gd&t-1notes

Geometric Dimensioning And Tolerance PGDPTD-2009

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Engineering Drawing: An engineering drawing is a type of technical drawing, used to fully and clearly define requirements for

engineered items. Sizes of Drawing Sheets.—Recommended trimmed sheet sizes, based on ANSI/ASME Y14.1-1980 (R1987),

G D & T: • GD&T is an international language that is used on engineering drawings to accurately describe the size,

form, orientation and location of part features. • It is also a design-dimensioning philosophy that encourages designers to define a part based on how it

functions in the final product or assembly. • GD&T is an exact language that enables design engineers to "say what they mean" on a drawing, thus

improving product designs and lowering cost. • Process engineers and manufacturing use the language to interpret the design intent and to determine the

best manufacturing approach. • Quality control and inspection use the GD&T language to determine proper set-up and part verification. • By providing company-wide uniformity in the drawing specifications and interpretation, GD&T reduces

controversy, guesswork, and assumptions throughout the design, manufacturing and inspection process. o Create clear, concise drawings o Improve product design o Create drawings that reduce controversy, guesswork, and assumptions throughout the

manufacturing process o Effectively communicate or interpret design requirements for suppliers and manufacturing

• However, because GD&T is such a precise language, it involves a great many symbols and terms. • Geometric dimensioning and tolerancing provides a comprehensive system for symbolically defining the

geometrical tolerance zone within which features must be contained. • It provides an accurate transmission of design specifications among the three primary users of engineering

drawings; design, manufacturing and quality assurance.

Terms & Definitions: Actual Local Size - The value of any individual distance at any cross section of a feature of size. Actual Mating Envelope of an External Feature of Size - A similar perfect feature counterpart of the

smallest size that can be circumscribed about the feature so that it just contacts the surfaces at the highest points. Actual Mating Envelope of an Internal Feature of Size - A similar perfect feature counterpart of the largest

size that can be inscribed within the feature so that it just contacts the surfaces at their highest points. All-Around Symbol - A circle placed on the bend of the leader line of a profile control. Angularity - The condition of a surface, center plane or axis being exactly at a specified angle Angularity Control - A geometric tolerance that limits the amount a surface, axis, or centerplane is

permitted to vary from its specified angle. ASME Y14.5M-1994 - The national standard for dimensioning and tolerancing in the United States.

ASME stands for American Society of Mechanical Engineers. The Y14.5 is the standard number. "M" is to indicate the standard is metric, and 1994 is the date the standard was officially approved.

Size Inches Metric Size mm

A 8.5x11 D 22x34 A0 841x1149 A3 297x420

B 11x17 E 34x44 A1 594x841 A4 210x297

C 17x22 F 28x40 A2 420x594


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Axis Theory - The axis (or center plane) of a feature of size must be within the tolerance zone Basic Dimension - A numerical value used to describe the theoretically exact size, true profile, orientation,

or location of a feature or datum target. Between Symbol - A double ended arrow that indicates the tolerance zone extends to include multiple

surfaces. Bi-Directional Control - Where the location of a hole is controlled to a different tolerance value in two

directions. Bilateral Tolerance - A tolerance that allows the dimension to vary in both the plus and minus directions. Bonus Tolerance - An additional tolerance for a geometric control. Whenever a geometric tolerance is

applied to a feature of size, and it contains an MMC (or LMC) modifier in the tolerance portion of the feature control frame, a bonus tolerance is permissible.

Boundary - The word "BOUNDARY" is placed beneath the feature control frames to invoke a boundary

control. Cartoon Gage - A sketch of a functional gage. A cartoon gage defines the same part limits that a functional

gage would, but it does not represent the actual gage construction of a functional gage. Circularity - A condition where all points of a surface of revolution, at any section perpendicular to a

common axis, are equidistant from that axis. Circularity Control - A geometric tolerance that limits the amount of circularity on a part surface. Circular Runout - A composite control that affects the form, orientation, and location of circular elements

of a part feature relative to a datum axis. Circular Runout Control - A geometric tolerance that limits the amount of circular runout of a part surface. Coaxial Datum Features - When coaxial diameters are used to establish a datum axis. Coaxial Diameters - Two (or more) diameters that are shown on the drawing as being on the same

centerline (axis). Feature: The general term applied to a physical portion of a part such as surface, hole, slot etc. Feature of Size – One cylindrical or spherical surface or set of 2 opposed elements or opposed parallel

surfaces associated with a size dimension. E.g diameter of hole, width of slot etc.


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Regardless of feature size (RFS) – A term that indicates a geometric tolerance applies at any increment of size of feature within size tolerance, in other words a geometric tolerance applies at whatever size a feature is produced.

Rule # 1(Individual feature of size rule) – Where only a tolerance of size is specified, the limits of size of an

individual feature prescribe the extent to which variation in its form as well as its size are allowed. It is called as “envelope rule” or “perfect form at MMC”.

Virtual Condition - A worst case boundary generated by collective effects of feature of size with geometric

tolerances specified at MMC or LMC.

Geometric Characteristic Symbols:


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Feature Control Frame: It is a specification on a drawing that indicates the type of geometric control for the feature, the tolerance for the control and the related datums if applicable. It consists of:

• Type of control (geometric characteristics) • Tolerance zone sizeTolerance modifier denotes • Tolerance zone shape • The condition under which tolerance applies (M,L,F,P etc.) • Applicable datum references • Datum reference modifier


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General Rule: Rule 1(envelope rule) : “It states where only a tolerance

of size is specified the limits of size also control limits of form.”

• Surfaces shall not exceed beyond a boundary (envelope) of perfect form at MMC. No variation in form is permitted if feature is produced at its MMC limit of size

• Where the actual local size of a feature has departed from MMC toward LMC a variation in form is allowed equal to amount of such departure.

• There is no requirement for a boundary of form at LMC. Thus a feature produced at LMC is permitted to vary from true form to maximum variation allowed by boundary of perfect form at MMC.

• Rule 1 only applies to a size. • Rule 1 does not affect location, orientation or

interrelationships between features of size. Therefore relationship between individual features must be defined.

• Geometric tolerances are often used to control the location , orientation or interrelationships between features of size.

Basic Dimensions: • When GD&T is used, the geometric tolerances apply to the features - not the dimensions. Therefore when

BASIC dimensions are used with geometric tolerances, several different dimensioning schemes may be used without changing the meaning of the drawing.

The example shows baseline of dimensioning. The other two illustrate chain dimensioning. Since the position tolerance is related to datums A, B and C, all three drawings have the same meaning even though the dimensioning is different. This would not be true if the dimensions locating the holes were toleranced, rather than BASIC. Because the meaning of these drawings is the same, the designer should consider the needs of those who will read the print when placing dimensions.


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Direct Tolerancing: • If we don't have opposed points (180+ degrees of arc), you cannot reproducibly find the center (axis) of a

radius. Often a curved surface will be defined on a drawing by locating the center and specifying a radius. • There is nothing wrong with this type of dimensioning. The problem is that traditionally these dimensions

are toleranced directly or use the title block tolerance. • Once the part is made, someone has to determine where the center is out in space to determine if the

dimensions are in tolerance. • When we have opposed points, we can contact these points with an indicator or gage pin and calculate the

location of the center. • Without opposed points, you have to try to fit a radius to the surface and try to determine where the center

is. If we are taking sample points with a CMM or vision system, we will get an answer; but, if the radius is not perfectly round (and it never will be), contacting different points will yield a different size radius with a different center location.

• This problem can be avoided by using Profile of a Surface control. The dimensioning doesn't change but the dimensions are made BASIC and originate at the datums.

• The tolerance is on the surface not on the dimensions. We might think of the dimensions as the goal. we don't want to tolerance the goal. We do want to specify how much the actual surface of the part may deviate from the goal. Now measuring the part does not involve trying to determine where the imaginary center of the radius may be.

Toleranced Dimensions Still Have Their Place-

• Even though basic dimensions are being used today far more than in the past, there are still many appropriate applications for toleranced dimensions.

• Care should be taken to only use toleranced dimensions in situations where they may be verified, reproducibly, without the possibility of misunderstanding.

• This generally limits their use to: o Tangent radii o Some steps or depths o Wall and material thickness o Features of size that contain opposed points o Use of the dimension origin symbol clarifies the meaning of the toleranced dimension.


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Definitions:

• Boundary, Inner- A worst case boundary (that is, locus) generated by the smallest feature (MMC for an internal feature and LMC for an external feature) minus the stated geometric tolerance and any additional geometric tolerance (if applicable) from the feature's departure from its specified material condition.

• Boundary, Outer- A worst case boundary (that is, locus) generated by the largest feature (LMC for an internal feature and MMC for an external feature) plus the geometric tolerance and any additional geometric tolerance (if applicable) from the feature's departure from its specified material condition.

• Dimension- A numerical value expressed in appropriate units of measure and used to define the size, location, geometric characteristic, or surface texture of a part or part feature.

• Dimension, Basic- 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.

• Dimension, Reference- A dimension, usually without tolerance, used for information purposes only. A reference dimension is a repeat of a dimension or is derived from other values shown on the drawing or on related drawings. It is considered auxiliary information and does not govern production or inspection operations.

• Envelope, Actual Mating- This term is defined according to the type of feature, as follows: o For an External Feature- A similar perfect feature counterpart of smallest size that can be

circumscribed about the feature so that it just contacts the surface at the highest points. For example, a smallest cylinder of perfect form or two parallel planes of perfect form at minimum separation that just contact(s) the highest points of the surface(s).

o For an Internal Feature - A similar perfect feature counterpart of largest size that can be inscribed within the feature so that it just contacts the surface at the highest points. For example, a largest cylinder of perfect form or two parallel planes of perfect form at maximum separation that just contact(s) the highest points of the surface(s).For features controlled by orientation or positional tolerances, the actual mating envelope is oriented relative to the appropriate datum(s).

• Resultant Condition- The variable boundary generated by the collective effects of a size feature’s specified MMC or LMC material condition, the geometric tolerance for that material condition, the size


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tolerance, and the additional geometric tolerance derived from the feature’s departure from its specified material condition.

• Size, Actual - The general term for the size of a produced feature. This term includes the actual mating size and the actual local sizes.

• Size, Actual Local- The value of any individual distance at any cross section of a feature • Size, Actual Mating - The dimensional value of the actual mating envelope. • Size, Resultant Condition - The actual value of the resultant condition boundary. • Size, Virtual Condition. The actual value of the virtual condition boundary. • True Geometric Counterpart - The theoretically perfect boundary (virtual condition or actual mating

envelope) or best-fit (tangent) plane of a specified datum feature. • True Position- The theoretically exact location of a feature established by basic dimensions. • Virtual Condition- A constant boundary generated by the collective effects of a size feature's specified

MMC or LMC material condition and the geometric tolerance for that material condition.

Fundamental Rules: a) Each dimension shall have a tolerance, except for those dimensions specifically identified as reference,

maximum, minimum, or stock (commercial stock size). b) Dimensioning and tolerancing shall be complete so there is full understanding of the characteristics of each

feature. c) Each necessary dimension of an end product shall be shown. No more dimensions than those necessary for

complete definition shall be given. d) Dimensions shall be selected and arranged to suit the function and mating relationship of a part and shall

not be subject to more than one interpretation. e) The drawing should define a part without specifying manufacturing methods. f) It is permissible to identify as non-mandatory certain processing dimensions that provide for finish

allowance, shrink allowance and other requirements, provided the final dimensions are given on the drawing.

g) Dimensions should be arranged to provide required information for optimum readability. Dimensions should be shown in true profile views and refer to visible outlines.

h) Wires, cables, sheets, rods, and other materials manufactured to gage or code numbers shall be specified by linear dimensions indicating the diameter or thickness.

i) A 90° basic angle applies where center lines of features in a pattern or surfaces shown at right angles on the drawing are located or defined by basic dimensions and no angle is specified.

j) Unless otherwise specified, all dimensions are applicable at 20°C (68°F). Compensation may be made for measurements made at other temperatures.

k) ) All dimensions and tolerances apply in a free state condition. l) Unless otherwise specified, all geometric tolerances apply for full depth, length, and width of the feature. m) Dimensions and tolerances apply only at the drawing level where they are specified. A dimension specified

for a given feature on one level of drawing, (for example, a detail drawing) is not mandatory for that feature at any other level (for example, an assembly drawing).

Application of Dimensions: • Dimension Lines- A dimension line, with its arrowheads, shows the direction and extent of a dimension.

Numerals indicate the number of units of a measurement. NOTE: The following shall not be used as a dimension line: a center line, an extension line, a phantom line, a line that is part of the outline of the object, or a continuation of any of these lines.

• Alignment- Dimension lines shall be aligned if practicable and grouped for uniform appearance. • Spacing- Dimension lines are drawn parallel to the direction of measurement. The space between the first

dimension line and the part outline should be not less than 10 mm. • Crossing Dimension Lines- Crossing dimension lines should be avoided. Where unavoidable, the

dimension lines are unbroken.


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• Extension (Projection) Lines- Extension lines are used to indicate the extension of a surface or point to a location preferably outside the part outline.

• Crossing Extension Lines- Wherever practicable, extension lines should neither cross one another nor cross dimension lines.

• Locating Points- Where a point is located by extension lines only, the extension lines from surfaces should pass through the point.

• Limited Length or Area Indication - Where it is desired to indicate that a limited length or area of a surface is to receive additional treatment or consideration within limits specified on the drawing, the extent of these limits may be indicated by use of a chain line.

• Leaders (Leader Lines)- A leader is used to direct a dimension, note, or symbol to the intended place on the drawing. Normally, a leader terminates in an arrowhead. However, where it is intended for a leader to refer to a surface by ending within the outline of that surface, the leader should terminate in a dot.

• Overall Dimensions- Where an overall dimension is specified, one intermediate dimension is omitted or identified as a reference dimension.

Dimensioning Features: Various characteristics and features of parts require unique methods of dimensioning –

• Diameter • Radii • Center of Radius • Foreshortened Radii • Chords, Arcs and Angles • Grid System • Symmetrical Outlines • Counterbored holesCountersunk and Counterdrilled holes • Chamfered and Countersunk holes on curved surface • Chamfers • Keyseat • Rectangular & Polar coordinate dimensioning

Datums: • Datum Feature: The feature of a part that is used to establish a datum. • Datum Identifier: The graphic symbol on a drawing used to indicate

the datum feature • Datum Plane: A datum is a origin from which the location or other

geometric characteristics of features of part are established. • Datum Simulator: Formed by the datum feature contacting a

precision surface such as surface plate. Thus the plane formed by contact restricts motion and constitutes the specific reference surface from which measurements are taken and dimensions verified. The datum simulator is the practical embodiment of the datum feature during manufacturing and quality assurance.

Terminology: • Datum Target: A specified point, line or area on a part used to

establish a datum. • Degrees of Freedom: The six directions of movement or translation

are called degrees of freedom. They are up-down, left-right, fore-aft, pitch, roll and yaw.


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• Feature Control Frames – When feature control frames reference datums, they also specify sequence for contacting the part to the datums referenced. Sequence is determined by reading the frame from left to right.

Datum Reference Frame: • Sufficient datum features, those most important to the design of part, or designated portions of these

features are chosen to position the part in relation to a set of 3 mutually perpendicular planes, jointly called datum reference frame. This reference frame exists in theory only & not on the part.

• It isn't always possible or practical to select datum features that are mutually perpendicular to one another when establishing a datum reference frame.

• Notice that datum feature C is not nominally perpendicular to datum feature B.

• The datum feature simulator for C would be made at 35° to the datum feature simulator


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for B (shown here in red). The actual datum planes (shown in blue), which comprise the datum reference framework, would however be mutually perpendicular to one another as is illustrated in the last figure.

• The deviation of the hole from the 55mm BASIC location would be measured from the third datum plane-not from the sharp point on the actual part.


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Position Tolerance: “A Positional Tolerance defines, a zone within which the center, axis, or center plane of a feature of size is

permitted to vary from a true (theoretically exact) position; or (where specified on an MMC or LMC basis) a boundary, defined as the virtual condition, located at the true (theoretically exact) position, that may not be violated by the surface or surfaces of the considered feature.”

• Basic dimensions establish the true position from specified datum features and between interrelated features.

• A positional tolerance is indicated by the position symbol, a tolerance value, applicable material condition modifiers, and appropriate datum references placed in a feature control frame.

• A positional tolerance applied at MMC may be explained in either of the following ways – o In Terms of the Surface of a Hole - While maintaining the specified size limits of the hole, no

element of the hole surface shall be inside a theoretical boundary located at true position. o In Terms of the Axis of a Hole - Where a hole is at MMC (minimum diameter), its axis must fall

within a cylindrical tolerance zone whose axis is located at true position. The diameter of this zone is equal to the positional tolerance.

• This tolerance zone also defines the limits of variation in the attitude of the axis of the hole in relation to the datum surface. It is only where the hole is at MMC that the specified tolerance zone applies. Where the actual mating size of the hole is larger than MMC, additional positional tolerance results.

• This increase of positional tolerance is equal to the difference between the specified maximum material condition limit of size (MMC) and the actual mating size of the hole.

• Where the actual mating size is larger than MMC, the specified positional tolerance for a hole may be exceeded and still satisfy function and interchangeability requirements.


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Calculating Positional Tolerance: Figure below shows a drawing for one of two identical plates to be assembled with four 14 mm maximum

diameter fasteners. The 14.25 minimum diameter clearance holes are selected with a size tolerance as shown. Using conventional positional tolerancing, the required tolerance is found by the equation -

Zero Positional Tolerance MMC: Figure above shows a drawing of the same part with a zero positional tolerance at MMC specified. Note

that the maximum size limit of the clearance holes remains the same but the minimum was adjusted to correspond with a 14 mm diameter fastener.

This results in an increase in the size tolerance for the

clearance holes, the increase being equal to the conventional positional tolerance specified in Fig. Although the positional tolerance specified is zero at MMC, the positional tolerance allowed is in direct proportion to the actual clearance hole size as shown by the following tabulation:

Positional Tolerance LMC: LMC as Related to Positional Tolerancing-

Where positional tolerancing at LMC is specified, the stated positional tolerance applies where the feature contains the least amount of material permitted by its toleranced size dimension. Specification of LMC requires perfect form at LMC.

Perfect form at MMC is not required. Where the feature departs from its LMC limit of size, an increase in positional tolerance is allowed, equal to the amount of such departure.

LMC may be specified in positional tolerancing applications where MMC does not provide the desired control and RFS is too restrictive.

In the examples below, LMC is used to maintain a desired relationship between the surface of a feature and its true position at tolerance extremes. Boss and hole combination: Wall thickness is minimum where the boss and hole are at their LMC sizes and both features are displaced in opposite extremes. Since positional tolerances are specified on an LMC basis, as each feature departs from LMC, the wall thickness increases. This permits a corresponding increase in the positional tolerance, thus maintaining the desired minimum material thickness. LMC Applied to a Radial Pattern of Slots : Radial pattern of slots is located relative to an end face and a center hole. LMC is specified to maintain the desired relationship between the side surfaces of the slots and the true position, where rotational alignment with the mating part may be critical


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Multiple Patterns of Feature: Multiple Patterns of Features Located by Basic Dimensions Relative to Common Datums:

Where two or more patterns of features are located by basic dimensions relative to common datum features referenced in the same order of precedence, and at the same material conditions, the following apply. Simultaneous Requirement – RFS:

Where multiple patterns of features are located relative to common datum features not subject to size tolerances, or to common datum features of size specified on an RFS basis, they are considered to be a single pattern.

For example, in the next Fig.(A) each pattern of features is located relative to common datum features not subject to size tolerances. Since all locating dimensions are basic and all measurements are from a common datum reference frame, verification of positional tolerance requirements for the part can be collectively accomplished in a single setup or gage as illustrated by Fig. (B). The actual centers of all holes must lie on or within their respective tolerance zones when measured from datums A, B and C.

Simultaneous Requirement – MMC

Where any of the common datums in multiple patterns of features is specified on an MMC basis, there is an option whether the patterns are to be consideredas a single pattern or as having separate requirements. If no note is added under the feature control frames, the patterns are to be treated as a single pattern. Where it is desired to permit the patterns to be treated as separate patterns, a notation such as “SEP REQT” is placed beneath each feature control frame. This allows the datum features of size to establish a separate datum reference frame for each pattern of features, as a group. These datum reference frames may shift independently of each other, resulting in an independent relationship between the patterns.


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Composite Positional Tolerancing: a. Pattern-Locating Tolerance Zone Framework (PLTZF) :

Where composite controls are used, the upper segment is referred to as the pattern-locating control. The PLTZF is located from specified datums by basic dimensions. It specifies the larger positional tolerance for the location of the pattern of features as a group. Applicable datums are specified in a desired order of precedence, and serve to relate the PLTZF to the datum reference frame.

b. Feature-Relating Tolerance Zone Framework (FRZF): The lower segment is referred to as the feature-relating control. It governs the smaller positionaltolerance for each feature within the pattern (feature-to-feature relationship). Basic dimensions used to relate the PLTZF to specified datums are notapplicable to the FRTZF. Where datum references are not specified in the lower segment of the composite feature control frame, the FRTZF is free to be located and oriented (shift and/or tilt) within the boundaries established and governed by the PLTZF. If datums are specified in the lower segment, they govern the orientation of the FRTZF relative to the PLTZF. (See Figs. C and D). Where datum references are specified, one or more of the datums specified in the upper segment of the frame are repeated, as applicable, and in the same order of precedence, to govern the orientation of the FRTZF.

Primary Datum Repeated in Lower Segment: As can be seen from the sectional view of the tolerance zones below, since datum plane-A has been repeated in the lower segment of the composite feature control frame, the axes of both the PLTZF and FRTZF cylinders are perpendicular to datum plane A and therefore, parallel to each other. In certain instances, portions of the smaller zones may fall beyond the peripheries of the larger tolerance zones. However, these portion of the smaller tolerance zones are not usable because the axes of the features must not violate the boundaries of the larger tolerance zones. The axes of the holes must lie within the larger tolerance zones and within the smaller tolerance zones. The axes of the actual holes may vary obliquely (out of perpendicularity)


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only within the confines of the respective smaller positional tolerance zones (FRTZF). Figure (E) repeats the heretofore-described relationships for the four-hole pattern, and Fig. (F) for the six-hole pattern of features.


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When using composite tolerancing, the second segment tightens the location between features in the pattern. If datum references are repeated in the second (lower) segment, the orientation of the pattern is tightened to the value specified in that segment. In the assembly shown below, if you want the mounting holes to line up but are not concerned about the rotation of the parts relative to one another (as illustrated by the arrow), the datum references would not be repeated in the lower segment. This is illustrated on the drawing on next slide. To limit this rotation, the datum references should be repeated in the lower segment as illustrated on the drawing further. It is important to note that even though rotation between the parts has been limited to 0.3mm, the parts may still move 0.8mm relative to each other. Primary and Secondary Datums Repeated in Lower Segment:

The lower segment of the composite feature control frame repeats datums A and B. The axes of the actual tolerance cylinders of the FRTZF may be displaced from the true position locations (as a group) as governed by the tolerance cylinders of the PLTZF, while remaining perpendicular to datum plane A and parallel to datum plane B. Figure shows that the actual axes of the holes in the actual feature pattern must reside within both the tolerance cylinders of the FRTZF and the PLTZF.

Single Segment Feature Control Frame: Where it is desired to invoke basic dimension

along with the datum references, single segment feature control frames are used. Figure shows two single-segment feature control frames. The lower feature control frame repeats datums A and B.

Figure below shows that the tolerance cylinders of the FRTZF (as a group) are free to be displaced to the left or right as governed by the basically-located tolerance cylinders of the PLTZF, while remaining perpendicular to datum plane A and parallel to datum plane B. It also shows that the actual axes of the holes in the actual feature pattern must reside within both the tolerance cylinders of FRTZF and PLTZF.


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Tighten Location in One Direction With a Separate Requirements Callout: To restrict movement in one direction only, two single segments should be used. In the assembly shown

below, if you want to restrict the movement of the part up and down to 0.3 but may allow a movement of 0.8 left and right, datums A and B should be repeated in the second segment as illustrated.

Single VS Composite: Single segment simply means that there are two separate geometric callouts which should be read

independently. Both requirements must be met. Composite, on the other hand, is represented using a single geometric characteristic symbol with multiple

tolerances shown. The primary purpose of the lower segment of a composite position tolerance is to control the location of features within a pattern. If any datums are repeated in the lower segment, they refine the orientation (perpendicularity, parallelism or angularity) of the pattern relative to the referenced datums.

The holes may not be out of location relative to one another and datum A by more than 0.1. Relative to the complete datum reference frame, the pattern may move and rotate in a plane parallel to Datum A by 0.4mm.

The holes may not be out of location relative to one another by more than 0.1. And the pattern must remain parallel to Datum A (as the arrows indicate) even though it may be out of location relative to Datum A by as much as 0.4mm.


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The tolerance in the upper segment of a composite tolerance is located by all applicable basic dimensions.

On the drawing above, the red tolerance is located by the red dimensions. The lower segment of a composite tolerance does not use the basic dimensions which originate at the datums. Only the basic dimensions within the patter n are applicable. If a datum is repeated, it indicates that the orientation of the pattern must be held to the tighter tolerance. In this case, the perpendicularity to datum A must be within 0.2 and the pattern of two holes may not tilt more than 0.2 relative to datum B. Two position symbols being used. This callout is single segments. The upper segment has the same meaning as the upper segment of the composite callout shown earlier. The lower segment, however, improves the location as well as the orientationof the feature(s) relative to the datums referenced in the second sement. Notice that the 19mm dimension from datum B is shown in red. The pattern must be positioned at the 19mm dimension from B within 0.2 total even though the pattern may be out of position as much as 0.6 total relative to datum C.


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Counter-Bored Holes:

Where positional tolerances are used to locate coaxial features, such as counterbored holes, the following practices apply-

a. Where the same positional tolerance is used to locate both holes and counterbores, a single feature control frame is placed under the callouts specifying hole and counterbore requirements. Identical diameter tolerance zones for hole and counterbore are coaxially located at true position relative to the specified datums.

b. Where different positional tolerances are used to locate holes and counterbores (relative to common datum features), two feature control frames are used, One feature control frame is placed under the callout specifying hole requirements and the other under the callout specifying counterbore requirements. Different Diameter tolerance zones for hole and counterbore are coaxially located at true position relative to the Specified datums.

c. Where positional tolerances are used to locate holes and to control individual counterbore-to-hole relationships (relative to different datum features), two feature control frames are used as in (b) above. In addition, a note is placed under the datum feature symbol for the hole and under the feature control frame for the counterbore, indicating the number of places each applies on an individual basis.


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Bi-Directional Positional Tolerance: Where it is desired to specify a greater tolerance in one direction than another, bidirectional positional

tolerancing may be applied. Bidirectional positional tolerancing results in a non-cylindrical tolerance zone for locating round holes; therefore, the diameter symbol is omitted from the feature control frame in these applications. Rectangular Coordinate Method:

For holes located by rectangular coordinate dimensions, separate feature control frames are used to indicate the direction and magnitude of each positional tolerance relative to specified datums. The feature control frames are attached to dimension lines applied in perpendicular directions. Each tolerance value represents a distance between two parallel planes equally disposed about the true position.


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P olar Coordinate Method : Bidirectional positional tolerancing is also

applied to holes, such as gear-mounting centers located by polar coordinate dimensions relative to specified datums, where a smaller tolerance is desired in the direction of the line-of-centers rather than at right angles to the line-of-centers.

In this application, one dimension line is applied in a radial direction and the other at right angles to the line-of-centers. A further requirement of perpendicularity within the positional tolerance zone has been specified. The positional tolerance values represent distances between two concentric arc boundaries and two parallel planes, respectively, equally disposed about the true position. Coordinate hole-locating dimensions, indicated as reference, may be included on the drawing for manufacturing convenience.

Non Circular Features: a. In Terms of the Surfaces of a Feature :

While maintaining the specified width limits of the feature, no element of its side surfaces shall be inside a theoretical boundary defined by two parallel planes equally disposed about true position and separated by a distance equal to that shown for W in the figure.

b. In Terms of the Center Plane of a Feature : While maintaining the specified width limits of the feature, its center plane must be within a tolerance zone defined by two parallel planes equally disposed about true position, having a width equal to the positional tolerance. This tolerance zone also defines the limits within which variations in attitude of the center plane of the feature must be confined.

c. In Terms of the Boundary for an Elongated Feature: While maintaining the specified size limits of the elongated feature, no element of its surface shall be inside a theoretical boundary of identical shape located at true position. The size of the boundary is equal to the MMC size of the elongated feature minus its positional tolerance.

To invoke this concept, the tem BOUNDARY is placed beneath the feature control frames.

In this example, a greater positional tolerance is allowed for its length than for its width. Where the same positional tolerance can be allowed for both, only one feature control frame is necessary, directed to the feature by a leader and separated from the size dimensions.


25 Faiyaz Ahmed

Non-Circular Features

gd&t-1notes

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