MEL 110 Graphic Science Dimensioning Dr R Ribeiro

MEL 110 Graphic Science Dimensioning Dr. R. Ribeiro, Mechanical Engg. Dept. IITD

Dimensioning BIS (Board of Indian Standards SP 46: 2003) defines dimension as a numerical value expressed in appropriate units of measurement and indicated graphically on technical drawings with lines, symbols and notes. Units of Measurement: The most commonly used unit for length is the millimetre. In civil engineering and architectural drawing, inch or foot is often used as a unit of length. Angles are shown in degrees. Symbols are incorporated to indicate specific geometry wherever necessary. Notes are provided to give specification of a particular feature or to give specific information necessary during the manufacturing of the object.

Example on drawing sheet No dimension unit to be mentioned on the drawing At the bottom sheet mention “All dimensions in ……” DRAWING SHEET 50 MENTION UNIVERSAL DIMENSION HERE All dimensions are in mm TITLE BLOCK

Dimensioning Providing information on a drawing about q Distances (size or functional dimensions) q Sizes and positions (location or datum dimensions) of holes, grooves and other features. q Details relating to manufacture etc. Location 15 Manufacturing detail Roughness 0. 5 Location 15 30 Hole f 15 Size, functional Size specification, f – symbol for diameter

Different lines in dimensioning • Oultline - Forms the shape of the object in a view (drawn dark and continuous) • Extension line – An extension of an outline or centerline (drawn light and continuous, used to indicate the entity being dimensioned) • Dimension line – Thin continuous line terminated by arrowheads touching the outlines, extension lines or center lines (used to specify end points of a dimension) Dimension line 50 Outline Dimension Extension line

Leader lines and notes q. Leader (or pointer) line – Thin continuous line connecting a note or dimension figure with the feature to which it applies. One end of the leader terminates in an arrowhead or dot. q. The arrowhead touches the outline while the dot is placed within the object or on the outline q. The other end of a leader is terminated in a horizontal line underlining the note Leader line Roughness 0. 5 Material: mild steel Hole f 15 Note

Rules for leader lines q A leader line is never drawn horizontal, vertical or curved q It is drawn at an angle not less than 30 o to the line that it touches q When pointing to a circle or arc, it is drawn radially Material: mild steel Leader line Roughness 0. 5 > 30 o Hole f 15 Note

Arrowheads and dimension line positioning • • • A dimension line is placed at least 6 -8 mm away from an outline and from each other An extension line extends ~3 mm beyond a dimension line Arrowhead – Placed at each end of a dimension line, its pointed end touches an outline, extension line or a centerline. It is also placed at the end of a leader line 45 o Open Closed w 3 w ~3 mm Oblique stroke Size of the arrowhead should be proportional to the length of the dimension line 50 Arrowhead Closed and filled ~6 -8 mm

Systems of Dimensioning Aligned System In the aligned system, dimensions are aligned with the entity being measured. They are placed perpendicular to the dimension line such that they may be read from the bottom or right-hand side of the drawing sheet. Dimensions are placed at the middle and on top of the dimension lines. Unidirectional System In the unidirectional system, dimensions are placed in such a way that they can be read from the bottom edge of the drawing sheet. Dimensions are inserted by breaking the dimension lines at the middle.

Dimensioning systems Aligned system Unidirectional system (readable from bottom and right edge of sheet) (visible from bottom edge)

Dimensioning elements A line on the drawing whose length is to be shown is called an object line. The object line is essentially an outline representing the feature(s) of the object. While showing an angle, the two lines forming the angle will be the object lines. Dimensioning is often done by a set of elements, which includes extension lines, dimension lines, leader lines, arrowheads and dimensions. These are shown in the figure below.

Placing of dimension Within extension lines Outside extension lines With leader 7 o

Chain (continuous) dimensioning All the dimensions are aligned in such a way that an arrowhead of one dimension touches tip-to-tip the arrowhead of the adjacent dimension. The overall dimension is placed outside the other smaller dimensions

Parallel (progressive) dimensioning All the dimensions are shown from a common reference line. Obviously, all these dimensions share a common extension line. This method is adopted when dimensions have to be established from a particular datum surface Smaller dimensions should always be placed nearer the view. The next smaller dimension should be placed next and so on.

Combined dimensioning When both the methods, i. e. , chain dimensioning and parallel dimensioning are used on the same drawing, the method of dimensioning is called combined dimensioning 15

Circular holes A hole is usually dimensioned by giving its diameter instead of radius. The dimension indicating a diameter should always be preceded by the symbol ø They should be dimensioned in the view in which they appear as circles f 40

Cylindrical dimensions q Cylindrical features should be dimensioned by giving their diameters. q They should be dimensioned in the views in which they appear as rectangles. CYLINDER – OUTER DIMENSION IS ALSO CIRCULAR. CAN BE HOLLOW OR SOLID

Number of holes with same size q When more than one hole of the same size forms a pattern, it is not necessary to dimension each one q One hole is dimensioned and a note specifies the total number of holes with that dimension 3 x f 20 (or 3 HOLES, f 20)

Dimensioning arcs An arc is usually dimensioned by giving its radius. The dimension indicating radius should be preceded by symbol R The R can be placed after the dimension also The position of center of arc is denoted by a

19 FOR T. V. ORTHOGRAPHIC PROJECTIONS f 30 40 FV O X 10 10 Y 10 FO 10 R 25 TV 25 30 R f 20 O F. V. PICTORIAL PRESENTATION IS GIVEN DRAW FV AND TV OF THIS OBJECT BY FIRST ANGLE PROJECTION METHOD AND DIMENSION USING THE UNIDIRECTIONAL METHOD

Spherical features may be dimensioned by giving either the radius or diameter of a sphere. The symbols SR or Sø must precede the dimension for radius or diameter respectively.

Dimensioning of squares Square features (e. g. , a rod of square cross-section) are dimensioned using symbol or SQ as shown in (i) or (ii)

17 ORTHOGRAPHIC PROJECTIONS FRONT VIEW 20 F OR . V. S R O F. V X. 20 FO 10 50 40 L. H. SIDE VIEW SQ 30 Y 75 30 60 F. V. S. V. PICTORIAL PRESENTATION IS GIVEN DRAW FV AND SV OF THIS OBJECT BY FIRST ANGLE PROJECTION METHOD AND DIMENSION THE VIEWS USING THE ALIGNED SYSTEM

f 40 Dimensioning conical features Giving 2 diameters and the perpendicular distance between them Giving 1 diameter, distance between end faces and the taper (slope) using the conical taper symbol Giving 1 of the diameters, length of taper and taper angle

Dimensioning of flat tapered features Giving the height of one side, distance between flat ends and taper (slope) using a flat taper symbol Giving the height of one side, length of taper and slope of the tapered face

Dimensioning of threads 1. External metric threads are dimensioned by giving the threaded length and nominal diameter preceded by symbol ‘M ’ (Fig. a) 2. Internal metric threads are dimensioned by giving the threaded length, depth of drilled hole before threading and nominal diameter preceded by symbol ‘M’ (Fig. b)

General rules of dimensioning 1. Between any two extension lines, there must be one and only one dimension line bearing one dimension. One of the extension lines may be common to another dimension as in parallel dimensioning. 2. As far as possible, all the dimensions should be placed outside the views. Inside dimensions are preferred only if they are clearer and more easily readable. 3. All the dimensions on a drawing must be shown using either Aligned System or Unidirectional System. The two systems should not be mixed on the same drawing. 4. The same unit of length should be used for all the dimensions on a drawing. The unit should not be written after each dimension, but a note mentioning the unit should be placed below the drawing. 5. Dimension lines should not cross each other. Dimension lines should also not cross any other lines of the object. 6. All dimensions must be given. 7. Each dimension should be given only once. No dimension should be redundant.

25 Dimensioning, correct vs. wrong

Symbols and abbreviations used in Engg. drawing

Draw 3 orthographic views in 3 rd. Angle and dimension them using UNIDIRECTIONAL system 22 TOP VIEW f 32 LEFT SIDE VIEW f 12, 16 DEEP 24 38 57 R 22 36 19 6 38 22 24 22 FRONT VIEW 30

Draw Front and R. S. views in IIIrd. angle with dimensions in ALIGNED system Complete TV and dimension it for HW

Draw Front and R. S. views in IIIrd. angle with dimensions in ALIGNED system Complete TV and dimension it for HW 35 f 28 f 18 26 f 9 f 15 19 Project tangent point in other view

PICTORIAL PRESENTATION IS GIVEN DRAW FV AND TV OF THIS OBJECT BY FIRST ANGLE PROJECTION METHOD ORTHOGRAPHIC PROJECTIONS 20 f 30 FOR T. V. Dimension using UNIDIRECTIONAL system FV RECT. SLOT 50 35 10 X FO R F. V . O 10 30 f 60 f TV Y

ORTHOGRAPHIC PROJECTIONS 10 25 15 25 40 10 25 R FO FO . V. S R O 25 F. V O. 80 25 S. V. F. V. PICTORIAL PRESENTATION IS GIVEN DRAW FV AND SV OF THIS OBJECT BY FIRST ANGLE PROJECTION METHOD Dimension using UNIDIRECTIONAL system