Selecting the number of SPM workstations

In order to achieve the feasible layouts for SPMs, it is important to identify the following information:

 The number of surfaces to be machined on the specific workpiece.

 The number and type of machining features to be created on these surfaces.

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 The number of SPM workstations, and the number/type of SPM machining units.

To clarify how these points affect the determination of SPM layouts, the workpiece shown in Figure 3-4 is taken as an example.

Figure 3-4. A designed cylinder head for motorcycles.

The top surface of this workpiece contains two M6 tapping features and four 12 mm reaming features. In this case, a drilling operation is needed as a first step, and then tapping is performed. Therefore, two workstations are needed: the first station is for drilling the two holes, and the second station is for making the taps.

The same procedure is followed for the reaming operation and two workstations are needed: the first station is for making the four holes, and the second station is for performing the reaming operation for each hole. In total, and for the top surface of this workpiece, an SPM layout of four workstations is needed to perform the tapping and reaming operations: the first station is for drilling the two 6 mm holes, the second station is for making the two M6 taps, the third station is for drilling the four 12 mm holes, and the fourth station is for reaming each of the four holes as shown in Figure 3-5. Another layout for these operations can also consist of four workstations but in a different order: the first and the second stations are for drilling, the third station is for tapping, and the fourth station is for reaming (see Figure 3-6).

55 Figure 3-5. The workstations that are needed to perform tapping and reaming for the

selected workpiece.

Figure 3-6. Two sequences for the same machining operations for the selected workpiece.

The main feature of SPMs in performing several machining operations at one time is considered in defining the number of workstations. Therefore, the rules for selecting the number of workstation are as follows:

- If a drilling operation only is required to be performed on a specific surface of a workpiece, then only one workstation is needed. In this case, the number and type of the machining units that are used depend on the number and size of the holes required on this surface. The position of the machining units depends on the surface set up of the workpiece.

56 - If a tapping operation is required to be performed on a specific surface of a workpiece, then two workstations are needed. The number and type of the machining units in both stations depend on the number and size of the taps required on the surface.

- If a reaming operation is required to be performed on a specific surface of a workpiece, then two workstations are needed. The number and type of the machining units depend on the number and size of the holes required on the surface.

- If there is more than one machining operation to be performed on the same surface of the workpiece (e.g. drilling and tapping), then the number of the workstations is equal to the sum of the number of stations required for both machining operations.

For each of the rules mentioned above, more specified rules were created in this work to cover the maximum number of possible cases for each operation. For example, and for the drilling operation, the rules are extended as follows:

If drilling is required and only one hole is created on the surface, then one workstation is needed. The type of the machining unit required for this case is determined based on the rules that are created for the drilling operation for one hole in SPMs.

Else, If there is more than one hole on the surface with a similar size, then one workstation is needed. In this case, the rules for selecting the multiple spindle heads are followed to determine the pattern and the size of the holes, and to select suitable machining units.

Else, If there is more than one hole on the surface and those holes have different sizes, then a workstation is needed for each size. In this case, the sizes of the holes need to be determined in order to select suitable machining units.

End If

57 Figure 3-7 shows the process of defining the workstations for the drilling operation. In order to perform the drilling operation effectively, it is important to achieve a suitable number of and sequence for the workstations by selecting the most suitable machining units and other SPM elements. The same procedure is followed for tapping and reaming operations.

Figure 3-7. The process of selecting the number of workstations for the drilling operation in SPMs.

The selection of the number of workstations starts by defining the number of the machined surfaces on the workpiece. After that, the types of machining operations on each surface are determined, and then the number of the workstations is determined for each machining operation as shown in Figure 3-8.

58 Figure 3-8. The selection process of the number of workstations required in SPMs.