Geometric.Dimensioning.and.Tolerancing.pdf

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GEOMETRIC DIMENSIONING
AND
TOLERANCING
In order to properly plan the processes to make a part, you must first
understand the part designer’s intention. Geometric dimensioning and
tolerancing is a method by which designers specify the geometric form of
parts.
The definitions and convention used in this course are based on ISO
standard.
It is assumed that you have a basic understanding of dimensional metrology
and are familiar with measuring equipment, such as gauge blocks, dial
indicators, optical comparators, etc. If you need to revise dimensional
metrology then you can refer to:
Busch, T. 1989.
Fundamentals of Dimensional Metrology.
Delmar
Publishers: New York.
C
Professor Graeme Britton, 2000.
1
© Professor Britton 1996 -2000
Topics
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Introduction
Interpreting Geometric Tolerances
Principles of Inspection
The goal of this part of the course is to describe the different types of
geometric tolerances and how actual parts can be inspected to ensure
compliance.
At the end of this part of the course you will be able to interpret a drawing
or model containing GD&T symbols and know (in principle) how to verify
whether a part complies to the design specification.
C
Professor Graeme Britton, 2000.
2
© Professor Britton 1996 -2000
Types of geometric deviations
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surface discontinuities
roughness
waviness
edge deviations
size deviations
form deviations
orientation deviations
location deviations
depth of irregularity
0,0001 mm
0,01+ mm
GD&T
This slide lists the different kinds of geometric deviations. Only the last
three are part of Geometric Dimensioning and Tolerancing (GD&T).
Size deviations are controlled by normal dimensioning practices.
Roughness and waviness require separate specifications.
In addition to these requirements the designer also needs to specify the
material to be used and the special conditions relating to the material.
Thus complex engineering drawings and models may require you to
understand several different conventions, each controlling different aspects
of the design.
C
Professor Graeme Britton, 2000.
3
© Professor Britton 1996 -2000
Waviness and roughness
Waviness
(100:1-1000:1)
Traversing length
spacing
depth
Roughness
(5:1-150:1)
spacing
depth
Not to scale!
One technique for distinguishing between roughness, waviness and form
control is by the spacing to depth ratio. The slide illustrates this for waviness
and roughness.
Roughness is measure at a much smaller scale than waviness and is an
indication of very small local imperfections in a surface. Roughness is
produced by the direct effect of the cutting process (chip formation),
deformation from blasting, crystallization, corrosion and other chemical
processes. The spacing to depth ratio between successive peaks is of the
order 5:1 to 150:1.
Waviness refers to periodic regularities in the surface of a part, but at a scale
smaller than that which is controlled by GD&T. The spacing to depth ratio
between successive peaks is of the order 100:1 to 1000:1. It is produced by
eccentric fixturing, form deviations in the cutting tool and vibration.
C
Professor Graeme Britton, 2000.
4
© Professor Britton 1996 -2000
Size deviation
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deviation of actual local size from nominal linear
size or from nominal angular size
actual local linear sizes are assessed by 2-point
measurements
actual local angular sizes are assessed by angular
measurements of averaged lines
assessed over entire geometric element
produced mainly by imprecise adjustment of
machine tool and cutting or process conditions
Size deviation is controlled on engineering drawings and CAD models by
stating the nominal size and a tolerance which defines the maximum
permissible deviation from the nominal size. The nominal size and tolerance
define the permissible design limits (upper and lower) within which all size
measurements must lie.
The tolerance values either side of the nominal have the same va lues
(bilateral tolerancing) or different values (unilateral toleranc ing). Unilateral
tolerancing is used to bias a size towards one of the design limits in order to
optimise performance. On the other hand, as you will find out later,
manufacturing processes are controlled using bilateral tolerances. Hence it is
normal practice to convert unilateral design tolerances to bilateral tolerances
during process planning.
C
Professor Graeme Britton, 2000.
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