Manufacture of prismatic components like gear boxes, bulkheads, frames, brackets, casings, covers etc require milling, boring, drilling, tapping and many other related machining operations to be performed. Before the introduction of machining centers, these machining operations had to be carried out on different machines resulting in considerable lead time for production. Machining centers are very important types of CNC machine tools and are multifunction machines equipped with automatic tool changers and are capable of carrying out milling, drilling, reaming, tapping, boring, counter boring and allied operations without operator intervention for change of tools. Tool changing is carried out using an automatic tool changer and is accomplished in 0.5 to 6 seconds depending upon the machine.
An indexable tool magazine, which can store several tools, characterizes a machining center. The tool magazine may carry 16 to 100 tools depending upon its capacity. An automatic tool changer (ATC) is provided to pick up the programmed tool from the tool magazine and mount it on the spindle. The removed tool is put back into the magazine and the ATC picks up the next tool. The ATC is thus ready with the tool for the next operation and awaits the current operation to be over to replace the tool.
Machining centers are often provided with two or more work tables called pallets. In a 2-pallet machine while the job on one pallet is being machined, the operator can set up the next job on the free pallet. The automatic pallet changer (APC) then moves away the pallet with the finished job from the working zone and moves the other pallet with the new workpiece to the working zone. The machining operation on the new workpiece begins with a lapse of only a few minutes.
The operator can now unload the finished job from the first pallet and set up a new workpiece on it without interruption of the machining operations.
There are machining centers with six, eight or more pallets. Workpieces can be set up in all pallets and the machine can be programmed to accept a new pallet when work on the previous pallet is completed. The workpieces may be different and may require different part programs. The supervisory computer will do the selection of the right program. Thus the two major sources of non-productive time (tool change and setting up the workpiece) are reduced to very low values. This enables machining centers to have high productivity as well as considerable reduction in the lead-time of production of complex components. This also reduces work in process (WIP) appreciably.
Machining centers are classified according to the spindle configuration as:
a. Horizontal spindle machining centers
b. Vertical spindle machining centers
c. Universal machining centers
Horizontal Spindle Machining Centers
Horizontal spindle machining centers are generally single spindle machines with automatic tool changers. Some exceptions are machines with turret type tool magazine, combination horizontal/vertical spindles etc. Horizontal spindle machining centers are generally bed type machines with the structural configuration as shown in Fig. 12.11.
In this type of machining X-axis table or column provides traverse and spindle head provides Y-axis traverse. The saddle or column, or headstock or spindle head provides the Z-axis traverse. The axes of machining center are also shown in Fig. 12.11. These machines are invariably used with a rotary indexing table to facilitate multiphase machining at different angles in a single setup. The axis of rotary table is parallel to Y-axis and is called ‘B’ axis.
The rotation of the table can be used for machining contours on surfaces of workpieces located on the rotary table, if CNC control is available for B-axis. The provision of B-axis enables the machining of all the four sides of the component in one set up. Use of modular fixtures on horizontal machining centers permits machining of more or more components in one set up, thereby increasing productivity and reducing machining cost.
The present day trend is to build the machines with traveling column construction with all the movements behind the tool so that the work module is independent of the basic machine. This enables choice of work holders to suit the user’s requirements and also enables the use of machining center in NC transfer lines.
Vertical Spindle Machining Centers
Figure 12.12 shows a typical vertical spindle machining center.
Vertical spindle machining centers are also bed type machines with:
1. Single spindle and automatic tool changers
2. Multi spindle with turret head (Turret machining centers)
- X-axis traverse provided by table or column
- Y-axis traverse provided by the saddle or the column or ram
- Z-axis traverse provided by the headstock
Vertical spindle machines are not suitable for large widths as this increases the throat distance. For very large widths horizontal spindle configuration or bridge type configuration (double column planer type) is used.
Universal Machining Centers
These are similar to horizontal machining centers but with the spindle axis capable of tilting from horizontal to the vertical position continuously under computer control. This constitutes the fifth axis of the machine. In some cases tilting of the table instead of the spindle provides the movement. Such machines facilitate access to the top surface of workpiece mounted on a horizontal machining center so that all the five sides of a component can be machined in a single set up. The fifth axis facility is essential for machining of some components, which require the cutter axis to be perpendicular to the surface being machined. Machining centers can be easily integrated into a flexible work cell. Provision of bi-directional random memory tool magazines, automatic tool offset measurement, magazine changers, advanced tool management techniques, tool breakage detection techniques, rail guided work transport, palletizing stations, in-process gauging, devices for automatic workpiece loading and unloading etc., are useful options to improve productivity and to realize flexibility in operation and to achieve unmanned operation of machining centers.