1.2.2 Geometric Dimensioning and Tolerancing (GD & T)
• Specified in standard ANSI Y14.5 (1983).
• Combines rules and independent symbols in addition to the normal tolerancing symbols
• Allows old style tolerances, but adds new methods that cover geometrical forms.
• Allows easy specifications of datums, etc.
• Advantages of this method are,
- makes drawings clearer and more ambiguous
- allows separated features to be related
- uses symbols instead of words to reduce language translation problems
- the method helps specify manufacturing and metrology methods
• The main purpose of GD&T is to ensure,
size - the overall dimensions are as specified
form - the shapes specified must have the correct geometrical form
fit - two parts must mate as specified
function - the product conforms to performance specification
1.2.2.1 - Feature Control Symbols
• The basic of GD&T is the feature control symbol.
• This indicates what the tolerance is, its value, the reference datums, and any modifiers needed.
• An example of a feature control is given below,
• not all of these symbols/categories will be used on a regular basis, but they provide the designer added flexibility in how they specify tolerances.
1.2.2.2 - Symbols and Meaning
• The basic symbols are shown below,
• Flatness - basically, all the surface elements are constrained to lie within two parallel surface places, separated by the tolerance
• Straightness - basically, one the surface elements is constrained to lie within two parallel surface places, separated by the tolerance. In effect, this means that if any line across the surface is within two parallel lines, the part is acceptable. This can be tested by running a comparator across the surface (using a reference plane)
• Circularity - all of the points on a cylindrical surface are constrained to lie within two circles. This can be tested with a talyrond.
• Cylindricity - an extension to circularity that specifies the tolerance along the cylinder.
• Concentricity -
• Angularity - requires that all points on a specified feature must form an angle with a datum. This could be measured with a sine bar and a height comparator.
• Perpendicularity - this has the same meaning as angularity, but it is specifically applied to 90• angles. This could be measured with squares and reference plates.
• Symmetry -
• Parallelism - all points on a surface are to be parallel to a given datum, within a specified tolerance
• Line Profile - the amount of deviation that is allowed (typically for irregular lines)
• Surface Profile - the amount of deviation that is allowed for a surface
• Circular Runout - when dealing with a surface of revolution, this determines the amount of deviation allowed from the central axis. This specifically refers to a specific point
• Total Runout - similar to circular runout, but this applies to the entire part. In effect, circular runout uses two circles, whereas total runout uses two surface planes.
1.2.2.3 - Datums
• These are reference features, that other features are to be measured against.
• These can be used when setting up parts, for manufacturing or production
• Typical features used are,
- axes
- cylinders
- planes
- lines
- points
• A datum reference frame can be constructed with,
- three perpendicular planes
- 3 contact points in the primary plane, 2 in the secondary plane, and 1 in the tertiary
• A datum is specified with a boxed letter with two dashes,
1.2.2.4 - Modifiers
• to overcome shortcomings in symbols, modifiers can be added to change their meanings.
• in particular,
************ Include an example of tolerances using GD&T
