G D & T N A M E & D E F I N I T I O N S — T E R M S PER ANSI Y14.5
The names and definitions of many GD&T terms have very specific meanings. To be able to function, it is important for each GD&T practitioner to be very familiar with these terms.
GD&T: A means of dimensioning and tolerancing a part with respect to relationship and function of that part. GD&T is used to define how a part feature relates the other part features in the same part or in a mating part; it’s a way to dimension and tolerance with respect to part’s function, the way it works.
Free State: The condition of a part free of applied forces. Use the free state symbol for plastic, sheet metal and other flexible parts. In section 1.4 FUNDAMENTAL RULES of Y14.5-2009 paragraph (m) stated "Unless otherwise specified, all dimensions and tolerances apply in a free-state condition. This principle does not apply to nonrigid parts...Where the actual local size of a regular feature of size has departed from MMC toward LMC, a local variation in form is allowed equal to the amount of such departure". The default, then, is that all parts are considered rigid unless there is some indication that the part may be flexed or distorted for inspection. A way to indicate that a part is not rigid is to add a constraint note. If some tolerances are to be checked with the part not constrained, the free state symbol may be added to specific tolerances. Many usable sheet metal and plastic parts will pass inspection and be functional if clamped to a condition that simulates the assembled or functionale state. Usually when a constraint note is applied to a drawing, at least one tolerance should be inspected in the free state to limit the free state variation.
Nominal Size: The size of a feature of perfect form as defined by the technical drawing.
Deviation: The difference between a size and the corresponding nominal size.
Upper deviation: The difference between the maximum limiting size and the corresponding nominal size of a feature.
Lower deviation: The difference between the minimum limiting size and the corresponding nominal size of a feature.
Tolerance: The difference between the maximum and minimum size limits of a part.
Clearance Fit: A fit type where clearance exists between assembled parts under all tolerance conditions.
Interference Fit: A fit type where interference exists between assembled parts under all tolerance conditions.
Transition Fit: A fit type where clearance or interference can exist between assembled parts depending on tolerance conditions.
ACTUAL LOCAL SIZE & A C T U A L M A T I N G E N V E L O P E
Actual local size is the measurement where you measure it (local size). Atual mating envelope is the shape of a perfect form that encompasses (bao quanh) the actual feature. Actual values are derived (lấy được từ) from actual parts.
The actual mating envelope is a similar, perfect, feature(s) counterpart of smallest size that can be contracted (rút gọn) about an external feature(s) or largest size that can be expanded within an internal feature(s) so that it coincides with the surface(s) at the highest points. It's the smallest perfect shape that will fit around an external Feature of Size or the largest perfect shape that will fit inside an internal Feature of Size. Two types of actual mating envelopes are described below.
* Note: The Actual Mating Envelope must be oriented relative to the specified Datums. When an inspector merely uses the size of a feature to calculate the bonus tolerance, out of spec parts may be accepted.
D I M E N S I O N & B A S I C D I M E N S I O N
Dimension: Is the numerical value that defines the size or geometric characteristic of a feature.
Basic Dimension: Title block tolerances do not apply to basic dimensions. The tolerance associated with a basic dimension usually appears in a feature control frame or a note. A basic dimension is a theoretically exact dimension. Basic dimensions define the true position of the toleranced Feature os Fize (FOS) relative to the datums referenced in the feature control frame. In certain cases, the basic dimensions in a TOP application are not specified; they are implied. There are two types of implied basic dimensions common in TOP applications:
Reference Dimension: Is the numerical value enclosed in parentheses provided for information only and is not used in the fabrication of the part.
Plus and Minus Dimensioning: Is the allowable positive and negative variance from the dimension specified.
Limits of Size: Is the largest acceptable size and the minimum acceptable size of a feature.
B O N U S T O L E R A N C E
Bonus tolerances can reduce manufacturing costs significantly. Bonus tolerance is an additional tolerance for a geometric control. Whenever a geometric tolerance is applied to a feature of size, and it contains an MMC (or LMC) modifier in the tolerance portion of the feature control frame, a bonus tolerance is permissible.
Bonus tolerance equals the difference between the actual mating envelope and the MMC sizes of a feature. The bonus tolerance is added to the geometric tolerance specified in the feature control frame. Of the three material condition modifiers, the MMC modifier is the most common and is typically used for features on parts that are to be fastened together in a static assembly.
Bonus Tolerance Applies to An Internal Feature: Example of a hole with a diameter 1.000/.995 and having a straightness at MMC as .003. If actual size is —»
Bonus Tolerance Applies to An External Feature: Example of a pin with a diameter .750/.747 and having a straightness at MMC as .001. If actual size is —»
DEGREES of FREEDOM — D A T U M and D A T U M F E A T U R E
Degrees of Freedom: (page 5-72).
Six Degrees of Freedom: 6DoF refers to the freedom of movement of a rigid body in three-dimensional space. Specifically, the body is free to move forward/backward, up/down, left/right (translation in three perpendicular axes) combined with rotation about three perpendicular axes, often termed pitch, yaw, and roll.
Datum: A datum is a theoretically exact point, axis, line, plane, or combination thereof derived from the theoretical datum feature simulator (used as a reference for tabular dimensioning). A datum is the origin from which the location or geometric characteristics of features of a part are established.
Implied Datum: An assumed plane, axis, or point drom which a dimensional measurement is made.
Datum Feature: A datum feature is a feature that is identified with either a datum feature symbol or a datum target symbol (page 5-61).
Datum Feature Selection: (page 5-61).
Datum Feature Simulator (Theoretical): A datum feature simulator (Theoretical) is the theoretically perfect boundary used to establish a datum from a specified datum feature.
Datum Feature Simulator (Physical): A datum feature simulator (Physical) is the physical boundary used to establish a simulated datum from a specified datum feature. Physical datum feature simulators are represented by inspection or manufacturing tooling
Datum Reference Frame: A datum reference frame consists of three mutually perpendicular intersecting datum planes.
True Geometric Counterpart or TGC: (page 5-67).
D E R I V E D E L E M E N T S
Derived Elements: A multitude of geometric elements can be derived from any feature. A geometric tolerance RFS applied to a feature of size controls one of these five.
A Level 2 (straightness or flatness) tolerance nulifies (huỷ bỏ; làm thành vô hiệu) Rule #1's boundary of perfect form at MMC. Instead, the separate tolerance controls overall feature from constraining the derived median line or derived median plane, according to the type of feature.
A cylindrical feature's derived median line is an imperfect line (abstract) that passes through the center points of all cross sections of the feature. These cross sections are normal to the axis of the actual mating envelope. The cross section center points are determined as per ANSI B89.3.1.
A width-type feature's derived median plane is an imperfect plane (abstract) that passes through the center points of all line segments bounded by the feature. These line segments are normal to the actual mating envelope.
Tolerance zone for straightness control RFS. In Fig. a, the absence of a material condition modifier symbol means the straightness tolerance applies RFS by default. This specifies a tolerance zone bounded by a cylinder having a diameter equal to the tolerance value, within which the derived median line shall be contained.
Tolerance zone for flaness control RFS. In Fig. b, the flaness tolerance applies RFS by default. This specifies a tolerance zone bounded by two parallel planes separated by a distance equal to the tolerance value, within which the entire derived median plane shall be contained. Both size limits are still in force, but neither the spine for the MMC size boundary nor the spine for the LMC size boundary need be perfectly formed. A straightness or flatness tolerance value may be less than, equal to, or greater than the size tolerance.
F E A T U R E a n d F E A T U R E o f S I Z E
Feature: A feature is a physical portion of a part, such as a surface, pin, hole, tab, slot, or its representation on drawings, models, or digital data files.
Feature of Size: A feature of size encompasses two types.
L I M I T S o f S I Z E (M M C) a n d (L M C)
Maximum Material Condition (MMC): The maximum material condition is the condition in which a feature of size contains the maximum amount of material within the stated limits of size. For example, the minimum hole diameter/slot width and the maximum pin/shaft diameter.
Least Material Condition (LMC): The least material condition is the condition in which a feature of size contains the least amount of material within the stated limits of size. For example, the maximum hole diameter/slot width and the minimum pin/shaft diameter.
M A T E R I A L C O N D I T I O N M O D I F I E R S
A material condition modifier is specified in a feature control frame, associated with the geometric tolerance of a feature of size or a datum feature of size. The material condition modifiers are shown in the table and figure below.
T R U E P O S I T I O N and T R U E P R O F I L E
True Position: True position is the theoretically exact location of a feature of size, as established by basic dimensions. Expressed as the total permissible variation that a feature can have from its “true” position. Depending on how it is called out, true position can mean several different things. It can be used with MMC (Max Material Condition), LMC (Least Material Condition), projected tolerances (P), and tangent planes (T). Tolerance zones are located at true position (examples).
True Profile: True profile is the theoretically exact profile on a drawing defined by basic dimensions or a digital data file. Tolerance zones are located about the true profile.
Resultant Condition: The resultant condition (điều kiện tổng hợp) is the state of a toleranced feature at the other boundary (ranh giới), the boundary the design engineer believes they care less about. Obviously, the design engineer must be cognizant (biết, hiểu biết, biết rõ) of both the MMB and LMB for every feature, but in many cases, there is one boundary that is more functionally significant. The resultant condition of a feature of size specified with a MMC modifier is the single worst-case boundary generated by the collective effects of the LMC limit of size, the specified geometric tolerance, and the size tolerance. The size tolerance is the bonus tolerance at LMC. Features specified with a least material condition modifier also have a resultant condition.
Resultant condition calculations for features toleranced at MMC:
For a part dimensioned and toleranced in accordance with ASME Y14.5, the resultant condition is either:
V I R T U A L C O N D I T I O N
Virtual Condition is the boundary generated by the collective effects of MMC, size limit of a feature and any associated geometric tolerance, virtual condition must be considered in determining the fit between mating parts. Features specified with a least material condition modifier also have a virtual condition (virtual size). Virtual Condition of the Hole is .992 and Virtual Condition of the Pin is ,751
W O R S T C A S E B O U N D A R Y
The worst-case boundary of a feature is a general term that describes the smallest or largest boundary generated by the collective effects of the MMC or LMC of a feature and any applicable geometric tolerance.
Features specified with an LMC modifier also have worst-case boundaries.