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Operation chaining is the method used to force two or more successive operations on the same routing to always be scheduled to run together on the same machine. As an example suppose that we have a routing which declares operations 80, 90, 100, and 110 as a chain. In general when PROSPAC's scheduler schedules operation 80, it successively creates schedules for operations 90, 100 and 110 so that together they form an unbroken, uninterrupted sequence of four operations on whatever machine operation 80 is scheduled. A necessary condition for operations to be chained together is that they must have a common alternate machine. The most often observed case is when two operations are chained. Such operations are usually bi-symmetrical or "mirror images" of each other. For example operation 50 calls for a "left side" operation, while operation 60 calls for the corresponding identical "right side" operation on the same part. Another rather common case is where the same machine can perform two or more quite different operations on the same part and save most or all of the setup in doing so. Suppose we have a routing with operation 20 drill hole(s), operation 30 countersink hole(s), and operation 40 chamfer holes(s). A six-spindle Burgmaster might be setup with the necessary tool inserts on each of three spindles. Once the part is secured in the holding fixture, you've got the machine "centered" to probably best perform all three operations one right after the other on the same piece without removing it from the holding fixture. Although the scheduler produces the same kind of result for both cases above, note that the first case is where the entire work order quantity is scheduled first through operation 50, then the entire work order quantity through operation 60. However in the second case the machine operator performs operations 20, 30 and 40 one right after the other on the same piece before performing the same three operations on the next piece. The scheduled result will look as if the full quantity of the work order goes through operation 20, then through operation 30, and then operation 40, even though the machine operator won't run them that way. In the end the schedule for the chained operations as a group are the same regardless of which way the operator actually perform the operations. The usual motivation for chaining operations is to save most or all of the setup for the succeeding operation(s) in the operation chain. However there is sometimes a second motivation for chaining operations: quality assurance. In these cases setting up a machine or process to consistently produce parts within required tolerances or acceptable quality limits may be technically difficult. Multiple operation chains may be declared in terms of "nests" or "clusters" of successive operations on the same routing. For example it may be that a routing has three different groups of operations to be chained: operations 40 and 50, operations 120 and 130, and operations 300, 310 and 320. These operation "clusters" are designated by arbitrary, but unique chain indices: e.g., operations 40 and 50 are assigned chain index 7, operations 120 and 130 are assigned chain index 2, and operations 300, 310, and 320 are assigned chain index 5. The chain indices serve no other function that to identify which operations belong to which chain "cluster". |
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