Engineering Problem Solving · 2011-01-04 · Dimensional Tolerances • The difference between the...
Transcript of Engineering Problem Solving · 2011-01-04 · Dimensional Tolerances • The difference between the...
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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
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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
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It is always advantageous to
select coarse tolerances
without compromising the
functional requirements of
the part
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Indicating maximum and minimum limits of dimension
Limits
0.000.0550φ
+−
05.005.050
+−φ
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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
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Fits
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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
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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
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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
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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
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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