Ipec limits fits

Limits, Fits, Tolerances & Surface

Roughness

Manoj Yadav

Tolerance Dimensioning

Tolerance is the total amount that a specific

dimension is permitted to vary.

It is the difference between the maximum and the

minimum limits for the dimension.

For Example a dimension given as 1.625 ± .002

means that the manufactured part may be 1.627”

or 1.623”, or anywhere between these limit

dimensions.

Tolerances

The Tolerance is 0.001” for the Hole as well as for the Shaft

Unilateral Tolerance The upper and lower deviations are all positive or all

negative.

Bilateral Tolerance

The tolerance is split above and below the basic size

LIMITS & FITS

When parts are assembled together, engineers have

to decide how they will fit together and the

economics associated with it.

• How they will fit together?

– Clearance fit

– Transition fit

– Interference fit

• Economics?

– Interchangability

Limits and Fits - Definitions


• Basic size /nominal size - the reference size of

hole/shaft to which the limit of size are fixed, the

basic size is the same for both members of a fit.

• Limits of size - the maximum and minimum sizes

permitted for a feature.

• Maximum limit of size - the greater of the 2 limits

of size.

• Minimum limit of size - the smaller of the 2 limits

of size.


Tolerance is the difference between the maximum

limit of size and the minimum limit of size. It is

always a positive quantity.

Maximum Material Condition- A part is said to be in

MMC when its dimensions are at the limits that give

max. amount of material in part.

Least Material Condition- A part is said to be in LMC

when its dimensions are at the limits that give least

amount of material in part.


Hole - designate all INTERNAL features of a

part, including parts which are not cylindrical.

Shaft - designate all EXTERNAL features of a

part, including parts which are not cylindrical.

Upper deviation - difference between the

maximum limit of size and the corresponding

basic size.

This is designated ‘ES' for a hole and 'es'

for a shaft.


Lower deviation - difference between the

minimum limit of size and the corresponding

basic size. This is designated ‘EI' for a hole and

'ei' for a shaft.

Fundamental Deviation - It shows the minimum

difference in size between component and basic

size.

*Basic shaft and basic hole have zero fundamental dimensions.

Deviation

Max Hole size – Basic Size = Upper

Deviation

Min Hole size – Basic Size = Lower

Deviation

HOLE Max shaft size – Basic Size = Upper

Deviation

Min shaft size – Basic Size = Lower

Deviation

SHAFT

Size Designations

Basic Size or Basic dimension: It is the

theoretical size from which limits of size are

derived by the application of allowances and

tolerances.

Actual Size: is the measured size of the finished

part.

Allowance: is the minimum clearance space (or

maximum interference) intended between the

maximum material condition of mating parts.

Fits Between Mating Parts

Fit is the general term used to signify the range of tightness or looseness that may result from the application of a specific combination of allowances and tolerances in mating parts.

There are three types of fits between parts

1. Clearance Fit: an internal member fits in an external member (as a shaft in a hole) and always leaves a space or clearance between the parts.

Minimum air space is 0.002”. This is the

allowance and is always positive in a

clearance fit

CLEARANCE FIT

Maximum shaft dimension < Minimum hole

dimension

2. Interference Fit: The internal member is larger than the

external member such that there is always an actual

interference of material. The smallest shaft is 1.2513”

and the largest hole is 1.2506”, so that there is an actual

interference of metal amounting to at least 0.0007”.

Under maximum material conditions the interference

would be 0.0019”. This interference is the allowance,

and in an interference fit it is always negative.

INTERFERANCE FIT

Maximum Hole size < Minimum Shaft size

3. Transition Fit: may result in either a clearance or

interference condition. In the figure below, the smallest

shaft 1.2503” will fit in the largest hole 1.2506”, with

0.003” to spare. But the largest shaft, 1.2509” will have

to be forced into the smallest hole, 1.2500” with an

interference of metal of 0.009”.

TRANSITION FIT

Obtained by overlapping of tolerance zones of shaft and hole

……Does not guarantee neither clearance nor interference fit

Basic Hole System

Minimum hole is taken as the basic size, an allowance is

assigned, and tolerances are applied on both sides of and

away from this allowance.

1. The minimum size of the hole

0.500” is taken as the basic size.

2. An allowance of 0.002” is decided

on and subtracted from the basic

hole size, making the maximum

shaft as 0.498”.

3. Tolerances of 0.002” and 0.003”

respectively are applied to the

hole and shaft to obtain the

maximum hole of 0.502” and the

minimum shaft of 0.495”.

Minimum clearance: 0.500”-

0.498” = 0.002”

Maximum clearance: 0.502” –

0.495” = 0.007”

Basic Shaft System

Maximum shaft is taken as the basic size, an

allowance is assigned, and tolerances are applied on

both sides of and away from this allowance.

1. The maximum size of the shaft

0.500” is taken as the basic size.

2. An allowance of 0.002” is decided

on and added to the basic shaft

size, making the minimum hole as

0.502”.

3. Tolerances of 0.003” and 0.001”

respectively are applied to the

hole and shaft to obtain the

maximum hole of 0.505” and the

minimum shaft of 0.499”.

Minimum clearance: 0.502”-

0.500” = 0.002”

Maximum clearance: 0.505” –

0.499” = 0.006”

Hole & Shaft Basis System HOLE BASED SYSTEM-

Size of hole is kept constant,

shaft size is varied

to get different fits.

SHAFT BASED SYSTEM-

Size of shaft is kept constant,

hole size is varied

to get different fits.

FITS

A fit is indicated by the basic size common to both components, followed by symbol

corresponding to each component, the hole being quoted first.

E.g. 45 H8/g7

Specifications of Tolerances

1. Limit Dimensioning

The high limit is placed above

the low limit.

In single-line note form, the low limit

precedes the high limit separated by a

dash

Specifications of Tolerances

2. Plus-or-minus Dimensioning

• Unilateral Tolerance

• Bilateral Tolerance

Cumulative Tolerances

Fundamental Deviations

The 27 deviations for HOLES are:

A B C CD D E.EF F G H JS J K M N P R S T U V X Y

Z ZA ZB ZC

The 27 deviations for SHAFTS are:

a b c cd d e ef f g h js j k m n p r s t u v x y z za zb zc

(As per IS:919-1963)

H : lower deviation of

hole is zero

h : upper deviation of

shaft is zero

E.S. – upper deviation

E.I. – lower deviation

Representation of

Tolerance

1) Letter Symbol

The selection of letter freezes one

limit of hole / shaft

(how much away from Basic size)

45 E8/e7 Basic Size

One can have different

possible combinations; eg.

45H6g7, 45H8r6, 45E5p7

Grades of Tolerances

There are 18 grades of tolerances

IT01, IT0 and IT1 to IT16. (IT - ISO series

Tolerances)

IT01 and IT0 are very fine grades

IT16 is the most coarse grade reflecting the

precision of the process.

The degree of error increases with:

– the precision of the process (IT grade), and

– the size of the component.

RANGE IN A GIVEN TOLERANCE GRADE

H : lower deviation of

hole is zero

Representation of Tolerance

The selection of Tolerance

grade number freezes the

other limit of hole / shaft

Together (Letter & Grade) on both

mating components decide quality

of fit

H7 : Tol Grade 7 mean 21μ variation (H

means upper deviation zero)

Representation of Fit

0.021

0.022 0.013

Φ30.000

Φ30.021

Φ30.022

Φ30.035

p6 : Tol Grade 6 means 13μ variation

(p means upper deviation is 22 μ)

INTERFERENCE FIT

Tolerances Related to Machining Processes

Grades of Tolerances

Limits and Fits Designation

• A hole tolerance with deviation 'H' and tolerance

grade IT7 is designated 'H7'.

• A shaft tolerance with deviation 'p' and tolerance

grade IT6 is designated 'p6'.

• Appropriate tolerance designation for a feature of

45 mm, e.g. 45H7 or 45p6.

• A fit combines the basic size of both features and

their designations. The designation of hole limits

should always be quoted first. E.g. 45H7-p6 or

45H7/p6.

International Tolerance Grade (IT)

They are a set of tolerances that varies according to the basic size

and provides a uniform level of accuracy within the grade.

Definitions

Tolerance Zone: refers to the relationship of the tolerance to basic size. It is established by a combination of the fundamental deviation indicated by a letter and the IT grade number. In the dimension 50H8, for the close running fit, the H8 specifies the tolerance zone.

The hole-basis system of preferred fits is a system in which the basic diameter is the minimum size of hole. For the generally preferred hole-basis system, the fundamental deviation is specified by the upper-case letter H.

The shaft-basis system of preferred fits is a

system in which the basic diameter is the

maximum size of the shaft. The fundamental

deviation is given by the lowercase letter h.

An interference fit results in an interference

between two mating parts under all tolerance

conditions.

A transition fit results in either a clearance or an interference condition between two assembled parts.

Tolerance symbols are used to specify the tolerance and fits for mating parts.

For the hole-basis system ,the 50 indicates the diameter in millimeters; the fundamental deviation for the hole is indicated by the capital letter H, and for the shaft it is indicated by the lowercase letter f.

The numbers following the letters indicate this IT grade.

Note that the symbols for the hole and shaft are separated by the slash.

Tolerance symbols for a 50-mm-diameter hole may be given in several acceptable forms. The values in parentheses for reference only and may be omitted.

Example - Clearance Fit

Example - Transition Fit

Example - Interference Fit

FITS APPLICATIONS

Interchangeability:- When components are mass

produced, unless they are interchangeable, the

purpose of mass production is not fulfilled. By

interchangeability, we mean that identical

components, manufactured by different personnel

under different environments, can be assembled and

replaced without any further rectification during the

assembly stage, without affecting the functioning of

the component when assembled.

Limit Gauges

1. Plug Gauge: It is used for gauging a hole.

Go End

(lower limit of hole)

No- Go End

(Upper limit of hole)

2. Snap Gauge:

Used to Gauge Shafts.

3. Thread Gauge

Taylor states that the 'GO' gauge should check all the

possible elements of dimensions at a time and the 'NO GO'

gauge should check only one element of the dimension at a

time.

'Go' and 'No GO' gauges should be designed to check the

maximum and the minimum material limits of components.

Taylor’s Principle of Gauge Design

Go end No Go end

Least Mat. condition Max. Mat. Condition

'Go' Limit ‘Go' limit is the one between the two size limits

which corresponds to the maximum material limit

the upper limit of a shaft and the lower limit of a

hole

'GO' gauge can check one feature of the

component in one pass

'NO GO' Limit

‘No Go' limit is the one between the two size

limits which corresponds to the minimum material

condition, the lower limit of a shaft and the upper

limit of a hole.

Design of Limit Gauges In practice gauge can not be manufactured to exact

size. Some variation in size can not be eliminated

due to imperfection in process and different

variables.

Some allowance is then provided to gauge maker

which is known as Gauge Maker’s Allowance or

Gauge Tolerance. Limit gauges are usually

provided with a gauge tolerance of 1/10th of work

allowance.

Another allowance is provided to Go side of gauge

only to compensate wear during the checking. It is

known as wear allowance, which is approximately

1/5th of work allowance.

Gauge Tolerance (≈ 1/10th of work tolerance)

Not

Snap Plug

Wear Allowance (≈1/5th of work tolerance)

Not Go

Go

Wear Allowance

Gauge Tolerance Work

Tolerance

Upper Limit

Lower Limit

Measurement of surface

roughness

Profilometer

Portable Surface Roughness Tester.mp4

Specification of surface

roughness

1. Peak-to-valley height method

2. Centre Line Average Method

3. Root Mean Square Method

Ipec limits fits

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