Using Overlap with Next Step

Overlap is a method to attain a more realistic and accurate lead time by reducing the calculated lead time (for a definition of lead time, see “lead time” below) based on the assumption that, in a series of sequential processes, the next process can begin before the last process is finished.

NOTE: Each step is compared to the next step in a Bill Of Material (BOM). The time to complete one step doesn’t change, but the determination of when to begin a step in relation to the previous step has a significant effect on the total manufacturing time for a BOM.

The use of overlapping is optional. You can flag any or all processes within a Bill Of Material (BOM) as overlapped; however, you can use overlapping only within a given BOM level. Each individual assembly or subassembly lead time can be calculated using overlap, but it is not extended to the entire BOM.

For example, the last process of a subassembly can’t be overlapped with the first step of the next assembly or subassembly in the process.

Example: The BOM calls for painting 60 pieces of wood. Each piece requires 10 minutes to sand and an additional 10 minutes to paint. You need 20 hours to complete the process if you sand each one, and then once you complete the 60th piece, you begin painting each one. If you sand one piece and pass it on to be painted (overlap), the entire process can be reduced to 10 hours and 10 minutes.

Lead Time

Lead Time is the amount of time required from the point of ordering or the point of entering the production line to the completion of the order. Each requirement of an order, such as a given raw material, subcontracted process, or labor or machine process, has a point in time where it should be introduced into the flow of the order to ensure an on-time delivery of the product. If materials are made available too early, there may not be room for them, and they may simply add congestion to the process. If brought in too late, delays may result. The problems are the same for labor, machine use, and subcontracting.

Lead time is in terms of hours. It assumes a perfect world in which all materials, machines, and personnel are available when needed. It indicates the number of hours until t the order will be finished after the materials are brought to the floor or the process begins.

Lead time is calculated by first establishing the process time of each individual process requirement, adding the queue time, setup time (uses labor setup time or machine setup time, whichever is longer), wait time, and move time.

To calculate the lead time, the system then starts at the top of the bill and works its way down, calculating the hours of lead time required at each step by accruing the process time for each step. Lead time is only calculated for processes. It is then assigned to materials, based on which routing step the material is assigned to. If the Bill of Material is more than one level deep, the lead times of the first level become the starting point of lead times for the second level, and so on. Overlap factors come into play here, which can complicate things considerably if being used.