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POLAR COORDINATE DIMENSIONS
POLAR COORDINATE DIMENSIONING is a
method of locating a point, line, or surface with a
linear distance and an angular measurement from a
fixed point of two intersecting perpendicular planes.
The fixed point of the intersecting planes is the
pole. The fixed direction from the pole is the ray.
The distance from the pole is the radius. The angle
from the ray is the polar angle. See Fig. 4-12. The
common use for polar coordinate dimensioning is
for locating features on circular objects.
TOLERANCING
A TOLERANCE is an acceptable variance
of a dimension for a part or feature. Tolerances
are extremely important in providing sufficient
specifications to allow the proper assembly of units
and the interchangeability of parts. Tolerances
control the size, form, orientation, location, shape,
function, and movement of a part or any of its
features. In addition, tolerances compensate for
deviations that occur during manufacturing. Since
it is impossible to make a part to its theoretical size,
tolerancing is required to control the quality and
precision of a part.
All dimensions have tolerances except for
nominal, referenced, minimum, and maximum
dimensions. All dimensions begin with a basic size.
BASIC SIZE is the specified theoretical value from
which limits of size are applied.
Basic size should not be confused with “basic
dimensioning” as described in Unit 12—Geometric
Dimensioning and Tolerancing. A basic dimension
is similar, but it differs by relating to the geometric
tolerancing of a feature or datum. “Basic size”
is used with dimensions, such as a part size, or
determining the sizes for hole and shaft systems.
For example, a hole with a basic size of 1.500 may
require a shaft with a basic size of 1.499, but an
allowable maximum variation of .001 on both parts.
See Fig. 4-13.
There are many ways to specify tolerances. Most
tolerances are applied to a dimension or a feature.
Tolerances can also be included in a note, a table,
or a tolerance block. A separate document can also
specify tolerances for a specific part, function, or
feature.
There are three types of tolerances—limits, plus
and minus, and geometric tolerances. Limits or
plus and minus tolerances are direct tolerances.
DIRECT TOLERANCING is the method of
specifying tolerances directly to dimensions that
control location or size. Geometric tolerances
apply to features to control form, profile, orientation,
location, and runout. Geometric tolerances are
discussed in Unit 12—Geometric Dimensioning
and Tolerancing. This section explains the
tolerancing of dimensions.
Fig. 4-11. Coordinate dimensioning is common on parts that
use numerical control for machining processes.
Fig. 4-12. Polar coordinates are useful for locating features on
circular parts or in a circular position.
DATUM
DISTANCE FROM
THE POLE
POLAR ANGLE
POLE
POLE
POLAR ANGLE
Dimensions and Tolerances