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GD&T F U N D A M E N T A L   R U L E S   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.


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.


R U L E   N U M B E R   1


Rule #1: At MMC the feature is on perfect form. (ex: diameter .997/1.000 shaft) then its MMC is 1.000. It's in perfect form that it's should have perfect straightness, circularity and cylindricity.


R U L E   N U M B E R  2


Rule #2: RFS automatically applies. (ex: diameter 1.000 / .900 / .800 steeped shaft). The bigger diameter is asigned as (datum B) and the smaller diameter .800 is controlled by a concentric geometric tolerance and this tolerance is applied without any dimensional size constraints ie regardless of feature size (RFS).


LEVEL 1 — SIZE LIMITS • SIZE LIMIT BOUNDARIES


Size Limits (Level 1 Control): For every feature of size, the designer shall specify the largest and the smallest the feature can be. Size Limit Bounday The standards provide three options for specifying size limits on the drawing: symbols for limit and fits, limit dimensioning (Ø.250/.245 or Ø.245/.250) or (25.45/25.00 not 25.45/25 or 32 +0.25/–0.10 not 32 +0.25/–0.1), and plus and minus tolerancing (.247 +.003/-.002 or .250 +/–.005) or (32 0/–.02 or 32 +0.02/0). Where tolerances directly accompany a dimension, it's important to coordinate the number of decimal places expressd for each value to prevent confusion. The rules depend on whether the dimension and tolerance values are expressed in inches (.500 +.005/–.000 not .500 +.005 /0) or millimeters (0.7 not .7 or 25.1 not 25.10 or 12 not 12.0).

Size Limit Boundaries: For every feature of size, the designer shall specify the largest and the smallest the feature can be (see Size Limits). With size limit boundary, we're concerned with the exact requirements these size limits impose (áp đặt) on a feature. It starts with a geometric element called a spine. The spine for a cylindrical feature is a simple (nonself-intersecting) curve in space that nay be straight or wavy, and we take an imaginary solid ball whose diameter equals the small size limit of the cylinde feature, and sweep its center along the spine. This generates a "wormlike" 3-dimensional bounday for the feature's smallest size. Then, we may create a secon spine, and sweep another ball whose diameter equals the large size limit of the cylindrical feature. This generates a second 3-D boundary, this time for the feature's largest size. Wwhether it's an internal or external feature, both feature surfaces shall contain the smaller boundary and be contained within the larger boundary.


LEVEL 2 ADJUSTMENT — ACTUAL LOCAL SIZES


Level 2 – Overall Feature Form: For feature of size that must achieve a clearance fit in assembly, such as this image, we calculate the size tolerances based on the assumption that each feature, internal and external, is straight.

Level 2 tolerances are intended to control feature form. Thus, the tolerance zone must interact with actual feature size independently at each cross section of the feature. Though the effective control is reduced from 3D down to 2D, inspection is more complicated.


LEVEL 4 CONTROL – POSITIONAL TOLERANCE – VIRTUAL CONDITION BOUNDARY FOR LOCATION


Level 4 – Positional Tolerance: How does it work? A positional tolerance may be specified in an RFS, MMC or LMC context.

Level 4 – Virtual condition Boundary for Location: For two mating features of size, Level 3's virtual condition boundary for orientation can only assure asemblability in the absence of any location restraints between the two features. , foe example, where no other mating features impede optimal location alignment between pin and hole. In Fig-c, we've moved the pin and hole close to the edges of the flanges and added a larger bore and boss mating interface at the center of the flanges. When the flange faces are bolted together tightly and the boss and bore are fitted together, the pin and the hole must each still be very square to their respective flange faces. However, the parts can no longer slide freely to optimize the location alignment between the pin and the hole. Thus, the pin and the hole must each additionally be accurately located relative to its respective boss or bore.

A position tolerance applied to a feature of size, modified to MMC or LMC, takes the virtual condition boundary one step further to Level 4. How to apply it?

 

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