GEOMETRIC DIMENSIONING AND TOLERANCING Contact : Octagon Octagon Consultant Consultants s – Pune Contact Contact No. No. – 020 25439282 25439282.. Mobile - 9325084622.
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Geometry
• It is is the the scie scienc ncee of of the the prop propert erties ies and and rela relatio tions ns of lines, surfaces and solids.
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Various Geometric Shapes
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Dimension
• It is is a measu measura rable ble exten extent, t, as as leng length, th, bread breadth th and and depth.
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Tolerance •
It is a all allow owab able le vari variat atio ion n in in any any meas measur urab able le property.
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The The tota totall amo amoun untt tha thatt fea featu ture ress of of the the part part are are permitted to vary from the specified dimension.
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The The tole tolera ranc ncee is is the the diff differ eren ence ce betw betwee een n the the maximum and minimum limits.
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Two Two com commo mon n met metho hods ds to spec specif ify y tole tolera ranc nces es – limit tolerances – plus plus-m -min inu us tole tolera ran nces 5
Limit tolerances
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Plus Minus Tolerances
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Dimension Anatomy
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What is GD&T • ASME Y14.5M-1994 - The national standard for dimensioning and tolerancing in the United States. • ASME stands for American Society of Mechanical Engineers. • The Y14.5 is the standard number. "M" is to indicate the standard is metric, and 1994 is the date the standard was officially approved. 9
What is GD&T Geometric Dimensioning and tolerancing (GD&T) is a language used on mechanical engineering drawings composed of symbols that are used to efficiently and accurately communicate geometry requirements for associated features on component and assemblies. A method to specify the shape of a piece of hardware on an engineering drawing.
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What is GD&T •
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A set of fourteen symbols used in the language of GD&T. It consists of well-defined of symbols, rules, definitions and conventions, used on engineering drawings to accurately describe a part. GD&T is a precise mathematical language that can be used to describe the size, form, orientation, and location of part features. GD&T is also a design philosophy on how to design and dimension parts.
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Why GD & T ?
Since GD & T is a language of communication on engineering drawing it is better to understand by the study of engineering drawing with and without GD & T symbols.
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Advantages of GD&T •
Use of this language or tool “can provide economic and technical advantage” stated the ASME.
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Maximizes quality of the products.
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Provides uniformity of specification and interpretation (reducing guesswork and controversy)
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Geometric dimensioning dramatically reduces the need for drawing notes to describe complex geometry requirements on a component or assembly by the use of standard symbology.
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Ensures the design requirements are carried out.
GD&T facilitates an efficient means to communicate specific datums on a part. Without the use of a datum system (zero reference) on a part, it is not clear to manufacturing or quality where to manufacture or measure from. Additionally, the use of datums dramatically simplifies the design and specification of parts for use in manufacturing and quality verification steps. .
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Provides maximum production tolerances (Bonus & Round tolerances always provided)
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Provides maximum production tolerances (Bonus & Round tolerances always provided)
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Create a part design that focuses on the product function. Convert product requirements into dimensional specifications. Better define parts without the need for assumptions. Document the design for future use. Discover problems in the design stage. Ensure that parts will assemble. Have less "hand fitting" at assembly. Ensure that parts are inspected as intended. Inspect parts more quickly. Reduce scrap or rework. Make a replacement that fits into the assembly. 25
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Have multiple sources on various parts of an assembly. Make valid engineering calculations. Have common parts across similar assemblies. Design subassemblies in different locations and have them function correctly. Do tolerance analysis to study the effect of part tolerances on the assembly. Use state of the art software tools to analyze parts in an assembly. Use state of the art software tools to inspect the parts. Reduce the risk caused by vague specifications.
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And finally saves money.
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Key Terms Feature The general term applied to a physical portion of a part, such as a surface, pin, hole, or slot.
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Key Terms Feature Of Size One cylindrical or spherical surface, or a set of two opposed elements or opposed parallel surfaces, associated with a size dimension. Examples: Cylinder, sphere, slot, etc.
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Key Terms Actual Local Size The value of any individual distance at any cross section of a FOS
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Key Terms Actual Mating Envelope (AME) A similar perfect feature counterpart that can be circumscribed/inscribed about/within the feature so it just contacts the surfaces at the highest points (Envelope is an imaginary, mathematical surface of a perfect form which establishes a boundary for a physical feature.) It is derived from an actual part 30
Actual Mating Envelope (AME)
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Actual Mating Envelope (AME)
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Tolerance Stack-up The additive rule for tolerances is that tolerances taken in the same direction from one point of reference are additive.The consequence is that tolerances to the same point taken from different directions become additive. The effect is called tolerance stack up. 33
Tolerance Stack-up TOOL APPLICATION Tolerance stacks are an important tool used to evaluate the variable features and dimensions of a single part or an assembly of parts. Tolerance stacks are used in the calculation of fits and fit-ups, tolerance accumulation/allocation, datum selection and the use of datum and feature modifiers.
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GD&T Concepts Basic Dimension • A numerical value used to describe the theoretically exact size, profile, orientation, or location of a feature or datum target. • A basic dimension is always associated with a feature control frame or datum target. Block tolerance does not apply and the applicable tolerance will be given to the feature control frame. • Specified by enclosing the numerical value in a rectangle
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Basic Dimension
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Regardless of Feature Size (RFS) “Regardless of feature size”, a statement to the effect that the size of the considered feature may not influence the tolerance or the datum reference frame under consideration. Indicates a geometric tolerance applies at whatever size the part is produced.
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Maximum Material Condition (MMC)
The condition in which a feature of size contains the maximum amount of material everywhere within the stated limits of size.
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Least Material Condition (MMC)
The condition in which a feature of size contains the least amount of material everywhere within the stated limits of size.
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Maximum Material Condition (MMC)
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Maximum Material Condition (MMC)
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Bonus Tolerance • An additional tolerance for a geometric control permitted with MMC or LMC modifiers
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Bonus Tolerance
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Bonus Tolerance
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Bonus Tolerance
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Bonus Tolerance
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Bonus Tolerance
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Bonus Tolerance
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Bonus Tolerance
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Bonus Tolerance • When the MMC modifier is used, it means: – the stated tolerance applies when the FOS is at MMC – as part size goes away from MMC, an increase in the stated tolerance is permitted – the increase is equal to the departure from MMC called the bonus tolerance – geometric characteristic can be verified with a fixed gage 50
Bonus Tolerance
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Virtual Condition • The boundary generated by the collective effects of MMC, size limit of a feature and any associated geometric tolerance.
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The Virtual Condition of a feature is a concept used to describe the worst-case envelope which either of two features must lie within in order to mate acceptably. For a shaft that fits into a hole, the shaft virtual condition must be smaller than the hole virtual condition. 52
For an external feature of size, such as a shaft, the virtual condition is equal to the size at MMC plus the size of the tolerance zone. For the shaft in Figure , the diameter of the virtual condition is the diameter of the MMC shaft plus the diameter of the position tolerance zone
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For an internal feature of size, such as a hole, the virtual condition is equal to the size at MMC minus the size of the tolerance zone. For the hole in Figure , the diameter of the virtual condition is the diameter of the MMC hole minus the diameter of the position tolerance zone.
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Figure shows the shaft and hole virtual conditions superimposed. Since the shaft virtual condition is smaller than the hole virtual condition, the two parts will always mate.
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Virtual Condition
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Virtual Condition
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Virtual Condition
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Virtual Condition
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Virtual Condition
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In summary, the way to calculate virtual condition (VC) for a shaft and hole is: SHAFT VC = MMC diameter + Position Tolerance Zone Diameter HOLE VC = MMC diameter - Position Tolerance Zone Diameter
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Application Of Virtual Condition
Virtual condition is extremely useful in the design of functional gauges. A functional gauge made to virtual condition will ensure that a part will always mate with its counterpart.
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Functional gauge design •
GDT functional gauges designed according to virtual condition ensures part fit. Virtual condition is the envelope for worst-case part fit. A part which fits on such a functional gauge is guaranteed to fit to all mating parts.
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Virtual condition is extremely useful in the design of functional gauges. A functional gauge made to conform to virtual condition will ensure that a part will always mate with its counterpart. In summary, the way to calculate virtual condition (VC) for a shaft and hole is: SHAFT VC = MMC diameter + Position Tolerance Zone Diameter HOLE VC = MMC diameter - Position Tolerance Zone Diameter 63
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Symbols and Modifiers Feature Control Frame A rectangular box that is divided into compartments within which the geometric characteristic symbol, tolerance value, modifiers, and datum references are placed.
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Symbols and Modifiers Geometric Tolerances are divided into five categories 1. 2. 3. 4. 5.
Form control Orientation control Location control Composite control Profile controls
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Symbols and Modifiers Form control
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Symbols and Modifiers Orientation Control
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Symbols and Modifiers Location Control
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Symbols and Modifiers Composite Control
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Symbols and Modifiers Profile Controls
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Summary of control tolerances
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Symbols and Modifiers Modifiers
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Free State • •
Free state variation is a term used to describe distortion of a part after removal of forces applied during manufacture. This distortion is principally due to weight and flexibility of the part and the release of intern1 stresses resulting from fabrication. A part of this kind, for example, a part with a very thin wall in proportion to its diameter, is referred to as a non-rigid part. In some cases, it may be required that the part meet its tolerance requirements while in the free state. In others, it may be necessary to simulate the mating part interface in order to verify individual or related feature tolerances. This is done by restraining the appropriate features. The restraining forces are those that would be exerted in the assembly or functioning of the part.
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Projected Tolerance Zone When it is desired to project a stated tolerance zone beyond the boundaries of a part the projected tolerance zone symbol is used. The symbol is placed after the stated tolerance and any modifiers and is followed by a value for the projection distance. The use of this symbol in local and general notes is prohibited.
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Projected Tolerance Zone The Projected tolerance zone modifier is generally used for threaded holes and holes that will receive a pin (usually a press fit) and the designer is concerned that the screw or pin projecting from the hole might interfere with the surface of the clearance hole in the mating part. In these cases it is more important to know where the stud or fastener will be rather than where the hole or tapped hole is. The projected value is usually the maximum thickness of the mating part or the maximum height of the pin or stud.
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GD&T Rules As the purpose of the GD&T is to describe the engineering intent of the item, there are some fundamental rules need to be applied. •
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All dimensions must have a tolerance.Nothing can be made to a perfection;therefore, an appropriate tolerance must be available.Tolerances may come in the form of limits of size applied to basic dimensions, ± style tolerance applied directly to dimensions as a tolerance block or a general note. The only exception is when a dimension is marked as minimum, maximum,stock or reference. All dimensions necessary to exactly reproduce the shown geometry must be present.Measurement and scaling should not be required. In order to avoid ambiguities, only the minimum dimensions required should be present. If additional dimensions would be helpful but not required, they should be marked as reference.
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GD&T Rules •
Dimension should be applied to features and arranged in such a way as to represent the function of the individual features.
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Descriptions of manufacturing methods should be avoided. The geometry should be described without explicitly defining the method of manufacture.
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If certain sizes are required during the processing, but are not required in the final geometry (due to shrinkage or other causes) they should be marked as NON-MANDATORY.
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All symbols should be arranged for maximum readability. They should be applied to visible lines in true profiles whenever possible.
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When geometry is normally controlled by gage sizes or by code, the dimensions should still be included but should with the gauge or code number in parentheses following or below the dimension. 80
GD&T Rules •
Angles of 90° are assumed when basic dimensions or centerlines are used but no angular dimension is explicitly shown.
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Dimensions and tolerances are valid at 20° Celsius unless stated otherwise.Unless explicitly stated, all dimensions and tolerances are valid when the item is in a free, unconstrained state.
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Dimensions and tolerances apply to the full length, width, and depth of a feature.
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Dimensions only apply at the level of the drawing where they are placed. If the intention is for them to apply at multiple levels, this must be stated.
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GD&T Rules •
Rule #1
Standard Rules
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Where only a tolerance of size is specified, the limits of size of the individual feature describe the extent to which variation in the geometric forms as well as size are allowed.
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When Rule #1 applies, the size limits define the size and form of an FOS (not orientation, location or relation between FOS)
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When Rule #1 Applies • Form of feature: • Surfaces must not extend beyond a boundary of perfect form at MMC • Where actual size has departed from MMC toward LMC, a variation in form is allowed equal to that amount of departure 83
Standard Rules •
Rule #1 •
The control of geometric form based on size is not applicable to :
Stock such as bars
Sheets
Tubings
Structural shapes
The forms of these shall be as per the industry standard norms. 84
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Rule #1 boundary: The part must be within the MMC envelope.
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Standard Rules
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Standard Rules •
Rule #2a •
For all applicable geometric tolerances, RFS applies with respect to the individual tolerance, datum reference or both, where no modifying symbol is specified.
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Modifiers for MMC and LMC must be specified on the drawing where it is required.
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Standard Rules •
Rule #2a •
Characteristics and controls which can be applicable to size features and thus to which RFS applies under Rule#2 unless modified to MMC or LMC are:
Straightness
Perpendicularity
Angularity
Parallelism
Position 88
Standard Rules •
Rule #2a •
Characteristics and controls which are always applicable at RFS under Rule #2 and due to the nature of the requirement cannot be applied at MMC or LMC are: Circular Runout Total Runout Concentricity Symmetry Flatness Roundness Cylindricity Profile of line Profile of surface 89
Standard Rules •
Rule #2b Pitch Diameter Rule •
Each tolerance of orientation or position and datum reference specified for a screw thread applies to the axis of the thread derived from the pitch cylinder. Where an exception to this is necessary, it has to be mentioned below as MAJOR or MINOR
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Each tolerance of orientation or location and datum reference specified for gears, splines must designate the specific feature of the gear to which it applies(PITCH,MAJOR DIA, MINOR DIA)
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Standard Rules •
Rule #2c Datum/Virtual condition Rule •
A virtual condition exists for a datum feature of size where its axis or center plane is controlled by a geometric tolerance. In such cases, the datum feature applies at its virtual condition even though it is referenced in a feature control frame as MMC or LMC.
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Datum
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Datum feature •
An actual feature of a part that is used to establish a datum.
Datum feature Symbol •
The symbolic means of indicating a datum feature consists of a capital letter enclosed in a square frame and a leader line extending from the frame to the concerned feature, terminating with a triangle. 93
Datum vs. datum feature
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Datum vs. datum feature
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Datum feature symbol
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Datum feature symbol
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Datum feature symbol
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Datum feature symbol
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Datum feature symbol
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Datum target •
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A specified point , line or area on a part used to establish a datum.
Datum target frame
The datum targets are indicated by a circular frame divided in two compartments by a horizontal line. The lower compartment is reserved for a letter and a digit.The letter represents the datum feature and the digit datum target number. The upper compartment is reserved for additional information, such as dimensions of the target area. If there is not sufficient space within the compartment, the information may be placed outside and connected to the appropriate compartment by a leader line. 101
Datum target •
A specified point , line or area on a part used to establish a datum.
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Common_datum •
A common_datum is a type of Datum that corresponds to a datum that is established from more than one datum feature. On technical drawing, a datum that is established from multiple datum features is indicated by by placing the identifying letters of the datum features, separated by a dash, within a single compartment in a feature control frame. There is no significance to the order of the datum feature identifying letters within a compartment of the feature control frame. 103
Datum system •
A group of two or more separate datums used as a combined reference for a toleranced feature.
Datum reference frame •
Datum reference frames are coordinate systems used to locate and orient part features.
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Datum reference frame •
A framework that consists of three mutually perpendicular datum planes, three datum axes(located at the intersection of each pair of datum planes), and a datum point (that is located at the intersection of the three datum planes). .
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A Cartesian coordinate system established using the Datums extracted from a set of Datum Features referenced in a Feature Control Frame.Datum Reference Frames serve to orient and locate tolerance zones.
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Datum reference frame
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Datum reference frame • •
Datums Are Mutually Mutually Perpendicular Perpendicular - Even When When the Datum Features Are Not. It isn isn't 't alw alway ays s poss possib ible le or or prac practi tica call to sel selec ectt datu datum m feat featur ures es tha thatt are mutually perpendicular to one another when establishing a datum reference frame. Notice that datum feature C is not nominally perpendicular to datum feature B. The datum feature simulator for C would be made at at 35° to the datum feature simulator for B (shown here in red). The actual datum planes (shown in blue), which comprise the datum reference framework, would however be mutually perpendicular to one another as is illustrated in the last figure. The deviation of the hole from the 55mm BASIC location would be measured from the third datum plane-not from the sharp point on the actual part.
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Datum reference frame
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Datum precedence •
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A dat datum um prec preced eden ence ce assi assign gnme ment nt corr corres espo pond ndss to to the the specification of the order in which a datum is established within a datum system. On tec techn hnic ical al dra drawi wing ngs, s, the the pre prece cede denc ncee of a datu datum m wit withi hin na datum system is typically specified in a feature control frame. The location of the compartment containing the letter(s) corresponding to the datum feature(s) from which the datum is established indicates the assigned precedence. The compartment for the primary datum (if it exists) is immediately to the right of the compartment containing the tolerance value. The compartment for the secondary datum (if it exists) is immediately to the right of the compartment for the primary datum. Lastly, the compartment for the tertiary datum (if it exists) is immediately to the right of the compartment for the secondary datum. 110
Datum precedence EXAMPLE—Figure (b) contains a feature control frame that specifies a datum system in which datum A is the primary datum, datum B is the secondary datum, and datum C is the tertiary datum.
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Datum precedence EXAMPLE—Fig. (c) contains a feature control frame that specifies a datum system in which datum A is the primary datum, datum C is the secondary datum, and datum B is the tertiary datum.
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Datums of size •
MMC and LMC on a Datum Reference do not Affect the Feature's Tolerance! Referencing a datum feature at MMC or LMC does not change the tolerance on the feature(s) being toleranced. These modifiers, however, may allow the part to float or shift relative to the datum reference frame. Notice that the 15mm diameter hole references both B and C at their MMC. Therefore, if both B and C are produced at their MMC, there is no shift of the datum feature relative to the datum reference frame established by A, B and C. If, however, datum feature B happened to be produced at 8.1 (its LMC size) and was perfectly perpendicular to the primary datum, the part could shift as much as 0.2mm total, relative to the datum reference frame. In other words, the part could be slid left or right 0.2mm total while maintaining contact with datum A in order to bring the 15mm hole into its allowable position tolerance zone. Some inspectors treat this 0.2mm shift as an additional bonus tolerance on the 15mm hole which would allow additional deviation in all directions. This would be incorrect since no movement of the part is allowed perpendicular to the primary datum. 113
Datums of size
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Datum simulators
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Datum simulators
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Datum simulators
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Datum simulators
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Datum simulators
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Composite Datum Features •
A composite datum feature is a datum feature that is composed of other features. Figures depict two examples of composite datum features.
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Composite Datum Features
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Dimension origin •
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When a dimension origin is specified for the distance between two features, the feature from which the dimension originates defines an origin plane for defining the tolerance zone. In such cases, the origin plane shall be established using the same rules as are provided for primary datum features. Signifies that the dimension originates from the plane established by the shorter surface and dimensional limits apply to the other surface. 123
Dimension origin
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Dimension origin
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Pattern locating tolerance zone framework(PLTZF) and Feature relating tolerance framework(FRTZF) •
The control of the location of the pattern as a group is called the patrem-locating tolerance zone framework
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When the pattern consists of two or more features, there is the possibility, through the use of composite tolerancing, to control the relative location of features within the pattern. This is done by specifying a secondary location tolerance, called the feature relating tolerance zone framework 126
Form control
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Flatness •
Flatness is the condition of surface having all of its elements in one plane.
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Flatness • •
A flatness error (out of flat condition) is the amount a surface can very from a flat plane. Flatness is the distance between two parallel planes, which includes all the elements (high and low points) of a surface.
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Flatness •
A flatness control zone is two parallel planes spaced apart by the flatness tolerance value. • In theory one plane of the tolerance zone is oriented by the high points of the surface. • The remaining plane is parallel to the first plane and offset by the flatness tolerance value.
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Flatness •
Size tolerance: The actual local size must be within the maximum and minimum limits of the size dimension.
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Flatness •
Rule #1 boundary: The part must be within the MMC envelope.
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Flatness •
Flatness Control: All surface elements of the tolerance feature must lie between two parallel planes 0.2 apart.
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Flatness •
Inspection of Flatness: One plane of flatness tolerance zone is established by a surface plate. When the part is brought into contact with the surface plate, the high points of the part contact the surface plate surface. Thus, the high points of the surface are contacting one plane of the tolerance zone. A dial indicator is placed in the hole of a surface plate. The dial indicator is calibrated to zero when it is line with surface plate. When the part is placed on the surface plate and moved in a random fashion over the dial indicator. The dial indicator is reading the difference between the surface high and low points. 134
Flatness •
Inspection of Flatness: The maximum dial indicator reading is the distance between the places that contain the high and low points of the surface. This is the flatness error of the surface.
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Flatness
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Flatness Applications •To ensure the integrity of mating or mounting surfaces •To ensure that surfaces seal properly •Appearance
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Flatness •
Examples: Sealing surface.
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Flatness •
Examples: Sealing surface.
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Flatness •
Examples: Flange mounting.
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Flatness •
Examples: Flange mounting.
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Flatness •
Examples: Flange mounting.
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Straightness •
A condition where an element of a surface or an axis is a straight line.
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Straight can be defined for
Straight of surface Straightness of Axis(RFS) Straightness of Axis(MMC)
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Straightness
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Straightness
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Straightness
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Straightness
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Straightness
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Straightness
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Straightness
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Straightness
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Straightness
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Straightness
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Circularity •
Circularity is a condition of a surface of revolution.
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Controls the roundness of any feature with circular cross section.
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Circularity
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The circularity tolerance is applied in either view of the drawing, whichever is most convenient.
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The tolerance zone for a circularity tolerance is bounded by two concentric circles.
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Circularity
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Circularity •
The tolerance value is determined by the radial distance between the circles, not the distance between the diameters..
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Tolerance specifies a tolerance zone within which the element or axis must lie.
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Circularity
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All elements of the surface are to be within the specified size tolerance and the boundary of a perfect form at MMC.
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The feature control frame is attached to the concerned surface by a leader.
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Tolerance value should equal to or less than, one half of the size tolerance(limits)
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Circularity •
Circularity of each circular element compares the form of each element to a circle. Since the control of the surface itself is of concern, its size variation is irrelevant to the form. Therefore, MMC or RFS principles cannot be applied.
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Part size may vary within its tolerance, yet the circularity tolerance remains the same.Where size of the produced part approaches LMC, the roundness tolerance proportionately diminishes.
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Circularity Inspection
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Circularity •
Circ Circul ular arit ity y tole tolera ranc nce e does does not not asso associ ciat ate e with with the the datum reference, each circular element relates to a perfect counterpart of itself, a circle thus no datum is needed.
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Circularity Circularity and the position
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Circularity Circularity Inspection
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Cylindricity Cylindricity describes a condition of a surface of revolution in which all points of a surface are equidistant from a common axis.
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Cylindricity •
Cylindricity tolerance specifies a tolerance zone bounded by two concentric cylinders within which the surface must lie.
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Cylindricity tolerance differs from circularity tolerance in that it applies to the total surface (entire length) simultaneously. 165
Cylindricity •
The cylindricity tolerance must be less than the feature size tolerance.
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Cylindricity tolerance is composite control of form which includes circularity, straightness and taper. 166
Cylindricity •
All the elements of the surface controlled are to be within the specified size tolerance and the boundary of perfect form at MMC.
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Cylindricity tolerance relates to the control of the surface form which is irrelevant to size, therefore RFS or MMC principle cannot be applied.
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Cylindricity tolerance is applicable only to cylindrical features, either inside or outside cylinders.
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Cylindricity •
Cylindricity tolerance does not associate with a datum reference, the cylindrical elements relate to a perfect counterpart of itself, a cylinder, thus no datum is needed.
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Cylindricity Cylindricity inspection
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Orientation Control
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Perpendicularity •
This could be considered flatness or straightness of an axis 90 degrees to a datum.
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Perpendicularity •
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Two common tolerance zones for a perpendicularity control are. -Two parallel planes. -A cylinder. Usually, when perpendicularity is applied to a surface or a planar feature of size, the tolerance zone is two parallel planes. When perpendicularity is applied to a cylindrical feature of size and a diameter symbol precedes the tolerance zone is cylinder.
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Perpendicularity
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Perpendicularity •
1. 2.
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When a perpendicularity is applied to a planar surface, the following conditions apply. The tolerance zone is two parallel planes. The tolerance value defines the distance between the tolerance zone planes. All elements of the toleranced surface must be within tolerance zone. The flatness of the surface is also controlled. 174
Perpendicularity •
When perpendicularity is applied to a feature of size, it controls the orientation of the axis (or center plane) of the actual mating envelope of the feature of size.
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Perpendicularity •
When a perpendicular control contains the MMC modifier, a fixed gauge may be used to verify the requirement. • The gauge size is equal to the worst case boundary of the tolerance feature. • Three points of contact must be maintained between the primary datum feature and the gauge surface. • The orientation of the toleranced feature may vary as long as the part will fit into the gauge. 176
Perpendicularity
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Perpendicularity
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Perpendicularity •
Examples: Assembly-Pilot diameter to mounting face.
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Perpendicularity •
Examples: Orientation-Controlling the orientation between datum features.
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Perpendicularity •
Examples: Orientation-Bracket mounting surface.
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Parallelism •The condition of a surface or axis which is equidistant at all points from a datum of reference.
•The considered feature surface must lie within a tolerance zone between two parallel planes, the stated tolerance apart, which is parallel to the datum plane. 182
Parallelism • Parallelism tolerance always requires a datum reference ; it is the control of a feature in its orientation to a datum feature.
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Parallelism •
The parallelism tolerance must be less than the associated size dimension.(and more appropriately less than one half the size tolerance.
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The parallelism tolerance and size tolerance is verified separately. The surface must be within the specified size limits.
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Parallelism Parallelism vs. Flatness •
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One common area of confusion within GDT is between parallelism and flatness. With, parallelism, a reference is made to a datum plane, while flatness is independent of a datum. Slide shows a block on a surface plate whose three points of contact define the -Adatum plane. As specified, the opposite face of the block must lie within two planes which straddle a plane parallel to and 2.0 from -A- and .125 to either side of it. 185
Parallelism Parallelism vs. Flatness •
Next slide shows a flatness specification on the same surface. No datum plane is involved. A "best fit" plane (3 point contact) defines the mid-plane, and all surface points must lie between two planes which are parallel to this plane, and .125 to either side of it. 186
Parallelism •
When a surface is to be parallel to a datum, the feature control frame is either connected by a leader to the surface or directly connected to the extension line of the dimension.
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Parallelism •
When parallelism is applied to an axis the axis of the hole may be specified within a tolerance zone that is parallel to a given datum. •
The feature control frame is place with the diameter dimension
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Parallelism – Parallelism to line elements. •
When it is desirable to control only individual line elements, rather than the entire surface, the note “EACH ELEMENT” or “EACH RADIAL ELEMENT”
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Parallelism
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Parallelism
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Parallelism
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Parallelism
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Parallelism
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Parallelism
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Angularity •
The distance between two parallel planes, inclined at a specified basic angle in which the surface, axis, or center plane of the feature must lie.
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The condition of a surface, center plane or axis being exactly at a specified angle (other than 90 degree) from a datum plane or axis.
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This could be considered flatness or straight of an axis at some angle to a datum.
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Angularity
Angularity tolerance always requires a datum reference, it is control of a feature in its orientation to a datum feature.
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Angularity –Figure 1 the application of angularity to a surface. –Figure 2 is the application of angularity to an axis. –Figure 3 is the application of angularity to a center plane
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Angularity •
Angularity tolerance is independent of the size tolerance and is verified separately. The part also meet all size requirements. Aspects of the controlled angular surface , which is also dimensioned and toleranced as a separate requirement, must also meet such requirements. 199
Angularity •
Inspection of Angularity:When setting up a part for the measurement of angularity on a sine bar, it is difficult to orient the part before performing the inspection.
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Angularity •
This problem is overcome if second datum is referenced in the angularity callout.
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Composite Control
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Circular Runout •
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Runout is a composite tolerance used to control the functional relationship of one or more features of a part to a datum axis. A composite control affects the form, location and the orientation. Runout controls are often used to control the co-axiality of the diameters. 203
Circular Runout •
There are two types of runout control, circular runout and total runout. The type used is dependent upon design requirements and manufacturing considerations. Circular runout is normally a less complex requirement than total runout.
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Circular Runout •
There are two two requirements when using a runout control 1. The datum reference must establish a datum axis. 2. Runout must be applied at RFS.
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Circular Runout •
There are three ways to establish a datum axis for a runout specification.
1. Use a single diameter of sufficient length as a datum feature. A single diameter can only be used when it is long enough to orient the part. If a diameter is too short to establish an axis for inspection, the diameter will not serve well as a primary datum feature for the part in its assembly. 206
Circular Runout 2. Use two or more coaxial diameters a sufficient distance apart as datum features to create a single datum axis. Two or more diameters are used to establish a datum axis when they serve an equal role in establishing the orientation of the part in its assembly.
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Circular Runout 2. Use a surface as a primary datum feature and use a diameter at a right angle as a secondary datum feature. A surface primary and a diameter secondary are used when the surface orients the part and the diameter locates the part in the assembly. When the surface is used as a primary datum feature, the diameter should be very short.
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Circular Runout • •
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A dial indicator is often used to verify a runout control First, the part is located in a chuck or collet to establish datum axis A. A dial indicator is placed on the surface being checked. As the part is rotated 360 degree, the dial indicator movement is the runout value of the circular element. Several independent dial indicator readings are made at different places along the diameter. 209
Circular Runout •
Summary: The dial indicator movement (runout error) can be a result of form error of the diameter.
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The dial indicator movement (runout error) can be a result of axis offset of the diameters.
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In industry, most parts have a combination of form error and axis offset. When the dial indicator reading is taken, it combines several types of part errors into a single dial indicator reading.
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Circular Runout •
Summary: Since the dial indicator reading contains several types of errors and provides a single out put (runout error) a runout control is considered a composite control.
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Circular Runout •
Examples: Locate coaxial diameters on a part.
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Circular Runout •
Examples: Create an axis from coaxial diameters.
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Total runout Runout •
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Surfaces Constructed at Right Angles to a Datum Axis
A total runout tolerance for a surface constructed at right angles to a datum axis specifies that all points of the surface must lie in a zone bounded by two parallel planes perpendicular to the datum axis and separated by the specified tolerance. 214
Total runout Runout •
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Surfaces Constructed Around a Datum Axis
A total runout tolerance zone for a surface constructed around a datum axis is a volume of revolution generated by revolving an area about the datum axis. Is the variation across the entire surface of a cylindrical feature.
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Total Runout • Total runout is used to control the combined variations of circularity, straightness, coaxiality, angularity, taper and profile when applied to surfaces around and at right angles to a datum axis. • Note that total runout cannot be applied to conical or curved surfaces as can circular runout.
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Runout circular vs. total
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Runout circular vs. total
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Location Control
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Position True Position • The theoretically exact location of a FOS as defined by basic dimensions.
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Position Position Tolerance defines a zone within which the axis or center plane of a feature is permitted to vary from true (theoretically exact) position.
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Position Location tolerances control position as well as orientation and form.Thus for a hole , a location tolerance will control the position of the hole axis relative to specified datums, tilt of the axis of the hole, and form deviations such as convexity or concavity of the axis.
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Position
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Position •
A position tolerance controls the position of the feature relative to one or more datums.
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All points on the axis of the hole must lie within the tolerance zone. The zone applies over the complete depth of the hole. 224
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Position
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Concentricity •
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Concentricity describes a condition in which two or more features (cylinders, cones, spheres, etc.) In any combination have a common axis. Concentricity is that condition where the median points of all diametrically opposed elements of a figure of revolution (or correspondingly located elements of 2 or more radially disposed features) are congruent with the axis (or center point) of a datum feature. 229
Concentricity •
Concentricity tolerance is a cylindrical (or spherical) tolerance zone whose axis (or centre point) coincides with the axis or centre point of the datum feature(s). The median of all correspondingly located elements of the feature(s) being controlled, RFS must lie within the cylindrical or spherical tolerance zone. The specified tolerance and the datum reference can only apply on RFS basis.
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Concentricity •
A Concentricity (coaxiality) tolerance controls the axis of one feature relative to the axis of another feature. In following case , the axis of the larger cylinder must lie within a cylindrical tolerance of 0.04 mm of the axis of the smaller cylinder (defined as datum A)
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Concentricity •
Concentricity tolerance is more restrictive and potentially costly requirement due to the possible need for detailed analysis of the part in verification. Before concentricity tolerance is selected, the options of position tolerance at MMC or runout tolerance should be considered.
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Concentricity tolerance considers in composite the effect of various surface errors such as out of straightness, out of circularity, out of cylindricity, etc as the median points are determined. 232
Concentricity •
Concentricity verification requires a form of differential measurement at opposed elements of the surface, to determine the resultant feature median point. Where precision spindle machine methods are used, polar graph printout and analysis with overlay gages will achieve the same results. Computerization analysis is also used where such capability is available. • Concentricity tolerance is always specified and implied on an RFS basis. If MMC principles are desired, consider position tolerance. • All size tolerance must be met independent of the concentricity tolerance. 233
Concentricity
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Concentricity
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Concentricity and position • • •
Position may use the MMC and LMC modifiers on the tolerance and Datum references. Concentricity ignores the size, roundness and cylindricity of the feature The next slide shows a possible gage (ignoring gage tolerance and wear allowance) that could be used when MMC is the modifier. If the sizes are within the size tolerances but the part doesn't fit the gage, the features position (coaxiality) is out of spec. This type of control works well when the main concern is the assembly of this part to another. The gage may usually be thought of as representing the worst mating part 236
Concentricity and position
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Runout vs. Concentricity •
Avoid specifying concentricity. Concentricity requires deriving the median line of a feature. All of the features shown below are concentric. Usually, designs require that a feature be round as well as concentric like example A. A better geometric control is usually circular runout. Circular runout controls circularity (roundness) as well as concentricity.
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Symmetry •
Symmetry is condition in which a feature (or features) is symmetrically disposed about a canter plane of a datum feature. • Symmetry tolerance is the distance between two parallel planes equally disposed about the center plane of the datum feature. • All size tolerances must be met independent of the symmetry tolerance.
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Symmetry
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Profile Controls
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Profile Tolerances • Profile tolerance – The outline of an object represented either by an external view or by a cross section through the object. – Profile tolerances may be used to control form, orientation and location. • There are two profile tolerance types, – Profile of a line and profile of a surface. – Note: when the profile is other than flat, basic dimensions must be used to describe the profile. » A basic dimension is depicted with a box around the dimension and is considered an exact dimension 242
Profile Tolerances • Profile of a Line – A two-dimensional cross section tolerance that extends along the length of the feature. – The line profile tolerance is used where parts have changing cross section along their length and do not have to be along the entire feature. – Figure shows the feature control frame for profile of a line geometric tolerance.
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Profile Tolerances
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Profile of a Line – Many automobile companies use an exact profile to gage sheet metal parts that have changing cross sections. • The hood of an automobile is a good example of such a cross section that has a changing profile. • The profile tolerance is assumed a bilateral tolerance, which is the tolerance both above and below the true profile line. • The following examples give usage of the profile of a line tolerance.
Cross section of a simple contour that is controlled by line profile tolerance.
profile of a line to control all around a cross section of a wing . 245
special case of using profile of a line for a unilateral specification.
Profile of a Surface
• Profile of a Surface
– Used to control all the elements of a surface and treats them as a single entity. – Profile of a surface, as its definition states, covers all points on a surface in three dimensions. – Figure shows the feature control frame for profile of a line geometric tolerance.
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Profile Tolerances
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Profile of a Surface – As with profile of a line, profile of surface can be measured with a mater gage to control features of the surface or profile. – The following examples give usage of the profile of a surface tolerance. • Figure 1 is a cross section of a plate with a slot. The slot is controlled by a profile of a surface tolerance. That means that the depth of the slot as well as it’s cross sectional profile is controlled. • Figure 2 uses profile of a surface to control the surface of a part similar to that seen in figure
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Profile of a Surface
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Profile of a Surface
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Profile
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