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CHAPTER 1ENGINEERING DRAWING

AND TOLERANCING

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GOAL

• Understand what an engineering drawing is.• Understand why geometric tolerancing is

superior to Coordinate tolerancing.

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ENGINEERING DRAWING• Engineering drawing is a document that communicates

a precise description of a part. This description consists of pictures, words, numbers and symbols.

• Engineering drawing information includes • Geometry• Critical function relationship.• Tolerance allowed for proper function.• Material, heat treatment, surface coatings.• Part documentation information (part number, revision

number)

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COMMUNICATION MODEL

Engineering drawings are legal documents.

They should be treated as formal, precise documents.

Each specification should be measurable.

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CONSEQUENCE OF DRAWING ERRORS

• Engineering drawing should communicate precisely, also need to be correct.

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• Drawing error can be very costly to an organization

• If a product liability lawsuit is involved, the costs that result from the drawing error can run into millions of dollars.

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DIMENSION AND TOLERANCES

• Dimension is a numerical value expressed in appropriate units of measure and used to define size, location and orientation, form and other geometrical characteristics.

• Tolerance is a total amount the feature of part are permitted to vary from specified dimension. The tolerance is a difference between maximum and minimum limits.

• Types of tolerance

Limit tolerance.

Plus-minus tolerance.

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• Plus –Minus Tolerance Equal bilateral tolerance. Unequal bilateral tolerance. Unilateral tolerance.

DIMENSION AND TOLERANCEScond..

+0.5

0

+0.3

- 0.2

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METRIC DIMENSION SPECIFICATION

• Common unit of measurement millimeter.• “UNLESS OTHERWISE SPECIFIED, ALL

DIMENSIONS ARE IN mm” should be added.• When a metric dimension is a whole number, the

decimal point and zero are omitted.

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• When a metric dimension is less than one millimeter, a zero precedes the decimal point. For example dimension “0.3” has a zero to the left of the decimal point.

• When a metric dimension is not a whole number, a decimal point with a portion of millimeter(10ths and 100ths) are specified.

METRIC DIMENSION SPECIFICATION (Contd..)

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When limit tolerances are expressed in a single line, the low limit is stated first, then a dash followed by the high limit.

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INTERPRETING DIMENSIONAL LIMITS

• All dimension are absolute.• Dimension are considered to be followed with

zeros after last specified digit.

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HISTORY OF GD&T

• Drawings existed as far back as 6000 B.C.• Unit of measurement at that time is royal cubit.• During 4000 B.C standardized to 18.24 inches.• Manufacturing started in 1800s .• The total process was conducted under one roof,

communication among craftsmen was immediate and constant.

• Nothing less than perfection was good enough.• There were variation, but back then measuring

instruments were not precise enough to identify them.

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• Now Engineers understand the variation is unavoidable.

• The variation acceptable without impairing the function of assembly is identified as tolerance.

• This led to the development of Coordinate dimensioning.

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ENGINEERING DRAWING STANDARDS

1935- American Standards Association (ASA) published “American

Drawing and Drafting Room Practices”.

World war II- Stanley Parker of Royal Torpedo Factory in Alexandria, Scotland

created positional tolerancing with cylindrical tolerance zone rather than square tolerance zone.

1940- Draftsmen’s handbook by chevrolet, U.S.

1944 &1948- British published “Dimensional Analysis of Engineering Design”.

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1946- ASA published II edition of “American Standard Drawing and Drafting Room Practice”.

1946- SAE published “Aeronautical Drafting Manual”

1949- U.S Military published MIL-STD-8

1952- “SAE Automotive Drafting Manual’

1953- MIL_STDA revised.

1982- American National Standards Institute (ANSI) published ANSI Y14.5.

1994- ANSI Y14.5 Revised.

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Fundamental Dimensional Rules

• ASME Y14.5M-1994 has defined 10 rules.

Regarding dimensioning convention

1. Each dimension shall have tolerance, except those specifically identified as reference, maximum, minimum, or stock size.

2. Dimensioning and Tolerancing shall be complete so there is full definition of part feature.

3. Dimension shall be selected and arranged to suit the function and mating relationship of a part and not to be subject to more than one interpretation.

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Regarding manufacturing method

specification

4. The drawing should specify a part without giving manufacturing methods.

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Regarding implied 90º angles 5. A 90° angle applies where centerlines and lines

depicting features are shown at right angles, no dimension is shown.

6. A 90° basic angle applies where center lines of feature in a pattern-or surfaces shown at right angles are located and defined by basic dimension and no angle is specified.

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Default condition for dimensions and tolerance

zones.

7. Unless otherwise specified ,all dimensions are specified at 20°C (68°F).

8. All dimension and tolerances apply in free-state condition.

This rule does not applies to non-rigid parts.

9. Unless otherwise specified, all geometric tolerances apply to full depth, length and width of the feature.

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Convention for which drawing level dimensions

and tolerance

10.Dimensions and tolerances apply only at the drawing level where they are specified. A dimension specified on the detail drawing is not mandatory for that feature on the assembly drawing.

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COORDINATE TOLERACING SYSTEM

• Part feature is located (or defined) by means of rectangular dimensions with given tolerances.

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THREE MAJOR SHORTCOMING OF COORDINATE DIMENSIONING

1. Square or rectangular tolerance zones.

2. Fixed-size tolerance zones.

3. Ambiguous instruction for inspection.

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1.Square (or illogical )Tolerance Zones.

• Diagonally more tolerance (0.707) than vertical and horizontal direction (0.5)

• More logical and functional approach is to allow same tolerance on all sides, creating cylindrical tolerance zone.

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2.Fixed-Size Tolerance zones

Function of a hole in assembly is , hole location is critical when the hole is at minimum limit (MMC).

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• If hole size is larger than its minimum size limit, its location tolerance can be correspondingly larger without affecting the part function.

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• Co-ordinate tolerancing does not allow for cylindrical tolerance zones and tolerance hole that increase with the hole size, lengthy notes have to be added. LMC

MMC

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3.AMBIGUOUS INSTRUCTION FOR INSPECTORS

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Different ways to hold the part for inspection, confusion for the inspector which surface to touch the gage equipment first, second and third.

Consequence:

Good parts could be rejected or,

Bad parts could be accepted.

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Type of dimension Appropriate

use

Poor use

Size √

Chamfer √

Radius √

Locating part feature

√

Controlling angular relationships

√

Defining the form of part feature

√

COORDINATE DIMENSIONING USAGE

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GEOMENTRIC DIMENSIONING AND TOLERANCING.

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COMPARISION BETWEEN GD&T AND COORDINATE TOLERANCING.

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• Sometimes designers wrongly think it is faster to dimension a part with coordinate tolerancing than by using geometric tolerancing.

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GREAT MYTH OF GD&T

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REVIEW OF FEW FACTS ABOUT GD&T

• Increases tolerances with cylindrical tolerance zones.

• Allows additional (Bonus) tolerances.• Allows the designer to communicate more

clearly.• Eliminates confusion at inspection.

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Questions & Answers

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• Dimension A contains a tolerance (limit/plus minus)

• Dimension B contains a tolerance (limit/plus minus)

• Dimension C contains a tolerance (limit/plus minus)

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• Dimension D contains a tolerance (limit/

plus minus)• Dimension B contains an bilateral tolerance

(equal/unequal)

• Dimension C contains a tolerance (unilateral/bilateral)

• Dimension E contain an bilateral tolerance

(equal/unequal)

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• What is the convention for a whole number metric dimension?

• What is a convention for a metric dimension less than one millimeter?

• What does ASME Y 14.5M-1994 stand for ? ASME _____________ Y 14.5 _____________ M _____________ 1994 _____________• What is coordinate tolerancing?• The three major shortcomings of coordinate

tolerancing are:_____________• The “Great Myth of GD&T” is __________

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Fill in the chart using the drawing

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