A machine tool can be considered to be a complex assembly consisting basically of the following:
1) Power units.
2) Speed control units for both work and movements.
3) A means of controlling the line of movement of the tool and /or work.
4) Tool or work movement actuating mechanisms with control on the amount of movement.
5) Tool and work holding devices.
6) A structure linking and aligning the individual units together, so that the tool and work are maintained in a controlled relationship(1).
Power units
There are three types of power units required:
1) For driving the main spindle (cutting speed).
2) For driving the saddle or carriages (feed)
3) Providing power for ancillary services.
The motors are of two kinds:
1) Electrical. Alternating current (AC), Direct current (DC) or stepper motors.
2) Fluid. Hydraulic or pneumatic motors.
Speed control units
The variation in speed of DC motors is obtained by varying either the armature voltage or the field voltage. The output signal generated by the pulse generator or tacho-generators is compared with the input value of the voltage to be applied to the DC motor, which represents the desired speed, and any difference between these voltages is used to vary the voltage to obtain the required speed.
Control of translational (line) movements
A slideway is used to control the direction, or line of action, of the translational movement of the carriage or table on which the tools or work are held.
As a safety measure, microswitches are positioned near the ends of the slideway. If the microswitches are activated (closed) by contact with the carriages, the power to the feed motors will be switched off. This will prevent the carriages traveling too far and striking the leadscrew bearing end housings, or the nut screwing off the leadscrew(2).
Slideways commonly used on machine tools have a number of different forms, such as cylindrical, vee, flat and dovetail.
Control of rotational movements
In a similar way as the translational movement of the carriage is controlled by the slideways, rotational movements of spindles are controlled by circular bearings. The accuracy of the roundness of the rotation is dependent on the quality of the bearings.
Actuating mechanisms
Of all the constructional that make up a numerically controlled machine tool, the efficiency and responsiveness of the actuating mechanisms (the drive unit) have the greatest influence on the accuracy of the work produced.
The actuating mechanisms for the carriage for tool or work found on numerically controlled machine tools are: screw and nut; rack and pinion; and ram and piston.
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