lesson Nineteen

Lesson Nineteen
N/C machines that use an open loop system contain no feedback signal to ensure that a machine axis has traveled the required distance .That is ,if the input received was to move a particular table axis 1.000 in.the servo unit generally moves the table 1.000 in. There is no means for comparing the actual table movement with the input signal,however. The only assurance that the table has actually moved 1.000in. is the reliability of the servo system used . Open loop systems are, of course, less expensive than closed loop systems. A closed loop system compares the actual output (the table movement of 1.000 in. ) with the input signal and compensates for any errors. A feedback unit actually compares the amount the table has been moved with the input signal. Some feedback units used on closed loop systems are transducers, electrical or magnetic scales, and synchros. Closed loop systems greatly increase the reliability of N/C machines.
The term measuring system in N/C refers to the method a machine tool uses to move a part from a reference point to a target point . A target point may be a certain location for drilling a hole， milling a slot，or other machining operation . The two measuring systems used on N/C machines are the absolute and incremental. The absolute (also called coordinate) measuring system uses a fixed reference point (origin) .It is on this point that all positional information is based . In other words，all the locations to which a part will be moved must be dimensions relating to that original
Lesson Nineteen
Figure B.23 On point to point system a 45 path is generated when equal values of x and y are programmed.
Lesson Nineteen
(Figure B.23). Point to point systems are generally found on drilling and simple milling machine where hole location and straight milling jobs are performed. Point to point systems can be utilized to generate arcs and angles by programming the machine to move in a series of small steps (Figure B.24). Using this technique, however, the actual path machined is slightly different from the cutting path specified .
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Lesson Nineteen
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Nineteen
N/C machine tool elements consist of dimensioning systems， control systems ， servomechanisms and open or closed loop systems，It is important to understand each element prior to actual programming of a numerically controlled part.
There are two types of control systems commonly used on N/C equipment point to point and continuous path. A point to point controlled N/C machine tool, sometimes referred to as a positioning control type , has the capability of moving only along a straight line . However, when two axes are programmed simultaneously with equal values (X2.000 in.Y2.000 in.)teen
Figure B.21 The absolute measuring system bases all its locations from a fixed origin
Figure B.22 The incremental measuring system establishes a new origin after each move
Lesson Nineteen
fixed reference point. Figure B.21 shows an absolute measuring system with x and y dimensions, each based on the origin. The incremental measuring system (also called delta) has a floating coordinating system . With the incremental system , the machine establishes a new origin or reference point each time the part is moved. Figure B.22 show x and y values using an incremental measuring system. Notice that with this system, each new location bases its values in x and y from the preceding location. One disadvantage to this system is that any errors made will be repeated throughout the entire program, if not detected and corrected .
Machine tools that have the capability of moving simultaneously in two or more axes are classified as continuous path or contouring. These machines are used for machining arcs, radii, circles and angles of any size in two or three dimensions. Continuous path machines are more expensive than point to point systems and generally require a computer to aid programming when machining complex contours.
Lesson Nineteen
N/C servomechanisms are devices used for producing accurate movement of a table or slide along an axis . Two types of servos are commonly used on N/C equipment：electric stepping motors and hydraulic motors. Stepping motor servos are frequently used on less expensive N/C equipment . There motors are generally high torque power servos and mounted directly to a lead screw of a table or tool slide. Most stepping motors are actuated by magnetic pulses from the stator and rotor assemblies .The net result of this action is that one rotation of the motor shaft produces 200 steps . Connecting the motor shaft to a 10 pitch lead screw allows 0.0005 in. movements to be made (1/200×1/10=0.0005 in.). Hydraulic servos produce a fluid pressure that flows though gears or pistons to effect shaft rotation. Mechanical motion of lead screws and slides is accomplished through various valves and controls from these hydraulic motors .Hydraulic servos produce more torque than stepping motors. However, they are more expensive and very noisy . Most larger N/C machines use hydraulic servos.