Chapter 5.Dimensions Tolerances and Surfaces

7/23/2019 Chapter 5.Dimensions Tolerances and Surfaces

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Dimensions Tolerances and Surfaces

The quality

o

a part is not only dependent on the physical and mechanical

properties o the material from which it

is

manufactured, but also upon the

geometrical attributes too. These attributes include the dimensions, tolerances,

flatness, roundness, angularity and surface finishes.

The American National Standards Institute (ANSI) defines a dimension as a

numerical value expressed in appropriate units

o

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,

o

a part or part feature.

The dimension represented on a part drawing

is

an almost ideal state that

is

generally unattainable

in

practice because

o

inaccuracies ... deviations ....

introduced by imperfections in the manufacturing processes. Tolerances

imposed on a dimension are specified to accommodate these imperfections, and

to specify the maximum admissible excursion from the desired perfect state.

Therefore parts with absolute dimensions lying outside this state are

unacceptable, they are rejected as scrap, and a financial penalty is incurred.

ANSI defines a tolerance as the total amount by which a specific dimension is

permitted to vary. The tolerance is the difference between the maximum and

minimum limits.

Surfaces

The quality

o

a part's surface

is

dictated by the manufacturing process employed

to create the two or three dimensional shape. Furthermore the surface quality

varies widely among processes and this part attribute is

o

importance for the

engineer because

o

a variety

o

technical and commercial reasons.

- Aesthetics

- Safety

- Friction and wear

- Assembly operations

- Electrical conductivity

- Mechanical and physical properties

When magnified, using n optical microscope, surface irregularities and

imperfections become clearly evident. Flaws, roughness, waviness and patterns

imposed by the manufacturing process responsible for creating the surface are

readily identifiable.

urface finish is a subjective term concerning the surface characteristics while

surface roughness can be defined as the average

o

the vertical deviations from

the nominal surface over a specified length. An arithmetic average can be

employed or a root-mean-squared average.



The decision of which manufacturing processes to employ to create a part in a

specific material with specific surface qualities and embodying specific

dimensions within prescribed geometrical tolerances is

n important engineering

consideration especially when it must be accomplished cost-effectively. To

facilitate this activity manufacturing processes can be codified based on the

dimensional accuracy and surface finish they can attain.

Generally the more accurate the part and the more refined the surface attributes

the more expensive the manufacturing processes needed to achieve this.

Therefore one should always specify the roughest surface with the largest

tolerances for a prescribed set of operating conditions.



Crater (flaw)

Roughness height

Roughness width

Surface texture

.......

Altered layer

/ ~ : I - - -

Substrate

Lay

direction

Crack (flaw)

FIGURE 5.5 Deviat ions

from nominal surface used

in

the

two

definitions

o

surface roughness.

Waviness spacing

Vertical deviations

y;)

Nominal surface

x

~ - - - - - - - - - - - - - - - L m - - - - - - - - - - - - - - - -

FIGURE 5.2 A magnif ied cross

section

o

a typical metallic part

surface.

Waviness

heigh1

FIGURE 5.1

Three ways to

specify tolerance limits or a

nominal dimension o

J

T

T

.500:

a)

bilateral,

b)

2.505

. 5 0 0 ~ : ~

Y 2 . 5 0 0 : g : g ~

nilateral, and c) limit

2.495

dimensions.

a)

b)

(c)



T BLE 5.4 Typical tolerance limits,

based

on process capabili ty Section

44 2lt

for various manufacturing processes.

Typical Tolerance Limits Typical Tolerance Limits

Process

mm

inches

Process

rom

inches

Sand casting

Cast iron

±1.3

±0.050

Abrasive processes:

Grinding

±0-008

±0.0003

Steel

±1.5

±0.060

Lapping

±0.OO5

±0.0002

Aluminum

±O.5

±0.020

Honing

±0.005

±O.0002

Die casting

Plastic molding:

Polyethylene

Polystyrene

Machining:

Drilling, diameter:

6 rom (0.250 in)

25 rom (1.000 in)

, ±0.12

±0.3

±0.15

+0.08, 0.03

+0.13, -O.OS

±0.005

±0.01O

±0.006

+0.003,

0.001

+0.006, 0.002

Nontraditional processes:

Chemical machining

Electric discharge

Electrochem. grind

Electrochem. machine

Electron beam cutting

Laser beam cutting

Plasma arc cutting

±0.08

±0.025

±0.025

± O ~

±O

•

±0.08

±1.3

±O.003

±O.OOl

±O.OOl

±0.002

±O.003

±0.003

±O.OSO

Milling

±0.08

±0.OO3

fuming

±0.05

±0.002

Compiled from

[4],

[5], and

other

sources.

T BLE 5.5 Surface roughness values produced by the various manufacturing processes.

a

Typical

Surface

Range

o Typical Surface Range

of

Process Fmish

Rougbness

b

Process

Finish

Rougbness

b

Casting:

Abrasive:

Die casting

Good 1 2

(30-65)

Grinding Very good

0.1-2 (S-75)

Investment Good 1.5-3 (SO-l00)

Honing Very good

0.1-1 (4-30)

Sand casting Poor 12-25 (SOO-l000) Lapping Excellent 0.05-0.5 (2-1S)

Metal forming:

Polishing Excellent

O.l-O.S (5-15)

Cold rolling

Good 1 3 (25-125)

Superfinish Excellent

0.02-0.3 (1-10)

Sheet metal draw

Good

1 3 (25-125)

Nontraditional:

Cold extrusion

Good 1 4

(30-1S0)

Chemical milling Medium

1.5-5 (50-200)

Hot

rolling Poor

12-25 (500-1000)

Electrochemical

Good

0.2-2 (10-100)

Machining:

Electric discharge Medium

1.5-1S (50-500)

Boring Good

O.S-6

(15-250)

Electron beam Medium

1.5-15 (50-500)

Drilling Medium

1.5-6 (60-2S0)

Laserbearn Medium

15 1S

(50-500)

Milling Good

1 6

(30-250)

Thermal:

Planing Medium

1.5-12 (60-S00)

Arc welding

Poor

5 25 (250-1000)

Reaming

Good

1 3

(30-12S)

Flame cutting

Poor

12-25 (500-1000)

Shaping Medium

1.5-12 (60-500)

Plasma

arc

cutting

Poor

12-25 (500-1000)

Sawing Poor

3 25

(100-1000)

Turning

Good

0.5-6 (15-250)

• Compiled from

m

[2J,

and o ther sources.

b Subjective description and typical range of surface roughness values are given, porn

po-in).

Roughness can vary significantly for a given process,

depending

on

process parameters.



Crater flaw)

Roughness height

Roughness width

~

Surface texture

~ ~ . : . . . ~ L . i J . . o I I - - - -

Altered layer

~ ~ ~ ~

/ 4 Substrate

Lay direction

Crack flaw)

FIGURE 5.5 Deviations

from nominal surface

used

in the two definitions o

surface roughness.

Waviness spacing

Actual surface

y

Vertical deviations

Nominal surface

~ - - - - - - - - - - - - - - - L m - - - - - - - - - - ~

FIGURE

5.2 A magnified cross

section o a typical metallic part

surface.

FIGURE

5.1

Three ways to

specify tolerance limits for a

nominal dimension

o

T

T

.500: a) bilateral, b) T

5 0 0 ~ g : ~

y

2 505

unilateral, and c) limit 2 500

Y

2 495

dimensions.

a)

b)

c)



T BLE

5.4 Typical tolerance limits, based on process capabili ty (Section 44.2), for various manufacturing processes.

Typical Tolerance Limits

1YPical Tolerance Limits

Process

mm

inches

Process

mI

inches

Sand casting

Abrasive processes:

Cast iron

±1.3

±0.050

Grinding

±0.008

±OJX)03

Steel

±1.5

±0.060

Lapping

±0.005

±0.0002

Aluminum

±O.S

±0.020

Honing

±O.OOS

±O.OOO2

Die casting

±0.12

±O.OOS

Nontraditional processes:

Plastic molding:

Chemical machining

±0.08

±O.003

Polyethylene

±0.3

±0.010

Electric discharge

±O.025

±0.00l

Polystyrene

±0.1S

±0.006

Electrochem. grind

±0.025

±0.001

Machining:

Drilling, diameter:

6 mm

0.250 in)

2S mm 1.000 in)

+0.08, 0.03

+0.13,

-O.OS

+0.003,

0.001

+0.006, 0.002

Electrochem. machine

Electron

beam

cutting

Laser

beam

cutting

Plasma arc cutting

±O.OS

±0.08

±0.08

±1.3

±0.002

±O.003

±O.003

±O.OSO

Milling

±0.O8

±0.003

Turning

±0.05

±0.002

Compiled from

[4],

[5], and other sources.

T BLE

5.5 Surface roughness values produced by the various manufacturing processes.

Typical Surface

Range

of

Typical Surface

Range

of

Process Finish

Roughness

b

Process Finish

Roughness

b

Casting:

Abrasive:

Die casting

Good

1 2 30-65)

Grinding

Very good

0.1-2 5-75)

Investment

Good

1.5-3 50-100)

Honing Very good

0.1-1 4-30)

Sand casting

Poor

12-25 500-1000)

Lapping

Excellent

0.05-0.5 2-15)

Metal forming: Polishing Excellent 0.1-0.5 5-15)

Cold rolling Good

1 3

25-125)

Superfinish Excellent

0.02-0.3 1-10)

Sheet metal draw

Good 1 3 25-125)

Nontraditional:

Cold extrusion Good

1 4

30-150)

Chemical milling Medium

1.5-5 50-200)

Hot rolling Poor 12-25 SOO-looo)

Electrochemical

Good 0.2-2 10-100)

Machining:

Electric discharge Medium

1.5-15 50-500)

Boring Good 0.5-6 15-250)

Electron beam Medium 1.5-15 50-500)

Drilling Medium

1.5-6 60-250)

Laser

beam

Medium

1.5-15 50-SOO)

Milling

Good

1 6 30-250)

Thermal:

Planing Medium

15 12 60-500)

Arc

welding Poor

5-25 250-1000)

Reaming Good 1 3 30-125)

Flame cutting Poor 12-25 500-1000)

Shaping Medium

1.5-12 60-500)

Plasma arc cutting

Poor 12-25 500-1000)

Sawing ,Poor

3-25 100-1000)

Thrning

Good

0.5-6 15-250)

• Compiled from 1], [2], and other sources.

b

Subjective description and typical range

of

surface roughness values a re given,

Lm

JL-in).

Roughness can vary significantly for a given process,

depending on process parameters.

Chapter 5.Dimensions Tolerances and Surfaces

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