manufacturing aspect.pdf

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SME 2713MANUFACTURING

ASPECTSAssoc Prof Zainal Abidin Ahmad

Dept of Manufacturing & Industrial Eng

UTM JB

Chapter 2

18-Jan-08 Assoc Prof Zainal Abidin Ahmad 2

Outlines Specification and standardisation for production

Introduction

Specification

Standardisation

Manufacturing of mating parts

Making to suit

Selective assembly

Interchangeable manufacture

Tolerancing

Limits and fits


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Specification and standardisation for

production - Specification

Definition (related to invention) : Part of a patentapplication, the specification is where the inventor specifies,describes, illustrates, and discloses the invention in so muchdetail that an experienced person could understand and usethat invention. The description, together with the claims, isoften referred to as the specification. As this word suggests,these are the sections of the patent application where youspecify what your machine or process is and how it differs fromprevious patents and technology.

A specification is an important part of your patent applicationand (later if granted) part of the legal definition of yourpatent. The specification must be described using clear detailswith full disclosures about the invention. Drawings are included

in the specification when required.



production - Specification Based on BS 7373 : 1991

Definition a statement of the attributes of aproduct, process or service

The attributes may be descriptive or requirements,depending on the use to which the specification is put. Theattributes may be descriptive of a product or service,identifying for the user or prospective user its likely fitnessto fulfill their purpose. If however, the purpose of thespecification is to establish a basis for conformity to design

or to manufacture, the attributes become requirements The attributes should be measurable and the limits of

acceptance and compliance with the specification shouldbe already defined.


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production - Specification

Discuss the importance of specification, in the

context of product manufacturing

Discuss how specifications are specified or

developed.



production - Standardisation

standardization - the condition in which astandard has been successfully established;"standardization of nuts and bolts had saved industrymillions of dollars"

standardize - cause to conform to standard ornorm; "The weights and measures were standardized

standardize - evaluate by comparing with astandard


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Standardization (or standardisation) is the process ofagreeing on standards, which are (usually voluntary, written)agreements on technical specifications that define parametersand properties of products (goods and services). The goals ofstandardization can be to help with or ensure independence ofsingle suppliers (commodification), compatibility,interoperability, safety, repeatability, or quality.

In the context oftechnologies and industries,standardization is the process of establishing a technicalspecification, called a standard, among competing entities in amarket, where this will bring benefits without hurting

competition




Refer Producibility principle in Chapter 1 -

Introduction to Mfg Processes

Discuss the importance of standardisation,

usage of standards with respect to product

manufacturing

List various Standard Bodies world wide

SIRIM

??

??


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Manufacturing of mating parts Most products consist of a few or many

components assembled of fixed together.

Give some examples of products please.

The problem of how to make the parts to givethe required assembly has three solutions;

Making to suit

Selective assembly

Using a system of limits and fits enablinginterchangeable manufacture


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Selective assembly - example


Selective assembly - example Selective assembly approach is generally used

where two conditions prevail;

The part cannot be made economically to the

required accuracy but can readily be measured

and graded

The assembly is replaced as a complete unit when

necessary, not repaired by replacing individual

parts.


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Selective assembly - example

Typical applications Ball and roller bearing manufacture

Cylinder bores and pistons

Pistons and gudgeon pins in motor & car engines.

Although selective assembly overcomes some of theproblems of making to suit i.e. allowing economicproduction methods, in most cases, unskilledoperators, the work involved in measuring, grading,storing in graded batches and selecting for assemblyall add to the cost of the finished assembly.


Interchangeable Manufacture Ideally a method of production is required in which

The most economic methods of manufacturing the part can

be used.

All parts are completely interchangeable, i.e. if one bin

contains 1000 shafts and another 1000 holes, any pair of

parts selected at random will go together to make a

satisfactory assembly

All assemblies produced will be uniformly acceptable. There

will be some variation in the fits produced but the amountof variation will have been predetermined by the designer

so that the assembly will function correctly throughout its

design life.


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Interchangeable Manufacture

Compared with making to suit or selective assemblythe advantages of such a system, especially withrespect to quality is enormous.

It is not even necessary to measure the parts. Limitgauges can be used to ensure that the parts arewithin the size limits fixed by the designer.

Skilled workers is not required, semi skilled operatorscan do the job. Part can be assembledinstead offitted,i.e they will not need final adjustments of a

skilled worker in order to produce satisfactoryassemblies.


Interchangeable Manufacture Assemblies so made can be serviced by a simple

system of replacement parts drawn from stock. This isconvenient for the user and is cheaper thanreconditioning involving the manufacture of newparts to special sizes.

Interchangeable manufacturing also means parts canbe made in widely separated localities and then bebrought together for assembly, where the parts will fittogether properly. This is an essential element of mass

production. Without interchangeable manufacturing, modern

industry could not exist.


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Interchangeable Manufacture

Before a system of interchangeable assemblycan be operated, certain fundamentalconditions must be met

The permissible variation (tolerance) of eachdimension must be fixed.

The mating condition (clearance, transition,interference) of each pair of parts assembled mustbe decided.

These conditions are generally satisfied by useof a system of limits and fits.


Tolerancing In order to ensure that assemblies function properly

their component parts must FIT TOGETHER in a

particular way.

No component can be manufactured to an exact size

(called the nominal or basic size), so the designer has

to decide on appropriate upper and lower limits for

each dimension Refer Fig 2.2)

Accurately toleranced dimensioned features usuallytake much more time to manufacture correctly and

therefore can increase production costs significantly.


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Tolerancing

Fig 2.2


Tolerancing


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Tolerancing

Good engineering practice fines the optimum balancebetween required accuracy for the function of thecomponent and minimum cost of manufacture. Thiswill depend upon the application and functionalrequirements. A manufacturer of childrens tricyclewould soon go out of business if the parts were madewith jet-engine accuracy, as no one would be willingto pay the price.

For cases where accuracy is not important a generalrange may be shown on a company drawing, e.g. Alldimensions to be within 0.2 mm unless otherwise

stated.


Tolerancing


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Dimension tolerances If a dimension is specified in millimeters, as 10 0.02. the part

will be acceptable if the dimension is manufactured to anactual size between 9.98 and 10.02

Below are some examples of ways of defining such limits for alinear dimension.


TOLERANCING - Definitions The basic size or nominal sizeis the size of shaft or hole that the

designer specifies before applying the limits to it. The basic sizeis the same for both members of a fit

The upper limitis the largest size allowed

The lower limitis the smallest size allowed

The toleranceis the difference between the upper and lowerlimit

Where variationeither size of the nominal dimension canoccur, the tolerance is called bilateral. Where one tolerance is

zaro the tolerance is said to be unilateral. Deviationis the algebraic difference between a size and the

corresponding basic or nominal size.


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TOLERANCING - Definitions Fitdescribes the working condition between a mating shaft

and hole.

Clearance shaft always smaller than hole allows movement

Transition may provide either clearance or interference, e.g. keyand keyway

Interference shaft always bigger than the hole provide fixing

International tolerance grade(IT) are numbers which for aparticular IT number have the same relative level of accuracybut vary depending upon the nominal or basic size

Hole basisis a system of fits relating to a basic or nominal holesize, i.e. starts with the basic hole size and adjusts shaft size tofit.

Shaft basisis a system of fits relating to a basic or nominal shaftsize, i.e. starts with the basic shaft size and adjust hole size to fit


Limits and FitsWhen parts are assembled together, engineers

have to decide how they will fit togetherand

the economicsassociated with it.

How they will fit together?

Clearance fit

Transition fit

Interference fit

Economics?

Interchangability

Standards

BS4500ANSI B4.1


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Limits and Fits - Definitions

Tolerance is the difference between the maximumlimit of size and the minimum limit of size.

Fit expresses the relationship between a mating

parts with respect to the amount of clearance or

interference which exists when they are assembled

together.

Hole - designate all INTERNAL features of a part,

including parts which are not cylindrical.

Shaft - designate all EXTFRNAL features of a part,

including parts which are not cylindrical.


Limits and Fits - Definitions Upper deviation - difference between the maximum

limit of size and the corresponding basic size. This isdesignated ES' for a hole and 'es' for a shaft.

Lower deviation - difference between the minimumlimit of size and the corresponding basic size. This isdesignated EI' for a hole and 'ei' for a shaft.

Grade of Tolerance - Group of tolerances with thesame level of accuracy for all basic sizes.

Clearance - difference between the size of the hole

and shaft (positive) Clearance - difference between the size of the hole

and shaft (negative)


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Limits and Fits - Definitions


Unilateral Tolerance

The upper and lower deviations are all positive

or all negative. 033.0030+hole 040.0 092.030

shaft


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Bilateral Tolerance

006.0

015.030+

hole

013.0

008.030+

shaft

The tolerance is split above and below thebasic size


Fundamental Deviations (BS4500)

The 27 deviations for HOLES are:

A B C CD D E.EF F G H J S J K M N P R ST U V X Y Z ZA ZB ZC

The 27 deviations for SHAFTS are:

a b c cdd e ef f g h js j k m n p r s t u v x y zza zb zc


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Fundamental Deviations (BS4500)

The 27 deviations for HOLES are:

A B C CD D E EF F G H J S J K M N P R S T UV 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 zazb zc


FundamentalDeviations

Grade 7 tolerance

zone for the

diameter range

6-10 mm

holes

shafts


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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.


Grades of Tolerances


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Limits and Fits Designation

A hole tolerance with deviation 'H' and tolerance gradeIT7 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. 4SH7-p6 or 45H7/p6.


Three Classes of Fit Clearance fit - A fit provides a clearance. The

tolerance zone of the hole is entirely above

that of the shaft.

Transition fit - A fit provides either a clearance

or an interference. The tolerance zones of the

hole and the shaft overlaps.

Interference fit - A fit provides an interference.The tolerance zone of the hole is entirely below

that of the shaft.


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Hole Basis System of Fit

Associating various shafts with a single hole

ISO - the lower deviation of the hole is zero


Hole Basis System of Fit More commonly used because holes are usually made

with standard tools such as drills and reamers, etc.

It is easier to manufacture shaft to the tolerancevalues and measure those values


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Shaft Basis System of Fit

Associating holes with a single shaft

ISO - upper deviation of shaft is zero


Shaft Basis System of Fit Less commonly used

Preferable when a shaft may have to accommodatea variety of accessories such as couplings, bearings,collars, gears, etc. - constant shaft diameter withvarying bores of accessories to obtain different typesof fits.


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Example - Clearance Fit


Example - Transition Fit


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ISO Fit (size 6-180 mm)

Easy running - H7/e8

Normal running - H7/f7

Slide - H7/g6

Location - H7/h6

Push - H7/k6

Light press - H7/p6

Heavy press - H7/s6

Shafts and

exposed

bores

should

have

chamfers


ISO Fit (size 6-180 mm)


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Example



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