Vertical Spindle Type

For a vertical spindle machining center, the most suitable type of work is flat parts, either mounted to a fixture on the table or held in a vise or chuck. Some multi-face machining of small parts can be done on a CNC vertical machining center equipped with a rotary table, Fig.36-2.

Horizontal Spindle Type

The horizontal machining center is used when the workpiece requires machining on two or more faces in a single setup. These machines are used widely in the automotive and aerospace industry, Fig. 36-3.

Parts of CNC Vertical Machining

Centers

The main parts of a vertical CNC machining center are the bed, saddle, column, table, servo motors, ball screws, spindle, tool changer, and the machine control unit (MCU), Fig. 36-4.

Bed - The bed is usually made of high-quality cast iron that provides for a rigid machine capable of high precision and heavy-duty machining. Hardened and ground ways on the bed provide rigid support for all linear axes.

Saddle - The saddle, mounted on the hardened and ground bedways, provides the machining center with the Y axis linear cross movement towards or away from the column.

Column - The column, mounted to the saddle, is designed to prevent distortion and deflection during machining. The column provides the machining center with the Z axis linear movement (up and down).

Table - The table, mounted horizontal on the bed, provides the machining center with the X axis linear movement (left and right).

Servo system - The servo system, provides fast, accurate movement and positioning of the XYZ axes slides.

Spindle - The spindle fastened to the column, provides the cutting tool with movement (up and down) along the Z axis.

Tool changers - The tool changer is capable of storing a number of preset tools that can be automatically called for use by the part program.

MCU - The MCU (machine control unit) allows the operator to perform a variety of operations such as programming, machining, diagnostics, and tool and machine monitoring.

Workholding Devices

When a workpiece is set up, it is important to ensure that the setup is safe. The workpiece must be securely fastened, and the setup must be rigid enough to withstand the forces that are present during the machining. If the workpiece or the holding device becomes loose during machining, damage can result to the tooling and/or the machine.

The machine operator should be sure that all workholding devices are free from chips and burrs before using. The workholding devices, generally specified by the programmer, should be located in the proper position on the machine table. Failure to follow these instructions may result in operator injury, damage to the machine, or scrapped workpieces.

Types of Workholding Devices

The CNC machining center and the conventional milling machine use similar workholding devices and cutting tools. A variety of cutting tools and workholding devices can be found in the Section on Milling Machines.

Cutting Tools

The selection of the proper cutting tools for each operation on a machining center is essential to producing an accurate part. Generally not enough thought or planning goes into the selection of cutting tools for each particular job. The CNC programmer must have a thorough knowledge of cutting tools and their applications in order to properly program any part.

Machining centers use a variety of cutting tools to perform various machining operations, Fig. 36-5. These tools may be made of high-speed steel, cemented carbide inserts, CBN (cubic boron nitride) inserts, or polycrystalline diamond insert tools. Some of the more common tools used on machining centers are end mills, drills, taps, reamers, and boring tools.

Tooling Systems

The machining center, a multifunction machine tool, uses a wide variety of cutting tools to perform various machining operations on a workpiece. For these cutting tools to be inserted and removed quickly and accurately from the machine spindle, they must all have the same taper shank. The most common taper used in CNC machining center spindles is a self-releasing taper, Fig. 36-5.

The toolholder must also have a flange or collar for the tool-change arm to grab, and a stud, tapped hole or some other device for holding the tool securely in the spindle by a power drawbar or other holding mechanism.