The single-station automated cell consists of a fully automated machine that can operate unattended for a time period longer than one machine cycle. The operator must load and unload the machine, and otherwise tend it, but is not required to be at the machine except periodically. See Figure 12.7. Advantages of single-station automated cells include:

·     Relatively inexpensive to implement and maintain compared to other automated manufacturing system types.

·     Production rates re higher than manned machines.

·     It can represent the first step towards implementing an integrated multi-station

automated system.

Auxiliary equipment and supporting machines may also be found in single-station automated cells.

The single-station automated cell consists of a fully automated machine that can operate unattended for a time period longer than one machine cycle.

Enablers for Unattended Cell Operation

Two categories of enablers can be deduced, one for unattended single model and batch model production, and one for mixed model production. The enablers for single model and batch model production are given in Table 12.4.

Table 12.4: Enablers of unattended cell operation in single model and batch model production

The enablers for mixed model production are recounted in Table 12.5; these are additional enablers required because f the unique traits of mixed model production.

Table 12.5: Enablers of unattended cell operation in mixed model production

Enablers for unattended cell operation include (for all models): a programmed cycle, a parts storage subsystem, the automatic transfer of work parts, periodic attention of worker, and built-in safeguards; and (for mixed models): a work identification subsystem; a programme downloading capability; and a quick set­up changeover capability.

Parts Storage Subsystem and Automatic Parts Transfer

The automated cell can theoretically operate unattended for a length of time given by:

where UT is the unattended time of the operation of the manufacturing cell; T_cj is the cycle time for part j that is held in the parts storage subsystem, for j = 1, 2, 3, ..., n_p, where n_p is the parts storage capacity of the storage subsystem, pc. This equation assumes that a work unit is processed each work cycle, and—if all of the parts are identical and require the same machine cycle—we can further simplify the equation to:

In reality the unattended time will be less than this amount; this is because the worker will need time to completely unload and to reload the machine for the start of the next work cycle.

The length of time that the automated cell can theoretically operate unattended can be calculated by considering storage capacity and the cycle time of the automated cell in question.

Generally, the time of unattended operation increases directly with storage capacity, so this should be taken into consideration in automated cell design. The objectives of designing sufficient storage capacity include:

·     Allowing a fixed time interval for an operator to tend a number of machines.

·     Scheduling time o allow for multiple tool changes during the same machine downtime.

·     Allowing a time period of one complete shift to occur.

·     To allow for unattended, over-night operation.

The minimum storage capacity of a parts storage system is one workpart. When the storage capacity is only one workpart, this usually means that the operator must be in full-time attendance. The overall cycle time of the single station with no storage is:

where T_c is the overall cycle time; T_m is the machine processing time; and T_s is the worker service time, typically required to load/unload the machine, or other tending duties. The overall cycle time of the single station with one part storage is:

T_c = Max{T_m  T_s}+ T_r

where T_r is the repositioning time to (1) move the completed workpart away from the machine workhead, and (2) to replace and position the next workpart in its stead. Machine utilization is high if the worker service time is less than the machine processing time; if, on the other hand, the machine processing time is less than the worker service time, then the machine will go through periods of forced idleness. This should be avoided.

The time of unattended operation increases directly with storage capacity.

In scenarios where the storage capacity is greater than one workpart, unattended operation is feasible when load/unload tasks are accomplished in less time than the machine processing time. Table 12.6 depicts several possible designs of parts storage subsystems for CNC machining centres.

Table 12.6: Parts storage subsystem designs for CNC machining centre

If storage capacity is greater than one workpart, unattended operation is feasible when load/unload tasks are accomplished in less time than the machine processing time. This often requires innovative parts storage system design.