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in addition to giving left- and right-hand views of the body surface. After the drawing has been approved, the die design can be completed, and numerical control tapes for die machining can be produced without waiting for model construction. If wood models are desired for a final review, they can be machined from the numerical control tapes simply by adding the machining commands.

Digitizing of Datum Values

Since clay model dimensions are presently extracted by manual methods, surface values for numerical design consist of a series of coordinate points obtained from the template profiles or body drafts. The datum points are available in digital form for the first time at the scanning of the aluminum draft or Van Dykes by a coordinate value measuring and digitizing device.

Some digitizers employ optical followers, such as the one shown in Figure VII-2, which operate on X, Y slides and rails stationed on a large table. Points along the drawing lines are measured and recorded at about one- to four-inch intervals, depending upon the angle of curvature and the tolerance limits. Since the optical system magnifies the line being measured by a ratio of 15:1, precise measurements are possible. The coordinate values of the points are immediately punched into data processing cards.

The data on the cards are merged onto magnetic tape for rapid entry into a computer. This surface data tape and a tape with surface development instructions are read into a computer which performs the mathematical operations necessary to curve-fit and expand the datum points into a tightly laced, continuous mathematical surface definition. The resulting dense array of point values provides the surface definition used in the preparation of numerical control tapes.

Before tapes are prepared and dies machined, however, final tool design must be completed. This phase includes designing die addenda and blankholder surfaces which hold the metal to be stamped, determining whether a double or triple action die is required, and establishing the extent to which the die set must be slanted or tipped in the press.

Tape Preparation

With the complete definition of the die, the computer produces a tape specifying the cutter paths for machining. The cutter location with dimensional offsets in three axes must be computed for each value in the surface mesh of points. The cutter location data is supplemented by appropriate machining instructions depending upon whether a die, a template, or a model is to be produced.

Tapes can also be prepared in two other ways. In the first, the die surface is delineated by several statements of a special numerical control part programming language. A manuscript of verbal, abbreviated

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statements is prepared and converted to coded information which the computer can use to compute the cutter's path. Although APT (Automatically Programmed Tools), the language often used for numerical die applications, has been successful for aerospace firms where parts are more easily defined, its value in making automotive dies has been restricted by the language's inability to convey non-conventional geometric shapes.

Consequently, an alternative method for tape preparation has been developed. A contour measuring device, similar in principle to the optical line follower, determines the dimensions of a three-axis model with a mechanical proximity probe and punches the coordinate values on tape. After the machining instructions have been added, the numerically controlled machine tool repeats the paths of the probe. Because right- and left-hand parts can usually be produced from the same tape, only one model need be digitized.

This second method of tape preparation has been especially useful for parts with non-conventional geometric shapes, but since it depends on a model it does not profit from the benefits of computerized surface development. As mathematical programming for non-conventional geometric shapes is refined, the need for the mechanical follower will de

crease.

Tape Verification

Tapes are usually verified on a numerically controlled drafting machine before machining. Two- and three-dimensional views are used to check the calculated cutter path. Tapes are also verified by first machining the programmed die in Polystyrene. Both are used to check shape conformity; neither adequately checks machining feed and speed rates.

Numerical Machining

With the correct numerical data machining can proceed with little delay. In addition to the time saved, numerical processing of templates, wood models, and dies also produces a surface with better symmetry and continuity, and with tighter tolerances. Hand finishing is greatly lessened for wood models and dies and is almost eliminated for templates.

Since the spacing of machining passes-identified as flow-lines-on three-dimensional work is easily regulated by the computer program, greater accuracy and smaller cusps can be achieved in curved areas by closer machining cuts. Wider machining passes can be used on flatter surfaces, where large cusps may be more readily removed by hand grinding. Machining cuts also can be made perpendicular to the main flow of cuts to provide a precise intersection for guidance in barbering. By making the perpendicular cuts correspond to the template locations, the machined surface can be checked more easily with the aluminum templates. The effects of numerical die processing on costs and schedules will

differ among the various toolmaking activities. For a specific evaluation, the one million toolmaking hours required for a typical model changeover may be divided into five major series of activities: template and die layout, models, machining, finishing, and construction and tryouts. The relative importance of each activity is indicated in Table VII-2 by proportional levels of effort. This comparison does not include lofting or surface development activities, where both the manual effort and the advantages of computerized and numerical methods are considerable.

Templates and Die Layout

Computerized lofting reduces the number of templates required, and numerical processing cuts their manufacturing time by about one-third. Surface development will be done more and more by computer methods.

TABLE VII-2

DISTRIBUTION OF TOOL AND DIE EFFORT TYPICAL BODY TOOLING PROGRAM FOR MODEL CHANGEOVER

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Numerically controlled drafting machines will accelerate the design evaluation process, thereby diminishing the need for cardboard and Masonite templates. Aluminum templates will never be completely eliminated, however, because of their suitability for gaging models and machined dies.

Tool design and layout require extensive experience, and manual layout should continue in use for some time. Survey respondents reported however, that more design decisions are being integrated into computeraided surface development and design.

Models

Numerical control produces more accurate wood models in hours or days than manual processing can produce in weeks. Wood machining rates up to 100 inches per minute are being achieved by the new procedure. Not only is little hand finishing required, but an unprecedented consistency between left and right-hand model members is also attained. Wood and plastic models for design mockup, die casting, etc. will con

tinue to be used; but the number required for machining and finishing will diminish significantly.

Machining

Tracer milling of body dies was reported to take 50-100% more time than numerical milling because of longer machine setup time, lower machining rates, and more machining interruptions. Numerical machining and subsequent operations save so much time and money that some companies start numerical die operations as late as the stage where a model's datum points are digitized.

Since machine vibrations and tool chatter disturb the tracing probe, the machining rates on tracer machines must be held lower than those on the more stable numerical die mills. The technically advanced multiaxis die machines ordered recently by automotive manufacturers will further increase productivity. Five-axis machines require fewer setups and make greater use of flat-end cutters that leave smaller cusps along contours and almost eliminate them on flat or gradually sloping work. The programming problems associated with five-axis machining of sculptured shapes, however, present a considerable obstacle.

Not all toolmaking activities are economical with numerical methods. Some dies and many fixtures may never be machined economically by numerical control. Up to now, body outer-skin dies and die shoes have been converted most frequently. Several users predict that approximately three-fourths of automotive toolmaking will eventually be numerically processed.

Die Finishing, Construction, and Tryout

Smaller cusps, machining cuts made perpendicular to the main flow of machining passes, improved die symmetry, and closer tolerances resulting from numerical processing reduce hand barbering and ease punch and die mating and die set construction by a reported 25-40%. One auto manufacturer indicated that dies requiring hand finishing take approximately 1.75 hours of hand work for every hour of actual body die surface machining time. Users of tracer equipment reported that their dies sometimes deviate from part specification by as much as 0.030 of an inch. Costly hand grinding is then required for the die to meet its specifications. The improvements in accuracy through numerical die processing reduce but do not eliminate barbering; some will always be necessary to produce the desired shapes in the stamped parts.

SUMMARY OF THE IMPACT ON THE
CHANGEOVER CYCLE

Computer-aided design and surface development has already cut the change-over cycle by two to five weeks. The toolmaking phase can also

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