Geometric Dimensioning and TolerancingObjectives What is GD&T How GD&T scores over limit type tolerancing Symbols and interpretation Concepts Datum features Tolerance zones Material condition modifiers Composite tolerancing Advantages of GD&T Glossary What is Geometrical Dimensioning and Tolerancing DEFINITION Geometric Dimensioning and Tolerancing (GD&T) is a universal language of symbols, used to efficiently and accurately communicate geometry requirements for features and components. It encourages designers to define a part based on how it functions (design intent) in the final product. ASME Y14.5M – 1994 is the accepted geometric dimensioning and tolerancing standard. Datum feature Basic dimension First glimpse: Part dimensioned using GD&T. Feature control frame Communicates complex geometrical descriptions not possible otherwise in language. rejection. Gives an acceptable range of values of an individual dimension (limits of dimension). in which the feature must lie in order to meet the design criteria. leading to successful end product. Eliminates guesswork. Limit Type Tolerancing Separates the specification of tolerance from the dimensioning. such as an area or a volume. Geometric Tolerancing Induces problems related to ambiguity. enables mfg. fit and function and not just size of the part. Results in deviation from design intent. Specifies a geometric region (tolerance zones). or how much a hole can tilt relative to a surface. according to design intent. thus reduces confusion. rework and loss of profits. Allows more flexibility and precise controls that relate directly to the form. No provision to specify how flat a surface needs to be. guesswork and multiple interpretation of part drawing.Tolerance can be specified only where a dimension is defined. . Parts of a feature control frame Each feature control frame can state only one requirement/message.Feature control frame in GD&T Basic sentence in GD&T is put in the form of a feature control frame. Only one set up or gage for one FCF. . It states the requirement for the feature to which it is attached. They are significant according to their precedence in the FCF.g. Broken down First compartmentGeometric characteristic symbol: Specifies the character to which the tolerance is to be applied.Feature control frame.contains one of the 14 geometric characteristic symbols. Third compartment – Datum system: Specifies datums if applicable. parallelism etc. First compartment. Flatness . angularity . Third compartment.contains the datum reference frame .profile . E. Second compartment. specifies a spherical tolerance zone. Broken down Second compartment. specifies a cylindrical tolerance zone.Feature control frame.g. Second compartment -Material condition modifiers: Features of size can be provided bonus tolerances using these modifiers. material condition modifier and other symbols.contains actual tolerance. and no modifier is specified. the default is RFS. If no symbol is given. or a total wide zone (like in profile tolerance). . E. If the feature being controlled is a feature of size.Symbol to specify shape of tolerance zone: This symbol precedes the tolerance and specifies the shape of tolerance zone. the default shape is parallel planes. 5.Other symbols in the FCF The symbols for projected tolerance zone. New datum feature symbol has been introduced in ANSI Y14. tangent plane. free state. 1994. . and statistical tolerance always follow the material condition modifier. point or axis from which a dimensional measurement is made. Diameter Symbol . etc. lines. block.A datum simulator a surface of adequate precision oriented to the high points of a designated datum from which the simulated datum is established. axes. planes. cylinders. Datums are points. .A datum feature is the actual component feature used (idealized) to establish a datum. a. indicates a circular feature when used on the field of a drawing or indicates that the tolerance is diametrical when used in a feature control frame The inspection equipment (or gage surfaces) used to establish a datum is the simulator. Datum Feature Simulator -..Concept of Datum System Datum . or geometric relationship of other part features may be established or related.the diameter symbol. Examples: gage pin. from which the location.A theoretically exact plane. and surface of granite block. Datum Feature . B Secondary Datum They are theoretically exact features (surfaces idealized to planes.Concept of Datum System Datums are specified in the third compartment of the feature control frame. . Primary Datum Feature constrained within said tolerance with respect to datums A. axes etc.) from which dimensional measurements are made. they may not be perpendicular to each other. Sample part: Surfaces are idealized to eliminate ambiguity about from where dimensions are to be measured But if both surfaces are idealized simultaneously. either of these two could be the correct position. .Precedence order in datum planes If we use a set of perpendicular datum references. the first side to be pressed against one of the edges (in this case. The part now has only one degree of freedom left. we have a completely constrained position and orientation. Final position depends on which side contacts first. Without precedence order. it can only slide back and forth along this edge. will make contact at the two highest points. Thus. they are specified in an order of precedence governed by the part function. datum A). either of the two positions could be right. Therefore. Once we butt the perpendicular side of the part with the corresponding straight edge (datum B). Concepts of Tolerance Zones Tolerances zones can be defined in GD&T instead of limits of dimension. These are geometric regions (3D or 2D) in which the feature must lie to be acceptable. Defined according to functional requirement of the part. Ensures closeness to real world requirements Enables specifications (like conical tolerance zones) not otherwise possible in limit type dimensioning. . Various symbols used to specify tolerance zones for: Form Position Profile Orientation Runout Symbol Description Circularity Cylindricity Flatness Straightness Concentricity Position Symmetry Profile Profile of a line Angularity Parallelism Perpendicularity Runout Total Runout Geometry Form Form Form Form Position Position Position Profile Profile Orientation Orientation Orientation Runout Runout .Symbols in GD&T GD&T has 14 geometric characteristic symbols. Straightness Straightness describes a condition where an element of a surface or an axis is a straight line. . Flatness Flatness is the condition of a surface having all elements in one plane.25 mm apart. . The surface must lie between two planes 0. 25 mm larger than the other. or sphere) where all points of the surface intersected by any plane (1) perpendicular to a common axis (cylinder. or (2) passing through a common center (sphere) are equidistant from the center. Each circular element of the surface in a plane perpendicular to the axis must lie between two concentric circles.Circularity Circularity describes the condition on a surface of revolution (cylinder. cone). . cone. one having a radius 0. .Cylindricity Cylindricity describes a condition of a surface of revolution in which all points of a surface are equidistant from a common axis.25 mm larger than the other. The cylindrical surface must lie between two concentric cylinders one with radius 0. .12 mm apart.Parallelism Parallelism is the condition of a surface. which are parallel to datum plane A. line. which is equidistant at all.2 mm dia cylindrical zone parallel to datum axis A. The feature axis must lie within a 0. points from a datum plane or axis. The surface must lie between two parallel planes 0. or axis. . or line. from a datum plane or a datum axis. which is 90 deg.2 mm apart.Perpendicularity Perpendicularity is the condition of a surface. axis. perpendicular to datum axis A. The feature axis must lie between two parallel planes 0. The surface must lie between two parallel planes 0.4 mm apart inclined at an angle of 300 to datum plane A.Angularity Angularity is the condition of a surface. 90. 180 or 270 deg. which is at a specified angle (other than 0. axis. or center plane. .) from a datum plane or axis. Profile of a surface Profile of a surface is the condition permitting a uniform amount of profile variation. either unilaterally or bilaterally. . on a surface. Profile of a line Profile of a line is the condition permitting a uniform amount of profile variation. Each line element of the surface must lie between two profile boundaries 0. Profile of a line is used in conjunction with profile of surface. either unilaterally or bilaterally. .006 mm apart in relation to the datum reference frame. along a line element of a feature. Profile of a surface defines the shape or location of a feature while profile of line refines it in one direction. each the circular element of the surface must be within the specified runout tolerance (0.Circular runout Circular runout gives the deviation from the desired form of a circular element of a part surface of revolution through one full rotation (360 deg) of the part on a datum axis At any position. .02 mm full indicator movement) when the part is rotated by 3600. Note: circular runout controls the circular elements of the surface. not the complete surface. the indicator fixed in a position normal to the true geometric shape. about the datum axis. The entire surface must lie within the specified runout tolerance zone (0. with the indicator at every location along the surface in a position normal to the true geometric shape without reset of the indicator. .02 mm full indicator movement) when the part is rotated by 3600 about datum axis A.Total runout Total runout is the simultaneous composite control of all elements of a surface at all circular and profile measuring positions as the part is rotated through 360. .Concentricity Concentricity describes a condition in which two or more features (cylinders.) In any combination have a common axis. The application of concentricity is complex and rare. Diametrically opposed dial indicators maybe used to check this. cones. This controls location. spheres. and can have some effect on the form and orientation of a feature. etc. . but applied to non-cylindrical features. Same concept as concentricity.Symmetry Symmetry is a condition in which a feature (or features) is symmetrically disposed about the center plane of a datum feature Controls opposing points (that form derived median plane). conical. rectangular.Position Position tolerance (formerly called true position tolerance) defines a zone within which the axis or center plane of a feature is permitted to vary from true (theoretically exact) position. . Position tolerancing is used to locate features of size (profile is used to locate features that don’t have a size associated with them). etc. median plane. These tolerance zones can be cylindrical. the axis. Defines a zone within which. or surface of a feature is permitted to lie. holes provide clearance for bolts.Positional tolerance for cylindrical zone Application Part mounts in assembly on surfaces shown. . Note that the diameter symbol is not present in the feature control frames indicating a distance between two parallel planes. Here the axes of the holes must lie in a 0.012X0.Position tolerance for rectangular zone Locates features with a greater tolerance in one direction than other.028 rectangular tolerance zone . .Positional tolerance for spherical zone The centre point of the spherical diameter must lie in a spherical zone of diameter 0. basically located to the DRF.03. .Positional tolerance for conical zone Application Used to control features such as a deep drilled hole. closer at one surface than another. 012 . Hole diameter Tolerance Zone diameter MMC of hole = . the amount of departure can be added to the position tolerance. then the specified tolerance holds only at maximum material condition.251 . The size of the tolerance zone is dependent on the size of the hole.255 (LMC) .254 .255 . centered according to the basic dimensions given. This feature control frame specifies the positional tolerance zone as a circle of diameter .013 .011 .Concepts – Material condition modifiers GD&T on holes (and shafts) provides a powerful method for increasing inspection yield without trial and error fitting or binning.010 at MMC.014 .250 (MMC) . If symbol appears after the tolerance. Used when the size of the feature interacts with its location.252 .253 .250 LMC of hole = . As feature departs from MMC.015 .010 . MMC is commonly used for clearance type applications. if symbol is used. the stated tolerance holds at least material condition (LMC). RFS is commonly used for pressed fits. (or symbol in past practice) then the stated tolerance holds regardless of material condition of feature. .Concepts – Material condition modifiers Similarly. LMC is commonly used for loose fits. If no modifier is specified. As the part departs from LMC. This is called RFS – regardless of size. the amount of departure is added to the position tolerance. Taking an example for MMC Position tolerance stated at MMC Obtained tolerance for hole at MMC . Worst case condition.Rationale behind bonus tolerances. Now the centre of the hole can shift further left in the worst case. The gap is now closed. With a larger hole. reduced. W/o compromising function. tolerance increased.Rationale behind bonus tolerances. cost of mfg. . the hole position is less stringent. and more parts can be accepted. The tolerance zone can therefore be enlarged by an equal amount in diameter. . orientation. and in some cases size. The upper segment controls location. form. The symbol is entered once. It does not control location. The lower segment controls mainly orientation and form. and is applicable to both horizontal entities.Concepts – Composite Tolerancing Can be used with profile and position tolerance. It however. The above specs allow the 0.005 tolzone to “float” up and down. .Example – Composite Profile Tolerancing. The upper entry controls location to the DRF (datum reference plane) The lower entry controls. must stay perpendicular to A Application: Used to provide loose location but restrictive orientation. size/shape and orientation (perpendicularity) to the specified datum. Eg. Pattern of holes to locate nameplate.030 tolzone. and tilt or rotate within the confines of the 0. and back and forth. Datum system for clarity in inspection / fixture mfg. Bonus tolerance by material condition modifiers. Reduces need for drawing notes.Advantages of GD&T Functional dimensioning philosophy Round Tolerance zones. provides more wieldable language for specifications. Supports Statistical process control (SPC) . Fig. Allows maximum tolerance to produce the part. or else the flange (or part) won’t fit.Functional dimensioning philosophy Tolerance and tolerance zones based on part function and requirement. Dimensioning matches function. which reduces manufacturing costs. shows bolts holes for mounting a flange onto a plate (function). the position of the holes with respect to each other is important. Functional dimensioning can often double or triple the amount of tolerance on many component dimensions. instead of the distances to the edge. Dimensioning does not match function. . When mounting the flange. Functional dimensioning leads to dimensioning the distance between the holes. Clearance critical .Case Study Tolerance analysis of gap b/w trunk lid and rear windshield in Indigo SR. Areas affecting tolerance 1 Position of holes for mounting of hinge on body Side. 2 . Position of mounting of rubber stopper. Areas affecting tolerance 5 Hem between inner and outer panel of trunklid Reinforcement plate connecting trunklid and hinge. . 4 3 Position of holes for mounting trunk lid on hinge. Sources / effect of variation no 1 Position of holes for mounting of hinge on body side have adjustment of +/.2mm 2mm . 5mm +/-1mm . hole for rubber stopper has an adjustment of +/.Sources / effect of variation no 2 Position of mtg.1mm 0. Sources / effect of variation no 3 Position of slots for mtg. trunk lid on hinge has an adjustment of + 4mm 4mm 4mm . Sources / effect of variation no 4 Variation in reinforcement plate connecting trunklid and hinge +/.5 .0.5mm +/.0. 5 +/.5mm .0.Sources / effect of variation no 5 Variation due to hem between inner and outer panel of trunklid +/-0. Analysis of variations S No. 1. 2. 3. 4. 5. Source of Variation Position of holes for mounting of hinge on body side. Position of mounting of rubber stopper. Position of slots for mounting trunk lid on hinge. Variation due to reinforcement plate. Hem of inner trunk lid to outer trunk lid. Total Amount +2 +0.5 +4 +0.5 +0.5 +7.5 -2 -0.5 -0 -0.5 -0.5 -3.5 Conclusion: Clearance values Max: 13.5mm Min : 2.5mm Recommendation 1: Not important to control. Important to control gap. Recommendation 2: Hole to slot edge distance to be controlled in component Dim to be controlled . 1mm adjustment .Recommendation 3: Design position to be at center with +/. Recommendation 4: Operator to ensure rearmost position after adjustment. before tightening bolts . 4. Hem of inner trunk lid to outer trunk lid. 2.5 -4.5 +1 +0. 5. Position of slots for mounting trunk lid on hinge.5 -2 -0. Analysis of variation after Source of Variation Position of holes for mounting of hinge on body side.5 +1. 1.5 +0.5 -1 -0. S No. 3.5 .recommendations. Total Amount +0 +0.5 -0. Variation due to reinforcement plate. Position of mounting of rubber stopper. . 005 and -.Round tolerance zones Four holes drilled through the block (1).007 (diameter) (3) Round tolerance zone over 2 square tolerance zone for the part given in 1. Center of each of the holes must fall within a square tolerance zone .007 and -. and each hole’s location relative to each other and the edges are specified using a limit tolerance of a distance and +. Actual worst scenario is . more capable process. Thus 57% increase in available tolerance.014 or + . 1 3 Resulting in more usable parts. reduced manufacturing costs .010 (2).010 x .005. the tolerance zone is always fixed in size at all hole conditions. and more tolerance is allowed for production. Parts that are functional are used. Increased Tolerance zone at largest hole dia. Tolerance zone at MMC (smallest hole dia) Material condition modifier for MMC Example explaining bonus tolerances. . GD&T allows tolerance to be increased without compromising function.Bonus Tolerances using material condition modifiers In coordinate tolerancing. resulting in lower operating costs. Datum symbol : . Datum Feature . Eliminates ambiguity in inspection.A theoretically exact plane. point or axis from which a dimensional measurement is made.A part feature that contacts a datum. Datums in GD&T provide a reference frame from which the dimensions are measured.The inspection equipment (or gage surfaces) used to establish a datum. Datum Feature Simulator .Datum System Datum . Clear communication of design intent.How Datum systems implement functional dimensioning Specified Datums and geometric tolerances based on functional requirements. . Leads to successful end product. .Improved wieldability of language Feature control frame specifies positional tolerance of hole. material condition. and datum system. bonus tolerance at max. Flatness of surface specified. .Improved wieldability of language Copious notes required to specify the same in conventional tolerancing. caused by some error in the manufacturing procedure. This can be easily detected and corrected. GD&T's Datum system provides the repeatable part measurements that are necessary for making a meaningful SPC chart. In special cases. . Thus SPC in GD&T helps optimize inspection costs and reduce waste via rework and scrap. following a normal distribution. variations in product are near the mean. How SPC works: Under normal conditions. SPC uses statistical tools to observe the performance of the production process and predicts significant deviations that may result in rejected product. quality was achieved by 100% inspection of product.Statistical process control Traditionally. accepting or rejecting based on how well it met its design specifications. the variations move away from this distribution. ?? .Questions….