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GD&T FUNDAMENTAL RULES PER ANSI Y14.5

Geometric Dimensioning and Tolerancing (GD&T) is a system for defining and communicating engineering tolerances. It uses a symbolic language on engineering drawings and computer-generated three-dimensional solid models that explicitly describes nominal (danh nghĩa) geometry and its allowable variation (sự thay đổi, sự khác nhau).

W H A T   A R E   R U L E S   O F   G D & T   ?

To help conform to this standard of measurement, a system for defining tolerances known as Geometric Dimensioning and Tolerancing (GD&T) was developed. Below are the rules of GD&T.

  • Each dimension shall have a tolerance, except for those dimensions specifically identified as reference, maximum, or stock. The tolerance may be applied directly to the dimension (or indirectly in the case of basic dimensions), indicated by a general note, or located in a suplementary block of the drawing format.
  • Dimensioning and tolerancing shall be complete so there is full understanding of the characteristics of each feature. Neither scaling nor assumtion of a distance or size is permitted, except as follows: Undimensioned drawing, such as loft, printed wiring, templates, and master layout prepared on stable (không có khả năng di động, không có khả năng thay đổi) material, are excluded provided the necessary control dimensions are specified.
  • Each necessary dimension of an end product shall be shown. No more dimension than those necessary for complete definition shall be given. The use of reference dimensions on a drawing should be minimized. Dimensions should NOT be duplicated, or the same information be given in two different ways.
  • Dimension shall be selected and arranged to suit the function and mating relationship of a part and shall not be subject to more than one interpretation..
  • The drawing should define a part without specifying manufacturing methods. Thus (theo cách đó, như vậy, như thế), only the diameter of the hole is given without indicating whether it's to be drilled, reamed, punched, or made by any other operation. However, in those instances (yếu tố cần thiết) where manufacturing, processing, quality assurance, or environmental information is essential to the deffinition of engineering requirements. It shall be specified on the drawing or in a document referenced on the drawing.
  • It's permissible (chấp nhận được, dùng được) to identify as nonmandatory (không có tính cách bắt buộc) certain processing dimensions that provide for finish allowance, shrink allowance, and other requirements, provided the final dimensions are given on the drawing. Nonmandatory processing dimensions shall be identified by an appropriate note, such as NONMANDATORY (MFG DATA).
  • Dimension should be arranged to provide required information for optimum (tốt nhất, tối ưu) readability. Dimension should be shown in true profile views and refer to visible outlines. Dimensions should be placed at finished surfaces or important center lines. Never cross dimension lines. Never cross extension lines.
  • Wires, cables, sheets, rods, and other materials manufactured to gage or code number shall be in parentheses following the dimension.
  • A 90° angle applies where center lines and line depicting features are shown on a drawing at right angles and no angle is specified.
  • Unless otherwise specified, all dimension are applicable at 20°C (68°F). Compensation (sự đền bù, sự bồi thường) may be made for measurements made at other temperatures.
  • All dimensions and tolerances apply in a free state condition. This principle doesn't apply to nonrigid parts as defined below.
  • Unless otherwise specified, all geometric tolerances apply for full length, depth, and width of the feature.
  • Dimensions and tolerances apply only at the drawing level where they are specified. A dimension specified for a given feature on one level of drawing, (for example, a detail drawing) is not mandatory for that feature at any other level (for example, an assembly drawing).

N O N R I G I D   P A R T S

A nonrigid part is a part that can have different dimensions while restrained (kiềm chế) in assembly than while relaxed (thanh thản) in its "free state." Rubber, plastic, or thin-wall parts may be obviously nonrigid. Other parts might reveal themselves as nonrigid only after assembly or functioning forces are applied. That's why the exemption of "nonrigid parts" from Fundamental Rule is meaningless. Instead, the rule must be interpreted as applying to all parts and meaning. "Unless otherwise specified, all dimensions and tolerances apply in it free state condition." Thus, a designer must take extra care to assure that a suspected nonrigid part will have proper dimensions while assembled and functioning. To do so, one or more tolerances may be designated to apply while the part's assembly and/or functioning.

N O N R I G I D   P A R T S — S P E C I F Y I N G   R E S T R A I N T

A nonrigid part might conform (thích nghi với) to all tolerances only in the free state, only in the restrained state, in both states, or in neither state. Where a part, such as a rubber grommet, may or may not need the help of restraint for conformance (sự phù hợp, sự thích hợp), the designer may specify optional restraint. This allows all samples to be inspected in their free states. Parts that pass are accepted. Those that fail may be reinspected — this time, while restrained. Where there is a risk that restraint could introduce unacceptable distortion, the designer should specify mandatory (có tính cách bắt buộc) restraint instead.

Restraint may be specified by a note such as UNLESS OTHERWISE SPECIFIED, ALL DIMENSIONS AND TOLERANCES MAY (or SHALL) APPLY IN A RESTRAINED CONDITION. Alternatively, the note may be directed only to certain dimensions with flags and modified accordingly (phù hợp với điều đã được nhắc đến hoặc biết đến). The note shall always include (or reference a document that includes) detailed instructions for restraining the part. A typical note (example: THIS TOLERANCE APPLIES WHEN DATUM FEATURE A IS MOUNTED AGAINST A FLAT SURFACE USING FOUR .250-28 BOLTS TORQUED TO 10 FOOT POUNDS.) identifies one or two functional datum features (themselves nonrigid) to be clamped into some type of gage or fixture. The note should spell out any specific clamps, fasteners, torques, and other forces deemed necessay to simulate expected assembly conditions.

 
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