TOLERANCES of LOCATION — POSITION TOLERANCE (Level 4 Control) ASME Y14.5-2009
Position (xác định vị trí) is a versatile (nhiều tác dụng; đa năng) tolerance (kích cỡ, khối lượng.. của một bộ phận có thể dao động mà không gây tác hại) that can be used to control location, coaxiality (đồng trục), orientation or axis offset of a part feature or axis. Position should be specified whenever the design requirements permit. This control provides an opportunity to utilize (dùng, sử dụng; tận dụng) many of the advantages of GD&T.
How Does It Work? A positional tolerance defines either of the following:
How to Apply It? A positional tolerance is specified using a feature control frame displaying the "position" characteristic symbol followed by a compartment containing the positional tolerance value. Within the compartment, the positional tolerance value may be followed by an MMC or LMC modifying symbol. Any additional modifiers, such as "statistical tolerance," and/or "projected tolerance zone" followed by one, two or three separate compartments, each contaning a datum reference letter. Each datum reference may be followed by an MMC or LMC modifying symbol, as appropriate to the type of datum feature and the design.
Dimensions for True Position: For each individual controlled feature, a unique true position shall be established with basic dimensions relative to a specified DRF (Datum Reference Frame). True position is the nominal or idal orientation and location of the feature and thus, the center of the virtual condition boundary or positional tolerance zone. The basic dimensions may be shown graphically on the drawing, or expressed in table form either on the drawing or in a document referenced by the drawing.
Datums for Positional Control: Every positional tolerance shall reference one, two or three datum features. The DRF need not restrain all six degrees of freedom, only those necessary to establish a unique orientation and location for true position. Ex: The DRF established in the right Fig. restrains only four degrees of freedom. The remaining two degrees, rotation about and transtaltion along the datum axis, have no bearing on the controlled feature's true position. Thus, further datum references are meaningless and confusing.
For many positional tolerances, such as those in the right Fig., the drawing view makes it quite obvious which part features are th origins, even if they weren't identified as datum features and referenced in the feature control frame. Although we may agree the part's left and lower edges are clearly datum features, we might disagree on their precedence in establishing the orientation of the DRF. In another example, where a part has multiple coaxial diameters, it might be obvious to the designer, but very unclear to the reader, which diameter is supposed to be the datum feature. For these reason, Y14.5 no loger alloweds implied datums.
Positional Tolerance and Angled Features: Positional tolerancing is especially suited to angled feature over it entire length. This presumes (cho là, coi như là) the feature has no functional interface (mặt phân giới) beyond its own length and breadth (bề ngang, bề rộng, khổ). However, in the right figure, a pin is pressed into the controlled hole and expected to mate with another hole in a cover plate. The mating feature is not the pin hole itself, but rather the pin, which represents a projection of the hole. Likewise, the mating interface is not within the length of the pin hole, but above the hole, within the thickness of the cover plate. See Figure. Establishing true positions from an implied datum – a common error.
If the pin hole were perfectly perpendicular to the plannar interface between the two parts, there would be no difference between the location of the hole and the pin. Any angulation (hình có góc), however, introduces a discrepancy (sự khác nhau) in location. This discrepancy is proportional (tương ứng về cỡ, số lượng hoặc mức độ ; có tỷ lệ đúng; cân xứng) to the length of projection. Thus (do đó, theo đó, vì thế, vì vậy), directly controlling the location of the pin hole itself is inadequate (không tương xứng, không xứng, không thích đáng, không thoả đáng) to assure assemblability. Therefore (bởi vậy, cho nên, vì thế, vậy thì), the location of the hole's projection (hình chiếu) needed to be control, which could be thought of as a phantom pin. this is accomplished with a positional tolerance modified with a projected tolerance zone.
Positional Tolerance and Projected Tolerance zone. The application of this concept is recommended where the variation inperpendicularity of threaded or press-fit holes could cause fastener could cause fasteners, such as screws, studs, or pins, to interfere with mating parts. In this example fig. (7-19 - pg-120), an interference can occur where a tolerance is specified for the location of a threaded or press-fit hole, and the hole is inclined within the positional limits. Unlike the floating fastener application involving clearance holes only, the attitude of a fixed fastener id governed by the inclination of the produced hole into which it assembles...(continue pg-119).
A projected tolerance zone is specified by placing the symbol P with a circle after the tolerance value in the position feature control frame. This establishes a constant-size (kích thước, cỡ, khổ không thay đổi) central tolerance zone bounded either by two parallel planes separated by a distance equal to the specified tolerance, or by a cylinder having a diameter equal to the specified tolerance. For blind holes and other applications where the direction of projection is obvious, the length of projection may be specified after the synbol in the feature control frame. This means the projected tolerance zone terminates at the part face and at the specified distance from the part face (away from the part, and parallel to the true position axis or center plane). The projection length should equal the maximum extension of the mating interface. In Figure. a (pin and cover plate example). the projection length must equal the cover plate's maximum thickness .38. Where necessary, the extent and direction of the projected tolerance zone are shown in a drawing view as a dimensioned value with a heavy chain line drawn next to the center line of the feature as in Figure. b.
Positional Tolerance at MMC: A positional tolerance applied at MMC may be explained in terms of the surface or the axis of the feature of size. In certain cases of extreme form deviation (within limits of size) or orientation deviation of the hole, the tolerance in terms of the axis may not be exactly equivalent to the tolerance in terms of the surface. See Im_1-13. In such cases, the surface interpretation shall take precedence. In some instances, the additional tolerance may indirectly benefit features other than the one that departed from MMC.
Specifying the Position Tolerance at MMC: Where the maximum material condition (MMC) symbol is specified to modify the tolerance of a feature of size in a feature control frame, the following two requirements apply:
C A L C U L A T I N G P O S I T I O N A L T O L E R A N C E
The right image shows a drawing for one of two identical plates to be assembled with four 14-mm maximum diameter fasteners. The 14.25 minimum diameter clearance holes are selected with a size tolerance as shown. The required positional tolerance is found by the equation and other considerations as given in Nonmandatory Appendix B. The shown formula does not accommodate factors other than hole and fastener diameter tolerances.
Datum Reference Frame Concept: Inspecting the part.
Establishing Multiple Datum Reference Frames: Datum X, Y, and Z establish one datum reference frame. Datums L and M establish second datum reference frame.
Datum Features Specified Individually: Place a note next to datum features symbols indicating how many datum features to consider separately. Place the note "2X INDIVIDUALLY" next to datum feature symbols of two separate datum features identified by same letter.
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 —»
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. Tolerance zones are located at true position.
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: