Geometric dimensioning and tolerancing

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STRAIGHTNESS OF LINES & SURFACES
Training By
Kishor Bhatia

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STRAIGHTNESS OF LINES & SURFACES
STRAIGHTNESS SYMBOL

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DEFINITION:
Straightness is a condition where an element of a surface or
an edge is in a straight line.
TYPICAL USE:
Typically used to control the form of cylindrical or conical
surfaces. The straightness requirement applies to the entire
surface.
STRAIGHTNESS

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Straightness tolerance is specified on a drawing by means of
a feature control symbol, which is directed by a leader to the
line requiring control.
STRAIGHTNESS

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INTERPRETATION
The lines should be straight
within 0.05 mm.
This means that the line
should be contained within
a tolerance zone consisting
of the area between two
parallel straight lines in the
same plane , separated by
the specified tolerance.
0.05

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STRAIGHTNESS
STRAIGHTNESS CHECKING WITH A STRAIGHT EDGE
Straightness can be measured by bringing a straight edge in contact
with the line, and determining the space between the straight edge
and the line.
MEASURING

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If the feature tends towards convexity the height of the tolerance zone
is measured where the least possible straightness error results.
MEASURING
Straightness includes all errors of form, such as concavity, convexity,
waviness, tool marks and other such imperfections.

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STRAIGHTNESS
STRAIGHTNES ERRORS IN A CYLINDRICAL SURFACE
ON CYLINDRICAL SURFACES

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STRAIGHTNESS
o A straight tolerance when applied to the surface, controls surface elements only.
o It controls bending or a wavy condition of the surface, or a “barrel” shaped part.
o Straightness not necessarily control the straightness of the centerline, norconicity
of the cylinder.
ON PROFILED SURFACES

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MEASUREMENT OF STRAIGHTNESS
MEASURING PRINCIPLE
The part is set up by suitable means, such as V-blocks on a surface
plate, with the surface to be evaluated closely parallel to the plate.
Readings are then taken at various points along the surface.
STRAIGHTNESS

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INTERPRETATION
Each line element of the surface shall be contained with in a tolerance zone
consisting of the space between two parallel planes, separated by the width of the
specified tolerance, when the part is rolled along one of the planes.
STRAIGHTNESS

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ON CONICAL SURFACES
STRAIGHTNESS

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A straightness tolerance indicates in one direction only.
ON FLAT SURFACES
STRAIGHTNESS

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The straightness tolerances may all be shown on a single
view by indicating the direction with short lines terminated by
arrow heads.
FLAT SURFACES
STRAIGHTNESS

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MEASURING PRINCIPLE
Straightness of a flat surface could be checked by laying a
straightedge along the part, and measuring any resultant space.
STRAIGHTNESS

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ON REGULAR POLYGONS
Straightness tolerance applies to all of the longitudinal surfaces.
For all other shapes it applies only to those surfaces indicated unless
otherwise specified.
STRAIGHTNESS

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STRAIGHTNESS IN A SPECIFIED LENGTH
o The expression 0.2/50 does not mean 0.2mm per 50mm of length.
o It means 0.2 mm in any 50mm long portion of the part.
STRAIGHTNESS

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Indicates straightness within 0.2mm for the full length.
Not to exceed 0.05mm in any 25mm length.

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THANK
YOU