) of the object, there are three axes known as ISOMETRIC AXES. To start an isometric drawing, a reference line (horizontal line) and the three axes (X, Y Z) are drawn by taking an angle of 30° from the reference horizontal line as drawn in Fig 7.2. Z axis is a vertical line to the horizontal line drawn from intersection point of X and Y axes.
Oblique projection may be illustrated in different ways, according to the choice of axes, length of inclined side and direction of looking the side. As compared with isometric projection, in oblique projection, one side of the object is horizontal, second side is vertical and the third side is inclined at 30Oor 45O to the horizontal. The lengths of the horizontal and vertical sides are equal to the actual lengths, but the length of the inclined side is taken as three-fourth or half of the actual length.
In oblique projection, an object is placed with its front face parallel to a vertical plane of projection and the visual rays parallel to each other pierce the plane of projection obliquely (Oblique means inclined). The projection represents the front face of the object in its true shape and size. The rest of the object is not projected true in its shape and size. Both the isometric and oblique projections are the methods of representing the object pictorially. But the oblique projection is preferable to the isometric projection in representing the objects of circular shapes, because the front face is in actual shape and size. It is not distorted.
Perspective projection or perspective drawing is the representation of an object on a plane surface, called the picture plane, as it would appear to the eye, when viewed from a fixed position. It may also be defined as the figure formed on the picture plane when visual rays from the eye to the object cut the picture plane. Perspective is mainly used in architecture. By means of perspective, the architecture is able to show how an object would appear when constructed.
It is not always possible to visualize the object merely by looking at the drawing. It requires the help of certain rules and convention of engineering drawing. It can be charts, maps, lay-outs, aerial photographs and orthographic projections.
The purpose of a workshop drawing is to convey the instructions of the designer to the mechanic concerned, in such a manner that the work to be done can be completed with accuracy and as rapidly as possible. The drawing will show all necessary measurements, which are expressed clearly without the need for any calculation. The limits of accuracy or the class of fit and the raw materials from which the parts are to be made, are also indicated.
The drawing may not be of actual size, but for convenience may be made larger or smaller than the actual job, i.e. it will be drawn to scale. The scale is indicated in the title block. Owing to usage and the possibility of being torn, it is undesirable that the original drawing should be used in the workshop. Instead, a number of prints are reproduced.
Types of Workshop Drawing
There are three types of workshop drawing:
(a) Detail Drawing. This type of drawing provides all necessary dimensions and working instructions for the production of an item, e.g. one part of a hydraulic component.
(b) Assembly Drawing. This type of drawing shows the component or assembly in an assembled state, e.g. the assembly of the various parts of a hydraulic component.
(c) General Arrangement Drawing. This type of drawing shows a number of components in an assembled condition to form a self- contained unit, e.g. the complete hydraulic system of an aircraft.
Requirement of a Good Workshop Drawing
Workshop drawings are intended to convey the requirements of thedesigner to the tradesman in such a way that the intended work can be carried out accurately and rapidly. To facilitate its reading, a good workshop drawing should satisfy the following requirements:
(a) It should show all the necessary measurements without superfluous data or repetition.
(b) It should not entail any kind of calculations.
(c) Clearly indicate the raw material from which parts are to be made.
(d) Clearly indicate the limit of accuracy or class of fit as applicable.
(e) Provide a key to machining and other symbols.
(f) Any other information not provided in the above clauses that may be required for satisfactory completion of work.
The use of properly composed notes often adds clarity to the presentation of dimensional information involving specific operations. Notes are also used to convey supplementary instruction about the kind of material, kind of fit, degree of finish, etc. It is a good practice to specify information representing a specific tool operation or a series of tool operations by notes rather than by figured dimensions. Brevity in form is desirable for notes of general information or specific instructions.