ieng 248 d. h. jensen 10/2/2015engineering graphics & 3-d modeling1 lecture 06 dimensioning...

IENG 248

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

Lecture 06

Dimensioning & Fastener Specification

IENG 248


Assignment: HW 06

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

IENG 248


Dimensioning

Why dimension drawings? Historical practicality Production inaccuracies Clarity of:

SizePositionGeometric Form

How to dimension Know dimensioning techniques Select dimension placement Choose dimensions shown to convey:

Functionality of part (very similar to design intent)Manufacturing steps for part

IENG 248


Dimensioning Terms

Nominal Size The approximate size used for naming purposes

Limits The extreme allowable sizes for a feature

Basic Size The theoretical exact size from which limits for

permissible variation are derived Basic Dimension

The theoretical exact location, to which tolerances are applied so as to limit permissible geometry variation

Datum A feature that is assumed to be exact for the purposes

of manufacture and inspection Tolerance

The total amount the feature is allowed to vary(upper limit - lower limit)

IENG 248


Dimensioning Terms

Allowance The minimum space between mating parts

Purposes of Dimensions Specify the size of a feature Specify the location of a feature

Purpose of Tolerances Specify the allowable variation of a feature

Variation in sizeVariation in positionVariation in form (shape)

Dimensions & Tolerances specify relationships Functional relationships Manufacturing relationships Inspection relationships

IENG 248


Dimensioning Lines

Dimensioning Line Types Dimension Line

Thin, dark, solid, with arrowheads at ends

Extension (Witness) LineThin, dark, solid, starting a small distance from the

feature and running to just past the dimension line

CenterlineThin, dark, long-short-long pattern; may be extended

for dimensioning

LeaderThin, dark, solid, with a single arrowhead/dot, at a

large angle with respect to feature being referenced

IENG 248


Dimensioning Systems

Dimension Text Directions Aligned Unidirectional

Dual Dimensioning Metric on top / left, English on bottom / right Metric in square brackets, position consistent

Decimal Dimensioning Systems Complete decimal dimensioning Combination dimensioning

mixed decimal and nominal dimensions Number of decimals shown reflects tolerance

IENG 248


Dimension Rounding

Three part rule:1. If the number following the rounding position

is less than 5, the positional digit is unchanged

2. If the number following the rounding position is exactly 5, round the position to the closest even digit

3. Otherwise, increment the positional digit

Exception: Diameters and other dimensions likely to be

divided by 2 should be rounded to an even digit, unless the precision is critical for strength, fit, etc.

IENG 248


Dimensional Values

Clarity is most important Decimal point is bold Avoid crowding Always break lines crossing dimension values

Metric Has leading zeros on fractions less than one mm There are no right zeros in fractions unless it is

expressing the tolerance

English Has NO leading zeros on fractions less than 1 inch The number of right zeros in fractions are the same as

the feature tolerance

IENG 248


Placement

Dimension lines may NOT cross other dimension

lines

Extension lines may cross other extension lines

and visible lines, but must be broken if crossing a

dimension line. There is a small gap between an

extension line and the object it references

Centerlines may be used as extension lines for

dimensioning purposes

Leaders may be used to specify radii and hole

diameters, but NOT protrusion diameters

IENG 248


Rules for Dimensions

Prime Directive: Dimensions should not be shown on a view

unless they promote the clarity of the drawing Important Rules:

Show dimensions where shapes are shown in profile and contours are shown true size

Align dimension lines and group dimensions as much as possible for reading ease

Prefer placing dimensions between views Dimension lines are drawn at right angles to

extension / centerlines whenever possible, but always parallel to the measured distance in any case

IENG 248


Standard Symbols

Counterbore / Spotface

Countersink (not a finish mark)

Depth (blind hole)

Diameter / Round shape

Radius

Places / times / by

Reference Square shape

R

( )

IENG 248


Manufacturing Considerations

Dimension from datums (finished measuring surfaces) Finish Mark (examples)

Avoid tolerance stacks Chain vs. Datum dimensioning

Do not over-dimension Leave one dimension missing in a locational chain

Do not over-specify tolerances / conditions Over specification can result in excessive cost

Design for manufacturability / assembly / X See Figure 9.51, p. 314 (old text, only)

Append notes to clarify intent

IENG 248


Assignment: HW 06

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

IENG 248


Fastener Terminology

Screw Thread a helical ridge of uniform section on the

curved surface of a cylinder (internal or external)

External Thread a thread on the outside of a member (shaft)

Internal Thread a thread on the inside of a member (hole)

Major Diameter the largest diameter of a screw thread

Minor Diameter the smallest diameter of a screw thread

IENG 248



Angle of Thread the angle between the two sides of a thread,

measured on a plane through the axis of the screw

Crest the top surface/edge joining the thread sides

Root the bottom surface/edge joining the thread

sides

Side the surface connecting the crest with the root

IENG 248



Pitch the distance from one thread to the next,

measured parallel to the axis. The reciprocal of the number of threads per inch.

Pitch Diameter the diameter of a cylinder if it passed through

the point on the thread where the area of the threads and the space between threads is equal.

Lead the distance a thread advances axially in one

turn.

IENG 248



Axis of Screw the longitudinal centerline through the screw

Depth of Thread distance from crest to root measured normal

to the axis

Form of Thread cross section of thread cut by a plane through

the axis

Series of Thread standard number of threads per inch and

based on the screw diameter

IENG 248


Thread Forms

Sharp V Whitworth Standard American National Unified Metric Standard Worm Square Acme Knuckle Buttress

IENG 248


Thread Notes

Thread Series Coarse Fine Extra Fine 8-pitch 12-pitch 16-pitch

Thread Fits Class 1 Class 2 Class 3

Internal / External A - external B - internal

Thread Engagement S - short N - normal L - long

Multiple Threads DOUBLE TRIPLE QUADRUPLE

Left / Right Hand RH assumed LH must be noted Rule of Thumb:

point thumb into material, and fingers will curl in turning direction

IENG 248


Coding a Thread Note

American National1. Major Diameter

2. Threads Per Inch

3. Form & Series

4. Fit Class

5. LH/RH

6. Thread Depth

Metric1. Metric Designation

2. Major Diameter

3. Pitch

4. Tolerance Grades

5. LH/RH

Unified National1. Major Diameter

2. Threads Per Inch

3. Form & Series

4. Fit Class

5. LH/RH

6. Thread Depth

Examples 3/4-10NC-2A LH-1.00

.75-8UNF-3B DOUBLE-.50 DEEP

M10X1.5-6H/5g6g LH

IENG 248


Thread Representation

Detailed shows thread profile approximate helix by straight lines between crests

Simplified shows thread depth by hidden lines parallel to the axis length is determined by thread length

Schematic show alternating crest (thin) lines and root (thick) lines

perpendicular to the axis length is determined by thread length

IENG 248


Standard Fasteners

Screws Nuts & Bolts Washers Keys Rivets (note spelling!)

Pins Comments:

Text gives typical shapes & dimensions for the heads

Appendices give standard dimensions for nominal sizes of fastener geometry

IENG 248


Specifying a Fastener

Bolts & Nuts: Nominal body size Thread spec/note Length Finish Head style Name*

CAP SCRMACH SCRSET SCRNUTEtc.

Examples: M10 X 1.5 X 12 HEX HD SCR M10 X 1.5 X 12 HEX SOC HD .375 X 1.25 SQ HD SS 10-32 X 5/8 SLOT PAN HD MACH

SCR

Abbreviations: See Appendix 4,

pp. a21 - a 24

ieng 248 d. h. jensen 10/2/2015engineering graphics & 3-d modeling1 lecture 06 dimensioning...

Documents