Introduction to Engineering Drawing
A technical person can use the graphic language as powerful means of communication with others for conveying ideas on technical matters. an engineer should posses good knowledge, not only in preparing a correct drawing but also to read the drawing correctly.
Classification of Drawing
1) Machine Drawing- These drawings are about machine parts.
2) Production Drawing- A production drawing, also referred to as working drawing, should furnish all the dimensions, limits and special finishing processes such as heat treatment, honing, lapping, surface finish etc., to guide the craftsman on the shop floor in producing the component.
3) Part Drawing- Component or part drawing is a detailed drawing of a component to facilitate its manufacture.
4) Assembly Drawing- Detail drawing of assembly.
Subtypes are: Design Assembly Drawing, Detailed Assembly Drawing, Sub- Assembly Drawing, and Installation Assembly Drawing.
5) Machine Shop Drawing: A machine shop drawing frequently gives only the information necessary for machining.
6) Patent Drawing: This drawing is drawn in the patent report with details and assembly of a parts. In this drawing numbers like 1,2,3,4 are given to Sub parts.
Most of engineering Students get confused about scale reduction and enlargement. Scale is the ratio of the linear dimension of an element of an object as represented in the drawing, to the real linear dimension of the same element of the object itself. Wherever possible, it is desirable to make full size drawings, so as to represent true shapes and sizes. If this is not practicable, the largest possible scale should be used. While drawing very small objects, such as watch components and other similar objects, it is advisable to use enlarging scales.
How to Specify a Scale:
The complete designation of a scale should consist of the word Scale, followed by the indication
of its ratio as:
SCALE 1: 1 for full size,
SCALE ×: 1 for enlarged scales,
SCALE 1: × for reduced scales.
The designation of the scale used on the drawing should be shown in the title block.
Recommended Scales: The recommended scales for use on technical drawings are given in Table. The scale and the size of the object in turn, will decide the size of the drawing.
If all drawings are made to the same scale, the scale should be indicated in or near the title block. Where it is necessary to use more than one scale on a drawing, the main scale only should be shown in the title block and all the other scales, adjacent to the item reference number of the part concerned or near the drawings.
Tips for Dimensioning:
1. As far as possible, dimensions should be placed outside the view.
2. Dimensions should be taken from visible outlines rather than from hidden lines.
3. Dimensioning to a centre line should be avoided except when the centre line passes through the centre of a hole.
4. Each feature should be dimensioned once only on a drawing.
5. Dimensions should be placed on the view or section that relates most clearly to the corresponding features.
6. Each drawing should use the same unit for all dimensions, but without showing the unit symbol.
7. No more dimensions than are necessary to define a part should be shown on a drawing.
8. No features of a part should be defined by more than one dimension in any one direction.