A drawing of a component, in addition to providing complete shape description, must also furnish information regarding the size description. These are provided through the distances between the surfaces, location of holes, nature of surface finsih, type of material, etc. The expression of these features on a drawing, using lines, symbols, figures and notes is called dimensioning.
Dimension is a numerical value expressed in appropriate units of measurment and indicated on drawings, using lines, symbols, notes, etc., so that all features are completely defined.
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.
Method of Execution
The elements of dimensioning include the projection line, dimension line, leader line, dimension line termination, the origin indication and the dimension itself. The various elements of dimensioning are shown below. The following are some of the principles to be adopted during execution of dimensioning:
1. Projection and dimension lines should be drawn as thin continuous lines.
2. Projection lines should extend slightly beyond the respective dimension lines.
3. Projection lines should be drawn perpendicular to the feature being dimensioned. Where necessary, they may be drawn obliquely, but parallel to each other (Fig. a). However, they must be in contact with the feature.
4. Projection lines and dimension lines should not cross each other, unless it is unavoidable (Fig. b).
5. A dimension line should be shown unbroken, even where the feature to which it refers, is shown broken (Fig. c).
6. A centre line or the outline of a part should not be used as a dimension line, but may be used in place of projection line (Fig. b).
Dimension lines should show distinct termination, in the form of arrow heads or oblique strokes or where applicable, an origin indication. Two dimension line terminations and an origin indications are shown in this figure. In this,
- the arrow head is drawn as short lines, having an included angle of 15°, which is closed and filled-in.
- the oblique stroke is drawn as a short line, inclined at 45°.
- the origin indication is drawn as a small open circle of approximately 3 mm in diameter.
The size of the terminations should be proportionate to the size of the drawing on which they are used. Where space is limited, arrow head termination may be shown outside the intended limits of the dimension line that is extended for that purpose. In certain other cases, an oblique stroke or a dot may be substituted.
Where a radius is dimensioned, only one arrow head termination, with its point on the arc end of the dimension line, should be used. However, the arrow head termination may be either on the inside or outside of the feature outline, depending upon the size of feature.
Methods of Indicating Dimensions
Dimensions should be shown on drawings in characters of sufficient size, to ensure complete legibility. They should be placed in such a way that they are not crossed or separated by any other line on the drawing. Dimensions should be indicated on a drawing, according to one of the following two methods. However, only one method should be used on any one drawing.
Dimensions should be placed parallel to their dimension lines and preferably near the middle, above and clear-off the dimension line. An exception may be made where super- imposed running dimensions are used.
Dimensions may be written so that they can be read from the bottom or from the right side of the drawing. Dimensions on oblique dimension lines should be oriented as shown in Fig. “Oblique dimensioning”. Angular dimensions may be oriented as shown in Fig. “Angular dimensioning”.
Dimensons should be indicated so that they can be read from the bottom of the drawing only. Non-horizontal dimension lines are interrupted, preferably near the middle, for insertion of the dimension.
Angular dimensions may be oriented as in Fig. “Angular dimensioning”.
Dimensions can be, (i) above the extension of the dimension line, beyond one of the terminations, where space is limited or (ii) at the end of a leader line, which teminates on a dimension line, that is too short to permit normal dimension placement or (iii) above a horizontal extension of a dimension line, where space does not allow placement at the interruption of a non-horizontal dimension line. Values of dimensions, out of scale (except where break lines are used) should be underlined as shown in this figure.
The following indications (symbols) are used with dimensions to reveal the shape identification and to improve drawing interpretation. The symbol should precede the dimensions.
Arrangement of Dimensions
The arrangement of dimensions on a drawing must indicate clearly the design purpose. The following are the ways of arranging the dimensions.
- Chain Dimensions – Chains of single dimensions should be used only where the possible accumulation of tolerances does not endanger the functional requirement of the part.
- Parallel Dimensions – In parallel dimensoning, a number of dimension lines, parallel to one another and spaced-out are used. This method is used where a number of dimensions have a common datum feature
- Super-imposed Running Dimensions – These are simplified parallel dimensons and may be used where there are space limitations.
- Combined Dimensions – These are the result of simultaneous use of chain and parallel dimensions.
- Co-ordinate Dimensions – The sizes of the holes and their co-ordinates may be indicated directly on the drawing; or they may be conveniently presented in a tabular form.
- Diameters – Diameters should be dimensioned on the most appropriate view to ensure clarity. The dimension value should be preceded by Ø. Below shown the method of dimensioning diameters.
- Chords, Arcs, Angles and Radii – The dimensioning of chords, arcs and angles should be as shown in figure “Dimensioning of chords, arcs and angles. Where the centre of an arc falls outside the limits of the space available, the dimension line of the radius should be broken or interrupted according to whether or not it is necessary to locate the centre.
Where the size of the radius can be derived from other dimensions, it may be indicated by a radius arrow and the symbol R, without an indication of the value.
- Equi-distant Features – Linear spacings with equi-distant features may be dimensioned as shown below.
- Chamfers and Countersunks – Chamfers may be dimensioned and countersunks as shown below.
- Screw Threads – Screw threads are always specified with proper designation. The nominal diameter is preceded by the letter M. The useful length of the threaded portion only should be dimenioned. While dimensioning the internal threads, the length of the drilled hole should also be dimensioned.
- Tapered Features – Tapered features are dimensioned, either by specifying the diameters at either end and the length, or the length, one of the diameters and the taper or the taper angle (Conical taper). A slope or flat taper is defined as the rise per unit length and is dimensioned by the ratio of the difference between the heights to its length (Flat taper).
- Notes – Notes should always be written horizontally in capital letters and begin above the leader line and may end below also. Further, notes should be brief and clear and the wording should be standard in form. The standard forms of notes and the method of indication, for typical cases is shown in below figure. The meaning of the notes is given in table below.
|1.||DIA 25 DEEP 25||Drill a hole of diameter 25 mm, to a depth of 25 mm.|
|2.||DIA 10 CSK DIA 15||Drill a through hole of diameter 10 mm and countersink to get 15 mm on top.|
|3.||4 HOLES, DIA 12 C BORE DIA 15 DEEP 8||Dirll through hole of f 12 mm, counterbore to a depth of 8 mm, with a f 15 mm, the number of such holes being four.|
|4.||6 HOLES, EQUI–SP DIA 17 C BORE FOR M 16 SOCKET HD CAP SCR||Drill a through hole of f 17 and counterbore to insert a socket headed cap screw of M 16. Six holes are to be made equi-spaced on the circle.|
|5.||KEYWAY, WIDE 6 DEEP 3||Cut a key way of 6 mm wide and 3 mm depth.|
|6.||KEY SEAT, WIDE 10 DEEP 10||Cut a key seat of 10 mm wide and 10 mm deep to the length shown.|
|7.||U/C, WIDE 6 DEEP 3||Machine an undercut of width 6 mm and dpeth 3 mm.|
|8.||(a) DIAMOND KNURL 1 RAISED 30°
|Make a diamond knurl with 1 mm pitch and end chamfer of 30°.|
|(b) M 18 × 1||Cut a metric thread of nominal diameter 18 mm and pitch 1 mm.|
|9.||(a) THD RELIEF, DIA 20 WIDE 3.5
|Cut a relief for thread with a diameter of 20.8 mm and width3.5 mm.|
|(b) NECK, WIDE 3 DEEP 1.5||Turn an undercut of 3 mm width and 1.5 mm depth
|(c) CARB AND HDN||Carburise and harden.|
|10.||(a) CARB, HDN AND GND||Carburise, harden and grind.|
|(b) MORSE TAPER 2||Morse taper No. 1 to be obtained.|
|11.||DIA 6 REAM FOR TAPER PIN||Drill and ream with taper reamer for a diameter of 6 mm to suit the pin specified.|
|12.||6 ACME THD||Cut an ACME thread of pitch 6 mm.|