2

Dimensioning• Orthographic and Isometric Views define the

shape and general features of the object

• Dimensioning adds information that specifies – Size of the object– Location of features (e.g. holes)– Characteristics of features (e.g. depth and diameter

of hole)

• Dimensions also communicate the tolerance (or accuracy) required

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Some General Guidelines

• Start by dimensioning basic outside dimensions of the object.

• Add dimension for location and size of removed features

• Add general and specific notes – such as tolerances

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Dimensioning Basic Shapes – Assumptions

• Perpendicularity

• Symmetry

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• ALWAYS give DIAMETER" " for full circles (360 degrees) and RADIUS "R" for arcs (less than 360 degrees)

Dimensioning Shows: A) Size B) Location and Orientation

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Dimensioning – Terminology

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Principles of Good Dimensioning

• The overriding principle of dimensioning is CLARITY

• Principles – not an infallible rule set, need to apply good judgment.

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Principles of Good Dimensioning

• Each feature dimensioned once and only once

• Dimensions should suit the function of the object

Objectives

• Understand description and control of variability through tolerancing

• Understand various classes of fits• Introduce multiple part tolerancing

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ToleranceTolerance is the total amount a dimension may vary.

It is the difference between the maximum and minimum limits.

Ways to Express:1. Direct limits or as tolerance limits applied to a

dimension2. Geometric tolerances3. A general tolerance note in title block4. Notes referring to specific conditions

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ANSI/ASME Standard

ANSI/ASME Standard Y14.5

Each dimension shall have a tolerance, except those dimensions specifically identified as reference, maximum, minimum, or stock. The tolerance may be applied directly to the dimension or indicated by a general note located in the title block of the drawing.

Variation is Unavoidable

• No two manufactured objects are identical in every way. Some degree of variation will exist.

• Engineers apply tolerances to part dimensions to reduce the amount of variation that occurs.

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Three basic tolerances that occur most often on working drawings are: limit dimensions, unilateral, and bilateral tolerances.

Tolerances

Tolerances

Three basic tolerances that occur most often on working drawings are: limit dimensions, unilateral, and bilateral tolerances.

Limit Dimensions

Limit dimensions are two dimensional values stacked on top of each other. The dimensions show the largest and smallest values allowed. Anything in between these values is acceptable.

These are limit dimensions,because the upper and

lower dimensional sizes arestacked on top of each other.

Limit Dimensions

Unilateral Tolerance

A unilateral tolerance exists when a target dimension is given along with a tolerance that allows variation to occur in only one direction.

This tolerance is unilateral, becausethe size may only

deviate in one direction.

Unilateral Tolerance

Bilateral Tolerance

A bilateral tolerance exists if the variation from a target dimension is shown occurring in both the positive and negative directions.

1. Direct Limits and Tolerance Values

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1. Direct Limits and Tolerance Values – Plus and Minus Dimensions

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2. Geometric Tolerance System

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Geometric Dimensioning and Tolerancing (GD&T) is a method of defining parts based on how they function, using standard ANSI symbols.

Feature Control Frame

Concentricity Symbol

3. Tolerance Specifications in Title Block

AU 2008 25

General tolerance note specifies the tolerance for all unspecified toleranced dimensions.

4. Notes Referring to Specific Conditions

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General Tolerances could be in the form of a note similar to the one shown below:

ALL DECIMAL DIMENSIONS TO BE HELD TO .002"

Means that a dimension such as .500 would be assigned a tolerance of 0.002, resulting in a upper limit of .502 and a lower limit of .498

Important Terms – Single Part

• Nominal Size – general size, usually expressed in common fractions (1/2" for the slot)

• Basic Size – theoretical size used as starting point (.500" for the slot)• Actual Size – measured size of the finished part (.501" for the slot)

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Important Terms – Single Part

• Limits – maximum and minimum sizes shown by tolerances (.502 and .498 – larger value is the upper limit and the smaller value is the lower limit, for the slot)

• Tolerance – total allowable variance in dimensions (upper limit – lower limit) – object dimension could be as big as the upper limit or as small as the lower limit or anywhere in between

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Important Terms – Multiple Parts

• Allowance – the minimum clearance or maximum interference between parts

• Fit – degree of tightness between two parts– Clearance Fit – tolerance of mating parts always

leaves a space– Interference Fit – tolerance of mating parts always

results in interference– Transition Fit – sometimes interferes, sometimes

clears29

Fitting Multiple Parts

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Part A

Tolerance of A Part B

Tolerance of B Fit Tolerance: Clearance or Interference

Fitting Multiple Parts

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Shaft and Hole Fits

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Clearance Interference

Shaft and Hole Fits

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Transition

CLEARANCE FIT

+ .003

Standard Precision Fits: English Units

• Running and sliding fits (RC)• Clearance locational fits (LC)• Transition locational fits (LT)• Interference locational fits (LN)• Force and shrink fits (FN)

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Basic Hole System or Hole Basis

• Definition of the "Basic Hole System":

The "minimum size" of the hole is equal to the "basic size" of the fit

• Example: If the nominal size of a fit is 1/2", then the minimum size of the hole in the system will be 0.500"

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Basic Hole System – G28-A

• Clearance = Hole – Shaft• Cmax = H____ – S____• Cmin = H____ – S____

Fill in the subscripts (min, max) in the equations above.

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SMAX

SMIN

HMAX

HMIN

Basic Hole System

• Clearance = Hole – Shaft• Cmax = Hmax – Smin• Cmin = Hmin – Smax

Both Cmax and Cmin <0 – _________ fitBoth Cmax and Cmin >0 – _________ fitCmax > 0; Cmin < 0 – ___________ fit

What types of fits are these?

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SMAX

SMIN

HMAX

HMIN

Basic Hole System

• Clearance = Hole – Shaft• Cmax = Hmax – Smin• Cmin = Hmin – Smax

Both Cmax and Cmin <0 – Interference fitBoth Cmax and Cmin >0 – Clearance fitCmax > 0; Cmin < 0 – Transition fit

• System Tolerance = Cmax – Cmin• Allowance = Min. Clearance = Cmin

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SMAX

SMIN

HMAX

HMIN

Basic Hole System

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.490

.485

.510

.505

Calculate Maximum and Minimum Clearance

Clearance = Hole – Shaft

Cmax = Hmax – Smin

Cmax = .510 – .485 = .025

Cmin = .505 – .490 = .015

What type of fit is this? Cmax > Cmin > 0 Clearance

Cmin = Hmin – Smax

Metric Limits and Fits

• Based on Standard Basic Sizes – ISO Standard

• Note that in the Metric system:Nominal Size = Basic Size

• Example: If the nominal size is 8, then the basic size is 8

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Metric Preferred Hole Basis System of Fits

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Metric Tolerance Homework – TOL-1

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????

????

9.960

9.924

Free Running H9/d9Basic Size: 10

(1) Nominal Size: ?(1) Nominal Size: 10

(2) Shaft Limits:

(7) MinimumClearance: ????

(8) MaximumClearance: ????

(3) Shaft Tolerance: ????(3) Shaft Tolerance: 0.036

(4) Hole Limits: ????

????

10.036

10.000(5) Hole Tolerance: ????(5) Hole Tolerance: 0.036

(6) Ts: ????(6) Ts: 0.072

(7) MinimumClearance: 0.040

(8) MaximumClearance: 0.112

CHECK: Ts = Cmax – Cmin?CHECK: 0.072 = 0.112 – 0.040 = 0.072

Today's Assignment

• Tolerance Yellow Packet– All problems. – Due 4/16

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Objectives

• Introduce Surface Control terms and symbols

• Introduce Geometric Dimensioning and Tolerancing

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Surface Control

• Why do we need to control surface characteristics?

– Rough surfaces cause friction and wear– It is difficult to make accurate measurements from

rough surfaces

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Surface Characteristics

• Roughness

– Small hills and valleys found on a surface

– Defined as the arithmetic average of the deviations above and below a mean height of a surface

– Expressed in microinches or micrometers.

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Surface Characteristics

• Waviness

– Surface irregularities greater than roughness– Expressed in inches or millimeters– See Figure 7.21 on page 7-14 of TG

• Lay

– Direction of tool marks on a machined surface.– See Figure 7.20 on page 7-13 of TG.

49Autumn 2009

Surface Texture Symbols

.25 inches.125 inSurface Control Symbol

Material must be removed

Material must not be removed

Material removal not specified

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Lay Symbols

M

Parallel to line representing surface

Perpendicular to the line representing the surface

Both directions to the line representing the surface

Multidirectional marks

Circular

Radial

Lay particulate, non-directional, protuberant

C

R

P

Bar added

Symbol location

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Examples with Roughness and Lay

24 Average roughness is 0.000024 inches, which is often referred to as 24 microinches or µ inches.Material must be removed.

2814 Maximum average roughness is 28 microinches.

Minimum average roughness is 14 microinches.Material must not be removed.

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R

Average roughness is 45 microinchesLines on the surface are radial with respectto the center of the surface

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Geometric Tolerancing

• Geometric Tolerancing includes specifications of form, profile, orientation, location, and runout.

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Geometric Tolerancing

What features of PART 1 need some constraints so that the assembly will work properly? Discuss with the people at your table.What happens if the two shafts are not parallel?

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Geometric Tolerancing – Definitions

• Basic Dimension – A numerical value for theoretical exact size or location

• True Position – The theoretically exact location of a feature established by basic dimensions

• Datum – A theoretically exact point, axis, or plane used as the origin from which location or geometric characteristics of features are located

• Datum Target – A specified point, line, or area on a part used to establish a datum

• Datum Feature – An actual feature of a part used to establish a datum

55Autumn 2009

Geometric Tolerancing – Definitions

• Maximum Material Condition (MMC) – The condition in which a feature of size contains the maximum amount of material with the stated limits of size. For example, minimum hole diameter and maximum shaft diameter

• Least Material Condition (LMC) – Opposite of MMC, the feature contains the least material. For example, maximum hole diameter and minimum shaft diameter

• Virtual Condition – The envelope or boundary that describes the collective effects of all tolerance requirements on a feature (See Figure 7-25 TG)

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Virtual Condition Envelope All Required Tolerances

20.06" MaximumEnvelope

20.00"MaximumAllowable Diameter

0.06"MaximumAllowable Curvature

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Basic Symbols for Geometric Characteristics

Individual Features Tolerance of Form Straightness Flatness Circularity (roundness) Cylindricity

Individual or Related Tolerance of Profile Profile of a line Profile of a surface

Related Features Tolerance of Orientation Angularity Perpendicularity Parallelism

Tolerance of Location Position Concentricity

Tolerance of Runout Circular Runout Total Runout

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Modifying Symbols for Geometric Characteristics

• At maximum material condition• At least material condition• Projected tolerance zone• Diameter• Spherical diameter• Radius• Spherical Radius• Reference• Arc length

M L

P

SR

SR( )

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Feature Control Frame

This feature must be parallel to Datum B within .007 at MMC (largest cylinder) as measured on the axis

.007 M B

Geometric Characteristic Symbol

Tolerance

Material condition

Datum Reference

B.007

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Tolerance of Form – Straightness

19.8919.86

0.03

0.03 Tolerance Zone

This cylinder must be straight within 0.03 mm.

What it means -

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Concentricity Tolerance Note

XX

A

YY

.007 A

This cylinder (the right cylinder) must be concentric within .007 with the Datum A (the left cylinder) as measured on the axis

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Concentricity Tolerance Note – What It Means

.007 Tolerance Zone