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Technology of watch production - part 8

T i m e f o r r e s t b r e a k s

(T br )

is established only for hard o r other

wis e s trenuous work. In all other cases only the time required for the

s atisfaction of natural needs is taken into account .

The sum of the basic machine time and the auxiliary time

( T

+ T

) is

called the o p e r a t i o n a l t i m e .

The technical - s e rvicing time , the o rganizational - s ervicing time and the

break time are calculated from norms established in dependence on the
type of work and the size and the design of the machine .

In order to simplify the calculations , the norms are pres ented as coef

ficients relating these time costs to the operational time :

Tt. s. = � (Tb + Ta ),

To.s.

� (Tb + Ta),

Tb, = r <Tb + Ta l,

whe re

�.

= are the coefficients taken from the corresponding norm s .

Thus , the piece norm can be express ed a s (allows :

The time norm should not include worktime losses stemming from

excessive machining allowance, increased hardne s s , etc. , nor should it

include time losses resulting from rej ects o r other organizational - technical

factors .

�

T h e c a l c u l a t e d t i m e defined as the total time required to p roduce

a part or a batch of parts can be expres s ed by the fo rmula

+ Ts. c.

p. c. = p

n '

whe re

Ts. c

= the s etup and concluding time;

= the number of parts in the batch.

The calculated time for a batch of parts is thus e qual to

T h e s e t u p a n d c o n c l u d i n g t i m e is the tim e spent by the ma

chine operator on familiarizing hims elf with the work and on preparing and

s etting up the machine and the tools and fixtures required, plus the time

spent on removing the tool and fixtures after the whole batch of parts has

been proces s e d .

The s etup and concluding time is calculated f o r the batch as a whole and

it does not enter into the piece norm . The size of the batch has no influence

on the magnitude of the s etup and concluding time .

I n mass production, and usually i n large - s eries production a s well, the

organization of the workplace is s uch that the machine is preset and the
m aterials , blanks , fixtures , tools and everything els e necessary to do the

job, are prepared in advance at the workplace .

I n such cases the s etup and concluding time i s not taken into account

at all in the machining norm .

The s etup and concluding time is c alculated from plant s tandards in

dependence on the type of p roduction, the work organization, the size of

the machines , the means of setting the fixtures and parts and the number

o f tools participating, etc.

By reducing the number of the individual components of the technical

norm , one increases the productivity. The basic machine time, for instanc e ,

can be reduced b y imposing the m o s t advantageous cutting conditions and

by simultaneously p rocessing s everal surfaces .

The auxiliary time can b e reduced by combining positionings and s etups,

liquidating unnecessary setups tolerated for any reasons , and by reducing

the time taken by any s etup .

In watch manufacture the auxiliary times constitute a considerable frac

tion (of the total machining time ], and the best way to reduce them is to

combine operations and to us e automatic machines .

TECHNICAL INSPECTION

Parts , assemblies or finished products which deviate for any reason

from the drawings or the p erformance specifications must b e regarded as

rejects .

Rejected parts and ass emblies c annot be used in the manufacture of the

p roduct, and rej ected p roducts cannot be put on the m arket.

Rej ects cause great damage to the plant and to the national economy,

leading as they do to increas ed expenditure m aterial s , unnecessary time

losses in p roduction, and breakdowns in the fulfilment o f the program.

An incessant struggle is accordingly waged agains t rejects and tho s e

responsible for rejects b e a r administrative and m aterial responsibility for

them .

E ve ry plant has its technical inspection department (TI D ) charge.d with

o vers e eing the quality of the plant ' s production at all stages , studying the
causes l eading to rejects , and formulating m easures to prevent their ap

pearanc e .

The TID s taff inspects t h e materials u s e d b y the basic and the auxiliary

(tool, m aintenance ) s hops . They are also charged with checking all measur

ing instruments and tools used in production.

Distinction is m ade between operation inspection, s ampling inspection,

and final inspection. In s ome cases the process conditions are inspected

as well .

p e r a t i o n i n s p e c t i o n is conducted after each operation for p arts

which undergo s everal operations .

Operation inspection is also carried out for important ass embly opera

tions .

I n s a m p l i n g i n s p e c t .i o n a fixed percentage of the parts in a batch

is inspected, and the results of the inspection are c onsidered as applying

to the entire batch.

T h e i n s p e c t i o n o f p r o c e s s c o n d i t i o n s is a preventative

m easure for operations where the conditions directly influence the final

process result (such as heat-treatment operations ).

The inspection of process c onditions consists mainly in observing instru

ment readings .

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In f i n a l i n s p e c t i o n , important finished parts or finished products

(completely assembled) are thoroughly inspected and tested, and the degree

to which they correspond to the drawings (in the case of parts ) or specifi

cations (in the case of products ) is ascertained .

When the inspection has been c ompleted, a certificate is issued attesting

to the quality of the accepted products .

One of the main objects of production inspection is the prevention of

rejects .

The prevention of rejects is of particular importance in mas s -production

operations becaus e the damage caus ed by rejects is particularly s evere in

such processes .

In high- output mass -production operations , such as c old s tamping or

turning on automatic lathes , the continuous operational inspection is fre

quently more labo

consuming than is the process its elf .

I t is , therefore, usual i n such operations t o inspect only s ome of the

parts rather than all of them . This is called s t a t i s t i c a l i n s p e c t i o n .

Statistical inspection, based on the methods of mathematical statistics,

makes it possible to determine the quality of an entire batch on the basis

of a limited number of inspected parts .

Statistical inspection is pos s ible only if the machining process is inhe

rently stable . By means o f s tatistical inspection techniques the quality

can be checked during production and accumulation of rejects is thus pre-

vented.

�

If the spread of dimensions conforms to the law of normal dis tribution,

the dimensions of 9 5 % of the parts in the batch may differ from the average

by no more than

±2 a ,

and the dimensions of 9 9 . 7 % of the parts by no more

than

± 3 a

(see above ).

Therefore, should the m easured dimensions of several of the parts in

spec-ted differ by more than

3 a

from the average, this would mean that the

s tability of the process has been ups et, and that a systematic error leading

to rejects has appeared .

Reject zone

Inspection zone

Acnon zone

zone

8 .9

No. of batches

FIGURE 17. Control chart

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Upper specification limit

Upper control limit

Line of central tendency

Lower control limit

A change in the machine setting, the blunting of the cutting tools o r the

dis turbance of the working c onditions c ould lead to such systematic errors .

The alarm signal thus given permits the timely detection and elimination

of the cause of the disturbance thus preventing the mass production of reject s .

I n statistical inspection, the inspector measures, a t definite time inter

vals , a small number of parts (sample ), taken from the machine in the

course of its operation.

He then plots the results on a control chart (Figure 17 )

where the X

axis  gives  the  sample  number,  and  the Y-axis  the  value  of the  average for
the  sample  of the  magnitude  measured .  The  line  of  central  tendency is
drawn  through  the middle  of the diagram  corresponding to the average  value

of the measured magnitude .

A t a distance o f

o n either s ide o f this line the upper and lower c ontrol

limit lines are drawn . Lines corresponding to the upper and lower speci

fication limits are added (beyond the control limits ) .

The field o f the diagram enclosed b etween the control limits i s called

t h e i n s p e c t i o n z o n e .

The s e ctors enclosed between the upper and lower specification limits

and their corresponding c ontrol limits are called the a c t i o n z o n e s .
Beyond the specification limits are what are known as the r e j e c t z o n e s .

If the points plotted by the inspector lie within the limits of the inspection

zone, the process proceeds normally and the dimensions of the parts are

within the range limits

(±3cr

) .

If points fall i n the action zone , a factor has appeared which disturbs

the normal course of the proces s . Although th e specification limits have

not yet been exceeded and there are the refore no rejects , the dis turbing

factor must be s ought out and eliminated in order to p revent the poss ible
production of rejects .

Points falling outs ide the specification limits indicate the complete break

down of the process and nec essitate the immediate stoppage of work.

The m ethod of statistical inspection desc ribed is called t h e c o n t r o

1 -

c h a r t m e t h o d f o r- a v e r a g e s i z e .

Other methods of s tatistical inspection exist, such as the method of the

c o n t r o l c h a r t f o r r a n g e �' .

The statistical inspection methods are des cribed in greate r detail in the

specialized literature.

Statistical inspe ction m ethods considerably reduce the size of the inspec

tion staff a plant requires , shorten the production cycle (the parts pas s
directly from operation to operation), reduce measuring-instrument outlay

(especially for gages ), and makes available to the engineer valuable material ,

in the form of control charts , for the analysis of the technological process .

On the basis of thes e charts , norms for the economic accuracy of equipment

and tool - cost norms are es tablished .

•

[In American practice, average-size and range-control charts are usually combined and are not considered

separate methods. )

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