Lecture Notes 6 - Metrology, GD T-new PDF

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lecture notes from 2018, me270, lecture notes 1....

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Roughness

Tolerances

Lecture Notes 6: Dimensional Tolerances & Product Quality

Geometric dimensioning and Tolerancing Quality Assurance

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Surface Texture and Roughness Why surfaces are important: - Surfaces affect safety - Aesthetic reasons - Friction and wear depend on surface characteristics - Surfaces affect mechanical and physical properties - Assembly of parts is affected by their surfaces - Smooth surfaces make better electrical contacts

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ME 270, Leon Liebenberg

Surface texture Repetitive and/or random deviations from the nominal surface of an object:

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Surface texture  Roughness - small, finely-spaced deviations from nominal surface Determined by material characteristics and processes that formed the surface

 Waviness - deviations of much larger spacing Waviness deviations occur due to work deflection, vibration, tooling, and similar factors

 Roughness is superimposed on waviness

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ME 270, Leon Liebenberg

Surface Roughness and Surface Finish  Surface roughness - a measurable characteristic based on roughness deviations  Surface finish - a more subjective term denoting smoothness and general quality of a surface  In popular usage, surface finish is often used as a synonym for surface roughness  Both terms are within the scope of surface texture

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Surface Roughness Average of vertical deviations from nominal surface over a specified surface length



1   +  +  +. . . + 1  = =   =  ||        

Rq 

y 2a  y b2  y c2  ...  y n2  n

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ME 270, Leon Liebenberg

1 n 2  yi  n i 1

1 L

L

y 0

2

dx

Surface Roughness Example: Consider a sample contour of a machined surface. Listed values are actual deviations of the surface from the reference plane, measured in μ-inches. Calculate the arithmetic average of the roughness and also the RMS value.

Ra = ……………………………………… Rq = ………………………………………

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Measuring Surface Roughness  Stylus-type instrument  Stylus head traverses horizontally across surface, while stylus moves vertically to follow surface profile

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ME 270, Leon Liebenberg

Typical surface finishes

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Surface texture & cut-off length  A problem with the Ra computation is that waviness may get included  To deal with this problem, a parameter called the cutoff length is used as a “filter” to separate waviness from roughness deviations  Cut-off length is a sampling distance along the surface  A sampling distance shorter than the waviness eliminates waviness deviations and only includes roughness deviations  These values are normally obtained from manufacturer’s tables, e.g.

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ME 270, Leon Liebenberg

Surface texture Standard terminology and symbols used for describing surface texture and surface finish (quantities in μm):

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Surface texture Common surface lay symbols (ISO 1101):

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ME 270, Leon Liebenberg

Surface texture

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Dimensional tolerances

Factors that determine the performance of a manufactured product, other than mechanical and physical properties, include:  Dimensions - linear or angular sizes of a component specified on the part drawing  Tolerances - allowable variations from the specified part dimensions that are permitted in manufacturing

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ME 270, Leon Liebenberg

Dimensions (ANSI Y14.5M-1982)  A dimension is “a numerical value expressed in appropriate units of measure and indicated on a drawing and in other documents along with lines, symbols, and notes to define the size or geometric characteristic, or both, of a part or part feature” • The dimension indicates the part size desired by the designer, if the part could be made with no errors or variations in the fabrication process

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Tolerances (ANSI Y14.5M-1982):  A tolerance is “the total amount by which a specific dimension is permitted to vary. The tolerance is the difference between the maximum and minimum limits”  Variations occur in any manufacturing process, which are manifested as variations in part size  Tolerances are used to define the limits of the allowed variation, usually with reference to a nominal value (or datum level)  ISO 286 + ISO 2768, or ANSI Y14.5M-1982

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ME 270, Leon Liebenberg

Tolerances

(a)

(b)

(c)

 Three ways to specify tolerance: (a) bilateral, (b) unilateral, and (c) limit dimension  Tolerances, include both accuracy and precision, and can be used to specify lengths, angles, concentricity, surface roughness, etc. 17

Tolerances

https://www.gdandtbasics.com/true-position/

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ME 270, Leon Liebenberg

Tolerances

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Tolerances & Surface Roughness for a 25 mm workpiece

The larger the part, the greater the obtainable tolerance range becomes!

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ME 270, Leon Liebenberg

Tolerance “Worst Case” Determination What is the effective dimension and tolerance between the two holes?

3.00 ± 0.01 7.00 ± 0.01

The furthest apart is: The closest is:

7.01 - 2.99 = 4.02 6.99 - 3.01 = 3.98

4.00 ± 0.02 ← TWICE the 4 +/- 0.02

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Proper Way of Dimensioning It is important to specify the dimension and tolerance at the location most important to function. Case one:

4.00±0.01 22

ME 270, Leon Liebenberg

Case two:

3.00±0.01

1.50±0.01

Will it Fit? Will this pin with variable position and diameter always fit into the “perfect” part below?

10 ± 0.1

Start with the largest diameter pin and add 2 times the ± position number (total tolerance):

dia 4.5 ± 0.2

4.5 + 0.2 + 0.2 = 4.9 dia 5 ± 0.0

4.9 < 5.0 Yes it will always fit!

10 ± 0.0

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Tolerance Stack-Up Problems due to tolerance stack-ups include:  Failure to assemble  Interference between parts  Failure of parts to engage  Failure to function as intended

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ME 270, Leon Liebenberg

Tolerance Stack-up Assume a stack of 10 parts. If the 1.0-inch thick parts have a Gaussian distribution with a tolerance of 0.1-inch, what is the stack thickness and stack tolerance? 10.0 ±

0.1



+ 0.1



+ 0.1



+ 0.1



+ …

10.0 ± 0.32 in.

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Measurement Measurement: Procedure in which an unknown quantity is compared to a known standard (or reference value), using an accepted and consistent system of units

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ME 270, Leon Liebenberg

Measurement Devices

Micrometer

Go – No go gage

Caliper

Diameter, Surface Flatness, Concentricity, etc.

Protractor

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Measurement Devices Go – No-go gage:

If the GO end of the gage fits the hole, we know that the hole has been bored large enough If the NO-GO end cannot be inserted, the hole has not been bored too large.

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ME 270, Leon Liebenberg

Measurement Devices Precision Gage Blocks

Blocks are stacked by wringing them together

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Measurement Devices Precision Gage Blocks

Example: To obtain a dimensional standard of 14.715 mm

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ME 270, Leon Liebenberg

Accuracy and Precision  Accuracy - the degree to which a measured value agrees with the true value of the quantity of interest A measurement procedure is accurate when it avoids systematic errors (positive or negative deviations that are consistent from one measurement to the next)

 Precision - the degree of repeatability (consistency) in the measurement process Good precision means that random errors in the measurement procedure are minimized

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ME 270, Leon Liebenberg

Geometric Dimensioning & Tolerancing

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Geometric Dimensioning and Tolerancing GD&T is an international way to specify tolerances in modern engineering drawings Basic Dimension

Feature Control Frame

Datum

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ME 270, Leon Liebenberg

How to Read Feature Control Frame?

• Reads as: the position of the feature must be within a .003 diametrical tolerance zone at maximum material condition relative to datums A, B, and C. 35

Geometric Characteristics

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ME 270, Leon Liebenberg

Form Features (commonly used)

Flatness

Straightness

Circularity

Cylindricity

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Datums – related to part use • • • •

Part is first located using surface A Then located using surface B Finally using surface C Part is fully constrained

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ME 270, Leon Liebenberg

MMC vs LMC • Maximum Material Condition = when the part weight is the most: – Largest allowable shaft or pin size – Smallest allowable hole size 0.42 – MMC for a shaft Ø 0.40±0.02? 0.38 – MMC for a hole Ø 0.40±0.02? • Least Material Condition = when the part weight is the least. – Smallest allowable shaft or pin size, – Largest allowable hole size – LMC for a shaft Ø 0.40±0.02? 0.38 0.42 – LMC for a hole Ø 0.40±0.02? 39

Bonus Tolerance?? • The “M” in the circle after the tolerance designation:

– The tolerance is 0.010 if the hole size is the MMC size. If the hole is bigger we get a bonus tolerance equal to the difference between the MMC size and the actual size. – For a hole, the larger the hole, the more it can deviate from its true position and still fit a mating pin or shaft. 40

ME 270, Leon Liebenberg

Greatest Position Tolerance for Each Case? 6.0

MMC

8.0

8.0

±1.0

±1.0

6.0

9.0

±1.5

2.0

LMC

8.0

±2.0

6.0

MMC

8.0

4.0

±3.0

6.0

LMC

10.0

±2.0

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Bonus Tolerance Example for a Hole 0.500 ±.003

Actual Hole Size Ø .497 (MMC) Ø .499 (.499 - .497 = .002)

Bonus Tolerance 0 .002

Ø of Tol. Zone .010

Ø .500 (.500 - .497 = .003)

.003

.013

Ø .502 Ø .503 (LMC)

.005 .006

.015 .016

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ME 270, Leon Liebenberg

.012

Will it Fit? Will this pin with variable position and diameter always fit into the “perfect” part below?

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Diameter Tolerance

4.5 ± 0.2

4.7

+

0.2...