Increase Production Efficiency – Part 2

Queue Time

Queue time is the normal length of time a job must spend waiting at a work center before work is done on it. This includes the wait time after completion of the prior operation, move time and wait time at the operation. There are several reasons for planning queue time such as an extremely expensive machine like an NC Machining Center being used for a critical operation and if that machine runs out of work is sits idle at a cost of 10’s of thousands of dollars. In addition, and on rare occasions, an operation for a specific item can have a queue time value. For example a new operation or employee may need some buffer time until the bugs are worked out.

Total queue time between work centers is a combination of this work center table queue time plus the queue time on the operation record. Normally, the operation queue time is zero. The combination thus yields the standard queue time. If a work center has a standard queue of 5 days, but on a particular operation, the queue time is to be reduced to 2 days, the operation queue time would then be set at -3.

In cases where the previously scheduled operation for the order used over one-half of the hours in the last period in which it was scheduled, one additional day is added to the queue time for the next operation.

Obviously, there is a tremendously important balance between granting the shop a schedule that provides flexibility so efficiency is high and generating a schedule which causes customers to leave because we are not delivering fast enough.

Queue times can actually be used to improve production efficiency by automatically compressing if a past due situation is imminent.

Queue Compression

Queue compression is designed to aid manufacturers in keeping shop orders on time after they are initially created/scheduled. As time progresses, queue compression is used as a pre-processor to backward schedule by adjusting queue time on a particular shop order so the original due date can still be met. As queues are being adjusted, operation priorities are raised as well.

Shop order queue time that consumes a high percentage of routing time can be adjusted/fine-tuned automatically as time progresses to prevent or eliminate past-due situations. Priorities will be set higher on operations that have queue consumed.

When active orders get behind schedule, a Queue Compression algorithm can lower queue and raise priority of the remaining operations. This ensures critical operation dates are met.
Shop Floor Control provides other scheduling tools to allow the scheduler to monitor the queue time and loaded capacity at each work center on an exception basis.

Backward/One Path Routing Example Queue Compression Summary

Queue will be compressed as time forces the shop order past due. This automated option will then adjust the shop order to meet its due date. Further compression will continue as time goes on until the minimum queue for the work center is reached and/or if the shop order is totally compressed.

Queue compression is an automatic priority manager that attempts to prevent a past due situation from occurring as well as a warning source to the scheduler using alarm mail messages for inactivity.

Summary

If we expect the shop to execute efficiently to the standards that we use to quote delivery dates to our customers, we must ensure that those standards include real-world provisions to adjust to the dynamics of the factory floor.

If we choose to ignore these realities, the factory will adjust only outside the system, which results in a huge amount of work in process inventories, and longer lead times.

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