Geometric Dimensioning and Tolerancing
Objectives
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
Objectives
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 par t based on h ow it functions (design intent) i ntent) in the final product. ASME Y14.5M Y14.5M – 1994 is the accepted geometric dimensioning and tolerancing standard.
Datum feature Basic dimension First glimps g limpse: e: Par Partt dimension dimensioned ed using using GD& GD&T. T.
Feature control frame
Limit Type Tolerancing
Tolerance can be specified only where a dimension is defined. Gives an acceptable range of values of an individual dimension (limits of dimension).
No provision to specify how flat a surface needs to be, or how much a hole can tilt relative to a surface.
Induces problems related to ambiguity, guesswork and multiple interpretation of part drawing. Results in deviation from design intent.
Geometric Tolerancing
Separates the specification of tolerance from the dimensioning. Specifies a geometric r egion (tolerance zon es), such as an area or a volume, in which the feature must lie in order to meet the design criteria. Communicates complex geometrical descriptions not possible otherwise in language. Allows more flexibility and precise controls that relate directly to the for m , fit and fu n ct io n and not just size of the part, leading to successful end product. Eliminates guesswork, enables mfg. accor ding to design intent, thus reduces confusion, rejection, rework and loss of profits.
Feature control frame in GD&T
Basic sentence in GD&T is put in the form of a feature control frame. It states the requirement for the feature to which it is attached.
Parts of a feature control frame
Each feature control frame can state only one requirement/message. Only one set up or gage for one FCF.
Feature control frame, Broken down
First compar tmentGeometric characteristic symbol: Specifies the character to which the tolerance is to be applied. E.g. Flatness , angularity ,profile , parallelism etc.
Third compartment – Datum system: Specifies datums if applicable. They are significant according to their precedence in the FCF.
First compartment- contains one of the 14 geometric characteristic symbols.
Third compartment- contains the datum reference frame
Feature control frame, Broken down
Second compartment- Symbol to specify shape of tolerance zone : This symbol precedes the tolerance and specifies the shape of tolerance zone. E.g. specifies a cylindrical tolerance zone, specifies a spherical tolerance zone. If no symbol is given, the default shape is parallel planes, or a total wide zone (like in profile tolerance).
Second com partm ent -Material condition m odifiers: Features of size can be provided bonus tolerances using these modifiers. If the feature being controlled is a feature of size, and no modifier is specified, the default is RFS.
Second compartment- contains actual tolerance, material condition modifier and other symbols.
Other symbols in the FCF
The symbols for projected tolerance zone, free state, tangent plane, and statistical tolerance always follow the material condition modifier.
New datum feature symbol has been introduced in ANSI Y14.5, 1994.
Concept of Datum System
Datum - A theoretically exact plane, point or axis from which a dimensional measurement is made. Datums are points, lines, planes, cylinders, axes, etc., a, from which the location, or geometric relationship of other part features may be established or related. Datum Feature - A datum feature is the actual component feature used (idealized) to establish a datum. Datum Feature Simulator -- A datum simulator a surface of adequate precision oriented to the high points of a designated datum from which the simulated datum is established.
Examples: gage pin, block, and surface of granite block. Diameter Symbol - the diameter symbol, 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.
Concept of Datum System
Datums are specified in the third compartment of the feature control frame. Primary Datum
Secondary Datum
Feature constrained within said tolerance with respect to datums A,B
They are theoretically exact features (surfaces idealized to planes, axes etc.) from which dimensional measurements are made. Sample part: Surfaces are idealized to eliminate ambiguity about from where dimensions are to be measured
But if both surfaces are idealized simultaneously, they may not be perpendicular to each other.
Precedence order in datum planes If we use a set of perpendicular datum references, either of the two positions could be right.
Without precedence order, either of these two could be the correct position. Final position depends on which side contacts first.
Thus, they are specified in an order of precedence governed by the part function.
Therefore, the first side to be pressed against one of the edges (in this case, datum A), will make contact at the two highest points. The part now has only one degree of freedom left, it can only slide back and forth along this edge. Once we butt the perpendicular side of the part with the corresponding straight edge (datum B), we have a completely constrained position and orientation.
Concepts of Tolerance Zones
Tolerances zones can be defined in GD&T instead of limits of dimension.
Defined according to functional requirement of the part.
These are geometric regions (3D or 2D) in which the feature must lie to be acceptable.
Ensures closeness to real world requirements
Enables specifications (like conical tolerance zones) not otherwise possible in limit type dimensioning.
Symbols in GD&T
GD&T has 14 geometric characteristic symbols. Various symbols used to specify tolerance zones for:
Form Position Profile Orientation Runout
Symbol Description
Geometry
Circularity
Form
Cylindricity
Form
Flatness
Form
Straightness
Form
Concentricity
Position
Position
Position
Symmetry
Position
Profile
Profile
Profile of a line
Profile
Angularity
Orientation
Parallelism
Orientation
Perpendicularity
Orientation
Runout
Runout
Total Runout
Runout
Straightness
Straightness describes a condition w here 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.
The surface must lie between two planes 0.25 mm apart.
Circularity
Circularity describes the condition on a surface of revolution (cylinder, cone, or sphere) wh ere all points of the surface intersected by any plane (1) perpendicular to a common axis (cylinder, cone), or (2) passing th rough 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, one having a radius 0.25 mm larger than the other.
Cylindricity
Cylindricity describes a condition of a surface of revolution in w hich all points of a surface are equidistant from a common axis.
The cylindrical surface must lie between two concentric cylinders one with radius 0.25 mm larger than the other.
Parallelism
Paralle lism is the condition of a surface, line, or axis, w hich is equidistant at all, points from a datum plane or axis.
The surface must lie between two parallel planes 0.12 mm apart, which are parallel to datum plane A. The feature axis must lie within a 0.2 mm dia cylindrical zone parallel to datum axis A.
Perpendicularity
P erpendicularity is the condition of a surface, axis, or line, wh ich is 90 deg. from a datum plane or a datum axis.
The feature axis must lie between two parallel planes 0.2 mm apart, perpendicular to datum axis A.
Angularity
Angularity is the condition of a surface, axis, or center plane, which is at a specifie d angle (other than 0, 90, 180 or 270 deg.) from a datum plane or axis.
The surface must lie between two parallel planes 0.4 mm apart inclined at an angle of 30 0 to datum plane A.
Profile of a surface
Pro file of a surface is the condition permitting a uniform amoun t of profile variation, either unilaterally or bilaterally, on a surface.
Profile of a line
Profile of a line is used in conjunction with profile of surface. Profile of a surface defines the shape or location of a feature while profile of line refines it in one direction. Each line element of the surface must lie between two profile boundaries 0.006 mm apart in relation to the datum reference frame.
Pr ofile of a line is the condition permitting a uniform amoun t of profile variation, either unilaterally or bilaterally, along a line element of a feature.
Circular runout
Circular runout gives the deviation from the desired form of a circular elem ent of a part surface of revolution through one full rotation (360 deg) of the p art on a datum axis
At any position, each the circular element of the surface must be within the specified runout tolerance (0.02 mm full indicator movement) when the part is rotated by 3600 , about the datum axis, the indicator fixed in a position normal to the true geometric shape . Note: circular runout controls the circular elements of the surface, not the complete surface.
Total runout
Total runout is the simultaneous composite control of all elements of a surface at all circular and profile measurin g positions as the part is rotated through 360.
The entire surface must lie within the specified runout tolerance zone (0.02 mm full indicator movement) when the part is rotated by 3600 about datum axis A, with the indicator at every location along the surface in a position normal to the true geometric shape without reset of the indicator.
Concentricity
This controls location, and can have some effect on the form and orientation of a feature. The application of concentricity is complex and rare. Diametrically opposed dial indicators maybe used to check this.
Concentricity describes a condition in which tw o or more features (cylinder s, cones, spheres, etc.) I n any combination have a common axis.
Symmetry
Symmetry is a condition in w hich a feature (or features) is symmetrically disposed about the center plane of a datum feature
Controls opposing points (that form derived median plane). Same concept as concentricity, but applied to non-cylindrical features.
Position
P osition tolerance (formerly called true position tolerance) defines a zone w ithin 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). Defines a zone within which, the axis, median plane, or surface of a feature is permitted to lie. These tolerance zones can be cylindrical, conical, rectangular, etc.
Positional tolerance for cylindrical zone
Application Part mounts in assembly on surfaces shown, holes provide clearance for bolts.
Position tolerance for rectangular zone Locates features with a greater tolerance in one direction than other. 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 0.012X0.028 rectangular tolerance zone
Positional tolerance for spherical zone
The centre point of the spherical diameter must lie in a spherical zone of diameter 0.03, basically located to the DRF.
Positional tolerance for conical zone
Application Used to control features such as a deep drilled hole, closer at one surface than another.
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. Used when the size of the feature interacts with its location. If symbol appears after the tolerance, then the specified tolerance holds only at maximum material condition. As feature departs from MMC, the amount of departure can be added to the position tolerance. MMC is commonly used for clearance type applications.
This feature control frame specifies the positional tolerance zone as a circle of diameter .010 at MMC, centered according to the basic dimensions given. The size of the tolerance zone is dependent on the size of the hole. Hole diameter Tolerance
Zone diameter
MMC of hole = .250 LMC of hole = .255
.250 (MMC)
.010
.251
.011
.252
.012
.253
.013
.254
.014
.255 (LMC)
.015
Concepts – Material condition modifiers
Similarly, if symbol is used, the stated tolerance holds at least material condition (LMC). As the part departs from LMC, the amount of departure is added to the position tolerance. LMC is commonly used for loose fits. If no modifier is specified, (or symbol in past practice) then the stated tolerance holds regardless of material condition of feature. This is called RFS – regardless of size. RFS is commonly used for pressed fits.
Rationale behind bonus tolerances. Worst case condition.
Taking an example for MMC
Position tolerance stated at MMC
Obtained tolerance for hole at MMC
Rationale behind bonus tolerances.
The tolerance zone can therefore be enlarged by an equal amount in diameter.
Now the centre of the hole can shift further left in the worst case. The gap is now closed. With a larger hole, the hole position is less stringent, and more parts can be accepted.
W/o compromising function, tolerance increased, cost of mfg. reduced.
Concepts – Composite Tolerancing
Can be used with profile and position tolerance. The symbol is entered once, and is applicable to both horizontal entities. The upper segment controls location, orientation, form, and in some cases size. The lower segment controls mainly orientation and form. It does not control location.
Example – Composite Profile Tolerancing.
The above specs allow the 0.005 tolzone to “float” up and down, and back and forth, and tilt or rotate within the confines of the 0.030 tolzone. It however, must stay perpendicular to A Application: Used to provide loose location but restrictive orientation. Eg. Pattern of holes to locate nameplate.
The upper entry controls location to the DRF (datum reference plane) The lower entry controls, size/shape and orientation (perpendicularity) to the specified datum.
Advantages of GD&T
Functional dimensioning philosophy Round Tolerance zones. Bonus tolerance by material condition modifiers. Datum system for clarity in inspection / fixture mfg. Reduces need for drawing notes, provides more wieldable language for specifications. Supports Statistical process control (SPC)
Functional dimensioning philosophy
Tolerance and tolerance zones based on part function and requirement. Fi g . s h o w s b o l t s h o l es f o r m o u n t i n g a f l a n g e o n t o a p l a t e ( f u n c t i on ) . W h e n m o u n t i n g t h e f l a n g e , t h e p o s it i o n o f t h e h o l e s w i t h r e sp e ct t o e a ch o t h e r i s i m p o r t a n t , o r e ls e t h e f l a n g e ( o r p a r t ) w o n ’ t f i t . Fu n c t i o n a l d i m e n s i o n i n g l e ad s t o d i m e n s io n i n g t h e d i s t a n ce b e t w e e n t h e h o l e s, i n st e a d o f t h e d i s t a n ce s t o t h e e d g e .
Allows maximum tolerance to produce the part.
Dimensioning matches function.
Functional dimensioning can often double or triple the amount of tolerance on many component dimensions, which reduces manufacturing costs. Dimensioning does not match function.
Case Study Tolerance analysis of gap b/w trunk lid and rear windshield in Indigo SR. Clearance critical
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
Reinforcement plate connecting t ru nk li d a nd hinge.
Hem between inner and outer panel of trunklid
3 Position of holes for mounting trunk lid on hinge.
4
Sources / effect of variation no 1 Position of holes for mounting of hinge on body side have adjustment of +/- 2mm
2mm
Sources / effect of variation no 2 Position of mtg. hole for rubber stopper has an adjustment of +/- 1mm
0.5mm
+/-1mm
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
+/- 0.5mm +/- 0.5
Sources / effect of variation no 5 Variation due to hem between inner and outer panel of trunklid +/-0.5 +/- 0.5mm
Analysis of variations S No.
Source of Variation
Amount
1.
Position of holes for mounting of hinge on body side.
+2
-2
2.
Position of mounting of rubber stopper.
+0.5
-0.5
3.
Position of slots for mounting trunk lid on hinge.
+4
-0
4.
Variation due to reinforcement plate.
+0.5
-0.5
5.
Hem of inner trunk lid to outer trunk lid.
+0.5
-0.5
Total
+7.5
-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
Recommendation 3: Design position to be at center with +/- 1mm adjustment
Recommendation 4: Operator to ensure rearmost position after adjustment, before tightening bolts
Analysis of variation after recommendations. S No.
Source of Variation
Amount
1.
Position of holes for mounting of hinge on body side.
+0
-2
2.
Position of mounting of rubber stopper.
+0.5
-0.5
3.
Position of slots for mounting trunk lid on hinge.
+1
-1
4.
Variation due to reinforcement plate.
+0.5
-0.5
5.
Hem of inner trunk lid to outer trunk lid.
+0.5
-0.5
Total
+1.5
-4.5
Round tolerance zones
Four holes drilled through the block (1), and each hole’s location relative to each other and the edges are specified using a limit tolerance of a distance and +.005 and -.005. Center of each of the holes must fall within a square tolerance zone .010 x .010 (2). Actual worst scenario is .014 or + .007 and -.007 (diameter) (3) Round tolerance zone over 2 square tolerance zone for the part given in 1. Thus 57% increase in available tolerance.
Resulting in more usable parts, more capable process, reduced manufacturing costs
1
3
Bonus Tolerances using material condition modifiers
In coordinate tolerancing, the tolerance zone is always fixed in size at all hole conditions. GD&T allows tolerance to be increased without compromising function. Parts that are functional are used, and more tolerance is allowed for production, resulting in lower operating costs. Tolerance zone at M M C (smallest hole dia)
Material condition modifier for MMC
Example explaining bonus tolerances.
I n c r e as ed Tolerance zone at largest hole dia.
Datum System
Datum - A theoretically exact plane, point or axis from which a dimensional measurement is made. Datum Feature - A part feature that contacts a datum. Datum Feature Simulator - The inspection equipment (or gage surfaces) used to establish a datum.
Datums in GD&T provide a reference frame from which the dimensions are measured. Eliminates ambiguity in inspection.
Datum symbol :
How Datum systems implement functional dimensioning
Specified Datums and geometric tolerances based on functional requirements.
Clear communication of design intent. Leads to successful end product.
Improved wieldability of language Feature control frame specifies positional tolerance of hole, bonus tolerance at max. material condition, and datum system.
Flatness of surface specified.
Improved wieldability of language
Copious notes required to specify the same in conventional tolerancing.
Statistical process control
Traditionally, quality was achieved by 100% inspection of product, accepting or rejecting based on how well it met its design specifications. SPC uses statistical tools to observe the performance of the production process and predicts significant deviations that may result in rejected product. GD&T's Datum system provides the repeatable part measurements that are necessary for making a meaningful SPC chart. Thus SPC in GD&T helps optimize inspection costs and reduce waste via rework and scrap.
How SPC works: Under normal conditions, variations in product are near the mean, following a normal distribution. In special cases, caused by some error in the manufacturing procedure, the variations move away from this distribution. This can be easily detected and corrected.
