A. The linear tape transducer, which resembles a steel measuring tape without visible graduations, measures incrementally in a manner that produces pulses that are converted into digital values. This provides a much higher degree of resolution than the visually readable graduations. The unwinding or rewinding tape is in slippage-free contact with a pluggage made with a bore for being pressed on the shaft of a rotary encoder, whose rotating grating disc produces the pulses corresponding in number to the travel length of the tape. The tape contacts, although over 270 degrees, only the gage plug, and it is received by a separate constant-torque spring type wind-up; consequently, the diameter variations of the spool do not affect the accuracy of the measurement. A housing, containing the mechanical components and the rotary pulse generator as one unit, and the finger holding the free end of the tape as the mating unit are mounted on machine elements that move in relation to each other; that movement causes the tape to extend or to retract. The extended tape is shielded from dirt by an aluminum extrusion. A widely used make of linear tape type digital readout system is manufactured in two models; one allows a maximum travel of 120 inches and the other, with components in heavy-duty design, allows a maximum travel of 360 inches. Both types provide 0.001-inch and 0.0005-inch resolution. The maximum cumulative error of these systems is stated to be 0.0005 inch in 20 inches, a factor to be considered in determining the system’s field of applications.

B. The endless tape transducer is a product of more recent development and uses a precision stainless steel tape with ends connected to form a continuous circular tape. This system eliminates the use of a rewind mechanism and permits calibration prior to installation. The operating principles of that system can be seen in the diagram in Fig. 18-31B, and Fig. 18-32 shows several major members of an endless tape system, including the counter for two axes and the tape housing for one of the axes, together with the pertinent finger that must be attached to the moving machine slide. The illustrated make of endless tape readout system is manufactured in two different models: one for long displacement distances up to a maximum travel of 160 inches, with 0.0005-inch highest resolution, and the other for higher-accuracy requirements, with 12-inch maximum travel and 0.000050-inch resolution.

Fig. 18-32. Endless tape system—several major members, including the counter box and one of the two tape housings with encased tape and the finger, which is to be attached to the moving machine tool member for its positive connection with the tape.

The rack-and-pinion system is recommended particularly for applications with travel lengths exceeding the practical limits of glass scales. The transducer elements of that system comprise a rack installed on one member of the machine tool, and engaged by the pinions of the pulse-producing device mounted on the other machine tool member, the mutual movements of which are to be measured. Figure 18-33 shows the principal elements of an input system, with the racks mounted on a spar that is to be installed on a machine tool member, as well as the rotary encoder whose pinion engages the racks. Rack-and-pinion type transducer systems are effectively used on machine tools with very long slide movements, such as large lathes, jig borers, and horizontal boring mills, an example of which is shown in Fig. 18-34.

Fig. 18-33. Principal elements of a rack-and-pinion type input system, shown with the protective cover removed. The rotary encoder (at right) carries a pinion in engagement with the rack, for registering by precise pulses the relative movements between the two machine tool members on which these mating components of the system are installed.

Fig. 18-34. Gaging unit of a post-process gaging system serving the centerless through-feed grinding of tubing in great length. The straddingly arranged and flexibly mounted mechanical contacts transmit the momentary work-size conditions to an air gage with its nozzle directed against the head of an adjustable reference screw.

The racks are manufactured in 12- and 16-inch or longer sections and are installed in spars that may have lengths of 16 feet for a single unit: several of these units can be mounted abutted end to end to produce a total measuring length of 100 feet or more. The individual racks are usually precision ground to a high degree of accuracy, typically ± 0.00015 inch, and the installation of the individual racks on the spars is carried out in a manner to compensate for even very small inaccuracies, thus holding the nonaccumulating accuracy over the whole length of the system to typically ± 0.0004 inch, at 68°F (20°C) envelope temperature. The calibration of a set of racks installed on a spar is preferably carried out by laser interferometers, which have an accuracy better than 0.5 part per million. That calibration procedure can also compensate for some types of errors of the machine on which the rack-and-pinion type transducer is used.

The optical encoders, on the spindles of which the pinions in engagement with the racks are mounted, usually have a very high response speed, permitting traverse rates on the order of 2400 inches per minute. Because the racks are made of steel they have practically the same coefficient of thermal expansion as the machine tools on which that system of travel-distance measurement is installed.