The success of any manual machining exercise is dependent on many factors, not least of which is the experienced worker’s practical skills. These skills are most in evidence when they affect the accuracy of the finished product, such as when they are involved in positioning, via the machine slides, the cutting tool and workpiece in the correct relationship to each other. This aspect of machining skill is also a crucial factor when the machine is electronically controlled.
Slide movement on computer numerically controlled machines is achieved by:
- hydraulically operated pistons
- electric servo motors attached to screw drives
- magnetic linear motor drive systems.
The use of electric motors is by far the most common technique. The engine is either directly coupled, or connected via a toothed belt drive, to the slide leadscrew. The servo motor, in effect, replaces the conventional handwheel and this is illustrated in Figure 1.5, which shows classic machines, a center lathe, and a vertical milling machine, fitted with servo motors. A few machine designs have retained handwheels as an aid to set up or to provide for both numerical and manual control.
Machine tools have more than one slide, and so the slide required to move will have to be identified. The plane in which movement can take place may be longitudinal,
Conventional center lathe fitted with servo motors.
transverse, or vertical. These planes are referred to as axes and are designated by the letters X, K, Z, and sometimes U, V, W. Rotary axes A, B, and C can also be applied to a machine around a center axis mentioned previously. A rotary axis has as its centerline one of the three standard axes (X to A, Y to B, and Z to C). Their location on standard machine tools is shown in Figure 1.6. Note that the Z axis always relates to a sliding motion parallel to the spindle axis.
The direction in which a slide moves, in rotary motor drive systems, is achieved by the direction of rotation of the motor, either clockwise or counterclockwise and the movement would be designated as plus or minus about a given datum. The direction of travel is designated on standard machine tools. Hydraulic or linear drive systems produce motion through a push or pull (linear force) activation on one side or the other of the machine slide. Slide movement and relative tool and work movement occur from the push or pull. Slide movement and corresponding tool and work movement are discussed in more detail in Chapter 6.
The rate or speed at which slide movement takes place, expressed in feet/meters per minute or inches/millimeters per revolution of the machine spindle, will be proportional to the revolutions per minute of the servo motor; the higher the revolutions per minute, the faster the rate of slide travel.
Conventional milling machine fitted with servo motors.
On rotary drives the length of slide movement is controlled by either the number of revolutions or the number of part revolutions the motor is permitted to make, one complete revolution being equal to the lead of the leadscrew, in the same way as one turn of a handwheel is similar to the point of a leadscrew. In some cases, there may be reduction pulleys or gears between the motor and the leadscrew, as shown in Figure 1.7, in which case the linear movement obtained concerning the motor revolutions would be proportionally reduced. The length of travel made, or required to be made, by a slide is referred to as a parallel dimension. In the case of linear drive mechanisms such as hydraulic or electromagnetic drives, movement is not dependent on a rotation and accuracy of a screw. In these drives, progress is tracked through a measurement device such as a precision linear scale or laser feedback system.
Since the slide movement is caused by the servo motor (or drive system), control of that motor (or drive system) will in turn control the slide movement. The drive is controlled electronically via the machine control unit. All the relevant information, that is the axis, direction, feed rate, and length of movement, has to be supplied to the control unit in an acceptable numerical form. The input of information to the machine controller is achieved in a variety of ways: in the past perforated tape or magnetic tape was used. Today input is achieved via a direct computer link, computer disk, or manually entered.