Page 64 - Robot Builders Source Book - Gordon McComb

2.3 How to Determine the Productivity of a Manufacturing Process 53

However, in some cases the necessary information must be obtained experimen-
tally. Take bronze aneroids assembled by soldering (Example 3): the heating and cooling
times required to ensure proper coating with tin in automatic plating of the mem-
branes cannot be calculated analytically, with the necessary accuracy. The only way to
get reliable results is to carry out experiments under conditions closely approximat-
ing the machine under design.
Similarly, in the tin-plating of printed circuits after the electronic items have been
mounted on the bases (Example 4), the time of exposure of the circuits to the tin wave
as well as the cooling time of the circuit have to be determined experimentally.
The timing diagram indicates where time can be saved to increase productivity. In
Figure 2.14 the auxiliary time intervals denoted byr serve to prevent collisions between
tools or between kinematic elements and when precisely defined can be reduced.
Reducing the auxiliary times can be a very effective way of raising productivity. Another
time-saving device is to reduce operation time intervals. For instance, the rate of wire
input can be increased, thereby reducing the time fjj however, this entails increasing
the driving power of the feeding rollers. Similarly, we can shorten the strokes of the
punch 7, tools 5, etc., reducing t 3 and t 4 correspondingly. However, this makes it nec-
essary to apply additional driving power, which entails higher accelerations and decel-
erations and, in turn, heavy dynamic loads on parts in executing Ihe desired
movements. Thus, the timing diagram brings us to the next step in the design proce-
dure, namely, designing the kinematics of the automatic machine, or the kinematic
layout. The layouts for cyclic and continuous manufacturing processes are different.
The advantages of continuous processes were discussed in Section 1.4 of Chapter 1.
Example 2 illustrates a continuous process. Note, however, that certain processes
involve a mix of concepts. For instance, Example 2 (manufacturing of cylindrical
springs) illustrates the combination of cyclic mechanisms (wire cutting tools; Figure
2.4) and the continuous process of feeding the wire and bending it into a spring. The
pitch-controlling mechanism involved in the production of spiral springs of variable
pitch is also cyclic. It must be mentioned here that at the stage of manufacturing-
process design we are concerned neither with the means which carry out the dis-
placements, nor with speeds, forces, sequences, and durations; we just define what
these parameters should be and estimate their values. It is at the kinematics stage that
we deal with how to achieve our desired objective.
There are two ways to draw the timing diagram: one we have already discussed in
the above example of chain manufacture and we can call it the linear approach; it is
the one given in Figure 2.14. The other is to use the circular approach.
As one might expect from its name, this diagram is circular in shape. It is conve-
nient to use inasmuch as it graphically illustrates the breakdown of the time period
into specific operations, auxiliary actions, etc.; see the diagram for a washing machine
in Figure 2.15. However, the disadvantages of this kind of timing diagram are as follows:
• It is difficult to render displacements, speed changes, temperature changes, etc.
In fact, such diagrams are generally used to show on-off actions.
• Because of the different diameters of the circles that make up the diagram, the
arcs corresponding to equal angles are of different length. This psychologically
disturbing feature interferes with evaluation of the diagram.
After the duration of the sequence of operations has been in some way determined
TEAM LRN
theoretically or experimentally, we may still conclude that the production output is