[email protected] ENGR-22_Lec-15_Dimensioning-1.ppt 1 Bruce Mayer, PE Engineering 22 – Engineering Design Graphics Bruce Mayer, PE Registered Electrical

[email protected] • ENGR-22_Lec-15_Dimensioning-1.ppt1

Bruce Mayer, PE Engineering 22 – Engineering Design Graphics

Bruce Mayer, PERegistered Electrical & Mechanical Engineer

[email protected]

Engineering 22

StandardStandardToleranciToleranci

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Skill-Development GoalSkill-Development Goal

To learn how to effectively tolerance parts such that • The Parts Function Correctly

• Fabrication Cost Is Kept To A Minimum

Read & Create Limit Dimensions• Range & Deviation Forms

– Symmetrical

– BiLateral

– UniLateral



Tolerance IntroductionTolerance Introduction

The total amount a specific dimension can vary. (ANSI/ASME Y14.5M-1994)

Tolerances are assigned so that any two mating parts will fit together.

Highly accurate parts are extremely expensive, so tolerances should be as generous as possible while still maintaining proper function for the part.



TolerancingTolerancing Definition: Allowance for specific variation in

the size and geometry of a part Need for Tolerancing

• It is IMPOSSIBLE to manufacture a part to an EXACT size or geometry

• Since variation from the drawing is inevitable we must specify the acceptable degree of variation

• Large variation may affect part functionality• Small allowed variation affects the part cost

– requires precise manufacturing – requires inspection and potential

False-Rejection of otherwise Functional parts



Tolerance Follows FunctionTolerance Follows Function Assemblies:

• Parts will not fit together if their dimensions do not fall with in a certain range of values

Interchangeable Parts: • If a replacement part is used it must duplicate the

original part within certain limits of deviation

The relationship between functionality and size/shape of an object varies with the part• Automobile Transmission is Very Sensitive to the

Size & Shape of the Gears • A Bicycle is NOT Too Sensitive to the Size &

Shape of the Gears (sprockets)



Two Forms of Physical ToleranceTwo Forms of Physical Tolerance Size

• Limits specifying the allowed variation in each dimension (length, width, height, diameter, etc.) are given on the drawing

Geometry• Geometric Dimensioning & Tolerancing (GD&T)

– Allows for specification of tolerance for the geometry of a part separate from its size

– GD&T uses special symbols to control different geometric features of a part

Will Discuss GD&T Next Time



Cost SensitivityCost Sensitivity Cost generally increases with “tighter”

tolerances• There is generally a ceiling to this relationship

where larger tolerances do not affect cost – e.g.; If the Fabricator ROUTINELY Holds to ±0.5 mm,

Then a ±3 mm Specification will NOT reduce Cost

• Tolerances at the Limits of the Fabricator’s Capability cause an exponential increase in cost

• Parts with small tolerances often require special methods of manufacturing

• Parts with small tolerances often require greater inspection, and higher part-rejection rates

Do NOT specify a SMALLER Tolerance than is NEEDED



Tolerance Spec HierarchyTolerance Spec Hierarchy

Generally Three Levels of Tolerances• DEFAULT: Placed in the Drawing Title-

Block by The Engineering Firm– Typically Conforms to Routine Tolerance Levels

• GENERAL: Placed on the Drawing By the Design-Engineer as a NOTE– Applies to the Entire Drawing

– Supercedes the DEFAULT Tolerance

• SPECIFIC: Associated with a SINGLE Dimension or Geometric Feature



Default Tolerance ExampleDefault Tolerance Example



Range Limit Dimensions

Range Limit dimensions specify the upper and lower value.

An acceptable part may be at the upper limit, lower limit, or any value in between

Advantage: Fabricators Measure Total Distance, and Limit Dims show these



Allowance & Fit

The minimum clearance space or maximum interference is the ALLOWANCE.

Fit is the range of tightness or looseness between parts for their function.



Types of Fit

CLEARANCE Fit: internal member always has space or clearance.

INTERFERENCE Fit: internal member is always larger and has to be forced inside external member.

TRANSITION Fit: either clearance or interference. LINE Fit: clearance or surface contact results.



Basic Hole System

Hole will be machined with a standard sized tool.

Determine type of fit necessary. Use fit table or otherwise determine allowance.

Apply tolerances using hole size as the base (nominal) dimension.

Generally Easier to “Turn Down” a Shaft, Than to Make a Non-Standard Sized Hole



Basic Shaft System

Less commonly used than basic hole system.

Most commonly used when many parts will fit on a standard shaft.

Shaft size is the base dimension.



Deviation Limit Dimensions

Basic size of Dimension is Given With Tolerances Noted As A Plus/Minus (±) Range.



Symmetrical (Equal) Bi-lateral Tolerance Forms

Most Easily Fabricated • Most

Fabrication Processes Vary Randomly

• “Target” Value Given by BiLat Dims

Advantage: Target Value stated – Fabricator can set Machine to Target



Baseline vs. Continued

Baseline tolerances don’t “stack-up” Continued (chained) tolerances CAN

“stack-up”



Surface FinishesSurface Finishes Surface Finish Refers, Primarily, to the

“Roughness” of a Surface

Use of “Finish Marks” MicroScopic Definition of Finish Elements



Surface Roughness → RSurface Roughness → RAA

In the “Old” days the Surface Roughness was Stated in µ-inches “RMS”• Root of the Mean

Square (see Engr43)

Now we use µm or µin with the “Arithmetic Average” (AA or Ra) as the Roughness Metric

• Make “k” Rouhness Height, HR, Measurements Relative to the Mean Surface Height– HR can be Positive or Negative

• Find Ra as

nHRnk

kRa k

1



Surface Finish SpecificationSurface Finish Specification The Basic Surface

Finish “Check Mark” Can have a Number of• Metrics

• Modifiers

The Modifer List and how to Draw the Check Mark



Tool-Mark LayTool-Mark Lay

The “Lay” of the marks left by the Surface Finishing Tool Refers to the Mark Orientation Relative to the Surface or Some Reference Direction



Man

ufactu

ring

M

anu

facturin

g

Pro

cess Determ

ines R

Pro

cess Determ

ines R

aa



Tolerancing with AutoCADTolerancing with AutoCAD Say we have this

part Default Tol. (±0.5

mm) is OK except for mating features that need Tol. of ±0.2• Top Groove Width =

nominal 58-22 = 36– Position Relative to CL

• Top Groove Hgt = 27

• 105mm Base Width– Position Relative to CL

• Edge Notch Height = nominal 13

36 +0.4/-0.013

3mm

105 +0.0/-0.4

36 +0.4/-0.0



To

lerance D

emo

Start P

tT

oleran

ce Dem

o S

tart Pt

13

3mm

13

3mm

1313

3mm



Dem

o R

esult

Dem

o R

esult



All Done for TodayAll Done for Today

What’s yourTolerancingTolerance?



Bruce Mayer, PELicensed Electrical & Mechanical Engineer

[email protected]

Engr/Math/Physics 25

AppendiAppendixx

6972 23 xxxxf



Tolerance Demo - 1 Tolerance Demo - 1 1. Open File

• Tolerance_Demo_Lec-17_Start_0508.dwg– Contains F, T, B, RS

views

2. Make and Label w/ CL extended Ctr-Lines in F & RS views

3. Modify STD Dim Style for overall scale of 19

4. Adjust LTSCALE to 12.7

5. Make 3 more Dim Styles• Tol_BiLat with

Tolerance of 0.2• Tol_LowLim with

+0.4/-0.0• Tol_HiLim with

+0.0/-0.4



Tolerance Demo - 2Tolerance Demo - 2

6. Apply Bilateral (Symmetrical) Tolerances to 5 dims

7. Apply Deviation Tolerance• Slot Width = 36.0

+0.4.-0.0

• Base Length = 105.0 +0.0.-0.4

8. Apply std dims to balance of part• Note that

– Slot Ctrs located relative to MACHINED Surface

– Slots shown on CL ( -.5mm Tol)

– NOTE for Rounded edges

– For R29 may need to “Place Text Manually”



Demo - 1Demo - 1

Extend CL’s 10mm beyond part envelope

To make CL symbol• ROMANS style

• KeyBd Input = \U+2104



Demo - 2Demo - 2

Apply Symmetrical Tolerance with Style Tol_BiLat



Demo - 3Demo - 3 Apply ASymmetrical Tolerance with Styles• Tol_HiLIm

• Tol_LowLim



Demo - 4Demo - 4

Apply Nominal Tolerances with STD

Documents

[email protected] ENGR-22_Lec-15_Dimensioning-1.ppt 1 Bruce Mayer, PE Engineering 22 – Engineering Design Graphics Bruce Mayer, PE Registered Electrical