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Technology of watch production - part 5

REFERENCE SURFACES

Parts which are to be processed on a machine mus t be positioned rela

tive to the cutting tools and must then be held in pos ition by clamping de
vic es .

Selection of the reference s urfaces during the part-design s tage deter

mines how the part will be dim ens ioned. Depending upon the reference

surfaces s elected, a means for positioning and fastening the parts for ma

chining is adopted, and this in turn determines the basic design of the

clamping device .

The reference s urfaces also determine the pos ition o f the part i n the

as s embled product and its interconnection with the other parts :

Thus , the concept of "reference s urface " denotes the totality of surfaces ,

lines or points in elation to which are orientated those surfaces which ad-

join (in the finished product) other c omponents

or thos e s urfaces of a c omponent which are

machined at the given s tage of manufacturing

of said components .

We distinguish between m a i n r e f e r e n c e

s u r f a c e s , g u i d e s u r f a c e s , a n d s t o p

s u r f a c e s .

The pos ition of these s urfaces in a prisma

tic part is shown in Figure 9 .

In o rder to direct (orientate) the part o n the

main reference surface we have to set it on three

points which determine the position of the plane .

After the part is s et on the main referenc e

surface, its orientation along the guide surfac e

A-maln reference ; B-guide; e-stop

is determined by two points .

The part having been s et along the main and

FIGURE

9 .

Reference surfaces on

a prismatic part :

surfaces.

guide s urface s , one point suffices to fix it with

respect to the stop s urface. If orientation of a part in three directions is
not neces sary during ass embly or machining, the number of reference sur

faces reduces to two o r one .

Let it be required to machine the upper s urface and maintain the dimen

s ion a on the part shown in Figure 10, a .

FIGURE

1 0 .

Machining parts according to different

holding surfaces

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FIGURE

1 1 .

Reference surfaces of a cylin

drical part :

1 -double guide surface; 2-stop surface.

In this case the part to be machined is s et on the main reference sur

face A , while the guide and stop surfaces a r e of n o special importanc e .

If, o n the other hand, w e are required t o machine the dim ensions a and

c (Figure 1 0 , b ), we s et the part along the main and the guide reference

surfaces, the stop surface be ing of no spec ial importance.

In thes e examples the c oncept of " refe rence surface " includes one o r

two surfaces of the part, respectively .

The position of cylindrical parts , such as rolle rs o r shafts , is deter

mined by the position of the axis of the external cylindrical s urface in the

horizontal and ve rtical directions , and by the pos ition of an end surface.

It is sufficient to support cylindrical parts on four points of the external

cylindrical surface and on the end, as s hown in Figure 1 1 .

A cylindrical surface along which the p art is s et as in Figure 1 1 is called

a d o u b l e g u i d e s u r f a c e .

T h e s t o p s u r f a c e in this case is Jhe cylinder end.

Let it be required to turn a shoulder on a cylindrical blank (Figure 12 ).

When the blank is clamped, the cylindrical surface of the c ollet serves

simultaneously as main referenc e , guide and stop surface for obtaining the

dim ension 2 . s - 0 . 02m m . The cylindrical surface of the collet als o serves

as the guide surface for obtaining the dimension 8 . 5 - 0 . 1 0 , while the in

ternal shoulder of the collet serves as both the main reference surface and

the stop surface .

FIGURE 12. A cylindrical part held in a collet

Surfaces are class ified, according to their function, as being either

d e s i g n , a s s e m b l y , h o l d i n g o r m e a s u r i n g s u r f a c e s .

T h e d e s i g n s u r f a c e s of the part are the totality of surfaces , lines

or points relative to which the other parts of the product are o rientated in

the designe r ' s calculations .

A n a s s e m b

y s u r f a c e is the totality of surface s , lines or points

relative to which the other parts of the product are actually o rientated .

A h o l d i n g ( t e c h n o l o g i c a l ) s u r f a c e is the totality of s urface s ,

lines or points relative t o which a surface being machined i s orientated

during the manufacture of the part.

A m e a s u r i n g s u r f a c e is that surface (or s et of surfaces ) from

which the dimensions are measured .

Parts processed on machines are set and held in universal o r special

clamping (fastening ) ':!evices .

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Since all the parts of a type are located on the same surface in the same

manner, and are machined identically, their dimensions will be identical

within the process accuracy.

When working with l ocating fixtures , l o c a t i n g e r r o r s appear in

addition to the random errors described earlier.

FIGURE

13. Locating error for different holding surfaces

The following example will s erve to illustrate how the s election of the

holding surface influences the machining accuracy and the design of the

fixtures .

Example, It is required that the dimension

1 .2-0.010 be held in machining the watch plate shown in

Figure

13, a. This is achieved directly by means of the setup shown in Figure 13, a, where the dimension

1 .2_ 0.010 is measured from the holding plane

The variation

the dimension

1.2 in the parts will depend

only on random factors causing scatter in the dimensions.

In Figure

13, b, it is required to hold the dimension 0.80+ 0.01, measured not from the holding plane

but from the plane A .

I f the pan is set o n plane

the dimension

0.8 will depend both on random factors an<i o n the actual

magnitude of the dimension

2 mm which has been machined previously. In this case the dimension 0.8 will

have deviations of a magnitude of

0.01 + 0.02

0.03 mm. In order to increase the accuracy, the tolerance

for the dimension

2.0 mm would have to be narrowed. This, however, would render the machining more

complex and cost! y.

The best solution in the given case is to set the part on plane A .

This example shows that, all other conditions being equal , the highest

accuracy is achieved when the plane from which the dimension in question

is m easured is the holding surface, that is when the m e a s u r i n g and the

h o l d i n g s u r f a c e s coincide.

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Technolo gical processes are accordingly preferably designed s o that

the reference and design surfaces coincide . It is also des irable to use the
same s urface for as many operations as pos sibl e . The transition from one

surface to another is always bound up with suppleme ntary errors . The

selection of the holding surfaces must not only ensure the required machin

ing accuracy, but must als o allow the reliable fastening of the part in the

fixture.

The calculation of the locating error as a function o f the l ocating m ethod

for different cases is given in the specialized literature .

A LLOWANCES A ND INTEROPERATIONAL TOLERANCES

The metal layer which must be removed in an operation or suboperation

in order to obtain the specified dimension is called the a

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o w a n c e f o r

t h e g i v e n o p e r a t i o n o r p a s s .

The value of the allowances specified depends both on the method used

to make the premachined blanks and on the machining process in question. The

more accurate the blank, the smaller the machining allowance required.

Larger allowances are allowed for roughing operations than for finishing

operations .

The question of the magnitude of machining allowances is of great tech

nical and economical importance because the metal removed by machining

is as good as lost, s ince the value of s havings is much smalle r than that

of the o riginal metal stock.

In addition, the removal of an exces s ively large allowance requires time ,

thus lowering the productivity and the capacity of the equipment, and raising

tool- wear and power -expenditure rates .

Excessively large allowances are thus s een t o reduce the economic ef�

fectivenes s of the process .

Excessively small allowances are not desirable either, since they make

the s etting and adjustm ent of the parts on the machines more difficult and

require higher accuracy in the blanking operations , increasing their cost.

Machining allowances specified must therefore be as s mall as is poss ible

under the given process conditions .

Long production experience has made it possible to fix ave rage allowances

for blanks obtained by various methods or being prepared for various ma

chining p rocesses and thes e can be found in the handbooks or factory spe

cifications , o r can be calculated .

The allowances for machining external and internal cylindrical surfaces

are given on the diameter (for both side s ). The allowances on external

planes and end surfaces are given for one s ide; and their num erical values

are accordingly half as large as thos e for corresponding cylindrical surfaces .

The total allowance for the machining of a surface i s the sum o f the al

lowances for all the various operations .

In order to ensure that one operation will not remove the minimum al

l owance intended for the next operation, o p e r a t i o n a l t o l e r a n c e s a r e

s et on the nominal value of the allowance for each operation, and thus the
maximum and minimum values for each operational allowance are fixed.

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