ieng 248 d. h. jensen 10/1/2015engineering graphics & 3-d modeling1 lecture 06 basic geometric...

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D. H. Jensen04/19/23 Engineering Graphics & 3-D Modeling 1

Lecture 06

Basic Geometric Dimensioning & Tolerancing (GD&T)

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Assignment: HW 06 Due Today

Reading Skim CH 9, 11

Review pp. 380 – 381 (pp. 314 – 318 in old text) (Dimensioning Do’s & Don’ts)

Assignment: Ex. 9.2, p. 387 (Fig. 9.67, p.327 in old text):

Sketch dimensioned views for b and d, onlyScale drawing so that the parts are roughly double

size on your paper (smallest hole diameter is 2 squares in width)

Draw orthographic, multi-view with straight edge

(not CAD)Use English units – 1 square is .20 inches / side

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Last HW Assignment: HW 07

Reading: Finish CH 11 Read CH 10 and start CH 12

Assignment: CH 10, p. 429 (Project p. 390 in old text):

Exercise 10.2 (Fig. 11.49 in old text) using GD&TExercise 10.3 (Fig. 11.50 in old text) using GD&T

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Fit Purposes

Clearance Used to allow motion between parts

RunningSliding

Interference Used to mechanically join parts

ForceShrink

Locational Used to constrain the position between parts

Locational Clearance FitsLocational Transition FitsLocational Interference Fits

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Tolerancing Definitions

Clearance Fit the internal member always has a space

between it and the external member Interference Fit

the internal member is always larger than the gap in the external member

Transition Fit may result in either a clearance or interference

condition Line Fit

limits specified so that either a clearance or exact surface contact condition results

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Definitions

Tolerance The total amount the feature is allowed to vary

(upper limit - lower limit) Basic Size (Basic Dimension – GD&T)

the theoretical exact value that deviations are applied to, and tolerances are computed from, in order to achieve the desired fit

Deviation The amount that a feature may vary from the basic size

in one direction (limit – basic size) Allowance

the minimum space between mating parts the difference between the largest allowable shaft size

and the smallest allowable hole sizeClearance Fit has a positive allowanceInterference Fit has a negative allowance

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Hands-On 11.1

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Tolerance Systems

Basic Hole System Used to set tolerances when it is easier to

change size of the shaft than the size of the hole

Minimum hole is taken as the basic size Most common system

Basic Shaft System Used to set tolerances when it is easier to

change the size of the hole than the size of the shaft

Maximum shaft is taken as the basic size Least common system

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Extreme Conditions

Maximum Material Condition (MMC) Prevailing conditions when the most material

is contained in both features Occurs when you have the smallest hole and

the largest shaft, simultaneously Think of it as when the part weighs the most,

and still fits all constraints (perfect form)

Least Material Condition (LMC) Prevailing conditions when the minimum

material is contained in both features Occurs when you have the largest hole and the

smallest shaft, simultaneously - or when the part has perfect form and weighs the least

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Specifying Tolerances

General TolerancesSpecified by notes in the title block, and apply to all

feature sizes unless otherwise specified

Dimensional (Parametric) TolerancesSpecified for a specific feature size

Limit DimensioningBoth upper & lower limit dimensions are specified

Plus-or-Minus DimensioningBilateral - a positive and a negative deviation

Plus AND Minus - symmetric, bilateral deviationUnilateral - only a positive or only a negative deviation

Single Limit DimensioningMIN or MAX is placed after the dimension if the other

feature size deviation is controlled by another element Angular Tolerancing

Bilateral Plus-or-Minus in degrees, minutes, seconds

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General Tolerance Development

Problem: Develop a tolerance for a pneumatic cylinder guide

(slide). The guide is a mating feature consisting of a pin and a hole. The pin will run back and forth within the hole as the cylinder extends/retracts. The hole will be produced with a drill and the shaft will be turned on a lathe. The nominal size is 13/16”, and the allowance is 0.002”. The tolerances will be specified to the thousandths of an inch.

Q: Is the hole or the shaft the basis for this application?

A: It is a basic hole system. The hole will be produced with a standard size drill bit, which is difficult to vary in fine increments. The shaft diameter can be easily varied on a lathe.

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Q: What kind of fit is required? A: Since the nominal allowance is positive, a clearance

fit will result. Common sense also tells you that a clearance fit is required to allow the running motion. An interference or transition fit would/could cause binding.

Q: What is the feature size? A: The nominal hole size is 13/16”; converted to

decimal inches it is 0.81250. This value is rounded to .812, using the dimensional rounding rules.

Q: What is a reasonable tolerance for the hole? A: From Table 10.2 (Fig 11.13 old text), for a drilling

operation with a nominal feature size between .600” and .999” the middle of the range of tolerances is .004”.

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Q: What material condition should the tolerance be based upon?

A: Since the specified fit is a clearance fit, the worst case condition is when the hole is smallest and the shaft is largest. This is the Maximum Material Condition, as it will constrain the maximum material in either part.

Q: What is the minimum hole dimension? A: For a Basic Hole System, the basic size is the

minimum acceptable hole size, or 0.812”.

Q: What is the maximum hole dimension? A: The hole tolerance is the difference between the

largest and smallest hole. The reasonable tolerance from Table 10.2 (Fig. 11.13, old text) was 0.004”. Adding it to the minimum hole gives an upper limit of 0.816”.

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Q: How can we show the hole tolerance? A: Using limit dimensioning, and standard English unit

practices (no leading zeros) the following would work:

.816

.812

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Q: What is the maximum dimension for the shaft?

A: The smallest hole size is 0.812”. For a clearance fit, subtracting the allowance (0.002”) gives the shaft size at MMC, or 0.810”.

Q: What is a reasonable tolerance for the shaft? A: From Table 10.2 (Fig. 11.13), for a turning operation

with a nominal feature size between .600” and .999” the middle of the range of tolerances is .0025”.

Q: What is the lower limit for the shaft dimension?

A: Subtract the tolerance from the maximum dimension to get 0.80750”, then round the dimension to 0.808”.

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Q: How can we show the shaft tolerance? A: Using limit dimensioning, and standard English unit

practices (no leading zeros) the following would work:

.810

.808

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Std. Tolerance Development

Problem: Develop a tolerance for an enhanced pneumatic cylinder

guide. The hole will still be produced with a drill and the shaft will be turned on a lathe. The nominal size is still 13/16”, but the tolerances will be specified to ten-thousandths of an inch.

Q: How can I specify a tolerance when an allowance is not given?

A: Empirical design. Look for standard tables or (previous practices) that help. Start with Table 10.1 (11.1) and note that an RC fit is what is needed for a running clearance. Then Appendix 7 (5) shows that a Close Running Fit (RC 4) is most appropriate. Appropriate clearances would run from 0.8 to 2.8 thousandths of an inch for a nominal feature size between 0.71” and 1.19”.

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Q: What are the standard limits to be applied to the basic hole size?

A: The nominal size was 0.8125”, falling between 0.71 and 1.19 in the left-most column of Appendix 7(5). The standard hole limits for an RC 4 Class fit are + 1.2 and

- 0 thousandths of an inch, or + 0.0012” and - 0.0000”.

Q: What are the standard limits to be applied to the basic shaft size?

A: The nominal size is also 0.8125”, falling between 0.71 and 1.19 in the left-most column of Appendix 7(5). The standard shaft limits for an RC 4 Class fit are - 0.8 and - 1.6 thousandths of an inch, or - 0.0008” and - 0.0016”. This gives an allowance of + 0.0008” between the shaft and hole at MMC, and a clearance of + 0.0028 at LMC.

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Q: How can we show the mating tolerances? A: Using dimensions with a unilateral tolerance and English

unit practices (no leading zeros) the following would work (use computations similar to those for the hole example):

.8125 .8117+ .0012 - .0000

+ .0000 - .0008

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Q: How can we show the mating tolerances? A: Using dimensions with a symmetric bilateral tolerance,

the following would work (center the dimensional value and adjust the unilateral deviations):

.8131 .8113+ .0006 - .0006

+ .0004 - .0004

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Q: How can we show the mating tolerances? A: Using the basic size, ISO notation standards for fits, and English

unit practices (no leading zeros) the following would work. H8 and f7 are the fit designations from the column headings in Appendix 7(5):

.8125 H8 .8125 f7

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Lecture 07B

GD&T & Examples

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Geometric Tolerances

Geometric Dimensioning & Tolerancing Abbreviated GD&T

Controls feature form / location variations, NOT feature size variations (width, height, depth); examples include:how cylindricalhow flathow straighthow symmetrichow parallel

Specified using internationally recognized graphic symbols for geometric characteristics

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Symbols

Straightness Flatness Circularity Cylindricity Perpendicularity Parallelism Position Concentricity Material Conditions etc. ...

M L

See Table 11.4 p. 373

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GD&T Symbol Construction

Datums specify their capital letter label in a frame

(to distinguish them from section labels) and connect them to the feature by:a leader, terminated with a triangle, oran extension line, immediately adjacent to the frame

(in this case, there are dashes bracketing the letter)

Basic Dimensions specify basic dimensions between controlled

features (just as with size dimensions), but distinguish them with frames

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GD&T Symbol Construction

Feature Control Construct Feature Control Frames by:

specifying the symbol for the geometric

characteristic to be controlled (i.e. position) in a box

specifying the tolerance zone shape and the

tolerance (i.e. diameter of the tolerance zone) in an

adjacent box, modifying for material condition at

tolerance specification

specifying the relevant datum(s) in adjacent boxes,

modifying for the material condition at measurement

Append notes as necessary to clarify

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Why GD&T?

GD&T allows us to: control more of the important aspects of the

feature - the geometry as well as the size avoid tolerance stacking have a cleaner, clearer drawing specify tolerance zones in a manner more

similar to the way they will be verified – it identifies the datum surfaces from which a feature is to be dimensioned

helps specify how the part is to be inspected and manufactured – implies how the part is to be fixtured

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Example: Flatness (No Datum)

Flatness is a characteristic of a single surface: If a surface is sufficiently flat, then all points

on the surface will lay in-between two parallel planes separated by the tolerance distance

Tolerance Zone Depiction:

Feature Control Frame & Leader:

.002

.002

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Example: Identifying Datums

Datums are theoretically perfect: The datum is assumed to be exact for the

purposes of manufacture and inspection. For practical purposes, they need to be 10X

more accurately produced than any measurement that will be derived from them.

For manufacturing purposes, these are the first features to produce, since they control the remaining characteristics of the part.

Identification:

B

1.02A

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Example: Parallelism (One Datum)

Parallelism is a characteristic of two surfaces: If a surface is parallel, then it will lay in-

between two planes parallel to the datum and to each other, offset by the tolerance distance

Tolerance Zone Depiction:

Feature Control Frame:

.003

.003 A

2.62

A

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Example: True Position (Multi-Datum)

True Position is a relationship between at least three surfaces: If the centerline of the feature is positioned

accurately, then it will lay within a tolerance zone envelope sized by the tolerance value

True Position is a tolerance of location: Location is specified by BASIC DIMENSIONS The basic dimensions originate at DATUM

surfaces It may be affected by the size of the produced

feature, so design intent should be indicated by the MATERIAL CONDITION modifier

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Two-Dimensional Tolerance View

To place a hole in the part, we need to locate the center of the hole in the coordinate plane relative to the axis of the hole, and then size the hole (allowing a hole size tolerance)

15.500

10.000

11.200 ± .002

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Three-Dimensional Tolerance View

Then we add the GD&T information to control the location of the hole center

15.500

10.000

C

B

11.200 ± .002

.001 M A B C

Note: Datum A forms the bottom surface of the hole, and so the tolerance zone is a perfect, right cylinder – resting on Datum A and located from Datum B and Datum C.

A

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This tells us the DATUMS that we will measure from to locate or inspect the hole

15.500

10.000

C

B

11.200 ± .002

.001 M A B C

A

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It tells us the BASIC DIMENSIONS that control where the hole is located

15.500

10.000

C

B

11.200 ± .002

.001 M A B C

A

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It tells us the size and shape of the tolerance zone for the hole center

15.500

10.000

C

B

11.200 ± .002

.001 M A B C

A

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And it tells us the worst case material condition used to inspect the hole center

15.500

10.000

C

B

11.200 ± .002

.001 M A B C

In this case, when the hole is at its’ smallest permissible size, the feature location is in its’ most critical state.

A

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Inspection of the Tolerance

Example inspection gage for the hole

C

B

11.200 ± .002

.001 M A B C

In this case, when the hole is at its’ smallest permissible size and perfectly located, the feature is in its’ most critical state, and just fits about the red gage pin.

A

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Effect of Material Condition Modifiers with Feature Size and Geometric Tolerances

Produced Hole Size

Geometric Tolerance

Feature Size ‘Bonus’

Total Effective Tolerance

11.197 out of size tolerance

11.198 .001 0 .001

11.199 .001 .001 .002

11.200 .001 .002 .003

11.201 .001 .003 .004

11.202 .001 .004 .005

11.203 out of size tolerance

Effect of Feature Size with M Tolerance Modifier:

B

C

15.500

10.000

M

11.200 ± .002

.001 M A B C

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GD&T Tolerance Development

Problem: Develop a geometric tolerance to control the location of

the hole for the enhanced pneumatic cylinder guide. The hole is still drilled, the nominal size is still 13/16”, and the limits for an RC 4 class fit will still be used; but the positional tolerance will be specified to be within a .0001” circular diameter of the true position of the hole, located 2.250” and 5.500” from the bottom left corner.

Q: How do I specify a tolerance like that? A: Use GD&T. The locational parameters become basic

dimensions (so put a frame around them). These dimensions should originate with a datum, so label the each datum on the drawing. The feature size is called out with a leader, and a symmetric size tolerance can be developed, just as before. A feature control frame is needed for the positional tolerance. Fill it out with the symbol for true position, specify the shape and size of the tolerance zone at MMC, and call out each datum needed to locate the hole.

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Effect of Material Condition Modifiers with Feature Size and Geometric Tolerances

Produced Hole Size

Geometric Tolerance

Feature Size ‘Bonus’

Total Effective Tolerance

.8122 out of size tolerance

.8125 .0001 0 .0001

.8128 .0001 .0003 .0004

.8131 .0001 .0006 .0007

.8134 .0001 .0009 .0010

.8137 .0001 .0012 .0013

.8140 out of size tolerance

Effect of Feature Size with M Tolerance Modifier:

B

C

5.500

2.250

M

.8131 ± .0006

.0001 M A B C

ieng 248 d. h. jensen 10/1/2015engineering graphics & 3-d modeling1 lecture 06 basic geometric...

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