Production times of machining processes can be shortened by combining multiple operations onto one machine, where each operation can require different tools. This is possible by using tool turret magazines or for CNC lathe machines.
The tool turrets allow the machine to carry multiple cutting tools at the same time and automatically deliver a sequence of tools to the cutting position based on the process plan. This is done by turrets, rotating along a vertical axis switching from a tool placed on one of the turret's index (stations) to successive tools mounted on another index (station). The time it takes for a turret magazine to rotate from one tool slot to another one is known by tool-indexing time.
Since cutting operations can not be carried out during the time when the turret is rotating the turret-indexing time is hence called non-machining time. The eciency of machining processes can be further improved by minimizing the non-machining time and increasing the time spent in cut, which results in shorter cycle time. This is possible allocating the cutting tools on indexes of turret magazine in an optimal order to reduce the number of unit-rotations during a process.
The industrial case used in the present paper1, is the machining operations on crankshafts for 4-cylinder engines, called OP30 (operation 30) carried out at an automotive engine plant. The CNC machining center is a two axes lathe, performing in total of 19 turning operation using 15 dierent cutting tools on two circular bi-directional turret magazines each with a holding capacity of 45 tools, with the indexing time of approximately 0.2s, which can ultimately machine 1304 crankshafts. An illustration of a turret magazine with 8 indexing stations and the turret magazine used in OP30 is shown in Figure 1.
Figure 1: (a) Tool indexing on a 8-index turret magazine; (b) the tool turret magazine with 45 index positions used in OP30
The existing allocation of cutting tools on the indexes in the factory is based on experience of the production and tooling engineers, and will machine the 1304 crankshafts with 58398 unit-rotations. However, by using a proposed genetic algorithm with customized solution representation (encoding-decoding) and novel genetic operators and rearranging the positions of 36 out of 45 cutting tools, the same number of crankshafts can be machined with only 19560 unit-rotations, reducing the non-machining time from 9 to 3 seconds per crankshaft. The results are summarized in Table 1.
1K. Amouzgar, A. H. Ng, and G. Ljustina. Optimizing index positions on cnc tool magazines considering cutting tool life and duplicates. Procedia CIRP, 2020
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