GEOMETRIC DIMENSIONING TOLERANCING ANSI Y 14 5 M

GEOMETRIC DIMENSIONING & TOLERANCING ANSI Y 14. 5 M 1994 Based on fit, form and function of a part and to control the relationship between part features

Three Categories of Dimensioning can be divided into three categories: • general dimensioning, • geometric dimensioning, and • surface texture. The following provides information necessary to begin to understand geometric dimensioning and tolerancing (GD&T)

Geometric Dimensioning & Tolerancing (GD&T) n GD&T is a means of dimensioning & tolerancing a drawing which considers the function of the part and how this part functions with related parts. – This allows a drawing to contain a more defined feature more accurately, without increasing tolerances.

GD&T cont’d n GD&T has increased in practice in last 15 years because of ISO 9000. – ISO 9000 requires not only that something be required, but how it is to be controlled. For example, how round does a round feature have to be? n GD&T is a system that uses standard symbols to indicate tolerances that are based on the feature’s geometry. – Sometimes called feature based dimensioning & tolerancing or true position dimensioning & tolerancing n GD&T practices are specified in ANSI Y 14. 5 M-1994.

For Example n Given Table Height n However, all surfaces have a degree of waviness, or smoothness. For example, the surface of a 2 x 4 is much wavier (rough) than the surface of a piece of glass. – As the table height is dimensioned, the following table would pass inspection. n If top must be flatter, you could tighten the tolerance to ± 1/32. – However, now the height is restricted to 26. 97 to 27. 03 meaning good tables would be rejected.

Example cont’d. n You can have both, by using GD&T. – The table height may any height between 26 and 28 inches. – The table top must be flat within 1/16. (± 1/32)

WHY IS GD&T IMPORTANT n n n Saves money – For example, if large number of parts are being made – GD&T can reduce or eliminate inspection of some features. – Provides “bonus” tolerance Ensures design, dimension, and tolerance requirements as they relate to the actual function Ensures interchangeability of mating parts at the assembly Provides uniformity It is a universal understanding of the symbols instead of words

WHEN TO USE GD&T n n n When part features are critical to a function or interchangeability When functional gaging is desirable When datum references are desirable to insure consistency between design When standard interpretation or tolerance is not already implied When it allows a better choice of machining processes to be made for production of a part

TERMINOLOGY REVIEW n n n Maximum Material Condition (MMC): The condition where a size feature contains the maximum amount of material within the stated limits of size. I. e. , largest shaft and smallest hole. Minimum Material Condition (LMC): The condition where a size feature contains the least amount of material within the stated limits of size. I. e. , smallest shaft and largest hole. Tolerance: Difference between MMC and LMC limits of a single dimension. Allowance: Difference between the MMC of two mating parts. (Minimum clearance and maximum interference) Basic Dimension: Nominal dimension from which tolerances are derived.

LIMITS OF SIZE A variation in form is allowed between the least material condition (LMC) and the maximum material condition (MMC). Envelop Principle defines the size and form relationships between mating parts.

LIMITS OF SIZE ENVELOPE PRINCIPLE LMC CLEARANCE MMC ALLOWANCE

LIMITS OF SIZE The actual size of the feature at any cross section must be within the size boundary. ØMMC ØLMC

LIMITS OF SIZE No portion of the feature may be outside a perfect form barrier at maximum material condition (MMC).

Other Factors I. e. , Parallel Line Tolerance Zones

GEOMETRIC CHARACTERISTIC CONTROLS 14 characteristics that may be controlled TYPE OF FEATURE TYPE OF CHARACTERISTIC SYMBOL TOLERANCE FLATNESS INDIVIDUAL (No Datum Reference) STRAIGHTNESS FORM CIRCULARITY CYLINDRICITY INDIVIDUAL or RELATED FEATURES LINE PROFILE SURFACE PROFILE PERPENDICULARITY ORIENTATION ANGULARITY PARALLELISM RELATED FEATURES (Datum Reference Required) CIRCULAR RUNOUT TOTAL RUNOUT CONCENTRICITY LOCATION POSITION SYMMETRY

Characteristics & Symbols – – – cont’d. Maximum Material Condition MMC Regardless of Feature Size RFS Least Material Condition LMC Projected Tolerance Zone Diametrical (Cylindrical) Tolerance Zone or Feature – Basic, or Exact, Dimension – Datum Feature Symbol – Feature Control Frame

Feature Control Frame FEATURE CONTROL FRAME GEOMETRIC SYMBOL TOLERANCE INFORMATION DATUM REFERENCES COMPARTMENT VARIABLES THE RELATIVE TO OF THE FEATURE MUST BE WITHIN CONNECTING WORDS

Feature Control Frame n Uses feature control frames to indicate tolerance n Reads as: The position of the feature must be within a. 003 diametrical tolerance zone at maximum material condition relative to datums A, B, and C.

PURPOSE OF A DATUM n. TO REPEATABLY LOCATE A PART FOR INSPECTION OR MANUFACTURE n. TO COMMUNICATE FUNCTIONAL DESIGN INFORMATION ^ ^

Introduction n. Datum – theoretically perfect point, line or plane. n. Datum References – letters shown in feature control frames. n. Datum ^ Features – physical features on a part from which measurements are made. ^

Placement of Datums n Datums are generally placed on a feature, a centerline, or a plane depending on how dimensions need to be referenced. A OR A ANSI 1982 ASME 1994 Line up with arrow only when the feature is a feature of size and is being defined as the datum A

Example Datums n Datums must be perpendicular to each other – Primary – Secondary – Tertiary Datum

Precedence n. C n. B n. A ^ ^

Precedence n. Secondary n. Tertiary n. Primary ^ ^

Precedence n. Tertiary: 1 Contact “Point” n. Secondary: 2 Contact “Points” n. Primary: 3 Contact “Points” ^ ^

Reading Feature Control Frames n The zone. of the feature must be within a tolerance n The tolerance zone at Datum. n The of the feature must be within a tolerance zone relative to Datum. n The of the feature must be within a zone at relative to Datum. n The of the feature must be within a tolerance zone relative to datums. of the feature must be within a relative to

Placement of Feature Control Frames n May be attached at side, end, or corner of the symbol box to an extension line. n Applied to surface. n Applied to axis

Placement of Feature Control Frames Cont’d. n May be below or closely adjacent to the dimension or note pertaining to that feature. Ø. 500±. 005

Basic Dimension n A theoretically exact size, profile, orientation, or location of a feature or datum target, therefore, a basic dimension is untoleranced. Most often used with position, angularity, and profile) Basic dimensions have a rectangle surrounding it.

Basic Dimension cont’d.

Orientation Tolerances – Perpendicularity – Angularity – Parallelism Controls the orientation of individual features n n Datums are required Shape of tolerance zone: 2 parallel lines, 2 parallel planes, and cylindrical n

PERPENDICULARITY: n is the condition of a surface, center plane, or axis at a right angle (90°) to a datum plane or axis. Ex: The perpendicularity of this surface must be within a. 005 tolerance zone relative to datum A. The tolerance zone is the space between the 2 parallel lines. They are perpendicular to the datum plane and spaced. 005 apart.

PERPENDICULARITY n Cont’d. Location of hole (axis) This means ‘the hole (axis) must be perpendicular within a diametrical tolerance zone of. 010 relative to datum A’

ANGULARITY: n is the condition of a surface, axis, or median plane which is at a specific angle (other than 90°) from a datum plane or axis. The surface is at a 45º angle with a. 005 tolerance zone relative to datum A. n n Can be applied to an axis at MMC. Typically must have a basic dimension.

PARALLELISM: n n The condition of a surface or center plane equidistant at all points from a datum plane, or an axis. The distance between the parallel lines, or surfaces, is specified by the geometric tolerance.

Material Conditions Maximum Material Condition (MMC) n Least Material Condition (LMC) n Regardless of Feature Size(RFS) n

Maximum Material Condition n n MMC This is when part will weigh the most. – MMC for a shaft is the largest allowable size. » MMC of Ø 0. 240±. 005? – MMC for a hole is the smallest allowable size. » MMC of Ø 0. 250±. 005? n n n Permits greater possible tolerance as the part feature sizes vary from their calculated MMC Ensures interchangeability Used – With interrelated features with respect to location – Size, such as, hole, slot, pin, etc.

Least Material Condition LMC n This is when part will weigh the least. n – LMC for a shaft is the smallest allowable diameter. » LMC of Ø 0. 240±. 005? – LMC for a hole is the largest allowable diameter. » LMC of Ø 0. 250±. 005?

Regardless of Feature Size RFS n Requires that the condition of the material NOT be considered. n Tolerance is applied no matter what the size of the feature is. n Valid only when applied to features of size, such as holes, slots, pins, etc. , with an axis or center plane. n

Limits of Size Rule 1. 2. 3. A feature of size must be within the stated upper and lower size limits. At any cross-section the part must not exceed the MMC or LMC. The feature may not exceed a perfect form boundary at MMC. If the part were produced at MMC, it cannot have any deviation in form. The part may vary between the LMC and the MMC.

Location Tolerances – Position – Concentricity – Symmetry

Position Tolerance n n n A position tolerance is the total permissible variation in the location of a feature about its exact true position. For cylindrical features, the position tolerance zone is typically a cylinder within which the axis of the feature must lie. For other features, the center plane of the feature must fit in the space between two parallel planes. The exact position of the feature is located with basic dimensions. The position tolerance is typically associated with the size tolerance of the feature. Datums are required.

Coordinate System Position n Consider the following hole dimensioned with coordinate dimensions: n The tolerance zone for the location (axis) of the hole is as follows: 2. 000 n . 750 Center can be anywhere along the diagonal line. Several Problems: – Two points, equidistant from true position may not be accepted. – Total tolerance diagonally is. 014, which may be more than was intended.

Position Tolerancing n Consider the same hole, but add GD&T: n Now, overall tolerance zone is: MMC = n The actual center of the hole (axis) must lie in the round tolerance zone. The same tolerance is applied, regardless of the direction.

Bonus Tolerancing n Geometric tolerancing has on one major advantage built in; the bonus tolerance application. Bonus tolerances can be obtained on geometric controls where the maximum material condition (MMC) or least material condition (LMC) is used. Bonus tolerances are calculated as follows: Original Tolerance at MMC – Actual Size = Bonus Tolerance Total Possible Tolerance = Original Tolerance at MMC + Bonus Tolerance n

Bonus Tolerance n Here is the beauty of the system! The specified tolerance was: This means that the tolerance is. 010 if the hole size is the MMC size, or. 497. If the hole is bigger, we get a bonus tolerance equal to the difference between the MMC size and the actual size.

Bonus Tolerance Example This means that the tolerance is. 010 if the hole size is the MMC size, or. 497. If the hole is bigger, we get a bonus tolerance equal to the difference between the MMC size and the actual size. Actual Hole Size Bonus Tol. Φ of Tol. Zone Ø. 497 (MMC) 0 . 010 Ø. 499 . 002 (. 010 +. 002 =. 012) . 012 Ø. 500 (. 500 -. 497 =. 003) . 003 (. 010 +. 003 =. 013) . 013 Ø. 502 . 005 . 015 Ø. 503 (LMC) . 006 . 016 Ø. 504 ? ? (. 499 -. 497 =. 002) This system makes sense… the larger the hole is, the more it can deviate from true position and still fit in the mating condition!

. 503 -. 497 = BONUS. 006 BONUS + TOL. ZONE =. 016

Restrictions on Bonus Tolerancing n You can never go beyond print size dimensions to get more bonus tolerance. n Bonus tolerances can only be used in cases where if all possible bonus tolerance were used, it would not affect the function of the part assembly. n Bonus tolerancing applies only on geometric controls where MMC or LMC applies.

Virtual Condition (Actual) n The virtual condition (VC) is the collective effects of the size and any geometric tolerances of form, orientation, or position. The following are calculations for the virtual conditions of a shaft and a hole: – Shaft: VC = maximum diameter + geometric tolerance – Hole: VC = minimum diameter + geometric tolerance n. Geometric Tolerancing Dimensioning and

Application of Virtual Condition n A coat hanger wire has a tolerance of. 050 -. 060” diameter. If the wire was made at. 060”diameter and was bent in its axis by. 010”, the virtual size of the wire would be: . 060” +. 010” =. 070”

TIR, FIR, and FIM n Many dimensions are shown on drawings as TIR, FIR, or FIM. These abbreviations stand for: – TIR – total indicator reading – FIR – full indicator reading – FIM – full indicator movement n The preferred term is FIM, but many drawings still use TIR or FIR. The important thing is that they all mean the same thing. They mean the total distance that the indicator point travels from the start of the measurement to the finish.