Tolerance, Need for tolerancing, Functional dimensions, Tolerance allocation, Interchangeability, Universal or full interchangeability, Advantages of interchangeable manufacture, Selective assembly
Tolerance:
The permissible variation of a size is called tolerance. It
is the difference between the maximum and minimum permissible limits [sizes] of
the given dimension. This is called tolerance zone. The tolerance permitted on
a given size does not affect the functioning of the part when assembled and put
to use. The variation may be allowed on one side or both sides of the basic
size. When the variation is permitted on both sides of the basic size, it need
not be equally distributed. If the variation is provided on one side of the
basic size, it is termed as unilateral tolerance. Similarly, if the variation
is provided on both sides of the basic size, it is known as bilateral tolerance
as shown.
It may be noted that in bilateral system of limits, same fit
cannot be maintained between the mating parts, whereas, the type of fit is not
changed in the case of unilateral tolerance system, when the dimensions are
permitted to vary. Unilateral system of tolerancing has an advantage over the
other system example in limit gauging Go gauges have the same lower limit [for
hole] and same upper limit [for shafts] for different grades of tolerance. If
the tolerances are fixed very close to the basic size i.e., reducing the
tolerance zone, the cost of production goes up due to manufacture of components
with closer tolerances, as shown as Fig., due to selection of precision
machines, highly skilled operators and close supervision and control.
Need for Tolerancing:
An assembly of components will only function if its
constituent parts fit together in a predictable manner. As it is not possible
to manufacture to an exact size, the designer has to decide how close to the
ideal size is satisfactory.
Cost implications of tolerances:
There are considerable benefits available from being able to
manufacture to close tolerances, but there is also cost involved. The three
main characteristics affecting cost of achieving a given tolerance are:
1.Tolerence size:
The smaller the tolerance, the greater the cost to achieve
that tolerance on a given component. The tolerance may affect the type of
tolerance form, example a sand casting cannot be held to the same dimensional
accuracy as investment casting. The required tolerance is likely to influence
the choice of machine to manufacture the component. Thus the effect of tighter
tolerance is higher cost, likely to require more manufacturing operations, more
intensive inspection and probably higher level of scrap.
2.Component size:
Large components are more difficult to handle and require
bigger, more expensive machines to produce them. Any scrap produced is also
likely to be of higher value.
3.Detail of the feature:
Some features are easier to achieve the given tolerances
than others. Example : External circular features can be easily produced to the
required tolerance levels than internal circular features.
Functional Dimensions:
The addition of dimensions and tolerances to a drawing will
inevitably add to the production and inspection costs of the component. Hence
care should be taken to ensure to define the dimensions and the associated
tolerances to simplify the manufacturing process and keep the costs to a
minimum. This could be achieved by concentrating on functional dimensions. A
functional dimension is one that directly affects the function of the product.
Using functional dimensions as datums, will prevent the use of unnecessary
tight tolerances. Normally other datums will only be chosen if they allow a
simple method of manufacture/inspection.
Tolerance Allocation:
A drawing must contain enough dimensions to ensure that the
component is fully dimensioned, however, it should not be over dimensioned. If
more than the minimum number of dimensions are shown, some will be redundant
and cause confusion, particularly when tolerances are involved.
Tolerance on
the dependant dimension is equal to the sum of the tolerances on all the
dimensions that affect the dependant dimension. The method of dimensioning is a
better method and the auxiliary dimension included in parenthesis is not toleranced.
Interchangeability:
A part which can be substituted for the component
manufactured to the same shape and dimensions is known as interchangeable part.
The operation of substituting the part for similar manufactured components of
the same shape and dimensions is known as interchangeability. In earlier days,
components are produced to small number of units. To obtain desired fit, the
operator has to adjust the mat parts within the permissible limit. But today
the operator is responsible for manufacturing the component and assembly too.
Mass production techniques play an important role for breaking up of several
smaller activities there by leading to specialization. Any one randomly
selected component should assemble with any other randomly selected component.
Due to interchangeability, the increased production can be obtained by
minimizing production cost. In case of big assembly, several parts of that unit
may be produced in different country with respect to the availability of
trained labour, raw material, power and other facilities. But final assembly is
carried out at one place. So, each part should he manufactured under the
concept of interchangeability with other same mating part. Now-a-days, the
interchangeable parts are called as spare parts. The advantages of
Interchangeability are:
• Replacement of worn out parts is easy.
• Repair is carried out easily.
• Maintenance cost is less.
• Shutdown of machines having interchangeable components is
reduced.
Interchangeability
is not possible without using any standards. So, some standards should be
strictly followed. Mainly, two standards are followed such as International
standards and local standards. Local standards are followed for obtaining
spares from other source. At the same time, this standard is also based on
international standards. International standards are mainly used to obtain
Universal acceptance. It can be obtained in two ways namely;
1.Universal or full interchangeability
2.Selective assembly.
Universal or Full Interchangeability:
This means that parts, which go into assembly, may be
selected at random from a large number of parts. In this type of
interchangeability, any component will mat with any other mating component
without doing any minor alterations to mat them. But many times, this interchangeability
is not feasible because high accuracy and closer supervision are required at
each and every time. Full interchangeability is useful where process capability
is equal to or less than the manufacturing tolerance allowed for that part.
Process capability is defined as its ±3σ spread of dimensions of components
produced by the machine.
Where, σ - be a standard deviation.
Advantages of interchangeable manufacture:
• Components can be manufactured in large batches.
• Repair of existing machines or products is simplified
because component parts can be easily replaced.
• During assembly, individual fitting is not necessary.
This results in reduced production cost.
Selective Assembly:
A product's performance is often influenced by the clearance
or, in some cases, by the preload of its mating parts. Achieving consistent and
correct clearances and preloads can be a challenge for assemblers. Tight
tolerances often increase assembly costs because labour expenses and the scrap
rate go up. The tighter the tolerances, the more difficult and costly the
component parts are to assemble. Keeping costs down while maintaining tight
assembly tolerances can be made easier by a process called selective assembly,
or match gauging. The term selective assembly describes any technique used when
components are assembled from sub-components such that the final assembly
satisfies higher tolerance specifications than those used to make its sub-components.
The use of selective assembly is inconsistent with the notion of
interchangeable parts, and the technique is rarely used at this time. However,
certain new technologies call for assemblies to be produced to a level of
precision that is difficult to reach using standard high-volume machining
practices. To match gauge for selective assembly, one group of components is
measured and sorted into groups by dimension, prior to the assemble process.
This is done for both mating parts. One or more components are then measured
and matched with a presorted part to obtain an optimal fit to complete the
assembly. It results in complete protection against defective assemblies and
reduces the matching cost.
Consider the case of bearing assembly on shaft, done
by selective assembly method. Pick and measure a shaft. If it is bit big, pick
a big bearing to get the right clearance. If it is bit small, pick a small
hearing. For this to work over a long stretch, there must be about the same
number of big shafts as big hearings, and the same for small ones. By focusing
on the fit between mating parts, rather than the absolute size of each
component, looser component tolerances can be allowed. This reduces assembly
costs without sacrificing product performance. In addition, parts that fall
outside their print tolerance may still be usable if a mating part for it can
be found or manufactured, thus reducing scrap. Consider the example of a system
in the assembly of a shaft with a hole. Let the hole size be 25±0.02
and the clearance required for assembly be 0.14 mm on the diameter. Let the
tolerance on the hole and shaft be each equal to 0.04. Then, dimension range between hole diameter
[25±0.02
mm] and shaft diameter [24.88±0.02 mm] could be used. By sorting and
grading, the shafts and holes can be economically selectively assembled with
the clearance of 0.14 mm as combinations given as follows. Hole diameter and
shaft diameter pairs are 24.97 and 24.83, or 25.00 and 24.86, or 25.112 and
24.88, etc. Not all products are candidates for selective assembly. When
tolerances are broad or clearances are not critical to the function of the
final assembly, selective assembly isn't necessary. Selective assembly works
best when the clearance or preload tolerance between parts is tight. Selective
assembly is also a good strategy when a large number of components must be
stacked together to form the assembly, as with an automobile transmission
system. In that instance, holding tolerances tight enough for random assembly
while maintaining the correct clearance or preload would be impractical.
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