the clay model for engineering purposes. It also makes full size fiberglass models of the car for management reviews and advertising photos. Final approval of the clay model precedes the appearance of the first production models by 21 to 24 months.

Surface Lofting

The dimensions precisely delineating the shapes and contours of the new style can only be taken from the completed model. Artistic skill and cumbersome manual procedures are used to capture these dimensions. The model is sectioned with parallel planes perpendicular to the model flow lines, and template signatures at five- to ten-inch intervals are then made at each of the plane-model intersections.

Cardboard Templates

When the clay model has been approved, template makers, being very careful not to gouge or scrape the clay, begin their work. Each man cuts a piece of cardboard to approximate the gridline he wishes to define and then roughly fits it to the contour of the car. By moving a pair of dividers lightly over the surface of the model, he traces a line parallel to the model surface on the cardboard and, removing it from the model, cuts along this line. After a series of time-consuming fittings and cuttings, each template is tailored to fit perfectly. Some auto companies have used the electronic scanner to supplement these manual methods.

The degree of discontinuity in the body surface determines the frequency of the template stations. Where curvature is pronounced-for instance, around the windshield, the lights, or the grille-the template stations must be close together. Principal styling features or character lines are tracked separately to insure accuracy. The clay model is usually measured several times during the development phase to promote accuracy. Extensive effort is being devoted to mechanize the manual measuring process because a refined system is expected to reduce the dimension extraction time by as much as two-thirds and to eliminate many errors.

Surface Development

The completed cardboard templates are used by a draftsman who traces the finished templates' outlines onto linen vellum. Front and side views are drawn on top of each other to make it easier for him to visualize and develop the intervening surfaces in three dimensions. Wherever the lines are discordant, the draftsmen fairs or "sweetens" them so that all the contours are continuous and smooth. Sweetening requires considerable skill and is accomplished through manual or graphical curve-fitting techniques. It also is a source of considerable error; for example, a

3 Character lines are distinct ridges or sharp curvatures such as those usually located on the top of the front and rear fender sections.

sweetened deck-lid may not mesh properly with adjoining body surfaces.

A set of Masonite templates are made from the linen or mylar drawing and arranged over the clay model to "prove" the linen draft. After checking continuity and making the required adjustments, the information on the linen vellum is transferred line by line onto a large sheet of aluminum previously painted a dull white so that, by using a gold or platinum stylus, the draftsman may leave permanent, clearly visible black lines. Accuracy is especially important here because the finished aluminum loft is the permanent definition of the three-dimensional body surface from which the final die information is taken.

Preliminary body contour information will already have been released to the engineering groups. The preliminary loft information available one or two months after the approval of the clay model is used, for example, in the design of the car's interior and of the points of fusion between the inner and outer body sections. Contingencies of time force designers to use this preliminary information even though it is incomplete and subject to change.

Aluminum Templates

Draftsmen then prepare drawings for the production of aluminum templates and the construction of wood models. These drawings, called Van Dykes, are made for each section of the body and show character lines, surface flow lines, plane-model intersection lines, and any other data required for the complete definition of the surface. The aluminum templates produced from the Van Dykes are a permanent record of the body surface dimensions. They are continually referred to in die machining, spotting, and finishing.

There are two methods of making aluminum templates from the Van Dykes. In the older method, the Van Dyke is placed over a sheet of aluminum where the template maker prick-punches it at many points along the defining lines. After the vellum is removed and the points are joined in the proper curves, the templates are cut on a band saw and hand finished until they conform perfectly with the master body draft. In the more advanced method, a two-axis milling machine electronically directs the cutting action as its optical scanner follows the lines on the Van Dyke. The resulting template is more accurate and eliminates much of the hand finishing.

Mahogany Wood Model

Using the aluminum templates and the loft drawing, master modelmakers machine, hand carve, and sand body parts out of mahogany blocks or sheets laminated together for dimensional stability. The finished blocks are assembled and attached to a stacking fixture to verify model continuity again. Management is called in for yet another look at the proposed configuration.

The final approval of the wood model is a decisive step in the changeover cycle. As soon as all drawing changes are completed, a final set of dimensions and die models or plastic duplicates of the die models are released to manufacturing engineering for final tooling design. Any changes emanating from the management review are quickly made in the design of the dies and fixtures needed for manufacturing operations. At this point, about 14 to 16 months prior to production, manufacturing engineers begin reviewing the changes required at assembly plant facilities for making the switchover.

Plastic Reproduction of Wood Model

Upon finalization of designs, female plastic patterns are made, frequently of epoxy, from mahogany models. These casts are in turn used to make the male epoxy model duplicates. All of the inner and outer body parts which have a contour require a model for use in tracer controlled machining and casting operations. The plastic models are usually complete 9 to 15 months prior to the introduction of the new car.

Casting of Die Components

The wood model surface often is slightly modified and addenda surfaces are added in die design. Die designers also determine if the die set is to be tipped in the press and whether multiple action dies are required. These decisions are based upon forming die development analyses. Male and female plastic patterns-Polystyrene-are made from the revised surface designs. The advantage of using plastic patterns is that they can be left in the sand avoiding the possibility of damage to the walls of the sand mold as the patterns are removed. Polystyrene boils into gas and escapes as the molten metal is poured into the mold.

Three types of dies are cast; steel, close-grained iron, and Kriksite. The Kriksite dies are used as soft tooling to make parts for pre-production test vehicles. These soft dies help qualify the manufacturing feasibility of the draw dies. The steel and close-grained dies are the production tools.

Die Machining

Most of the production dies are machined on tracers with a plaster or plastic machining model. After the roughing and finishing cuts, the many cusps, or ridges of metal left on the die by the cutting tool, are worn away by tedious, expensive hand grinding and stoning.

A plastic surface checking fixture is used for die finishing and serves as a reference master for checking the die. Both the checking fixture and the machined punch are fitted together to mark discrepancies in the machined die. The punch is coated with a thick yellow paint, the checking

4 The use of a plastic material, usually Polystyrene, began in the early 1960's. This new process cuts time, greatly improves casting quality, and reduces cost by a third.

fixture with thick blue paint. As the fixture is lowered onto the punch, blue paint smudges the high points on the die's yellow surface. These elevations are then removed by careful hand grinding.

When the punch is properly finished, it is mounted on a spotting press. The punch is covered with blue paint and lowered into the yellow painted female die. "Barbering," or hand grinding, removes the elevations on the female die where the yellow paint has been fouled. Once the imperfections in the die set have been worn away, all die surfaces are hand rubbed with an abrasive stone so metals can be drawn over the die surfaces within specified friction rates. The die set is then tried out through many test stampings. Modifications are made with hand grinders wherever the stamped part reveals an imperfection in the die set. When the die set has passed this test, it is ready for shipment.

Pilot Assembly

The dies and associated tooling are sent directly to the production plants. The more important assembly fixtures are sent to a central pilot assembly plant where they are tested to identify tooling and assembly problems before the start of mass production two to five months later. Pilot assembly, which usually begins in May or June, occurs the first time vehicles are assembled with the 400 to 500 body components produced by the new tooling rather than by the handmade parts of the prototype models. The vehicles are tested while the new tooling is being installed at the production sites. The modifications suggested by the tests are completed in time for actual production of the new model in August or early September.

Die Quality

The final test of die quality is its performance on the production press where it must stamp thousands of the specified parts without imperfections in the stampings. As was indicated earlier, production presses frequently have minor operating peculiarities which differ from the try-out press. These differences sometimes necessitate additional die set adjustments, including gaging, remachining of the base, etc., at the stamping plant production line.

THE NEW PROCESS

Thus far, attempts to mechanize the extraction of dimensions from the clay model have been thwarted, leaving the early phases of the changeover cycle unaffected by numerical toolmaking. Clay models and the manual or mechanically-assisted delineation of surface dimensions continue to be used. Eventually, the manually operated height gages and templates will be replaced by electronic or photogrammetric measuring and recording devices, but both techniques need further development.

When the measuring breakthrough occurs and the power of the digital computer is applied fully to both the surface development and die design, the complete integration of product design and toolmaking will quickly follow. The concomitant increase in accuracy and decrease in scheduling time will antiquate many conventional toolmaking methods.

Computer-Aided Surface Development

The promise of numerical die processing lies in computer-aided design techniques which are able to generate complete mathematical surface definitions more rapidly than the relatively more elaborate, time-consuming, and inaccurate conventional methods. A sparse array of datum points or lines in space as well as the appropriate equations for expanding the points into a complete definition are required as basic information. Since the points or lines define the surface only at specific locations, the data on the intervening points must be developed mathematically."

Despite the extensive calculations which go into surface development, the computer, provided with the appropriate equations and algorithms, can write an analytical definition of the entire vehicle. The resulting mathematical model of the surface can be altered with a minimum of effort by changing the values of coefficients in the equations. This flexibility permits an earlier beginning for die design and avoids the costly delay and rework of the conventional method.

Preliminary surface data is available about one month after clay model approval. The constant pressure to shorten the changeover cycle forces the use of this preliminary information for die engineering even though final data may necessitate extensive modifications. In the old method where the dimensional accuracy of the machined parts depended on the accuracy of the plastic models, delays often occurred when modifications forced a rebuilding of the models.

A numerical surface definition does not require models but should be produced in some visual form for aesthetic evaluation. A numerically controlled drafting machine with taped information from the computer can translate the coordinate values into graphic pictures. These drawings, the result of considerable hand work by the old method, are prepared at speeds up to 500 i.p.m. One drafting machine can do the work of several draftsmen. The perspective and frequency of the contour and character lines are selected to suit the needs of the designer. The machine can draw at quarter, half, full, or double the scale of the programmed tape,

The sparse array of points usually are 1 to 4 inches apart, but close enough to graphically reproduce the curve within a tolerance of approximately 0.010 of an inch. The mathematically created dense array of points defines the curve in such a way that a numerically controlled machine will not deviate from the true curve by more than a specified tolerance-usually 0.001 of an inch. Around acute angles, such as those created by the headlights, the spacing of the dense array will seldom exceed one-half inch.