engineering problem solvingnitc.ac.in/dept/me/jagadeesha/mev303/chapter_5_1...dimensional tolerances...

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