Unit14 limits and tolerances

Limits and Tolerances

Dimensioning

The design of a machine includes many factors

other than

those of determining

the loads and stresses and selecting the proper materials. Before construction or

manufacture

can

begin,

it

is

necessary

to

have

complete

assembly

and

detail

drawings

to convey all necessary information to the shop men. The designer frequently is

called

upon

to

check

the

drawings

before

they

are

sent

to

the

shop.

Much

experience

and familiarity with manufacturing processes are needed before one can become

conversant

with all

phase

s of production drawings. [1]

Drawings should be carefully checked to see that the dimensioning is done in

a

manner

that

will

be

most

convenient

and

understandable

to

the

production

departments. It is obvious that a drawing should be made in such a way that it has

one

and

only

one

interpretation.

In

particular,

shop

personnel

should

not

be

required

to

make

trigonometric

or

other

involved

calculations

before

the

production

machines

can be

set up

. [2]

Dimensioning is an

involved

subject and long experience is required for its

mastery. [3]

Tolerance

s

must

be

placed

on

the

dimensions

of

a

drawing

to

limit

the

permissible

variation

s

in

size

because

it

is

impossible

to

manufacture

a

part

exactly

to

a

given

dimension.

Although

small

tolerances

give

higher

quality

work

and

a

better

operating

mechanism,

the

cost

of

manufacture

increases

rapidly

as

the

tolerances

are

reduced,

as indicated by the typical curve of Fig 14.1. It is therefore important that the

tolerances be

specified

at the largest values that the operating or functional

considerations permit.

Tolerances may be either

unilateral

or

bilateral

. In unilateral dimensioning,

one tolerance is zero, and all the variations are given by the other tolerance. In

bilateral dimensioning, a mean dimension is used which extends to the midpoint of

the

tolerance

zone

with

equal

plus

and

minus

variations

extending

each

way

from

this

dimension.

The

development

of

production

processes

for

large-volume

manufacture

at

low

cost

has been largely dependent upon

interchangeability

of component parts. Thus the

designer must determine both the proper tolerances for the individual parts, and

the

correct

amount

of

clearance

or

interference

to

permit

assembly

with

the

mating

parts

. The manner of placing tolerances on drawings depends somewhat on the kind

of

product

or

type

of

manufacturing

process.

If

the

tolerance

on

a

dimension

is

not

specifically

stated,

the

drawing

should

contain

a

blanket

note

which

gives

the

value

of

the

tolerance

for

such

dimensions.

[4]

However,

some

companies

do

not

use

blanket

notes on the supposition that if each dimension is considered individually, wider

tolerances than those called for in the note could probably be specified. [5] In

any

event

it

is

very

important

that

a

drawing

be

free

from

ambiguities

and

be

subject

only to a single interpretation.

Dimension and Tolerance

In

dimensioning

a

drawing,

the

numbers

placed

in

the

dimension

lines

represent

dimension that are only approximate and do not represent any degree of accuracy

unless

so

stated

by

the

designer.

[6]

To

specify

a

degree

of

accuracy,

it

is

necessary

to add tolerance

figure

s to the dimension. Tolerance is the amount of variation

permitted in the part or the total variation allowed in a given dimension. A shaft

might

have

a

nominal

size

of

2.5

in.

(63.5

mm),

but

for

practical

reasons

this

figure

could not be

maintain

ed in manufacturing without great cost. Hence, a certain

tolerance would be added and,

if a variation of±0

.003 in.

(±0

.08 mm) could be

permitted, the dimension would be stated 2.

500±

0.003 (63.

5±0

.08 mm).

Dimensions

given

close

tolerance

s

mean

that

the

part

must

fit

properly

with

some

other part. Both must be given tolerances

in keeping with

the allowance desired,

the manufacturing processes available, and the minimum cost of production and

assembly that will

maximize

profit.

Generally speaking

, the

cost

of a part

goes

up

as

the

tolerance

is

decreased.

If

a

part

has

several

or

more

surfaces

to

be

machined,

the cost can be excessive when little

deviation

is allowed from the nominal size.

Allowance,

which

is

sometimes

confused

with

tolerance,

has

an

altogether

different meaning. [7] It is the minimum

clearance space

intended between mating

parts and represents the condition of tightest permissible fit. If a shaft, size

1

.

498

?

0

.

000

?

0

.

003

, is to fit a hole of size l.500

±，

the minimum size hole is l. 500 and

the maximum size shaft is l. 498. Thus the allowance is 0. 002 and the maximum

clearance is 0.008 as based on the minimum shaft size and maximum hole dimension.

Tolerances may be either unilateral or bilateral. Unilateral tolerance means

that

any

variation

is

made

in

only

one

direction

from

the

nominal

or

basic

dimension.

Referring

to

the

previous

example,

the

hole

is

dimensioned

l.500

±

,

which

represents

a

unilateral

tolerance.

If

the

dimensions

were

given

as

l.500

±

0.003,

the

tolerance

would

be

bilateral;

that

is,

it

would

vary

both

over

and

under

the

nominal

dimension.

The

unilateral

system

permits

changing

the

tolerance

while

still

retain

ing

the

same

allowance or type of fit. With the bilateral system, this is not possible without

also changing the nominal size dimension of one or both of the two mating pats. In

mass

production,

where

mating

parts

must

be

interchangeable,

unilateral

tolerances

are

customary

. To have an

interference

or force fit between mating parts, the

tolerances must be such as to create a zero or negative allowance.

Tolerances

，

Limits and Fits

The

drawing

must

be

a

true

and

complete

statement

of

the

designer

’

s

requirements

expressed

in

such

a

way

that

the

part

is

convenient

to

manufacture.

Every

dimension

necessary

to

define

the

product

must

be

stated

once

only

and

not

repeated

in

different

views.

Dimensions

relating

to

one

particular

feature,

such

as

the

position

and

size

of a hole, should, where possible, appear on the same view.

There

should

be

no

more

dimensions

than

are

absolutely

necessary,

and

no

feature

should

be

located

by

more

than

one

dimension

in

any

direction.

[8]

It

may

be

necessary

occasionally

to

give

an

auxiliary

dimension

for

reference,

possibly

for

inspection.

When this is so, the dimension should be enclosed in a

bracket

and

mark

ed for

reference. Such dimensions are not

govern

ed by general tolerances. [9]