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Types of Dimensioning in Engineering drawing Linear Dimensioning
Chain Dimensioning
Angular Dimensioning
Dimensioning Diameters
Dimensioning Holes
Parallel Dimensioning
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What Is GD&T? GD&T stands for geometric dimensioning and tolerancing. It is a system of symbols, rules and definitions used to define the geometry of mechanical parts. GD&T is one of the most powerful tool available that can improve quality, reduce cost and shorten delivery time. All of this is possible when the concurrent engineering team is involved with the creation of the drawing. The drawing is a common thread that ties these groups together. They all are involved with the engineering drawing. GD&T on the drawing must first and foremost capture design intent. However, the best design in the world is worthless if it cannot be produced. That is why it is necessary for production/vendors and quality to be involved with the requirements that are placed on the drawing. When they are not involved, the drawings often have overly tight tolerances and result in non-producible parts. At least not producible at the quality level, cost and timeliness expected by industry. In short GD&T is ……….. •Symbols •Rules •Vocabulary •Mathematical definition (ASME Y14.5.1) •A National Standard (ASME Y14.5M-1994) •An International Standard (ISO 1101)
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Why Engineering Drawings? Engineering drawing is a formal and precise way of communicating information about the shape, size, features and precision of physical objects. Drawing is the universal language of engineering.
Drawing Standards Just like written language has standards, the “grammar” of technical drawing is defined by...the ASME Y14.5 or the ISO 1101 standard The ASME standards must be understood to read a drawing.
Basic Information Included in a Drawing • Projected Views: Show as many sides as needed for completeness. • Cross Sections: A view that is good for showing interior features. • Table: Lower right corner, with material information, part name, designer etc. and finally • DIMENSIONS!!!: These are the most important and most complicated part of the drawing. There is more to it than just the numerical values! 4
GD&T STANDARDS The ANSI standard was revised in 1982. In 1994, the standard was moved form ANSI to the American Society of Mechanical Engineers (ASME) and the name changed to ASME Y14.5. The standard was also revised into its current form of ASME Y14.5-1994.
GD&T – Geometric Dimensioning & Tolerancing Establishes the standard by which designers can communicate the intended function of the part to the machinist making the part and the inspector checking the part. This standard lets the designer inform the machinist, toolmaker or fabricator what are the important features of the design when they are making the part. The standard also lets the inspector know what are the important features to inspect form. GD&T uses symbols to communicate the information to those involved in making the part to eliminate any misunderstanding. Word explanations can be confusing especially with the global market and the translations of words into different languages. 5
A Dimensioning Example,showing that placement should match intent These drawings show bolts holes for mounting a flange onto a plate. When mounting the flange, the position of the holes with respect to each other is very important, or else the flange (or part) won’t fit. It makes sense to dimension the distance between the holes, instead of the distances to the edge.
Dimension placement matches intent
Dimension placement does NOT match intent
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Types of Dimensional Tolerances Limit Dimensioning
Plus & Minus Tolerancing
Both methods are acceptable.
Geometric Tolerancing Geometric Tolerancing is used to specify the shape of features. Things Like: Straightness, Flatness, Circularity, Cylindricity, Angularity, Profiles, Perpendicularity, Parallelism, Concentricity and more….. Geometric Tolerancing are shown on a drawing with a feature control frame. 7
Co-ordinate Tolerancing System
Geometric Dimensioning & Tolerancing
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Disadvantage of Co-ordinate Tolerancing System
Consequence : Good parts could be rejected. Hole location is critical when the hole is at Minimum limit (MMC) Co-ordinate tolerancing does not allow cylindrical tolerance zones and tolerance hole that increases with the hole size.
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Disadvantage of Co-ordinate Tolerancing System (A)
(B)
x
This method for part Measurement?
x
OR
This method for part Measurement?
Consequence : Bad parts could be accepted.
Benefits of Geometric Dimensioning & Tolerancing •Cylindrical tolerance zones. •Maximum material condition. •Datums specified in order to precedence. •Avoid tolerance stacking
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Feature & Feature control frame
Feature of size
Diameter symbol
Geometric characteristics symbol Datum Tolerance Maximum material condition
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Geometric Characteristic Symbols These are a set of fourteen symbols used in the language of Geometric tolerancing.
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Features A Feature is a general term applied to a physical portion of part, such as a surface, hole or slot.
A – Hole surface B – Back face of surface E C – Right face D – Bottom face E – Left face F – Top face G – Back face of surface C
The above part described with A,B,C,D,E,F&G are surfaces of the part. The part has seven features. 13
Feature of size This could be a cylindrical surface or a parallel surface associated with size dimension which has an axis, central line or plane contained within it. Feature of size are features which do have diameter or thickness. External FOS
External FOS
External FOS Non FOS
Internal FOS
Non FOS
Internal FOS
External feature of size are comprised of part surface that are external surface. Internal feature of size are comprised of part surface that are internal surface. 14
Feature of size Caliper Rule -- Opposed Points This rule is not found in the Standard but it is pretty helpful. The left end of the caliper shown is used if the feature being measured has opposed points. Since these features have a toleranced dimension and contain opposed points, they are features of size. Toleranced dimensions that are measured using the probe on the other end of the caliper do not have opposed points and are not features of size. This rule of thumb is intended to help you identify features of size and is not intended to suggest that calipers are the best way to measure size dimensions.
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Modifiers Modifiers convey additional information about the drawing or tolerancing of the part. These are common modifiers used in geometric tolerancing. TERM
ABBREVATION
MAXIMUM MATERIAL CONDITION
MMC
LEAST MATERIAL CONDITION
LMC
SYMBOL
PROJECTED TOLERANCE ZONE TANGENT PLANE DIAMETER
DIA
RADIUS
R
CONTROLLED RADIUS
CR
REFERENCE
()
REGARDLESS FEATURE OF SIZE
RFS
FREE STATE UNILATERAL
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Material conditions MMC vs. LMC
Largest shaft
Smallest shaft
Smallest hole
Largest hole
“Maximum Material Condition”
“Least Material Condition”
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Regardless feature of size - RFS In the below example Regardless of Feature Size is implied. This is the most restrictive (expensive) control. Although the size may not be less than 9.8mm or greater than 10.2mm, the hole may act like 9.5mm to 10.5mm. Any holes made from 9.5mm to 9.8mm or from 10.2mm to 10.5mm should be rejected for size even though they may function.
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Maximum material condition - MMC By using the MMC modifier, a bonus is allowed that could permit the hole to be out of position by as much as 0.7mm. Although the inner boundary did not change, the outer boundary has grown to 10.9mm since a 10.2mm size hole may be out of position by as much as 0.7mm due to bonus tolerance. This could cause shift problems if this hole is being used to locate parts in an assembly.
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Least material condition - LMC By using the LMC modifier, a bonus is allowed that could once again permit the hole to be out of position by as much as 0.7mm. This time the outer boundary did not change from the first RFS example, but, the inner boundary could be as small as 9.1mm since a 9.8mm size hole may be out of position by as much as 0.7mm due to bonus tolerance. This could cause fit problems in an assembly.
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Projected tolerance When a fastener mates with a threaded hole or a pin pressed in to a hole, or even when there is a very close fit, it is good idea to specify a projected tolerance zone. When specifying projected tolerance zone for a blind hole or a through hole, it should be clear which surface is the interface and should be shown in the drawing.
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Tangent plane Here the Disk contacts the high points of the flat surface on the Drive Shaft. The tangent plane modifier, the letter T in a circle, means that the tolerance applies to a plane established by the high points of the surface. The flatness deviation in the surface would be ignored when the surface profile tolerance is inspected. A flat plate placed against the surface would have to be in the profile tolerance centered at the 14mm basic dimension relative to the datum reference frame. This modifier works well where it is expected that the process will create a surface that is concave.
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Free state When a tolerance is defined under free state, the measurement is to be done after removal of all restraining forces on the part. A Free state tolerance is defined for fragile and thin sections parts which are susceptible to deformation under stress. Mainly used in plastic and rubber components
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Diameter Modifier - Ø The diameter symbol is used two ways. One inside the feature control frame to denote the shape of the tolerance zone and outside to denote as diameter for the feature of size.
Inside the feature control frame Outside – Feature of size
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Radius & Controlled radius – R & CR Radius - A straight line extending from the center of an arc or circle to its surface. Part contour must be within the tolerance zone. Flats and reversals are allowed. Max radius 15.4 Min radius 14.6 Controlled Radius - A radius with no flats or reversals allowed. The symbol for a controlled radius is "CR." Part contour must be within the tolerance zone. Flats and reversals are not allowed. Max radius 15.4 Min radius 14.6 25
Hole symbols Counter bore or Spot face Symbol
Depth symbol Counter sink symbol
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Why Datum system ? It allows the designer to specify …… which part surfaces are to contact the inspection equipment for the measurement of dimension …. which sequence the part is to contact the inspection equipment for the measurement of dimension. dimension 16.4 16.0
12.4 12.0
Method 1 16.4 16.0
Method 2
12.4 12.0
Surface plate
Surface plate
Benefits of datum system: It aids in making the dimensional measurement as intended by the designer It aids in communicating part functional relationship Consequences: Good parts are rejected and Bad parts are accepted. 27
Importance of Datum Two possible parts according to first drawing.
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Datum A Datum is theoretically exact plane, point or axis from which a dimensional measurement is made. It is a true geometric counterpart of a datum feature. A true geometric counterpart is the theoretical perfect boundary or best fit tangent plane of a datum feature.
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Datum representation Indication of datum feature
Filled or Unfilled is ok
Datum with letter
Datum with tolerance control frame
The datum triangle is placed on a feature surface and an extension line
Placement of datum feature symbols on feature of size. 30
Datum representation Datum reference in feature control frame Primary datum : A Secondary datum : B Tertiary datum: C
Basic dimension symbol
Placement of datum feature symbol in conjunction with a feature control frame
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Datum feature of part / Stimulated datum Datum plane A True geometric counterpart of datum feature A Datum feature – Actual part surface Part
Surface plate Or datum feature stimulator Stimulated datum is established by inspection equipment
Stimulated datum plane A Plane derived from the datum feature stimulator
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Datum reference frame A Datum reference frame is a set of three mutually perpendicular datum planes. The datum reference frame provides direction as well as an origin of dimensional measurement.
Primary datum establishes the orientation of the part to datum reference frame. It restricts three degree of freedom. Secondary datum locates the part within the datum reference frame. It restricts two additional degrees of freedom. Tertiary datum locates the part within the datum reference frame. It restricts last degree of freedom. 33
Degrees of freedom
34
Degrees of freedom
35
Degrees of freedom
36
Datum symbol placement The below drawing illustrates establishing datum center planes for datum B and datum C. To do this, the triangles are placed inline with the size dimensions. This approach would be used if the pattern of holes should remain centered on the plate regardless of the actual length and width.
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Datum symbol placement If the centering of the pattern of holes is not important, the datum triangles may be offset as shown in the below drawing. Datum planes are established by the sides of the part. Although, this approach is usually preferred by manufacturing, the symmetry of the part is lost and may cause confusion at inspection. Depending on the actual size of the part, the pattern will be controlled better to one side than the other.
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Datum symbol placement Placement of the new datum feature symbol (triangle) can be critical. In the first three views below the datum feature symbol is associated with the size dimension of a feature of size. They indicate that a datum axis should be established using the feature indicated.
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Datum symbol placement In the view below, the datum may be interpreted as a line lying in a plane tangent to the feature indicated. If line contact is desired a datum target line should be indicated.
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Datum center plane MMC primary A internal FOS as a datum feature
Stimulated gauge
An external FOS as a Datum Feature
Stimulated gauge Datum feature stimulator Gauge element width – 20.4mm.
Datum feature stimulator Gauge element width – 7.7mm.
Datum center plane C Center plane of gauge
Datum center plane C Center plane of gauge
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Datum axis MMC secondary
Datum axis B
Datum feature stimulator (90° to datum plane A Fixed s ize Ø17.8 diameter) Datum plane A Surface plate as datum feature stimulator
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Rules of GD&T Rule # 1 This rule is referred to as the “ Individual feature of size rule” In industry rule 1 is phrased as “Perfect form at MMC” or the “Envelope rule” When no geometric tolerance is specified, the dimensional tolerance controls the geometric form as well as the size. No element of feature shall extend beyond the MMC boundary of perfect form. The form tolerance increases as the actual size of the feature departs from the MMC towards LMC. In this rule, the word perfect form means perfect flatness, straightness, circularity and cylindricity. In other words parts produced at MMC have perfect form
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Rule 1 applied to………… External feature of size
MMC size & form boundary Size and form must allow the part to pass through the boundary
Ø10.2 boundary of perfect form at MMC
LMC size limit
Ø9.8
Each two-point measurement must be within the specified tolerance 44
Rule 1 applied to………… Internal feature of size
MMC size & form boundary Ø9.8 boundary of perfect form at MMC LMC size limit Ø10.2
Each two-point measurement must be within the specified tolerance 45
Rules of GD&T Rule # 2 (NEW) as per 1994 standard RFS automatically applies to individual tolerances and to datum feature of size. MMC & LMC must be specified where required.
RFS is the default for tolerance and datum reference (as appropriate)
Rule # 2a (OLD) as per 1982 standard It is an alternative practice of rule # 2 according to which RFS may be specified as a symbol in feature control frame if desired and applicable.
For a tolerance of position, RFS may be stated for the tolerance and datum references (as appropriate) 46
Rules of GD&T Rule # 3 (1982 standard) For all other geometric controls, RFS automatically applies.
Rule # 4 All geometric tolerance specified for screw threads apply to the axis of the thread derived from the pitch diameter. Exceptions must be specified by a note (such as major dia or minor dia). All geometric tolerance specified for gears and splines must designate the specific feature (such as major dia or minor dia) at which each applies
Rule # 5 When a datum feature of size is controlled by a geometric tolerance and is specified as secondary or tertiary datum, the datum applies at virtual condition with respect to orientation.
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Revision of Standard
48
Virtual condition The virtual condition of a part is the condition that defines the boundary of acceptability. This condition is the boundary established by the collective effect of size and geometric tolerance. Virtual condition is not a control but a condition of a feature as a result of size, geometric tolerance and the modifiers. Virtual condition is the boundary (locus) at which the features are no longer acceptable. The boundary (locus) may violate rule #1, which in part states that “the boundary of perfect for at MMC”. In certain applications, the combined effect of actual mating size, the geometric tolerance and the modifiers will exceed the boundary of perfect form. The letter denoted by VC.
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Virtual condition – Internal feature of size @ MMC The virtual condition for a internal feature is the constant value equals to its maximum material condition size minus its applicable tolerance of form or location Stimulated Gauge Plug gauge - GO Ø24.8 diameter
01 Plug gauge – NO GO Ø25.21 diameter
02 Functional gauge Ø24.75 diameter Virtual condition = MMC – Geometrical tolerance VC = 24.8 – 0.05 Virtual condition = 24.75
03
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Virtual condition – External feature of size @ MMC The virtual condition for a external feature is the constant value equals to its maximum material condition size plus its applicable tolerance of form or location Stimulated Gauge Snap gauge – Go Ø45.2 diameter No G o - Ø44.79 diameter 01 &
02
Functional gauge Ø45.40 diameter Virtual condition = MMC + Geometrical tolerance VC = 45.2 + 0.20 Virtual condition = 45.40
03
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Introduction to BONUS Tolerance When a MMC modifier is used in the feature control frame, it means that the stated tolerance applies when the FOS is at its MMC. When the actual mating size of FOS departs from MMC towards LMC, an increase in the stated tolerance - equal to the amount of departure is permitted. This increase or extra tolerance is called Bonus tolerance. 2.7 Gauge Interpretation opening
0.2 bow At MMC
Maximum bonus occurs when the actual mating envelope is at LMC
Feature AME
Straightness tolerance
Bonus tolerance
Straightness Tolerance zone
2.5 MMC
0.2
0.0
0.2
2.4
0.2
0.1
0.3
2.3
0.2
0.2
0.4
2.2
0.2
0.3
0.5
2.1
0.2
0.4
0.6
2.0 LMC
0.2
0.5
0.7 52
Example – Condition - MMC/LMC/RFS
AME
Geometric Tol.
12.39
‘Bonus’ Tol.
Total Tolerance
out of size tolerance
MMC - 12.40
0.05
0.00
0.05
12.45
0.05
0.05
0.10
12.50
0.05
0.10
0.15
12.55
0.05
0.15
0.20
LMC - 12.60
0.05
0.20
0.25
12.61
out of size tolerance
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Example – Condition - MMC/LMC/RFS
AME
Geometric Tol.
12.39
‘Bonus’ Tol.
Total Tolerance
out of size tolerance
MMC - 12.40
0.05
0.20
0.25
12.45
0.05
0.15
0.20
12.50
0.05
0.10
0.15
12.55
0.05
0.05
0.10
LMC - 12.60
0.05
0.00
0.05
12.61
out of size tolerance
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Example – Condition - MMC/LMC/RFS
AME
Geometric Tol.
12.39
‘Bonus’ Tol.
Total Tolerance
out of size tolerance
MMC - 12.40
0.05
0.00
0.05
12.45
0.05
0.00
0.05
12.50
0.05
0.00
0.05
12.55
0.05
0.00
0.05
LMC - 12.60
0.05
0.00
0.05
12.61
out of size tolerance
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FORM TOLERANCE
Straightness Flatness Circularity Cylindricity
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Straightness Straightness is a condition where one line element of a surface or axis is in a straight line. The tolerance zone when projected in a plane is limited by two parallel straight lines, a distance ‘t’ apart. The tolerance zone is limited by a cylinder of diameter ‘t’ if the tolerance value is preceded by the sign Ø
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Straightness On Drawing
Means this
Place the part on V-Blocks. At each measuring position along longitude, move dial indicator around circumference. Take lowest and highest reading.
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Straightness On Drawing
a. The maximum diameter of the pin with perfect form is shown in a gauge with a 16.04 diameter hole.
b. With the pin at max. dia 16.0, the gauge will accept the pin with up to 0.04 variation in straightness.
Part is held between centers. Measurements are taken along longitudinal sections. At least four sections must be measured. At least three points must be measured in each section. This holds good for RFS condition. For ‘M’ modifier use functional gauges. c. With the pin at min. dia 15.89, the gauge will accept the pin with up to 0.15 variation in straightness.
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Exercise On Drawing
Functional gauge
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Flatness Flatness is the condition of a surface having all of its elements in one plane. The tolerance zone for a flatness control is three dimensional. General representation Two parallel planes 0.2 apart
Surface plate Interpretation of Flatness tolerance: It consist of two parallel planes within which all the surface elements must lie. The distance between the parallel planes is equal to the flatness control tolerance value.
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Flatness In this example, the flatness has been applied to datum feature A. Three possible inspection methods are illustrated as below.
01
In the first case, the part is leveled on the surface plate. If the surface is convex, the part will rock making it difficult to determine the minimum indicator reading over the entire surface. In the second illustration, the surface is leveled by placing it on three equal height gage blocks. The indicator is then moved across the surface. Ideally, the gage blocks should be placed under the high points on the surface. Otherwise, the indicator movement may not be the lowest possible.
02
03
In the third illustration, the CMM will mathematically "level" the points of the surface contacted by the probe. Of course, a CMM will automatically align the points to evaluate the flatness error. Often insufficient points are taken to evaluate the flatness error. As a result, an out of spec surface may be accepted.
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Circularity Circularity is a condition, where all points on the periphery of a surface of revolution, at any cross section of shaft, which is perpendicular to its axis are equidistant from that axis. A circular tolerance is given to control a roundness of a feature, e.g, the circumference of shaft or hole.
General representation
For example a shaft may be bowed or bent yet still meets the requirement because it does not control relative positions of the tolerance zone.
Measuring principle of circularity Place the shaft on a V-Block which is kept on a surface plate. The dial indicator should be stationary. Measurement at one line on surface when part is rotated. The deviation or circular error is the difference between the highest and lowest reading. 63
Circularity - Interpretation The shaft diameter to be within Ø4.70 – Ø5.3 with a circular tolerance of 0.2mm. Here the inner boundary is Ø4.7 and outer boundary is Ø5.3. The size tolerance is 0.6. The circularity control tolerance must be less than the size tolerance.
(5.3)
The shaft diameter should be within size tolerance and form (out of round) should be within the circular tolerance (4.7)
Circular tolerance – (0.2)
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Cylindricity Cylindricity controls roundness of a feature over its entire surface. Cylindricity is a condition of a surface of revolution in which all points of the surface are equidistant from a common axis. Cylindricity controls deviations such as concavity and convexity of a shaft. It composite control that limits the circularity, straightness and taper of a diameter simultaneously.
Place the shaft on a V-Block which is kept on a surface plate. The dial indicator should be stationary. Measurement at all points on surface when part is rotated and moved length wise. The deviation or cylindrical error is the difference between the highest and lowest reading through out the surface. 65
Cylindricity In this example, the cylindricity has been applied to 5 features. Cylindricity is an individual control. Therefore, each feature is inspected independent of the others. An electronic probe gathers many points on the surface. A computer then evaluates the points to fit them between two concentric cylinders that may not have a radial separation greater than 0.02. The actual size, location and orientation of the feature are ignored. Cylindricity is a composite of circularity, surface straightness and taper.
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PROFILE TOLERANCE
Profile of any line Profile of any surface
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Profile of any line Profile of any line is a two-dimensional cross section tolerance. 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. The profile tolerance is assumed a bilateral tolerance, which is the tolerance both above and below the true profile line.
XY
Uses profile of a line to control all around of a cross section of a wing.
Cross section of a simple contour that is controlled by line profile tolerance.
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Profile of any Surface Most of us still use ± linear, general tolerances found in the titleblock of a drawing. We do not know why we use it, we just do. "That's the way we've always done it." This general note causes much confusion because it does not relate tolerances to a datum reference frame, allows tolerance accumulation and wedge shaped tolerance zones. To clear up the drawing's meaning, many practice to change to a general profile of a surface tolerance with datum referencing. This requires that the dimensions describing surfaces be basic. Since this will usually be most of the dimensions, it is becoming a common practice to add the note "UNTOLERANCED DIMENSIONS ARE BASIC". This note eliminates a lot of boxes on the drawing. Since the general profile tolerance controls location of the entire surface of each feature, the only reason to put any geometric tolerances on the field of the drawing is for refinements of orientation, size and form.
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Profile of any Surface MMC May Be Applied to Datum References in a Profile tolerance callout, but not to the Profile Tolerance. We cannot apply the MMC modifier to a profile tolerance because it is a surface control. We can, however, apply MMC or LMC to the datum references of a profile tolerance where the datum features are features of size. Here the secondary and tertiary datum features are features of size
70
Profile vs Direct tolerancing A better approach is shown on the revised drawing below. By stating that ALL UNTOLERANCED DIMENSIONS ARE BASIC and specifying a general profile tolerance relative to a datum reference frame, several improvements have been made to the drawing. Tolerance accumulation is reduced / avoided compared to conventional tolerancing. No need for all of those boxes around the dimensions. Better. Life made easy
71
Another advantage of profile tolerancing Imagine trying to inspect the R8 dimension if it had been toleranced without GD&T as is shown here. Try to determine if the center of the 4X R8±0.1 are on the Ø36±0.1 and at the implied 90°±0°30'. By the way, how centered does the Ø36 have to be on the Ø28. There are many ways to "interpret" this drawing. However, the drawing with GD&T has "one clear meaning." Bottom line, if you want to do an inadequate job of inspecting a part using calipers-go ahead, regardless of how the part is toleranced. Dimensioning without GD&T Dimensioning with GD&T
Better. Life made easy 72
Profile equal and unilateral According to the Y14.41 standard, the value that follows the U in a circle is the amount of tolerance zone that applies outward from the surface. This may also be used to indicate a unilateral profile tolerance. Of course, if you want to use this new symbol, you should reference ASME Y14.41-2003 as well as ASME Y14.51994 so everyone reading the drawing knows where to find the meaning.
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ORIENTATION TOLERANCE
Angularity Perpendicularity Parallelism
74
Angularity Control Angularity is a condition of a surface, center plane or axis being exactly at the specified angle. It limits the amount of surface, center plane or axis is permitted to vary from its specified angle. Angularity are applied to surface and cylindrical FOS. Tolerance zone for surface are two parallel planes and for cylindrical FOS will be a cylinder. Angularity tolerance to a surface given with two datum references. Measurement method – Placing the block on sine bar and measuring high and low limits which should fall under 0.2mm.
Use two datum reference for Angularity. 75
Angularity Control Angularity are applied to cylindrical FOS. Tolerance zone for cylindrical FOS will be a cylinder or parallel planes based on notation given to tolerance. Tolerance zone is oriented relative to the datum plane by a basic angle. Axis of hole must lie between the tolerance zone.
Axis can lie on Ø0.4 With ref. to datum A
Axis can lie between two planes 0.2 apart With ref. to datum A
76
Parallelism Control Parallelism is a condition of a surface, center plane or axis being exactly parallel to the datum. It limits the amount of a surface, center plane or axis is permitted to vary from being parallel to datum. It is applied to a surface and cylindrical FOS. Size to be within 29.7 to 30.3 mm.
Datum A
29.7 30.3
Parallelism applied to a surface
Parallelism to be within 0.1 with ref. to datum A
Datum A
77
Parallelism Control Bonus tolerance is permissible. A fixed gauge can be used to verify parallelism. Parallelism tolerance zone limits the flatness of the toleranced feature. Parallelism applied to a cylindrical FOS at MMC
Datum A
AME
Parallelism tolerance
Bonus tolerance
Total Tolerance zone
8.2 LMC
0.1
0.4
0.5
8.0
0.1
0.2
0.3
7.8 MMC
0.1
0.0
0.1
The axis can lie on Ø0.1 with ref. to datum A. Bonus tolerance allowed up to Ø0.5 at LMC Datum A
The axis can lie between two planes of 0.1 apart with ref. to datum A. Bonus tolerance allowed up to 0.5 at LMC 78
Perpendicularity Control Perpendicularity is a condition that results when a surface, axis or center plane is exactly 90 degree to a datum ref. It limits the amount of surface, axis or center plane is permitted to vary from being perpendicular to the datum. It is applied to surface, planar FOS and cylindrical FOS.
Surface to lie within parallel planes 0.2 apart and 90° with ref. to datum A
Center plane established from max. separation of parallel planes to lie within parallel planes 0.2 apart and 90° with ref. to datum A
Datum A Datum A
79
Perpendicularity Control Perpendicularity applied to cylindrical FOS. Bonus tolerance is permissible. Perpendicularity applied to a cylindrical FOS at MMC
AME
Perpendicularity tolerance
Bonus tolerance
Total Tolerance zone
50.2 MMC
0.1
0.0
0.1
50.1
0.1
0.1
0.2
50.0 LMC
0.1
0.2
0.3
Axis of Ø50 can lie between two planes 0.1 apart with ref. to datum A and 0.3 at LMC. Gauge ID Ø50.3
Axis of Ø50 can lie on Ø0.1 with ref. to datum A and Ø0.3 at LMC. Gauge ID Ø50.3 80
LOCATION TOLERANCE
Position Concentricity Symmetry
81
Position Control True position is the theoretically exact location of a FOS as defined by basic dimension. Tolerance of position (TOP) can control is a geometric tolerance that defines the location tolerance of a FOS from its true position. It can control distance between FOS (SLOTS), location of FOS (HOLES), coaxiality between FOS. Interpretation
True position of Ø11.8 – Ø12.2
Ø0.2 Cylindrical tolerance zone perpendicular to Datum A
From Datum C
From Datum B 82
Advantage of Position Tolerancing Cylindrical tolerance zone are 57% larger than square zones. Permits additional tolerance – Bonus and datum shift. Prevents tolerance accumulation. Permits the use of functional gauges. Protects part function and lowers manufacturing cost.
Co-ordinate Tolerancing
Co-ordinate tolerance allows 0.2 square
TOP Tolerancing
Cylindrical tolerance allows 57% more tolerance for the hole
Cylindrical tolerance allows Ø0.28
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Inspection of TOP applied at MMC A TOP applied at MMC can be verified in number of ways like Variable gauges, CMM and functional gauging. Functional gauge is a gauge that verifies functional requirements of a part feature. Also called attribute gauge or fixed gauge. Functional gauge only provides a pass or fail assessment of a part feature. i.e. does not provide numerical reading of a part parameter. It represents the virtual condition of the tolerance FOS. Functional Gauge – Used to check the function of part feature Plug gauge Ø12.0
AME
Position tolerance
Bonus tolerance
Total Tolerance zone
12.80 LMC
0.20
0.30
0.50
12.70
0.20
0.20
0.40
12.60
0.20
0.10
0.30
12.50 MMC
0.20
0.00
0.20
Datum C
Datum A
Datum B
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Benefits of Functional gauges •The gauge represents the worst case mating part. •Parts can be verified quickly. •A functional gauge is economical to produce. •No special skills are required to read the gauge or interpret the results. •In some cases a functional gauge can check several part characteristics simultaneously.
Functional Gauge
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Co-axial diameters controlled by TOP (RFS) Interpretation : The axis of Ø45 can lie on tolerance zone of Ø0.4 with reference to datum A regardless feature of size. Variable gauges are used. Functional gauges are not used on RFS condition. Axis of Ø20 (Datum A) Ø0.4 Tolerance of position
AME
Position tolerance
Bonus tolerance
Total Tolerance zone
45.10
0.4
0.0
0.4
45.00
0.4
0.0
0.4
44.90
0.4
0.0
0.4
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Bi – Directional TOP (Locating a hole in two directions) In this application the tolerance zone are parallel boundaries in the direction of TOP control. The shape of the tolerance zone is rectangular.
Tolerance zone
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Use TOP to locate an elongated hole For elongated hole – bonus tolerance is are permissible. The size should meet the functional requirements.
Outer boundary
Inner boundary
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Use TOP in fixed fastener assembly A fixed fastener assembly is where the fastener is fixed or restrained in to one of the components of the assembly. Often, the holes in one of the component in assembly are clearance hole and other would be threaded to held the fastener. This is called fixed fastener assembly
Calculating position tolerance: T – Position tolerance diameter ? H – MMC of clearance hole (Top plate) F – MMC of fastener
T = (H-F)/2 =(12.5-12)/2 =0.25 89
Concentricity Control A concentricity or coaxiality is a condition where the median points of all diametrically opposed elements of a cylinder are congruent with the axis of the datum feature. It is used when precise balance of part is required, equal wall thickness are required, equal distribution of mass. Datum axis A
Tolerance zone Ø 0.1 Median points of all diametrically opposed to be congruent with datum axis A 90
Symmetry Control Symmetry is a condition where the median points of all opposed elements of two or more feature surfaces are congruent with the axis or center plane of a datum feature. It is applied to features that are shown symmetrical to the datum center plane. It is difficult to measure, since it requires deriving the features median points to determine if they are contained within the specified tolerance zone which is centered on the datum axis or datum center plane
91
Symmetry Control Symmetry tolerance added to a line or axis. Here the tolerance is given to the axis of a lever. The axis of the hole to be contained between two parallel planes 0.1 apart and symmetrically disposed with respect to datum A,B&C.
The difference between dimensions A and B must not exceed 0.1.
Use a mandrel and measure the dimensions of A and B 92
Circular Runout Circular runout requires inspecting circular elements individually. Because of this, circular runout will not detect changes in size. Any barreling, waisting or taper should be ignored. These conditions will be controlled by the size tolerance. Circular elements that are contained within a cylindrical feature that is shown centered on the datum axis.
93
Total Runout Total runout requires determining the full indicator movement over the entire feature. Total runout, therefore, detects any changes in size, i.e. barreling, waisting or taper. Although changes in size are controlled, the actual size of the feature is determined and controlled by the size dimension and tolerance. A feature may be manufactured to a very tight size tolerance but due to eccentricity, the runout could be large. Total runout controls form, orientation and location but not size. Cylindrical features shown centered on the datum axis. Controls all geometric characteristics except size.
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Runout (Circular Vs Total) The worst circular runout error occurs at the slice with the greatest variation. In this case two slices vary a total of 0.03. Total runout is the difference between the highest and lowest readings found over the entire feature. The highest reading was +0.02 and the lowest reading was -0.09. Therefore, the total runout for the feature is 0.11, the difference between +0.02 and -0.09.
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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.
“When in doubt use Runout”
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