Factory floors, and the activity that happens there, simply put, will either make or break a manufacturing enterprise. In our Throwback series we have discussed how excellent customer service and reduced inventory costs can be achieved with sound techniques, and cooperation among and between the teams involved, even though their individual goals may conflict.
These customer service and inventory techniques will not be successful unless the same cooperation and overall compromise for the good of the enterprise extends to where “the rubber meets the road”, the factory floor.
It is critical that every manufacturer makes effective use of its assets. The investment in production capacity is often significant. Think about how much it costs to set up a factory; the production line with all its machinery and technology. Another good way to look at how efficiently a business operates is to look at “productivity”.
Productivity measures the relationship between inputs into the production process and the resultant outputs.
Productivity can be measured in several ways: e.g.
- Output per worker or hour of labor
- Output per hour / day / week
- Output per machine
- Unit costs (total costs divided by total output)
The unit cost measure is particularly important. A falling ratio would indicate that efficiency was improving.
Why is achieving high productivity important?
- Most importantly, a more efficient factory will produce lower cost goods than competitors. That means the business can either make a higher profit per unit sold (assuming that the product is sold for the same price as a competitor) or the business can offer customers a lower price than competitors (and still make a good profit) thus increasing volume.
- Investing in production assets (e.g. equipment, factory buildings) is expensive therefore a manufacturer must maximize the return it makes on these assets
An example of Tracking Production Efficiency
Courtesy: WorkWise, Inc.
What makes a manufacturing shop floor manager happy? A stable schedule and lead time “flexibility." There is an age old question, “Is lead time a yard stick, or a rubber band?"
Many ERP systems assume the former when in reality, in the world of machinery, skilled laborers, and management “priority” changes, “stuff” happens.
We have discussed in other articles how engineering techniques such as product configuration and operational techniques like Lean/Kanban can improve efficiencies. But at the end of the day, the Master Schedule must be translated into a production plan that is reasonable and has enough elasticity in it so that your key production can use their skills and experience to adjust on the fly.
For purposes of this discussion let’s stipulate that our planning system has provide a high level production plan with dates and quantities and due dates.
The science and “art” of translating that plan into a detail daily production schedule involves such things as operational scheduling, queue time compression, and resource assignment.
Operations scheduling is the process whereby a production requirement with either a due date or start date is translated into a step by step schedule in which each operation and its associated details such as machinery and labor skill required is considered.
Scheduling Methods
A number of methods can be used to schedule shop orders.
Some commonly used scheduling methods are:
In the above descriptions, the term queue time and its use is discussed. Queue time has had a clouded past and at times a bad reputation, primarily because of abuse and over use. It is our contention that although some production environments can drastically reduce or eliminate queue time, there are still many products and processes that require its use and we feel that it is better to educate our people on the proper use and measurement of queue times than to pretend they are not needed, and not fool the system, but fool yourself.
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.
The following is a basic example of the routing queue compression:
Summary
There is another old expression 'Plan your work, and work your plan' that applies to this discussion.
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 and the results will be huge work in process inventories, and longer lead times.
Remember; allow that rubber band to stretch when needed!