Basics of engineering drawing part 4 v4
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Dimensioning is providing numerical values and graphical indications on technical drawings to specify sizes, distances, and other geometric features of an object. It involves using extension lines, dimension lines, leader lines, arrowheads, and notes. Key rules include placing only one dimension between two extension lines, using consistent units of measurement, avoiding crossed lines, and providing all necessary dimensions once without redundancy. Common symbols denote features like diameters, radii, squares, tapers, and threads to clarify dimensional specifications. Dimensioning is essential for manufacturing objects to the correct specifications.
Basics of engineering drawing part 4 v4
- 1. Basic of Engineering Drawing Part-4 Dimensioning Rules By Prof. A.J.More Department of Engineering Sciences
- 2. 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. Dimensioning
- 3. Example on drawing sheet 50 TITLE BLOCK All dimensions are in mm No dimension unit to be mentioned on the drawing At the bottom sheet mention “All dimensions in ……” DRAWING SHEET MENTION UNIVERSAL DIMENSION HERE
- 4. Dimensioning Providing information on a drawing about Distances (size or functional dimensions) Sizes and positions (location or datum dimensions) of holes, grooves and other features. Details relating to manufacture etc. 15 15 30 Roughness 0.5 Location Location Size, functional Manufacturing detail Hole f15 Size specification, f – symbol for diameter
- 5. 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) Outline Extension line50Dimension line Dimension
- 6. Leader lines and notes Hole f15 Note 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. The arrowhead touches the outline while the dot is placed within the object or on the outline The other end of a leader is terminated in a horizontal line underlining the note Leader line Material: mild steel Roughness 0.5 Leader line
- 7. Rules for leader lines Hole f15 Note Material: mild steel Roughness 0.5 Leader line A leader line is never drawn horizontal, vertical or curved It is drawn at an angle not less than 30o to the line that it touches When pointing to a circle or arc, it is drawn radially > 30o
- 8. 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 ~3mm 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 ~3mm 50 Arrowhead 45o Open Closed Closed and filled Oblique strokew 3w Size of the arrowhead should be proportional to the length of the dimension line ~6-8mm
- 9. 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. Dimensioning elements
- 10. Placing of dimension Within extension lines Outside extension lines With leader 7o
- 11. 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
- 12. 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.
- 13. 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
- 14. 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 f40
- 15. Cylindrical dimensions Cylindrical features should be dimensioned by giving their diameters. They should be dimensioned in the views in which they appear as rectangles. CYLINDER – OUTER DIMENSION IS ALSO CIRCULAR. CAN BE HOLLOW OR SOLID
- 16. Number of holes with same size 3 x f20 (or 3 HOLES, f20) When more than one hole of the same size forms a pattern, it is not necessary to dimension each one One hole is dimensioned and a note specifies the total number of holes with that dimension
- 17. 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 Dimensioning arcs
- 18. O PICTORIAL PRESENTATION IS GIVEN DRAW FV AND TV OF THIS OBJECT BY FIRST ANGLE PROJECTION METHOD AND DIMENSION USING THE UNIDIRECTIONAL METHOD 19 ORTHOGRAPHIC PROJECTIONS FOR T.V. X Y FV O 40 10 10 TV 25 25 30 R 100 1010 f 20 f30
- 19. 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. Spherical features
- 20. Square features (e.g., a rod of square cross-section) are dimensioned using symbol or SQ as shown in (i) or (ii) Dimensioning of squares
- 21. 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
- 22. 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) Dimensioning of threads
- 23. 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. General rules of dimensioning
- 24. Dimensioning, correct vs. wrong 25
- 25. Symbols and abbreviations used in Engg. drawing
- 26. Thank You