Classification of Machining Centers
Machining centers are commonly classified according to the spatial orientation of their spindles into vertical machining centers, horizontal machining centers, gantry machining centers, and universal machining centers. A vertical machining center has its spindle positioned vertically in space, while a horizontal machining center has its spindle positioned horizontally.
(1) Vertical Machining Center: The structure is mostly fixed column type, with a rectangular worktable, suitable for machining disc, sleeve, and plate-type parts. Vertical machining centers typically have three linear motion axes: X, Y, and Z axes, and a fourth axis (A axis) can also be installed on the worktable. Advantages and disadvantages: Vertical machining centers are easy to clamp, operate, and observe the machining process, and the program is easy to debug. However, due to the influence of column height and tool changing mechanism, they cannot machine very tall parts. Vertical machining centers have a simple structure, small footprint, and low price.
(2) Horizontal Machining Center: Typically uses a moving column, with the spindle head between two columns, moving up and down along the guide rail. Horizontal machining centers typically have three linear motion axes: facing the machine tool, left and right movement is the X axis, forward and backward movement is the Z axis, and up and down movement is the Y axis. A fourth axis (A axis) can also be installed on the worktable. Horizontal machining centers can machine helical, cylindrical cam, and other parts. Advantages and disadvantages: Horizontal machining centers are inconvenient to observe during program debugging and trial cutting, and difficult to monitor during machining. Part clamping and measurement are also inconvenient, but pin removal is easier during machining. Compared to vertical machining centers, horizontal machining centers have a more complex structure, occupy a larger area, and are more expensive. (3) Gantry machining centers: Their spindles are mostly vertically set, equipped with an ATC system and replaceable spindle head accessories. The system software has many functions, allowing for multiple uses and making them suitable for machining large parts.
(4) Universal machining centers: Universal machining centers, also known as five-sided machining centers, have the functions of both vertical and horizontal machining centers. After a workpiece is clamped once, all sides and top surfaces except the mounting surface can be machined.
Common universal machining centers include:
- The spindle can rotate 90°, allowing it to work like a vertical machining center or a horizontal machining center.
- The spindle does not change direction, and the worktable rotates 90° with the workpiece to complete the machining of all five surfaces. In summary, based on the number and function of the worktables, machining centers can be categorized into single-worktable machining centers, dual-worktable machining centers, and multi-worktable machining centers.
Structural Components of a Machining Center
(1) Basic Components: These are the foundational structures of the machining center, consisting of the bed, column, and worktable. They primarily bear the static load of the machining center and the cutting load generated during machining, thus requiring sufficient rigidity. These large components can be cast iron or welded steel structures; they are the largest and heaviest parts in the machining center.
(2) Spindle Components: These consist of the spindle box, spindle motor, spindle, and spindle bearings. The spindle’s start, stop, and speed changes are controlled by the CNC system, and it participates in the cutting motion through the tools mounted on the spindle, serving as the power output component for cutting. The spindle component is a key component of the machining center, determining its machining accuracy and stability.
(3) CNC System: The CNC part of the machining center consists of a CNC device, a programmable logic controller (PLC), a servo drive device, and an operation panel.
(4) Automatic Tool Changer: This system consists of a tool magazine, a robotic arm, and a drive mechanism. When a tool change is needed, the CNC system issues a command, and a robotic arm (or other means) removes the tool from the tool magazine and inserts it into the spindle hole. This solves the problem of automatically storing, selecting, transporting, and exchanging tools between processes in continuous multi-process machining after a single workpiece clamping. The tool magazine (tool holder) is a device that stores all the tools used in the machining process. Tool magazines come in disc and chain types, with capacities ranging from a few to hundreds of tools. The structure of the tool arm varies depending on the relative position and structure of the tool magazine and spindle, such as single-arm and double-arm types. Some machining centers do not use tool arms and instead achieve tool changing directly through the movement of the spindle box or tool magazine.
(5) Auxiliary devices include lubrication, cooling, chip removal, protection, hydraulic, pneumatic, and detection systems. Although these devices do not directly participate in the cutting motion, they play a crucial role in ensuring the machining efficiency, accuracy, and reliability of the machining center, and are therefore an indispensable part of the machining center. (6) APC Automatic Pallet Exchange System Some machining centers use multiple automatic exchange worktables to store workpieces in order to achieve further unmanned operation or further shorten non-processing time. While one workpiece is being processed on the worktable, another one or several worktables can also load and unload other parts. After the processing of the parts on one worktable is completed, the worktable is automatically exchanged to process the new parts. This can reduce auxiliary time and improve processing efficiency.
Main Machining Objects of a Machining Center
(1) Box-type parts: Box-type parts generally refer to parts with one or more hole systems, internal cavities, and a certain proportion in the length, width, and height directions.
(2) Complex curved surfaces: Complex curved surfaces are difficult or even impossible to complete using ordinary machining methods.
(3) Irregularly shaped parts: Irregularly shaped parts are parts with irregular shapes, and most require multi-station mixed machining of points, lines, and surfaces, such as shift forks.
(4) Disc, sleeve, and plate-type parts: Disc, sleeve, or shaft-type parts with keyways, radial holes, or distributed hole systems on the end face, such as flanged bushings, keyway or square-headed shaft parts, and plate-type parts with many holes, such as various motor covers. Vertical machining centers are suitable for disc-type parts with distributed hole systems on the end face and curved surfaces, while horizontal machining centers are suitable for those with radial holes. (5) Parts produced periodically: When machining parts with a machining center, the time required mainly includes basic time and preparation time. Among them, preparation time accounts for a large proportion, such as process preparation, programming, and trial cutting of the first part. These times are very long. Using a machining center can store the data of these times for repeated use later. In this way, these times can be saved when machining the same part in the future. The production cycle can be greatly shortened.
