If a third-party, single rotary mechanism is placed on a 3-axis milling machine, it becomes a 4-axis machine. The most popular dedicated 4-axis machines are the horizontal types shown in Figure 2-38.

Figure 2-38: 4-axis horizontal machining center.

These machines are mostly used for tombstone work, where parts are clamped to all sides of the tombstone fixture and machined by rotating them into different positions. The chips don t collect on the work-piece because they fall away by gravity and are cleaned off by strategically-placed coolant nozzles.

The example in Figure 2-38 shows a pallet changer, which is positioned outside the machine s enclosure, allowing the operator to load workpieces and unload finished parts during the machine cycle. Elaborate pallet changer assemblies are also available, with multiple tombstones on which a variety of different jobs can be preloaded and made ready-to-run. This arrangement allows for quick changeover to a new job without stopping the machine.