By Ananta 
What is dimensioning in Engineering drawing?
Dimensions in engineering drawings are numerical values indicated graphically in a proper unit of measurement on engineering drawing with lines, symbols, and notes.
These are indicated on the engineering drawing to define the size characteristics such as length, height, breadth, diameter, radius, angle, etc.
The Bureau of Indian Standards (BIS) recommends the general principle of dimensioning in engineering drawing in its bulletin IS 11669:1986 ( Reaffirmed 1999).
Importance of dimensioning in engineering drawing
To provide a clear and complete description of the features of an object, proper dimensioning in engineering drawings is needed.
Types of Dimensioning in engineering drawings:
Dimensions are classified into the following types:
- Functional dimensions.
- Non-functional dimensions.
- Auxiliary dimensions.
Functional dimensions(F)
This is the dimensioning in engineering drawing that is essential to the function of the piece or space.
Non-functional dimensions(NF)
This is the dimensioning in engineering drawing that is not essential to the function of the piece or space.
Auxiliary dimensions (AUX)
The dimensioning in engineering drawing which is given for information purpose only. It is derived from other values shown on the drawing or in related documents.
An auxiliary dimension is given in parentless and no tolerance applied to it.
Dimensioning Terminology
The various terminologies related to the dimensioning in engineering drawing are as follows:
Dimension value
This is a numerical value that is assigned to the size, shape, or location of the feature being dimensioned. Dimensions are expressed in specific units on the drawing with relevant information.
Dimension Lines
Dimension lines are the thin continuous lines that show the extent and direction of the dimension. These lines should be placed 8mm to 10 mm away from the outline of the drawing and should be placed uniformly 6mm to 8mm from each other. The dimension values are preferably placed near the middle of the dimension lines.
Projection Lines
Projection lines are thin continuous lines that are stretched out from the outlines for dimensioning and extended 2mm to 3mm beyond the dimension lines. These should be drawn in a direction perpendicular to the feature to be dimensioned. The dimension lines and projection lines should not intersect other lines unless this is unavoidable.
But in the case of special circumstances, the projection lines may be drawn in an oblique way, but parallel to each other.
Leaders
Leaders are thin continuous lines and terminated by an arrowhead or dot referring to a feature and note. Figure and notes are written above the extended dimension line.
To avoid confusion leaders should not be –
- inclined at an angle less than 30°
- parallel to adjacent projection lines.
- parallel or adjacent dimensions.
Leaders are never drawn horizontal, vertical, curved, or freehand. They are generally drawn at any convenient angle 30°,45°, and 60°. Long leaders are generally avoided.
Arrowheads
The termination of dimension lines is indicated by arrowheads. The arrowheads may be open at a convenient angle of 30° to 90°, closed filled, or close blank. The closed filled arrowheads have a length of about three times the width/ depth and are generally preferred in the engineering drawing.
Generally, the length of the arrowhead for small drawing is 3 mm and for large drawing the length of the arrowhead is 5 mm.
When the space to accommodate the termination of arrowheads is insufficient, oblique stroke and points may be used.
Methods of Dimensioning in Engineering drawing:
The dimensions are done in a drawing as per one of the following methods as recommended in SP 46-2003.
SP 46 (2003): Engineering Drawing Practice for Schools and Colleges.
The two recommended system of placing the dimensions are as follows:
Aligned system of dimensioning in engineering drawing:
In this method of dimensioning, the dimension text is placed parallel to the dimension line and above it preferably in the middle.
The dimension texts are placed in such a way that they can be read either from the bottom or from the right-hand side of the drawing.
Linear dimensioning
Angular dimensioning
Unidirectional system of dimensioning in Engineering drawing:
In the unidirectional method of dimensioning, the dimensions are indicated so that they can be placed in the horizontal direction only.
Linear dimensioning
Angular dimensioning
Arrangements of Dimensions
When several dimensions are to be placed in an engineering drawing, those dimensions are to be arranged in such a way that it provides a unique explanation.
The classification of dimensioning on the basis of arrangement is as follows.
Chain dimensioning or Continuous dimensioning
In this method, successive dimensions are arranged in a continuous straight line.
Parallel dimensioning or progressive dimensioning
In this method, a number of single dimensions parallel to one another are placed from a common origin. This method is adopted where a number of dimensions have a common origin. The cumulative error can be avoided by following this method.
Running dimensioning or Superimposed dimensioning
In this method of dimensioning, all the dimensions begin from a common origin, which is indicated by a small circle of 3mm diameter, and terminated with arrowheads where individual dimension ends.
The dimension texts are rotated through 90° and placed in-line with the expansion line or above the dimension line near the arrowhead.
Combined dimensioning
In this arrangement of dimensions, there is the simultaneous use of parallel dimensioning, chain dimensioning, and running dimensioning in a single drawing.
Coordinate dimensioning
Whenever a part is having too many dimensions, it is easier to read the drawing by the coordinate dimensioning method.
Example: A plate with many many holes can be easily read using the coordinate dimensioning method. Other styles of dimensioning will provide an overly cluttered result in this case.
In this system, origin and other features are to be numerically leveled. A coordinate table contains all the dimensional details to be placed near the drawing.
Rules of Dimensioning in Engineering Drawing
To follow a set of standard dimensioning rules is very crucial as the dimensions are used to define the size characteristics of a drawn object such as length, height, breadth, radius, diameter, angle, etc with lines, notes, and symbols.
The Bureau of Indian Standards (BIS) suggests the rules of dimensioning in engineering drawing in its bulletin IS 11669:1986 ( Reaffirmed 1999).
IS 11669: 1986 (Reaffirmed 1999): General Principles of Dimensioning on Technical Drawings
Dimensioning Rules:
Following dimension rules should be observed while dimensioning:
- Mark the dimension outside the figure or view. However, in the case of Dimensioning the radius of an arc or circle, it may be shown inside.
2. Dimensioning rules for Circle: A circle has to dimensioned by its diameter symbol Ø. The symbol Ø has to place before the dimension value.
An arc has to dimensioned by its Radius symbol R.
3. Dimensioning rules for Centerline:
i. The axis or centerline shall not be used as a dimension line with arrowheads at its ends.
ii. The axes or center lines must extend approximately 3 mm beyond the boundary of the part whose details they indicate.
iii. Marking a dimension from a centerline is incorrect, except when the centerline passes through the center of a hole.
Also, an extended centerline may be served as an extension line.
4. Dimensioning rules for Holes:
The center-to-center distance between the holes has to dimension in the view in which the holes are visible.
5. Dimensioning shall be done to the visible lines and not to the invisible or hidden lines.
6.The dimension value must be placed approximately 2 mm above the dimension line.
7.The extension line must extend 2 mm beyond the dimension line.
8.Dimensioning rules for Conical taper section:
The conical taper on diameter must be dimensioned as shown.
Conical Taper = (D-d)/L= 1 in n
9.Dimensioning rules for Hatched portion:
When dimensioning within a hatched portion of a drawing, the hatching line should not cut the dimension text.
10.Dimensioning rules for Repeated features:
As far as possible dimensioning of repeated features of the same size is avoided.
The notes have to be written horizontally. The leader lines have to be inclined at an angle of 30°,45°, and 60° to the horizontal.
11.Overall dimensioning rules:
The overall dimensions have to be placed outside the intermediate dimensions.
The smaller dimensions must be placed nearer the view and the larger dimensions further away so that the extension lines or projection lines do not cross the dimension lines.
12. Dimension rules for a broken feature:
The dimension line should be shown unbroken where the figure to which it represents is shown broken.
13. When a dimension line cannot be completely drawn to its normal termination point, the free end must be terminated in a double arrowhead.
14. As far as possible, all the dimensions in a particular drawing should be in one unit only.
15.Dimensions indicated in one view need not be repeated in another view, except for the case of clarity and identification.
16. The dimensions must be placed on the view that most clearly shows the corresponding features.
17. The drawing line should never be used as a dimension line or it should not coincide with a dimension line. The dimension line should be uniformly spaced throughout the drawing. Dimension lines should be 8mm to 10 mm from the outline of the drawing and 6mm to 8mm from each other.
18. The dimension text should be placed preferably near the middle. If unavoidable due to lack of space, the dimension text may be placed above the extended portion of the dimension line beyond the arrowheads, preferably on the right-hand side.