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• Please sit with your team

• Hardcopy of SW parts 4 and 5 due W 9/26

• Read Chapters 2 and 14 - Quiz 4 on F 9/28

• Learning Factory machining training must be completed by ????

• Mid-term exam R 10/11 6:30-7:45 112 Kern

Lecture 08Geometric Dimensioning and Tolerances

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Geometric Dimensioning

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Tolerance Stacking

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Common Dimensioning Errors

• Drawing should not be cluttered• All dimensions including hole DIA, position

and depth must be shown• Use leader lines, no dimensions on the body of

the part• No redundant dimensions• Never dimension from the edge of a hole,

always from its center• No overlapping dimension lines or arrows• Dimension from one edge only

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Common Dimensioning Errors

• Title block must contain– Default tolerances– Name of drafter– Name of checker (if applicable)– Team ID and names of members– Date– Part Designation– Material

• Appropriate tolerances on dimensions balancing the need for accuracy with machining realities

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Common Dimensioning Errors

• Use ANSI dimension standard (Tools- Options - Document Properties)

• Do not place dimensions or notes on the isometric view

• A feature such as a hole should only have only one note attached to it

• Never use notes on more than one view.• Show hidden lines on the 3 planar views

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Recommendations and Tips

• Use heavy "Thick(2)" line weight for part edges to provide visible contrast with dimension lines  (to change, right click on drawing view, select Component Line Font, Visible Edges, Thick(2) line weight)

• Before adding a hole with Hole Wizard, sketch a point at the desired center position and smart dimension it immediately, then use hole wizard.

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Why Do We Need Tolerances?

• To produce interchangeable parts• Assure that mating parts can be assembled and

disassembled with ease• Assure that interference fits can sustain

operating loads without slip or separation

• Codified in ANSI Standard B4.1-1967, R1979• The choice of tolerance is a compromise:

– Loose tolerances = low manufacturing cost, easy assembly

– Tight tolerances = interchangeability, functionality, precision

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Standard Tolerances

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Impact of Tolerances on Machining Cost

Reference: J. Collins, Mechanical Design of Machine Elements and Machines

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Types of Fits

• Clearance– clearance is guaranteed under all conditions)

• Transition– could be clearance or interference

• Interference– guaranteed under all conditions

Designation of standard fits

RC Running or sliding clearance

LC Locational clearance

LT Locational transition clearance or interference

LN Locational interference

FN Force or shrink fit

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Selecting Clearance Fits

Class of Fit Intended Application

RC1 Close sliding fit; for accurate location with no perceptible play

RC2 Sliding fit; for accurate location, parts move and turn easily but are not intended to run freely. In larger sizes, parts may seize as a result of small temperature changes

RC3 Precision running fit; for precision work at slow speeds and light loads, the closest fit that can be expected to run freely

RC4 Close running fit; accurate machinery at moderate speeds and loads, provides accurate location and minimum play

RC5-6 Medium running fit; for higher speeds and/or higher loads

RC7 Free running fit; accuracy is not essential or large temperature changes are expected

RC8 Loose running fit; intended where larger commercial (as-received) tolerances may be advantageous or necessary

RC9 Loose running fit;

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Clearance Fits

Based on a 1” hole and shaft, ANSI Std B4.1-1967, R1979Ref: Machinery’s Handbook

For a reliable close running, rotating (RC4) fit, the dimensions of both parts must be specified so that there is a minimum (worst case) clearance of 0.0008”. The maximum clearance will be 0.0028”

For a close locational fit (LC4), the dimensions must insure that the minimum worst case clearance is 0.000”, the maximum clearance will be 0.0055”

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Example

• It is desired to have a nominal 1/2” shaft run freely, at slow speed, but with minimal play. How should the shaft and hole diameters be specified?

½” Shaft ? ?

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Running and Sliding Fits

0.50”

Precision Running Fit

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Force and Shrink Fits

Class of Fit Intended Application

FN 1 Light drive fit; light pressure required to assemble mating parts, relatively permanent assemblies

FN 2 Medium drive fit; suitable for ordinary steel parts, or shrink fits for light sections

FN 3 Heavy drive fit; suitable for heavier steel parts, or shrink fits for medium sections

FN 4 Force fit; suitable for parts that can be highly stressed, or shrink fits where the heavy pressing forces required for assembly would be impractical

FN 5 For even higher interference pressures

Force fits, or “press” fits result in permanent assemblies. They are typically assembled using some persuasion – (hammer, press) or by shrink fit (outer part is heated, inner is cooled, or both). A heavy press fit can carry substantial torque and is sometimes used in lieu of a keyway.

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Preferred Basic Sizes (English)

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Machine Screws

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Project 2 Part 1- Reverse Engineering

• Objective– Reverse engineer and document the part provided

such that an experienced machinist can reproduce the part without having the original

• Skills practiced– Use of reverse engineering as a design tool– Metrology (calipers, micrometers, hole gauge,

thread gauge) – CAD and dimensioning– Fits and tolerances

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Project 2 Part 1 - Reverse Engineering(continued)

• Procedure1. Sketch the part in your journal TODAY2. Measure all dimensions with appropriate accuracy and dimension

your sketch3. Consider appropriate tolerances to facilitate cost-effective

machining and proper fit between mating parts. Specify the intended ANSI fit and tolerance values at all interfaces with mating parts.

4. Produce a professional quality CAD drawing using SolidWorks

• Deliverable1. Hard copy of your individual SolidWorks drawing –A-size, three

standard views plus isometric view2. Due F 9/28

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For Next Class

• SW parts 4 and 5 due W 9/26

• Read Chapters 2 and 14 - Quiz 4 on F 9/28

• Read handout on “Razor Wars” for W 9/26

• Please bring your journals to class on W 9/26

• Individual SW drawing due on F 9/28