geometrical dimensioning and tolerancing
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Geometrical Dimensioning and TolerancingBy: Mahender Kumar
ANSI Y14.5 1994 Standard Y14 5-1994This standard establishes uniform practices for defining and interpreting dimensions, and tolerances, and related requirements for use on engineering drawings.
What is a good level of tolerance? good tolerance ?
Designer: tight tolerance is better i h l i b (less vibration, less wear, less noise) Machinist: large tolerances is better g (easier to machine, faster to produce, easier to assemble)
Tolerancing application: an example The type of fit between mating features Designer needs to specify: basic diameter and the tolerance of shaft: Ss/2 basic diameter and the tolerance of hole: Hh/2 Allowance: a = Dhmin Dsmax.
Tolerancing Definition: Allowance for specific variation in the size and geometry of a p g y part Need for Tolerancing It is IMPOSSIBLE to manufacture a part to an EXACT size or geometry Since variation from the drawing is inevitable we must p y p g specify the acceptable degree of variation Large variation may affect part functionality Small allowed variation affects the part cost requires precise manufacturing i i f t i requires inspection and potential rejection of parts
Tolerance Follows Function Assemblies: Parts will not fit together if their dimensions do not fall with in a certain range of values
I t h Interchangeable P t bl Parts: If a replacement part is used it must duplicate the original p within certain limits of deviation g part
The relationship between functionality and size/shape of an object varies with the part Automobile Transmission is Very Sensitive to the Size & Shape of the Gears A Bicycle is NOT Too Sensitive to the Size & Shape of the Gears (sprockets)
Two Forms of Physical Tolerance Size Limits specifying the allowed variation in each dimension (length, width, height, diameter, etc.) are given on the drawing
GeometryG Geometric Dimensioning & T l t i Di i i Tolerancing (GD&T) i Allows for specification of tolerance for the geometry of a part separate from its size GD&T uses special symbols to control different geometric features of a part
Cost Sensitivity Cost generally increases with tighter tolerances Th There is generally a ceiling t thi relationship i ll ili to this l ti hi where larger tolerances do not affect cost e.g.; If the Fabricator ROUTINELY Holds to 0.5 mm, Then Th a 3 mm S Specification will NOT reduce Cost ifi ti ill d C t
Tolerances at the Limits of the Fabricators Capability cause an exponential increase in cost Parts with small tolerances often require special methods of manufacturing Parts with small tolerances often require greater inspection, and higher part-rejection rates
Do NOT specify a SMALLER Tolerance than is i NEEDED
Tolerance Spec Hierarchy Generally Three Levels of Tolerances DEFAULT: Placed in the Drawing Title-Block by The Engineering Firm Typically Conforms to Routine Tolerance Levels
GENERAL: Placed on the Drawing By the Design-Engineer as a NOTE Applies to the Entire Drawing Supercedes the DEFAULT Tolerance
SPECIFIC: Associated with a SINGLE Dimension or Geometric Feature
Fit Between Parts Clearance fit: The shaft maximum diameter is smaller than the hole minimum diameter. Interference fit: The shaft minimum diameter is larger than the hole maximum diameter. Transition fit: The shaft maximum diameter and hole minimum have an interference fit, while the shaft , minimum diameter and hole maximum diameter have a clearance fit Clearance Fit Interference Fit Transition Fit
Classes of FitThe limits to sizes for various types of fit of mating parts are defined by the standard ANSI B4.1
There are five basic classes of fit: 1. Running and sliding clearance (RC) 2. 2 Location clearance (LC) 3. Location transition (LT) 4. Location interference (LN) 5. Force fits (FN)
Unilateral and Bilateral Tolerances:nominal dimension 1.00 + 0.05 tolerance means a range 0.95 - 1.05
+ 0.10 0 10 - 0.00
+ 0.00 0 00 - 0.10
1.00 + 0.05 -
Overview of Geometric TolerancesGeometric tolerances define the shape of a feature as opposed to its size.
We will focus on three basic types of dimensional tolerances: 1. 1 2. 3. Form tolerances: straightness, circularity, flatness cylindricity; straightness circularity flatness, Orientation tolerances; perpendicularity, parallelism, angularity; and Position tolerances: position, symmetry, concentricity.
COMMON TERMS AND DEFINITIONS
Basic Dimension A numerical value used to describe the theoretically exact size, profile, orientation, or location of a feature or datum target. It is the basis from which permissible variations are established by tolerances on other dimensions in notes or in feature control frames dimensions, notes, frames.
Datum A theoretically exact point, axis, or plane derived from the true geometric counterpart of a specified datum feature. A datum is the origin from which the location or geometric characteristics of features of a part are established.
Datum Target A specific line, or area on a part used to establish a datum.
Maximum Material Condition (MMC) The condition in which a feature of size contains the maximum amount of material within the stated limits of size-for example, minimum hole diameter, maximum shaft diameter.
Least Material Condition (LMC) The condition in which a feature of size contains the least amount of material within the stated limits of size-for example, maximum hole diameter, minimum shaft diameter.
Regardless of Feature Size (RFS): The term used to indicate that a geometric tolerance or datum reference applies at any increment g pp y of size of the feature within its size tolerance. Full Indicator Movement The total movement of an indicator when appropriately applied to a surface to measure its variations (formerly called total indicator reading-TIR). reading TIR) Virtual Condition The boundary generated by the collective effects of the specified MMC limit of size of a feature and any applicable geometric tolerances.
Feature Control Frame The feature control frame consists of: A) type of control (geometric characteristic), B) tolerance zone, C) tolerance zone modifiers (i e MMC or RFS) D) datum references if applicable and any datum reference (i.e., RFS), modifiers.
Profile of a Line A uniform two dimensional zone limited by two parallel zone lines extending along the length of a feature.
Profile of a Surface A uniform three dimensional zone contained between two envelope surfaces separated by the tolerance zone across the entire length of a surface.
Angularity A l it 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.
Perpendicularity (squareness) The condition of a surface axis median plane or line which is exactly at 90 degrees with respect to a datum plane or axis surface, axis, plane, axis.
Parallelism The condition of a surface or axis which is equidistant at all points from a datum of reference.
True Tr e Position A zone within which the center, axis, or center plane of a feature of size is permitted to vary from its true (theoretically exact) position.
Concentricity A cylindrical tolerance zone whose axis coincides with the datum axis and within which all cross-sectional axes of the feature being controlled must lie. (Note: Concentricity is very expensive and time-consuming to measure. Recommended that you try position or runout as an alternative tolerance.)
Runout A composite tolerance used to control the relationship of one or more features of a part to a datum axis during a full 360 degree rotation about the datum axis. Circular Runout Each circular element of the feature/part must be within the runout tolerance.
Total Runout All surface elements across the entire surface of the part must be within the runout tolerance.
Flatness A two dimensional tolerance zone defined by two parallel planes within which the entire surface must lie.
Straightness A condition where an element of a surface or an axis is a straight line.
Circularity A condition on a surface of revolution (cylinder, cone, sphere) where all points of the surface intersected ( y , , p ) p by any plane perpendicular to a common axis (cylinder, cone) or passing through a common center (sphere) are equidistant from the axis of the center.
Cylindricity A condition on a surface of revolution in which all points of the surface are equidistant from a common axis.
Feature Control FrameA geometric tolerance is prescribed using a feature control frame. It has three components: 1. the tolerance symbol, 2. the tolerance value, 3. the datum labels for the reference frame.
Order of PrecedenceThe part is aligned with the datum planes of a reference frame using 3-2-1 contact alignment. 3 points of contact align the part to the primary datum plane; 2 points of contact align the part to the secondary datum plane; 1 point of contact aligns the part with the tertiary datum plane
Straightness of a shaft
Straightness of a Shaft A shaft has a size tolerance defined for its fit into a hole. A shaft meets this tolerance if at every point along its length a diameter measurement fall within the specified values. This allows the shaft to be bent into any shape. A straightness tolerance on the shaft axis specifies the amount of bend allowed.
Add th straightness tolerance to th maximum shaft size (MMC) t obtain a i t l the t i ht t l t the i h ft i to bt i virtual condition Vc, or virtual hole, that the shaft must fit to be acceptable.
Straightness of a Hole
The size tolerance for a hole defines the range of sizes of its diameter at each point along the centerline. This does not eliminate a curve to the hole. The straightness tolerance specifies the allowable curve to the hole. Subtract the straightness tolerance from the smallest hole size (MMC) to define the virtual condition Vc, or virtual shaft, that must fit the hole for it to be acceptable.
Straightness of a Center Plane The size dimension of a rectangular part defines the range of sizes at any cross-section. The straightness tolerance specifies the allowable curve to the entire side. Add the straightness tolerance to the maximum size (MMC) to define a virtual condition Vc that the part must fit into in order to meet the tolerance.
FlatnessTolerance zone defined by two parallel planes.0.0 01
p ar al l e l p lanes 0.0 01
Flatness, Circularity and CylindricityCircularity
The flatness tolerance defines a distance between parallel planes that must contain the highest and lowest points on a face. The circularity tolerance defines a pair of concentric circles that must contain the maximum and minimum radius points of a circle. The cylindricity tolerance defines a pair of concentric cylinders that much contain the maximum and minimum radius points along a cylinder.
CYLINDRICITY Tolerance zone bounded by two concentric cylinderswithin which the cylinder must lie.0.01
1.00 ' 0.05
Rotate in a V
Rotate between points R t t b t i t
Parallelism ToleranceA parallelism tolerance is measured relative to a datum specified in the control frame. If there is no material condition (ie. regardless of feature size), then the tolerance defines parallel planes that must contain the maximum and minimum points on the face. p If MMC is specified for the tolerance value: If it is an external feature, then the tolerance is added to the maximum dimension to define a virtual condition that the part must fit; If it is an internal feature, then the tolerance is subtracted to define the maximum dimension that must fit into the part part.
Perpendicularity A perpendicular tolerance is measured relative to a datum plane plane. It defines two planes that must contain all the points of the face. A second datum can be used to locate where the measurements are taken.
Perpendicular Shaft, Hole, and Center PlaneShaft Hole Center Plane
Shaft: The maximum shaft size plus the tolerance defines the virtual hole hole. Hole: The minimum hole size minus the tolerance defines the virtual shaft. Plane: The tolerance defines the variation of the location of the center plane.
An angularity tolerance is measured relative to a datum plane. It defines a pair planes that must 1. contain all the points on the angled face of the part, or 2. if specified, the plane tangent to the high points of the face.
Concentricity Concentricit Tolerance Note.007 007
.007 Tolerance Zone
This cylinder (the right cylinder) must be concentric within .007 with the Datum A (the left cylinder) as measured on the diameter d th di t
What It Means
TRUE POSITIONDimensional tolerance1 .0 0 0 .0 1
1 .2 0 0 .0 1
O .8 0 0 .0 2 O 0 .0 1 M A B
Hole center tolerance zone
True position tolerance t l1 .0 0
Tolerance zone 0 .0 1 dia 0
Position Tolerance for a Hole The position tolerance for a hole defines a zone that has a defined shape, size, location and orientation. It has the diameter specified by the tolerance and extends the length of the hole. Basic dimensions locate the theoretically exact center of the hole and the center of the tolerance zone. Basic dimensions are measured from the datum reference frame.
Position Tolerance on a Hole PatternA composite control frame signals a tolerance for a pattern of features, such as holes. p ,
The first line defines the position tolerance zone for the holes. The second line defines the tolerance zone for t e pattern, w c s ge e a y s a e . the patte , which is generally smaller.
Virtual Condition Envelope All Required Tolerances20.06 Maximum Envelope
0.06 0 06 Maximum Allowable Curvature
20.00 20 00 Maximum Allowable Diameter
A uniform boundary along the true profile within whcih the elements of the surface must lie.0 .0 05 A B
RUNOUTA composite tolerance used to control the functional relationship of one or more features of a part to a datum axis. Circular runout controls the circular elements of a surface. As the part rotates 360 about the datum axis, the error must be within the tolerance , limit.A 1.500 " 0.005 0 .0 0 5 A 0.361 " 0.002
Dat um ax is
Deviat ion on each circular check ring is less t han t he t olerance.
TOTAL RUNOUTA 1.500 " 0.005 0 .0 0 5 A 0.361 0 361 " 0.002 0 002
Dat um ax is
Deviat ion on t h D i ti he t ot al swept when t he part is rot at ing is less t han t he t olerance.
Geometric Tolerancing Definitions Maximum Material Condition (MMC) The condition in ( ) which a feature of size contains the maximum amount of material with the stated limits of size, - fore example, minimum hole diameter and maximum shaft diameter Least Material Condition (LMC) Opposite of MMC, the feature contains the least material. For example, maximum hole diameter and minimum shaft diameter Virtual Condition The envelope or boundary that describes the collective effects of all tolerance requirements on a feature (See Figure 7 25 TG) 7-25
Material Condition ModifiersRFSIf the tolerance zone is prescribed for the maximum material condition (smallest hole). Then the zone expands by the same amount that the hole is larger in size size. Use MMC for holes used in clearance fits.
No material condition modifier means the tolerance is regardless of feature size. Use RFS for holes used in interference or press fits.
MMC HOLELMC hole MMC hole hole axis t olerance zone
MMC peg will f it in t he hole , axis must be in t he t olerance zone
Given th same peg (MMC peg), when th Gi the ) h the produced hole size is greater than the MMC hole, the hole axis true position tolerance zone can be enlarged by the amount of difference between the produced hole size and the MMC hole size.
TOLERANCE VALUE MODIFICATIONO 1 .0 0 0 .0 2 O 0 .0 1 M A B
1 .0 0
Produced hole size1 .2 0
True Pos tol M L 0.01 0 01 0.02 0.03 0.04 0.05 0.05 0 05 0.04 0.03 0.02 0.01
S 0.01 0 01 0.01 0.01 0.01 0.01
0.97 MMC 0.98 0 98 0.99 1.00
out of diametric tolerance
The default modifier for true position is MMC.
1.01 LMC 1.02 1.03
out of diametric tolerance
the allowable tolerance = specified tolerance + (produced hole size - MMC hole size)