Geometric Dimensioning and Tolerancing (GD&T), when made an integral part of the Product Development/Delivery Process through drawing previews/reviews, is the tool that can have the greatest impact on improving quality, cost and delivery of your product. GD&T is not new, but there is newness about it. Recent developments in CAD and CMM technology, ISO Standards and Statistical Process Control have brought GD&T once again to the forefront of design, quality and manufacturing.
GD&T is a common language that may be ud to improve communication between design, quality, manufacturing, suppliers and customers.
herebyFor nearly two hundred years, industry has been struggling with the problem of clear part definition. The latest and most successful approach has been to employ Geometric Dimensioning and Tolerancing (GD&T). After nearly sixty years of refinement, industry now has a Standard which will handle virtually every situation. The Standard is published by the American Society of Mechanical Engineers and is designated ASME Y14.5M-1994.
Some of the advantages of using GD&T on engineering drawings and product data sheets are that it:
•Us datum and datum systems to define dimensional requirements with respect to part interfaces.
阿尔法人格•Specifies dimensions and related tolerances bad on functional relationships.
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•Removes ambiguity by applying universally accepted symbols and syntax.
•Provides information that can be ud to control tooling and asmbly interfaces.
•Express dimensional tolerance requirements using methods that decrea tolerance accumulation
Straightness - a condition where an element of a surface or an axis is a straight line.
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Flatness - is the condition of a surface having all elements in one plane.
Roundness - describes the condition on a surface of revolution (cylinder, cone, sphere) where all points of the surface intercted by any plane.
Cylindricity - describes a condition of a surface of revolution in which all points of a surface are equid
istant from a common axis.
Profile of a Line - is the condition permitting a uniform amount of profile variation,
ether unilaterally or bilaterally, along a line element of a feature.
Profile of a Surface - is the condition permitting a uniform amount of profile
variation, ether unilaterally or bilaterally, on a surface?
All Around Symbol - indicating that a tolerance applies to surfaces all around the part.
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Angularity - is the condition of a surface, axis, or center plane, which is at a specified angle from a datum plane or axis.
Perpendicularity - is the condition of a surface, axis, or line, which is 90 deg. From a datum plane or a datum axis.
Parallelism - is the condition of a surface, line, or axis, which is equidistant at all, points from a datum plane or axis.
Position Tolerance - defines a zone within which the axis or center plane of a feature is permitted to vary from true (theoretically exact) position.
Concentricity - describes a condition in which two or more features , in any
combination, have a common axis.
Symmetry - is a condition in which a feature (or features) is symmetrically dispod about the center plane of a datum feature.
Run out - is the composite deviation from the desired form of a part surface of revolution through on full rotation (360 deg) of the part on a datum axis.
Total Run out - is the simultaneous composite control of all elements of a surface at all circular and profile measuring positions as the part is rotated through 360. Maximum Material Condition (MMC) - is that condition of a part feature wherein it
contains the maximum amount of
material within the stated limits of size. That is: minimum hole size and maximum shaft size.
Least Material Condition (LMC) - implies that condition of a part feature of size
wherein it contains the least (minimum) amount of material, examples, largest hole size and smallest shaft size. It is opposite to maximum material condition.
Regardless Of Feature Size (RFS) - the condition where the tolerance of form, run out or location must be met irrespective of where the feature lies within its size tolerance.
Projected Tolerance Zone - applies to a hole in which a pin, stud, screw, etc., is to be inrted. It controls the perpendicularity of the hole to the extent of the projection
from the hole and as it relates to the mating part clearance. The projected tolerance zone extends above the surface of the part to the functional length of the pin, stud, and screw relative to its asmbly with the mating part.
calm downTangent Plane - indicating a tangent plane is shown. The symbol is placed in the feature control frame following the stated tolerance.
Free State Variations - is a term ud to describe distortion of a part after removal of forces applied during manufacture.
Diameter - indicates a circular feature when ud on the field of a drawing or indicates that the tolerance is diametrical when ud in a feature control frame. Basic Dimension - ud to describe the exact size, profile, orientation or location of
a feature. A basic dimension is always associated with a feature control frame or datum target. (Theoretically exact dimension in ISO)
Reference Dimension - a dimension usually without tolerance, ud for information
purpos only. It does not govern production or inspection operations. (Auxiliary dimension in ISO)
Datum Feature - is the actual component feature ud to establish a datum.
Dimension Origin - Signifies that the dimension originates from the plane established by the shorter surface and dimensional limits apply to the other surface.
Feature Control Frame - is a rectangular box
containing the geometric characteristics symbol, and
the form, run out or location tolerance. If necessary,
datum references and modifiers applicable to the
feature or the datums are also contained in the box.
Conical Taper - is ud to indicate taper for conical tapers. This symbol is always shown with the vertical leg to the left.
Slope - is ud to indicate slope for flat tapers. This symbol is always shown with
the vertical leg to the left.
Counterbore/Spotface - is ud to indicate a counter bore or a spot face. The
symbol precedes the dimension of the counter bore or spot face, with no space
Countersink - is ud to indicate a countersink. The symbol precedes the
dimensions of the countersink with no space.
Depth/Deep - is ud to indicate that a dimension applies to the depth of a feature.
This symbol precedes the depth value with no space in between.
Square - is ud to indicate that a single dimension applies to a square shape. The
symbol precedes the dimension with no space between.
Number of Places - the X is ud along with a value to indicate the number of times a dimension or feature is repeated on the drawing.
Arc Length - indicating that a dimension is an arc length measured on a curved
outline. The symbol is placed above the dimension.
Radius - creates a zone defined by two arcs (the minimum and maximum radii). The
part surface must lie within this zone.
Spherical Radius - precedes the value of a dimension or tolerance.
Spherical Diameter - shall precede the tolerance value where the specified tolerance value reprents spherical zone. Also, a positional tolerance may be ud
to control the location of a spherical feature relative to other features of a part. The symbol for spherical diameter precedes the size dimension of the feature and the positional tolerance value, to indicate a spherical tolerance zone.
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Controlled Radius - creates a tolerance zone defined by two arcs (the minimum and maximum radii) that are tangent to the adjacent surfaces. Where a controlled radius is specified, the part contour within the crescent-shaped tolerance zone must
be a fair curve without flats or reversals. Additionally, radii taken at all points on the part contour shall neither be smaller than the specified minimum limit nor larger than the maximum limit.
Between - to indicate that a profile tolerance applies to veral contiguous features, letters may designate where the profile tolerance begins and ends. The letters are
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referenced using the between symbol (since 1994) or the word between on drawings made to earlier versions of the Standard.
Statistical Tolerance – i s the assigning of tolerances to related components of an asmbly on the basis of sound statistics (such as the asmbly tolerance is equal to the square root of the sum of the squares of the individual tolerances). By applying statistical tolerancing, tolerances of individual c
omponents may be
incread or clearances between mating parts may be reduced. The incread tolerance or improved fit may reduce manufacturing cost or improve the product's performance, but shall only be employed where the appropriate statistical process control will be ud. Therefore, consideration should be given to specifying the required Cp and /or Cpk or other process performance indices.
Datum Target - is a specified point, line, or area on a part that
is ud to establish the Datum Reference Plane for
manufacturing and inspection operations.
Target Point - indicates where the datum target point is dimensionally located on
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the direct view of the surface.
Why Do I Need GD&T?
The craftsmen of old could fashion parts in a way that would allow them to slide together and give the impression that the parts fit "perfectly." Today, with the concept of interchangeable parts, credited to Eli Whitney, it is expected that parts will asmble the first time and perform their intended function. Interchangeability does not apply only to mass produced parts. Whenever two parts are expected to fit together and function without rework or adjustment, the parts must be clearly defined. Parts that have been made in other departments, plants, cities or even countries must consistently fit and function even though slight variation from the intended shape and size will exist in every part.
All parts go through a manufacturing process. There is variation in all manufacturing process. The variations are reflected in the parts. In addition, there must be a way to inspect a part to assure that it was made to the required specifications. As Bob Traver says:
"You can't make what you can't measure becau you don't know when
you've got it made!" Most importantly, the part must perform its intended task or function. To accompl
ish all of this, the part must be clearly and totally defined. In most cas this definition is accomplished on a detail drawing or within a CAD file.chmm
When ud properly, GD&T will get the right questions asked early in the program, simplify the engineering drawing, and directly relate customer requirements to product specifications and process control.
Give Production the Dimensions They Need.
When GD&T is ud, the geometric tolerances apply to the features - not the dimensions.
Therefore when BASIC dimensions are ud with geometric tolerances, veral different dimensioning schemes may be ud without changing the meaning of the drawing. The first drawing below is an example of baline dimensioning. The other two illustrate chain dimensioning. Since the position tolerance is related to datums A, B and C, all three drawings have the same meaning even though the dimensioning is different. This would not be true if the dimensions locating the holes were toleranced rather than BASIC. Becau the meaning of the drawings is the same, the designer should consider the needs of tho who will read the print
when placing dimensions.
