A machining center is a highly automated machine tool capable of performing multiple machining operations under computer numerical control (CNC) in one setup with minimal human attention. Workers are needed to load and unload parts, which usually takes considerable less time than the machine cycle time, so one worker may be able to tend more than one machine. Typical operations performed on a machining center are milling and drilling, which use rotating cutting tools.
The typical features that distinguish a machining center from conventional machine tools and make it so productive include:
- Multiple operations in one setup – Most parts require more than one operation to completely machine the specified geometry. Complex parts may require dozens of distinct machining operations, each requiring its own machine tool, setup, and cutting tool. Machining centers are capable of performing most or all of the operations at one location, thus minimizing setup time and production lead time.
- Automatic tool changing – To change from one machining operation to the next, the cutting tools must be changed. This is done on a machining center under CNC program control by an automatic tool-changer designed to exchange cutters between the machine tool spindle and a tool storage carousel. Capacities of these carousels commonly range from 16 to 80 cutting tools.
- Pallet shuttles – Some machining centers are equipped with pallet shuttles, which are automatically transferred between the spindle position and the loading station. Parts are fixtured on pallets that are attached to the shuttles. In this arrangement, the operator can be unloading the previous part and loading the next part while the machine tool is engaged in machining the current part. Nonproductive time on the machine is thereby reduced.
- Automatic part positioning – Many machining centers have more than three axes. One of the additional axes is often designed as a rotary table to position the part at some specified angle relative to the spindle. The rotary table permits the cutter to perform machining on four sides of the part in a single setup.
Machining centers are classified as horizontal, vertical, or universal. The designation refers to spindle orientation. Horizontal machining centers normally machine cube shaped parts, in which the four vertical sides of the cube can be accessed by the cutter. Vertical machining centers are suited to flat parts on which the tool can machine the top surface. Universal machining centers have workheads that swivel their spindle axes to any angle between horizontal and vertical.
The success of CNC machining centers led to the development of CNC turning centers. A modern CNC turning center, is capable of performing various turning and related operations, contour turning, and automatic tool indexing, all under computer control. In addition, the most sophisticated turning centers can accomplish part gaging (i.e. checking key dimensions after machining), tool monitoring (sensors to indicate when the tools are worn), automatic tool changing when tools become worn, and even automatic part changing at the completion of the work cycle.
Another type of machine tool related to machining centers and turning centers is the CNC mill-turn center. This machine has the general configuration of a turning center; in addition, it can position a cylindrical part at a specified angle so that a rotating cutting tool (e.g., milling cutter) can machine features into the outside surface of the part. An ordinary turning center does not have the capability to stop the part at a defined angular position, and it does not possess rotating tool spindles. Further progress in machine tool technology has taken the mill-turn center one step further by integrating additional capabilities into a single machine. The additional capabilities include combining milling, drilling, and turning with grinding, welding, and inspection operations, all in one machine tool; using multiple spindles simultaneously, either on a single workpiece or two different workpieces; and automating the part handling function by adding industrial robots to the machine. The terms multitasking machine and multifunction machine are sometimes used for these products.
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