2.3.1 Production Rate
Prior to determining production rate, one must determine the operational cycle time of a process. The operational cycle time includes all time element activities involved in producing one part.
Figure 2-0 Milling process example
Consider the machining process operation in Figure 2-0. This particular operation involves drilling two holes and milling a slot into the part shown. The operational cycle time is the time from the start of processing a part to the point at which the next part is started. The time elements for this operation include loading and unloading the part into the machine, machining the part, and changing tools as needed. Thus, the operational cycle time for this process can be given as
tc = to + twh + tth
where,
| t c | = | operational cycle time, expressed in min/part |
| t o | = | time of actual processing, expressed in min/part |
| t wh | = | workpiece handling time, expressed in min/part |
| t th | = | tool handling time, expressed in min/part. |
The actual processing time (to) and workpiece handling time (twh) occur for each part processed. The tool handling time (tth), however, may not occur for each part. Perhaps a tool can process 100 parts before it needs to be changed. Thus, the time it takes to change the tool must be divided or averaged over those 100 parts.
The above equation is valid not only for machining type processes but also for assembly, molding, or almost any type of discrete manufacturing process. Consider the following examples.
Example 2.3
The following table lists the steps for a machining process. The times listed are those needed to load, unload, and process one part. Calculate the operational cycle time (tc).
Solution
The governing equation is
tc = to + twh + tth.
From the table above, combine steps 2 and 4 to determine the total processing time per part (to). Thus,
to = 2 min/part + 3 min/part = 5 min/part.
The work handling time (twh) is a summation of results in steps 1, 3, and 5:
twh = 0.75 min/part + 0.5 min/part + 0.25 min/part = 1.5 min/part.
To determine the tool handling time (tth), find the average time it takes to change tools over the 20 parts:
tth = 5 min/20 parts = 0.25 min/part.
Thus the operational cycle time becomes
tc = 5 min/part + 1.5 min/part + 0.25 min/part = 6.75 min/part.
Example 2.4
An injection molding machine processes an 8-cavity mold in 2.6 min per cycle. The parts are automatically ejected from the mold and travel by conveyor to the next process. Every 200 cycles the mold is cleaned and sprayed with mold release. This takes 15 min to complete. Calculate the operational cycle time (tc).
Solution
The governing equation is
tc = to + twh + tth.
First, calculate the actual processing time per part (to). Recognize that the process produces 8 parts each cycle and that each cycle takes 2.6 min. Thus,
to = (2.6 min/cycle)(cycle/8 parts) = 0.325 min/part.
Since the parts are automatically ejected from the mold, the workpiece handling time is zero:
twh = 0.
Mold cleaning and reapplication of mold release is equivalent to changing a tool in a machining process. Thus, the time it takes to accomplish these tasks needs to be averaged over each part produced in those 200 cycles:
tth = 15 min/[(200 cycles)(8 parts/cycle)] = 0.0094 min/part.
Therefore, the operational cycle time becomes
tc = 0.325 min/part + 0 + 0.0094 min/part = 0.334 min/part.
Consider a process manufacturing system. In this system parts are produced in batches or lots. Each time a part is to be produced the machines that produce the part must be set up to process that particular part. This setup time needs to be captured in the production rate calculations. Therefore, for process manufacturing systems the time to process a batch of parts is calculated and then converted into average production time. The equation to calculate the batch processing time is
tb = tsu + Qtc,
where
| t b | = | batch processing time (min) |
| t su | = | time to set up machine to produce batch (min) |
| t c | = | operational cycle time per part (min/part) |
| Q | = | number of parts in batch (parts). |
The average production time then becomes
tp = tb /Q,
where
tp = average production time (min/part)
Q = number of parts in batch (parts).
Once the cycle time of the process is known, the production rate can be calculated. Note that the production rate depends on the manufacturing system employed. Hence, the variable R is used to represent the average production rate for the various processes discussed. The average production rate for a process can be determined by taking the reciprocal of the average production time:
Rp = 1/tp,
where
Rp = average production rate (parts/min)
tp = average production time (min/part).
Note that it is often more desirable to express the average production rate (Rp) in units of parts/hr.
Example 2.5
Calculate