22
10/30/2010 1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 17 Dimensioning and Tolerancing Homework Figure 17.88 Mounting Saddle
10/30/2010
1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chapter 17
Dimensioning
and
Tolerancing
Homework
Figure 17.88 Mounting Saddle
10/30/2010
2
What three elements does this drawing have?
What are drawings?
Communication Tools
1. Graphics
2. Dimensions/
(Symbols)
3. 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?
10/30/2010
3
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: 1. Dimension overall size first (H, W, D)
2. Dimension basic sizes and locations of features
3. Add notes describing details of construction for manufacture
10/30/2010
4
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
10/30/2010
5
What is the most precise way to
dimension a part? Datum Dimensioning-
Theoretical exact surface used as a reference for dimensionS
Tape Measure Rule
Contour Dimensioning •Dimension in their most descriptive view
•Never to a hidden line
10/30/2010
6
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!
10/30/2010
7
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
Standard Practices •Placement
•Spacing
•Not inside part
___________ is the overall objective!
10/30/2010
8
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?
10/30/2010
9
Use of Chain Lines •Needing another process
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
10/30/2010
10
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
10/30/2010
11
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?
10/30/2010
12
Dimensioning Threads & Grooves
Which groove method is easier to verify?
M8X2
M8X2
10/30/2010
13
Homework
Figure 17.88 Mounting Saddle
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
10/30/2010
14
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
10/30/2010
15
Tolerances expressed in
several ways: 1. Direct limits
2. Tolerance Values
• Unilateral
• Bi-lateral
• Symmetric
3. Geometric Tolerances (GD&T)
4. General tolerance note or title
block note
Tolerancing
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)
When does a
part have MMC
vs. LMC?
10/30/2010
16
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
10/30/2010
17
Tolerance
Stack up: Amount of Variation due to
tolerances dimensioned from
different sides or between
multiple features
Alternate method
BAD GOOD
Homework
10/30/2010
18
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)
10/30/2010
19
Applying
tolerances for
Clearance basic
hole system
Givens
Applying tolerances for
Interference basic hole system
Givens
10/30/2010
20
Metric Tolerancing
Use the tables Higher the number=looser the fit
Surface Texture
Symbols
10/30/2010
21
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
10/30/2010
22
Homework
