Scales in drawing

          Usually the word scale is used for an instrument used for drawing straight lines, but in Engineering language scale means '' The proportion  (or) ratio between the linear dimensions adopted for the drawing to the actual dimensions of the object". 

It can be indicated in three different ways.

1) For example a 75mm long pencil may be shown by a drawing of 75mm length, drawings drawn of same size of the objects, are called full-size drawings.

2) For example the actual dimensions of the room say 10m X 8m cannot be adopted on the drawing. In suitable proportion the dimensions should be reduced in order to adopt conveniently on the drawing sheet. If the room is represented by a rectangle of 10cm X 8cm size on the drawing sheet that means the actual size is reduced by 100 times. This is called reduced scale.

3) For example if we draw a small size object to an increased size of our convenience, then this is called enlarging scale.

PRACTICAL HINTS ON DIMENSIONING

 1) Dimension lines should be drawn at least 8 mm away from the outlines and from each other.

2) Dimensions in a series may be placed in any one of the following two ways:

a) Continuous or Chain dimensioning:

Dimensions are arranged in straight line. An overall dimension is placed outside the small dimension. Least important dimensions is generally omitted.

b) Progressive or parallel dimensioning:

All dimensions are calculated from a common base line. Cumulative error is avoided by this method. This method is preferable compared to other.

3) Smaller dimensions should be placed nearer the view and the larger further away that so that further away so that extension lines do not cross dimension lines. Extension lines may cross each other or the outlines.

4) When a number of parallel dimension lines are to be shown near each other, the dimensions should be stagged.

5) Dimension should be placed where the shape is easily identified.

UNIT OF DIMENSION AND GENERAL RULE FOR DIMENSIONING

UNIT OF DIMENSION:

          As far as possible all dimensions should be given in millimeters, omitting the abbreviation mm. Even when it is not convenient to give dimension in millimeters and another unit is used, only the dimension figures are written. But a foot note such as "All dimensions are present in centimeters/meters" is inserted in a prominent place near the title block. The decimal point in a dimension should be quite distinct and written in line with dimension line. A zero must always precedes the decimal point when the dimension is less than unity.

GENERAL RULE FOR DIMENSIONING:

1) Every dimension of a part/segment must be given, but none should be given more than once.

2) Every dimension should be given so completely that further calculation or assumption of any dimension, or direct measurement from the drawing is not necessary.

3) A dimension should be placed on the view where it use is shown more clearly.

4) Dimension should be placed outside the views, unless they are clearer and more easily read inside.

5) Dimensioning between hidden lines and mutual crossing of dimension lines should be avoided. 

6) Dimension lines should not cross any other line of the drawing.

7) An outline or centre line should never be used as a dimension line. A centre line may be extended to serve as an extension line.

8) In general we use two types of dimensional systems (i.e, Unidirectional and aligned), in those Aligned dimensioning system is recommended.

 

 

   

PLACING OF DIMENSIONS

          It is necessary to indicate the size, and other details of the object for the construction and function, using lines, numerals, symbols, notes etc, we indicate the details in a drawing by proper dimensioning. Dimensions are written either above the dimension lines or inserted at the middle by breaking the dimension lines.

Normally dimensions are placed according to either of the following two systems:

1) Aligned System and

2) Unidirectional system

1) Aligned system:

          In Aligned system, the dimension is placed perpendicular to the dimension line in such a way that it may be read from the bottom edge or the right-hand edge of the drawing sheet. The dimension should be placed near the middle and above, clear of the dimension lines.

2) Unidirectional system:

          In unidirectional system, all the dimensions are placed in one direction such that they can be read from the bottom of the drawing. In this system, the dimension lines are broken near the middle for inserting the dimensions. This system is mainly used on large drawings -- as of automobiles etc., where it is inconvenient to read dimensions from the right-hand side.