[email protected] • ENGR-22_Lec-15_Dimensioning-1.ppt1
Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
Bruce Mayer, PERegistered Electrical & Mechanical Engineer
Engineering 22
StandardStandardToleranciToleranci
ngng
[email protected] • ENGR-22_Lec-15_Dimensioning-1.ppt2
Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
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
[email protected] • ENGR-22_Lec-15_Dimensioning-1.ppt3
Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
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.
[email protected] • ENGR-22_Lec-15_Dimensioning-1.ppt4
Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
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
[email protected] • ENGR-22_Lec-15_Dimensioning-1.ppt5
Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
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)
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Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
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
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Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
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
[email protected] • ENGR-22_Lec-15_Dimensioning-1.ppt8
Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
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
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Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
Default Tolerance ExampleDefault Tolerance Example
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Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
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
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Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
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.
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Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
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.
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Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
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
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Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
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.
[email protected] • ENGR-22_Lec-15_Dimensioning-1.ppt15
Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
Deviation Limit Dimensions
Basic size of Dimension is Given With Tolerances Noted As A Plus/Minus (±) Range.
[email protected] • ENGR-22_Lec-15_Dimensioning-1.ppt16
Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
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
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Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
Baseline vs. Continued
Baseline tolerances don’t “stack-up” Continued (chained) tolerances CAN
“stack-up”
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Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
Surface FinishesSurface Finishes Surface Finish Refers, Primarily, to the
“Roughness” of a Surface
Use of “Finish Marks” MicroScopic Definition of Finish Elements
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Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
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
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Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
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
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Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
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
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Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
Man
ufactu
ring
M
anu
facturin
g
Pro
cess Determ
ines R
Pro
cess Determ
ines R
aa
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Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
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
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Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
To
lerance D
emo
Start P
tT
oleran
ce Dem
o S
tart Pt
13
3mm
13
3mm
1313
3mm
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Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
Dem
o R
esult
Dem
o R
esult
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Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
All Done for TodayAll Done for Today
What’s yourTolerancingTolerance?
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Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
Bruce Mayer, PELicensed Electrical & Mechanical Engineer
Engr/Math/Physics 25
AppendiAppendixx
6972 23 xxxxf
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Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
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
[email protected] • ENGR-22_Lec-15_Dimensioning-1.ppt29
Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
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”
[email protected] • ENGR-22_Lec-15_Dimensioning-1.ppt30
Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
Demo - 1Demo - 1
Extend CL’s 10mm beyond part envelope
To make CL symbol• ROMANS style
• KeyBd Input = \U+2104
[email protected] • ENGR-22_Lec-15_Dimensioning-1.ppt31
Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
Demo - 2Demo - 2
Apply Symmetrical Tolerance with Style Tol_BiLat
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Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
Demo - 3Demo - 3 Apply ASymmetrical Tolerance with Styles• Tol_HiLIm
• Tol_LowLim
[email protected] • ENGR-22_Lec-15_Dimensioning-1.ppt33
Bruce Mayer, PE Engineering 22 – Engineering Design Graphics
Demo - 4Demo - 4
Apply Nominal Tolerances with STD