CAD/CAM Testimonial | CAD/CAM Advantages

Ken M. Gettelman

CAD/CAM Beats Cheap Labor

It's no secret that many mold and other tooling orders have left for the low-cost labor areas of Southest Asia.

How is the business won back by domestic shops where skilled mold makers and toolmakers earn ten times that of their Third World counterparts? The answer, as shown by Beach Mold & Tool of New Albany, Indiana, is not found in working harder by using traditional methods. It comes from completely changing the process by which such tooling is designed and made. The new processes instituted by Beach wring out direct labor hours and excessive lead times while adding new levels of quality assurance and product performance.

The key has been integrating computer-aided design (CAD), computer-aided manufacturing (CAM), and electrical discharge machining (EDM), both wire and ram, into a unified and disciplined production process.

The company was founded in 1972 by Bill Beach. It had three employees. Today, the organization has more than 250 people, in three functional groups, under one roof. The first group makes molds for plastic injection machines. The second operates a battery of 23 injection molding machines with capacities ranging from one to 165 ounces per shot. The third group coordinates an extensive assembly area where complex products, including complete sub-assemblies for the computer industry, are put together.

Contoured Surfaces

A pair of plastic speaker grilles for portable radios demonstrates the strategic value of Beach's integrated manufacturing capability. These grilles feature a curved surface that flows smoothly across the face of the radio and merges into its case. The clean, sweeping lines were an essential aesthetic element of this design.

Achieving the smooth surface contour and having it cleanly blend into the contour lines of the case were the prime challenges in designing and building the mold set. The finished grille, after shrinkage, had to have dimensional tolerances within a [+ or -]0.002-inch band around the entire curved periphery for easy assembly and an acceptable fit. Without CAD/CAM, these objectives could not be met (Figure 1).

Phil Kiesler, Chief Engineer, and Gary Wilson, CAD Manager, recall that the original drawing received in the request for a quote did not contain detailed dimensioning. Several points, radii, and matching curves were described and detailed, but smooth blending of surfaces was left to this mold shop.

The design work was done on a Calma CAD system by starting with the defined dimensional data and then using a series of splines and Bezier curves (curves drawn through points so no sharp junctures or cusps appear where they meet). One spline line must flow smoothly into the next. It turned out that this was no small task, because the rate of curvature slightly changes across the face of the grilles. In addition, blending had to be smooth in all directions. To make matters worse, the original specification called for hundreds of holes in each grille that had to be normal to the surface of the workpiece. The pattern easily could be defined, but some sophisticated math routines were needed to orient each one normal to the grille surface. This orientation would also greatly increase the mold cost. After a discussion, the customer agreed that the holes could be parallel. Again, the computer assisted in working out the various patterns for study and comparison.

Programming

The design database was generated in about four days, but it was only the beginning. The design showed the final shape of the workpiece and gave a thorough description of it in coordinate axes dimensions. But two mold halves also were needed, with the appropriate cavity, mounting surfaces, ejector pins, coolant connections, plastic flow channels, and all the other features inherent in the complete mold set.

One of the principal considerations in plastic mold design is allowing for shrinkage. Although some mathematical routines can be worked out, they are not infallible. There is no substitute for experience in this area.

Plastic shrinkage during cooling is not the only problem. A decision had to be made about machining the critical cavity areas. Mr. Wilson and Mr. Kiesler chose to machine all mold elements except for finishing the cavity, then use ram EDM to finish. The cavity, in 420 stainless steel, was roughed out to within 1/16 inch of its finish dimension, and then hardened. It was then turned over to the ram EDM unit for the final finish cut. Individual NC part programs had to be generated to rough the mold cavity, machine the EDM electrode, and even machine a prototype part for visual inspection and checking.

Working from the basic design resident in the CAD computer, Mr. Wilson entered the desired machining sequence (the logical sequence of machining operations), the tooling that was to be used, and the offsets and shrinkage factors that were critically important. For example, EDM requires an offset to allow for the dielectric fluid and spark gap. It must be stated in the proper direction for either the male or female half of the mold. When using a ball nose milling cutter to rough out a cavity, the cusp height allowances must be stated to leave enough material for the finish cuts.

Single Data Source

Once this information is entered, the CAD system will generate the appropriate NC party programs for the mold components and the prototype part. One CAD file is the source for all of these programs, including those for programming a coordinate measuring machine (CMM) and analyzing the data from inspection routines.

The CAD database also is often used to generate NC programs for the wire EDM. Some mold features require more than one electrode to sink them. Wire EDM may be used to shape these electrodes in either graphite or copper. For the grille mold sections, one shaped electrode was used to generate the female mold surface. Another, with holes drilled in it, was used to plunge down over a solid steel section to form the die pins.

EDM is used to generate the finish cut in the mold cavity because of its ability to generate surface finishes somewhere around an 8 AA (arithmetical average expressed in millionths of an inch) or better. This gets close to a mirror finish and greatly reduces the final job of mold polishing.

Before any mold work was done, a prototype was machined from hard plastic on one of the CNC machining centers. After the roughing cuts were taken, a finish pass was made with a fine diamond cutter to get the curve and finish of the final contour. The machined model provided an excellent example of how the molded part would look and function. The model was taken to the customer for approval. Next, the mold manufacturing process needed to be determined. The process centered around the CNC machining centers and the CNC ram and wire EDM units.

The heart of the mold is the shaped cavity. But a mold also includes the base, top, fittings, flow channels, cooling passages, and so on (Figure 2). Standard fitting designs used by Beach have been entered into the CAD software. Calculations for coolant passages and plastic passages have also been made part of the software base. Thus, these do not have to be reprogrammed for each job. They can be called from the file and entered into the workpiece program.

For the grilles, several workpiece programs were generated. One program was used to rough out the mold cavity, and others were generated to machine the EDM electrodes that would finish the cavity. Another program was generated to compare CMM data against the finished part and the design data to assure conformance.

In each instance, the basic CAD part data served as the reference. Skilled mold makers determined the allowances for tool offset, electrode gap allowance, and shrink factors. These people also had to enter feeds and speeds, the desired sequence of machining operations, methods of fixturing and setup, and cutting tool or electrode selection that are vital components of any working part program. The computer made the thousands of calculations and specified coordinate points.

The final results were just what the customer wanted. The mold produced the parts to specification. Total time from design to machined prototype was four weeks. It could have been done in one week if serveral days had not been spent in conferring with the customer and presenting the prototype for his study and approval.

Data Exchange

Beach Mold & Tool, Inc., notes that many of its major customers are doing their design work on CAD systems. They are also using their CAD systems for engineering analysis, quality determination, geometric dimensioning and tolerancing, and group technology. It is essential to have expertise in these areas in order to secure work from such customers. Beach has had to wrestle with the process of translating data, whether received by wire or by tape, from the customer's CAD system to their own.

Beach has two basic approaches to this problem. One uses an IGES (Initial Graphics Exchange System) software package that is currently a standard and bridges the gap between different CAD software packages. However, software development is moving so fast that the IGES effort is hard-pressed to keep up. As a result, it will handle about 80 percent of the conversion on the average. The second approach consists of specific conversion software or direct translator packages. These direct translators often will do a better job of handling Bezier curves and some of the more complex geometry. Translator software packages are available from either the CAD vendors or independent software houses.

Not only does the CAD/CAM approach generate part programs, but it also enables Beach to develop detailed engineering drawings for the customer. These drawings can be used for manufacturing mating parts, assembly fixtures, and so on.

By constantly adding to the CAD/ CAM database, Beach is building its own expert systems software. The complete catalog of one of the major mold component suppliers has been entered. Thus, the specifications for standard bases, fittings, ejector pins, and so on quickly can be called up and entered into a specific mold design. Such components easily can be ordered by the computer. In 15 minutes, Mr. Wilson can design a mold base from standard components. This allows him to devote his creative efforts to designing the critical cavity for each job. In addition, tooling and fixture components for Beach's own CNC machine tools have been entered into their database. Feeds and speeds for machining various mold and die steels have also been included. Standard processing routines are being added as they are developed.

EDM

CAD/CAM is not an isolated entity, but a method of integrating all shop functions into a coordinated whole. Rich Shirley, Superintendent of the EDM department, has offered a lot of feedback to the design people to help them implement the particular processing for each individual job. Mr. Shirley has taken both ram and wire EDM to their limits with two Mitsubishi EDM units (one wire, one ram). He can use the wire to shape complex electrodes for the ram unit. With automatic electrode changing on the ram unit, he can use several different electrodes to sink complex geometric features (Figure 3).

Modern EDM has advanced to the point that the shop can hold tolerances to within a [+ or -]0.0005-inch band with wire in normal cuts and even to within [+ or -]0.0002-inch tolerance with successive skim wire cuts. The Mitsubishi wire machine has control over the U and V axes (secondary axes movements parallel to the primary X and Y), which are employed to generate tapers. Although the machine builder rates the maximum taper capability at 20 degrees, Mr. Shirley has been able to achieve slopes as high as 26 degrees.

Many ram EDM electrodes, whether graphite or copper, are machined on one of the CNC machining centers. An Okamoto VMC 800 machine is equipped with an indexing head, for a fourth axis, and a powerful suction system that enables it to control the graphite dust (Figure 4).

Not A Cure-All

With the speed and versatility of CAD/CAM, the Beach organization pulls in a lot of prototype mold work. Appliance and other manufacturers often order molds made of aluminum or other lower cost alloys simply to build a prototype run. The important considerations are fast turnaround and design integrity. CAD/CAM provides them both.

CAD, CNC, and EDM are not remedies that will cure everything with the push of a button. The professionals at Beach have worked hard to master the benefits these individual technologies have to offer. All three technologies have gone through at least two generations of development. What Beach has done is integrate the people and the technology so that they can perfect a design and quickly develop it into a finished mold. The mold creates a precise image of the actual design.

Behind it all is the overall business strategy to which everyone is committed: Design it well in the shortest possible time and quickly translate that design into a working mold with an assurance of quality, integrity, and performance that wins business.

It is formula that defeats the low-cost labor challenge.

PHOTO : Fig. 1 - The design of this grille for a portable radio features clean sweeping lines. Beach Mold & Tool's CAD system allows them to view the contoured surfaces from any angle.

PHOTO : Fig. 2 - Mr. Kiesler with the female mold half and the part. CAD/CAM was the key to moving quickly from concept to finished design to NC programs that generated the mold.

PHOTO : Fig. 3 - A closeup of many standard mold cavity graphite electrodes used to sink standard cavity shapes on the ram-type EDM unit. Many of these electrodes were profiled with the wire EDM.

PHOTO : Fig. 4 - Some electrodes are produced on a CNC machining center equipped with a fourth axis. On this machine, an indexing unit mounted on the machine table provides that fourth axis. It is also equipped with a powerful suction system that controls the graphite dust.