FORM CONTROL – FLATNESS — ASME Y14.5-2009
There are four form controls (điều chỉnh hình thể). Flatness (mặt phẳng), Straightness (thẳng, không cong), Circularity (hình vòng tròn; dáng tròn), Cylindricity (hình trụ, cylindricity is a 3-Dimensional tolerance that controls the overall form of a cylindrical feature to ensure that it is round enough and straight enough along its axis). Practically form controls are the least used GD&T controls as other tolerances like location and orientation will have sufficient control to satisfy parts functional requirements and form.
Form Controls Never Need a Datum Reference: As a general rule, apply a form (only) tolerance to a non-datum (số lượng đã cho; điều đã cho biết luận cứ) feature only where there is some risk that the surface will be manufactured with form deviation (độ lệch) severe enough to cause problem in subsequent manufacturing operations, inspection, assembly, or function of the part.
Flatness: The condition of a surface or derived (bắt nguồn từ, lấy được từ) median (ở giữa) plane (mặt phẳng) having all of its elements in one plane is known as Flatness. Flatness is not the same as parallelism. Parallelism uses a datum to control a surface while flatness does not. Think of a table with two missing legs at an angle to the floor. The table top may be within flatness tolerance but would not be parallel to the floor.
Applications used flatness tolerance: A real-world application of a flatness tolerance is to limit flatness deviation (độ lệch) on mating surfaces in a gasket joint application. When you want to constrain the amount of waviness or variation in a surface without tightening the dimensional tolerance of said surface. Usually flatness is used to give a surface an even amount of wear or for sealing properly with a mating part. Commonly used on a fixture that must mate flush with another part without wobbling, but where orientation is not important.
Flatness tolerance is applied to a planar feature: Where a flatness tolerance feature control frame is placed on a plane or surface (leader-directed to a feature or attached to an extension line from the feature) the tolerance applies to a single nominally flat feature. All the surface elements must be within two parallel planes (indicated with a flatness tolerance) apart. Each actual local size must measure within the flatness tolerance specified. The flatness tolerance do not overrides Rule #1.
Flatness tolerance is applied to a width-type feature: A flatness tolerance feature control frame placed below or attached to a leader-directed callout or dimension pertaining to the feature or attached either side/end of the frame to an extension of the dimension line pertaining to a feature of size (associated with a width dimension) replaced Rule #1's requirement for perfect form at MMC with a separate tolerance controlling the overall flatness of the width-type feature. Where the tolerance is modified to MMC or LMC, it establishes a Level 2 virtual condition boundary
Flatness Tolerance: Specifies a tolerance zone of two parallel planes within which a surface or derived median plane must lie. Flatness tolerances do not use different tolerance zones for surfaces and derived median planes (đường, tuyến được hình thành từ mặt phẳng). A flatness tolerance cannot control whether the surface is fundamentally concave, convex, or stepped; just the maximum range between its highest and lowest undulations.
Flatness Tolerance Zone: A tolerance zone of two parallel planes within which a surface or derived median plane must lie. Flatness tolerance zone is the space between two parallel planes.
Rule #1 effect on flatness deviation. When Rule #1 applies to a planar feature of size, it limits the flatness deviation of the derived median plane to equal to the size tolerance.
Rule #1 effect on a flatness deviation of a surfaces. When Rule #1 applies to a surface, no flatness deviation permitted on both surfaces where the part is at MMC, maximum flatness deviation on both surfaces when part is at LMC.
Rule #1 effect on a flatness diviation of a feature of size. When Rule #1 applies to a feature of size, derived median plane must be perfectly flat, bonus tolerance increased as size
Determining Whether a Flatness Tolerance Is Applied to a Planar Surface or a Feature of Size Dimension. The location of the flatness tolerance indicates what it's applied to. Where the leader line of a flaness tolerance is directed to a surface, or attached to an extension line from the surface, it indicates that it applies to the surface (the tolerance applies to a single nominallyflat feature). Where a flaness tolerance is located beneath or beside a size dimension, it indicates that it applies to the feature of size.
Derived Median Plane (mặt bằng tìm thấy từ điểm giữa/trung tuyến được hình thành từ nhiều đường phân đoạn) is the term for an imperfect plane formed by the center points of all line segments bounded by the feature of size.
FLATNESS TOLERANCE APPLIED TO A PLANAR SURFACE OR A FOS
Flatness tolerance is applied to a feature of size dimension at RFS.
FLATNESS TOLERANCE FOR A WIDTH-TYPE FEATURE
Flatness Tolerance for a Width-Type Feature: A flatness feature control frame placed according to option (a) or (d) in (Table Geometric Characteristics and Their Attributes) associated with a width dimension replaces Rule #1's requirement for perfect form at MMC with a separate tolerance controlling the overall flatness of the width-type feature. Where the tolerance is modified to MMC or LMC, it establishes a Level 2 virtual condition boundary as described in (Level 2 – Overall Feature Form) and in Fig. 1a and Fig. 1b below. Alternatively, "the center method" described in (Level 2 Adjustment – Actual Local Sizes) may be applied to a flatness tolerance at MMC or LMC, but there's rarely any benefit to offset the added complexity. Unmodified, the tolerance applies RFS and establishs a central tolerance zone as described in (Tolerance Zone Shape), within which the feature's derived median line shall be contained.
This application of a flatness tolerance is an extension of the principles of (Straightness Tolerance for a Cylindrical Feature). Y14.5 suggests an equivalent control using the "straightness" characteristics symbol. However, where strict adherence to Y14.5 is needed, then straightness symbol should be used.
UNDERSTAND FLATNESS VERIFICATION PRINCIPLES
How to verified a flatness tolerance applied to a planar surface:
How to verify a flatness tolerance (at MMC) applied to a feature of size:
When should a functional gage be used to verify a flatness tolerance? Whenever a flatness tolerance is applied at MMC to a feature of size. Flatness measurement is a T.I.R (Total Indicator Reading) or F.I.M (Full Indicator Movement) that is achieved with a probe or dial indicator. Surface must be leveled (optimum plane) so that the probe sees only hills and valleys of flatness error. Alternative is (3) Pins. Three pins at exactly the same height within millionths. The pins level the surface.