Page 708 - Mechanical Engineers' Handbook (Volume 2)

3 Numerical Controllers 699

Linear transducers may include scales or distance coded markers. Linear transducers
have the advantage that they do not introduce backlash errors that a rotary transducer may
not be able to detect depending on its mechanical location.
Traditional input media for entering programs into an NC machine include (1) punched
cards, (2) punched tape, (3) magnetic tape, and (4) direct entry of the program into the
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computer memory of the numerical controller. More modern media include flash memory
cards.
Originated by Herman Hollerith in 1887, the punched card as an input medium is almost
obsolete. The standard ‘‘IBM’’ card’s fixed dimensions are 3.25 in. wide, 7.375 in. long, and
0.007 in. thick. Each card contains 12 rows of hole locations with 80 columns across the
card. To edit a part program, cards in the deck are replaced with new cards. With a deck of
cards, it is easy to lose sequence or have missing blocks due to the loss of a card. Also,
punched cards are a low-density storage medium with an input rate that is slower than most
other media. As a result, punched cards are now regarded as obsolete for NC program media.
Punched tape was for many years the most popular input medium for a numerical
controller. Although punched tape is mostly obsolete, many numerical controllers still pro-
vide a punched tape reader. The specifications of the punched tape are standardized by the
EIA (Electronic Industries Association) and the AIA (Aerospace Industries Association).
Tapes are made of paper, aluminum–plastic laminates, or other materials. Making editorial
changes to the punched tape is difficult; only minor editing is possible by splicing new data
into the tape. With the advent of online computer editing techniques, rapid editorial changes
to a program can be made on a computer screen. At the end of an editing session, a tape
can be automatically punched on command from the keyboard.
Magnetic tape is not used as much as punched tape because of its susceptibility to
pollutants in the NC environment. Dust, metal filings, and oil can cause read errors on the
tape. Sealed magnetic tapes overcome some of these problems.
Along with flash memory cards, direct entry of a part program into the controller mem-
ory is a common input medium for today’s NC. The programmer can either type in the NC
program from a keyboard and a video display terminal or generate the NC program from
an interactive graphics environment. Part programming with the aid of interactive graphics
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is discussed in more detail in Section 3.5.
Operation of a Numerical Controller: Machine Coordinate System
NC requires a point of reference and a coordinate system to express the coordinates of parts,
tools, fixtures, and other components in the workspace of the machine tool. The commonly
used coordinates are three orthogonal intersecting axes of a right-handed Cartesian coordinate
frame, as shown in Fig. 13. The rotations a, b, and c about the x, y, and z axes are used for
NC with more than three axes. In most older numerical controllers, a coordinate frame is
marked on the machine and all coordinates are with respect to this fixed frame. In the newer
numerical controllers, the machine tool user can program, or ‘‘teach,’’ a location for the
origin of the reference coordinate system. Since such a reference coordinate system is not
permanently attached to the machine, it is sometimes called a floating coordinate system.
To illustrate the use of an NC coordinate system, let us consider a simple example of
drilling a hole in a rectangular plate with a numerical controller. This example will illustrate
the steps in the initial setup and operation of an NC machine. Figure 14 shows the drawing
of a simple rectangular plate to be drilled by a drilling machine.
The first step in the programming of any NC operation is getting a drawing of the part.
The drawing is usually a blueprint with the dimensions and geometrical attributes of the
part. Figure 14 shows a rectangular part and the location of the center of the hole to be
drilled in the part. Let the lower left-hand corner of the part be the origin of a two-
dimensional Cartesian coordinate frame with the x axis and the y axis as shown in Fig. 14.