MACHINING CENTRES – CNC
History
The first machining centres emerged with the computerization of machines during the 1980s, making them Computer Numerical Control (CNC) machines. This is a broad category of machines whose specificities, capabilities, and roles can vary significantly.
Operating Principles
Generally speaking, a machining centre autonomously performs a sequence of predefined operations. Depending on the machining centre, raw material can be loaded in the form of a rectangular plate or as a bar, similar to a lathe. The digitized plan of the component to be manufactured and the list of necessary tools are input into the machine’s program. The CNC centre proposes the optimal sequence of operations, which the operator can adjust if needed. The various required tools (mills, drills, grinding wheels, etc.) are placed in a magazine within the machine, in positions corresponding to the operation sequence designated by the program. After the machine is programmed and the tools and material are loaded, it’s ready to operate, requiring the operator only to start the sequence. The CNC centre then carries out the machining operations, automatically switching tools as necessary. Most machining centres primarily perform milling operations, while machines loaded with bar material can also perform turning operations. Depending on the complexity of the machine, a machining centre may have one or two spindles (rotating tool holders).
Axes
The spindles of a machining centre are designed to move along different axes, the number of which can vary, starting from three-axis. The first three axes of a machine are linear (X, Y, and Z, or width, depth, and height). Five-axis machines have three linear axes (X, Y, and Z) plus two rotational axes for the spindle (longitudinal and vertical axes). Six-axis machines add a third rotational axis (lateral axis). Additionally, the arm holding the tool spindle can itself move along one or more axes, resulting in machines with seven, nine, or even twelve axes. It’s important to note that in many cases, the machine’s various axes are divided between the spindle and the table (the chassis where the plate is secured). For example, the spindle may move along the rotational axes while the table moves along the linear axes.
“Reprise” Machines
Some machining centres are capable of working on both sides of the component to be manufactured. These are known as “reprise” machines. For machines using the material in plate form, the machining centre will complete all operations on the first side, then flip the plate to continue working on the other side. Machines using bar material perform operations on one side of the bar with a specific spindle, after which the bar is gripped by a second spindle positioned 180° opposite the first. Then the bar is cut off, released from the first spindle, and further operations can be performed on the reverse side of the component. Reversal machines are much more complex due to the need for perfect positioning and centering of the component on the machine after the reversal.
Robotic Loading
Most machining centres can optionally be equipped with robotic loaders (for bars or plates), providing the machine with almost unlimited autonomy.
Eligible Materials
All machinable materials are eligible for machining centres (metals, plastics, synthetic corundums, composites, etc.).
Advantages, Disadvantages, and Applications
Machining centres are versatile, precise, and autonomous. However, they require relatively long programming and setup times, making them more suited to large production runs. In watchmaking, they can be used for the manufacture of almost all movement components, cases, and bracelets, as well as for the production of tools.