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Production Drawing
• Dimensioned drawing which describes a machine part completely, with the details of
– Quantity to be produced– Raw material type, kind or size– Dimensional tolerances – Geometrical tolerances
– Surface roughness or surface quality
– Manufacturing methods
– Applicable national standards– Special instructions
Geometrical and Dimensional Tolerances
• It is impossible to manufacture parts to the exact designed shape and size
• It should be possible for parts to be ‘assembled’ rather than ‘fitted’
• It should be possible to replace a defective component in the assembly, with a similar part
• Mass production and interchangeability• Parts should be produced with their shape and
size varying within certain ranges
It is always advantageous to
select coarse tolerances
without compromising the
functional requirements of
the part
Indicating maximum and minimum limits of dimension
Limits
0.000.0550φ
+−
05.005.050
+−φ
Dimensional Tolerances• The difference between the maximum and minimum limits of
dimension is known as tolerance
Terminology• Basic dimension (or nominal dimension)
– Dimension of the machine part obtained by design calculations• Upper deviation
– Algebraic difference between the maximum Limit and basic size• Lower deviation
– Algebraic difference between the minimum limit and basic size• Tolerance zone
– Algebraic difference between the maximum Limit and minimum limit
• Ex: For a basic size of 25mm, if the maximum limit is 25.05mm and minimum limit is 24.95mm,
Upper deviation = 25.05 – 25 = 0.05mmLower deviation = 24.95 – 25 = -0.05mm
Tolerance zone = Max limit – Min limit = Upper deviation - Lower deviation = 0.10mm
Fits
Geometrical Tolerances
• Indicates the conditions of – Straightness
– Flatness
– Parallelism
– Perpendicularity– Angularity – Symmetry – Concentricity
• Geometrical variations are sub-classified as– Form or shape variations– Position variations
Form Tolerances
• Deviations in flat surfaces– Tolerance of straightness
• Determined by the distance between two parallel lines in plane P right angle with the tested surface
– Tolerance of flatness• Expressed as the distance between two planes M and N
which enclose the tested surface between them
Form Tolerances• Deviations in cylindrical and conical surfaces
– In axial section
• Barrel form, bow form and Curvature of axes– Determined by the difference between the diameters of cross
sections at the midpoint and ends
• Taper– Ratio of the difference in diameters of two cross sections at
right angle to the axis and axial distance between them
Form Tolerances
• Deviations in cylindrical and conical surfaces– In transverse section
• Oval form– Decided by the difference between the major and minor axes
of the elliptical shape
• Lobed form– Cross section contour is made of arcs drawn from different
centres
Position Tolerances• Deviations in relative locations of surfaces
– Radial run out• Maximum difference of radial distance between two concentric
circles measured in the right angle plane to the axis of the work piece for one complete revolution
• This is due to misalignment of axes, ovality, lobed form or bending of axes
– Axial run out• Maximum deviation of end surface from the right angle plane
with the axis of the part