Dimensioning and Tolerancing Chapter 17 Dimensioning and Tolerancing Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Homework Figure 17.88 Mounting Saddle

What three elements does this drawing have? What are drawings? Communication Tools Graphics Dimensions/ (Symbols) Words (Notes) Dimensioning- process of adding size, location, and manufacturing information to a drawing (Find only in 1 place) Current standard is ASME Y14.5M-2009

Reading Direction of Dimensions Text is to read Unidirectional What industry uses aligned?

Text and Arrow Standards Inch- No 0 before decimal point Metric- 0 precedes decimal point Dimensions are always expressed to same number of decimal places as tolerance What if dimension is only ½” but the tolerance is to the thousandths?

Rules to Dimension by: Dimension overall size first (H, W, D) Dimension basic sizes and locations of features Add notes describing details of construction for manufacture

Units of Measure: America- all dimensions in inches unless otherwise noted If parts are over 72” switch to feet and inches Outside US- metric system or international system of measure (SI) What is required to be on drawing if metric units are being used? METRIC

Dimension Terminology Who Can define each of these dimension elements? Basic Dimensions- Numeric value defining theoretically exact size, location, profile, or orientation of a feature . Reference Dimension- Numerical value in parenthesis , for information purposes only, not to be used to fabricate part

What is the most precise way to dimension a part? Datum Dimensioning- Theoretical exact surface used as a reference for dimensionS Tape Measure Rule

Dimension in their most descriptive view Contour Dimensioning Dimension in their most descriptive view Never to a hidden line

Angular Dimensions- shown either in degree, decimal degree, or degree, minute, seconds How many minutes are in a degree? What would 45.5˚ be in degrees, minutes, seconds? 2 ways to dimension a chamfer

Dimension this part with a partner using methods we just discussed!

Size Location Machining Info How many views are needed? What information is necessary to fabricate this block with a hole drilled through it? How many views are needed? Size Height Width Depth Location X direction Y direction Machining Info Hole thru unless noted

___________ is the overall objective! Standard Practices Placement Spacing Not inside part ___________ is the overall objective!

Dimensioning Tricks Avoid over dimensioning Dimension in most descriptive view Avoid Chain line dimensioning Staggering if dimensions are too close

Centerline and Extension line Practices Why are center marks extended past largest concentric radius or circle?

Needing another process Use of Chain Lines Needing another process

Use multiples of occurrence in that view only Repetitive Elements Use multiples of occurrence in that view only What is the centerline going through the center of all of the instances? Leaders Pass through center

Dimension this part with a partner using methods we just discussed!

Symbols for drilling operations Symbol/ size/ Secondary operation symbol/ Symbol/ size Depth Symbol/ size

Dimensioning Blind Holes and Slots ________hole is assumed if no depth is given. What information is lacking to mill the slot?

Summary of current vs. previous standards If we have had the new standard since 2009 why should we know these?

Dimensioning Threads & Grooves Which groove method is easier to verify? M8X2 M8X2

Homework Figure 17.88 Mounting Saddle

With a partner dimension a parts with 5 holes With a partner dimension a parts with 5 holes. Use the two methods to dimension them 1” from each end and .50 apart with a tolerance of +/- .05. Calculate how far off the last hole could potentially be from the left edge of the part. Base line Chain line

Why do we have Tolerencing? Controlling the variation that exist on all manufactured parts (Total amount a dimension may vary) System for two or more mating parts Pen lids/ Markers

Functional Dimensioning Dimension and Tolerance functional features first

Tolerances expressed in several ways: Direct limits Tolerance Values Unilateral Bi-lateral Symmetric Geometric Tolerances (GD&T) General tolerance note or title block note

When does a part have MMC vs. LMC? Tolerancing When does a part have MMC vs. LMC? Maximum Material Condition (MMC)- when it contains the most material (Weighs the most) Least Material Condition (LMC)- when it contains the least material (Weighs the least)

Important Tolerance Terms Nominal size – General size ½” Basic size- Theoretical size .500” Actual size- measured size .498” Limits- Max and Min shown in tolerance Allowance- Min clearance or Max interference (Tightest fit) Tolerance- allowable variance difference between upper and lower limits Piece Tolerance- Difference between upper and lower limits of single part System Tolerance- Sum of all piece tolerances

Dimension these two parts in groups of 2 How many views are needed for each part? What is most important feature? Assembled View

Tolerance Stack up: BAD GOOD Alternate method Amount of Variation due to tolerances dimensioned from different sides or between multiple features BAD GOOD Alternate method

Homework

Fit Types Clearance- a space when assembled Interference- Interfere when assembled Transition- Sometimes clearance and sometimes interference What is the fit if the shaft is machined to its smallest diameter? What is the fit if the shaft is machined to its largest diameter?

Standard Precision Fits for English Units Running and Sliding Fits (RC)- Loosest Class where shaft must move freely Clearance Locational Fit (LC)- Shaft and Hole may be same size (Line to Line fit) Transition Locational Fit (LT)- Transition between LC & LN fits Interference Locational Fits (LN)- Shaft larger than hole (Alignment dowel pins) Force and Shrink Fits (FN)- Shaft is always bigger (Transmit torque)

Applying tolerances for Clearance basic hole system Givens

Applying tolerances for Interference basic hole system Givens

Higher the number=looser the fit Metric Tolerancing Use the tables Higher the number=looser the fit

Surface Texture Symbols

Practice Exercise

Homework: Tolerance Handout Use the tolerancing tables in appendixes 7 through 11 to calculate the limit dimensions between the shaft and hole for the given classes of fit

Homework