Since CAM technologies have been advancing throughout the past few years, opportunities for industrial designers to generate aesthetically pleasing sculptural surfaces and organic free-form designs have exploded. Until recently, product and packaging designers have been advised to adhere to the rules and limitations of manufacturing processes and to design for profitability. Throughout the past several years, advances in software and manufacturing technologies have changed the rules and removed many limitations, thus bringing the conventional design wisdom out of the 21/2-D box. The changes to the industry are awesome. No longer are there significant manufacturing limitations being imposed on the design process that may restrict the freeflow of creativity in product design. This article will focus on some of the latest advances in 3-D CAM software and high-speed machining (HSM) technology as well as the need to keep up with these "next generation" technologies that are affecting all manufacturers and designers.

In the frenetic high-speed environment at Utley's, Inc. (Woodside, NY) - a third-generation, family-owned prototype model shop producing precision appearance models - projects come in from clients for prototype cosmetics packaging design, vacuum form blister packs, new consumer products, giftware, awards, sculptures and statues thoroughly designed with multi-faceted contoured surfaces not unlike actual sculptural pieces.

With Utley's cross-generational mindset for speed and quality, it is no wonder that the company has embraced some of the latest advances in CNC and CAM technologies. With project deadlines in days instead of weeks, there is little time for typical machine shop or sculpture studio procedures. Customer projects that come in as IGES files early in the morning are often programmed and CNC machined the same day or, depending upon their complexity, overnight, then immediately handed off to a bench department of skilled model makers and to the finishing department to meet frequent next day deadlines.

In the past, jobs involving unusually complex surfaces were sometimes turned down because they couldn't be fabricated by standard sculptural or model making processes or would just take too long to complete. Today, with its ample CNC and CAD/CAM systems in place, Utley's takes on projects it previously could not.

Generating Machinable 3-D Models

The 3-D models usually are e-mailed in from a plethora of different 3-D surface/solid modeling systems used by the client's designers. Sometimes, 3-D models are created internally in Pro-Engineer or Mastercam Design from the client's 2-D concept sketches or Illustrator drawings. The CNC programmers at Utley's import the 3-D model files - usually in IGES format - into one of the CAM systems, depending upon the parameters of the project, such as deadlines and complexity of the surfaces.

Sometimes photos or sketches are scanned and Artcam is then used to machine delicately detailed sculptural reliefs from them, or to engrave artwork and corporate logos. Setting up good machinable surfaces to generate toolpaths is often a dimensionless and paperless process. There is no need to use multiview orthographic drawings to communicate dimensional data. Generally, the only use for dimensions is to determine the machining stock size from the wireframe bounding box, which is created around the boundaries of the 3-D CAD model. The best results are obtained from "watertight" NURBS surface models directly translated in IGES format from 3-D solid models. But this is not always possible.

There are times when complex 3-D models are imported with huge gaps, or untrimmed or missing surfaces. This is due to the inefficiencies of translating data into the CAM environment originating from the client's various CAD systems, which don't always speak the same language. The surface and solid design tools in Mastercam provide a quick way to visualize and fix these surfaces. Otherwise, converting imperfect 3-D models to STL format (triangular surface mesh file) and tightening up the tolerances is sometimes all that is needed to prepare the model for toolpath generation.

They don't have to be high-tolerance, "watertight" surface models in order to be machined within acceptable tolerances. Both Mastercam and Powermill are very forgiving and will generate quality toolpaths from non-mating surfaces with or without gaps, flaws or reversed normals. In most cases, there is precious little time to use healing software or to upload files to a contract web-based ASP (application service provider) that specializes in fixing the flaws in 3-D models. The CAM system's tolerant modeling capabilities are used to overlook gaps and flaws. In the world of high-speed sculpted surface prototyping, one advantage is that acceptable tolerance ranges may vary a considerable amount. Speed is often the key factor as long as high quality visual standards are maintained. Exceptions to the rule are precision prototypes and working models where high-tolerance machining and speed are both required.

Toolpath Strategies for HSM of Complex Contoured Surfaces

A reliable visual toolpath simulation capability to visualize and fix any errors or prevent crashes before generating the NC code is essential to machining without trial runs. Carefully verifying the toolpaths before actual cutting on the CNC machine tool is very important. As a habit, the WIZYWIG verification functions are used before any job goes to the machine tool. Figure 3 is a screen shot of a typical finishing toolpath simulation in the CAM environment.

The procedures for generating toolpaths from the 3-D model often follow a pattern of 3-D operations - such as roughing, finishing and rest machining - in addition to 2-D operations of contouring and detailing passes with appropriately-sized cutting tools. Since Utley's prototypes are mostly machined from acrylic, Renshape polyurethane foam and such soft metals as aluminum and brass, semi-finishing operations usually can be eliminated and finishing toolpaths with very fine step-overs, sometimes as tiny as .002", are programmed directly following roughing.

Mastercam generally is used for programming toolpaths for simple and complex 3-D models, which require 2½-D, 3-D, or four-axis rotary machining operations or a combination of operations. For huge model files with hundreds of surfaces, Powermill is used primarily for its super-fast calculation speed, which reduces toolpath generation time to seconds or minutes at the most. Sometimes both Mastercam and Powermill are used in concert on the same model file programming 2-D and 3-D operations simultaneously.

Offset roughing is a fast, efficient 3-D roughing strategy that can be seen in Figures 4 and 5. This toolpath operation is applied to rough out the model from a block of clear acrylic plastic in these cases. The cutter will hog out each Z level and step down by a programmed amount until all of the surplus material is machined away, leaving a specified amount of stock on the surface of the model for finishing.

One of the most timesaving, new 3-D finishing strategies is called 3-D offset finishing in Powermill or scallop finishing in Mastercam. With this strategy, fine stepovers are generated on both vertical and horizontal contoured surfaces at the same time, offset by the same amount no matter what angle the surface takes, which can be seen in Figure 6.

In other words, you can machine the flat and nearly flat surfaces and vertical walls of a model with one automatic toolpath operation. This saves a great deal of programming time where you would otherwise need to select the nearly horizontal surfaces separately from the vertical ones in a complex model and machine them individually in two or more toolpath operations. Another plus is that it eliminates additional seam lines and extra bench work. The 3-D offset finishing strategy is a high-speed CNC programming operation that eliminates these extra steps by providing the finest machined finish on the entire model in the shortest amount of time.

Another effective finishing strategy for machining fine details on multi-faceted surfaces is rest machining. Here, toolpaths are applied only to the areas of the model missed by larger tools, as can be seen in Figures 7 and 8, which show these fine machining strategies being applied to the Roto-Rooter bottle prototype and the crystal Star of David prototype. For the final finishing operations, the star was rest machined with tiny ball mills as small as .0156" diameter, to cut the fine inner surface details for the faceted crystal prototype. Figures 9 and 10 show the finished pieces after they were highly polished in the finishing department.

High-Speed Machining of Contoured Surfaces

Maintaining precision cuts while operating at high feedrates is a software or controller issue. One solution to the inaccuracies generated by higher feedrates is to use feedrate optimization functions provided by the CAM software or the CNC controller. These afford a toolpath analyzing function that "looks ahead" in the code and slows down the feed to a specified rate on sharp angle moves and speeds it up on the straighter linear moves.

This is the same scenario as a car going 100 mph that comes to a sharp curve in the road and has to slow down or it may veer off the road. In the same manner, a cutter moving at feedrates of more than 100 ipm has to slow down on sharp turns or it may violate the surface and cause gouges. These feedrate adjustments have increased machining productivity by decreasing the running time on certain finishing operations by as much as one-half of the time - even on such commonplace CNC machine tools as the Haas VMC.

Finish Machining

Generally speaking, the smaller the part, the less CNC machining time is involved. On the other hand, the more complex the surfaces, the more programming and toolpath calculation time is involved. Accordingly, more emphasis is placed on decreasing programming time as opposed to investing in expensive high-speed CNC equipment. Many jobs are run overnight, so it makes little difference whether it takes 10 hours or five hours to machine as long as it is finished by 7:00 a.m. Rather, the feedrate is in fact slowed down on the control, which increases machining time in order to produce a better surface finish.

In the overall mix, speed and quality are juggled along with the scheduling of jobs in order to meet deadlines. For example, if the finish stepovers are very fine - say .002" to .005" - then this reduces hand finishing, but increases machining time. If the job has to be machined quickly to free up the CNCs for other jobs, the finishing stepovers may be programmed at .010" or more, which would cut machining time in half.

Figure 12 shows a perfume bottle prototype being finish machined in aluminum. The faceted design in the center was so intricate that hand finishing and polishing would have been a long and tedious process unless it was machined with a very fine finish. In this case, toolpaths were programmed with such small stepovers that virtually no handwork on the inner facets was required at all by the finishing department.

For thin-walled parts to be machined accurately on both sides, they have to be held rigidly in a suitable fixture in order for machining to be done on the second side without destroying the part. Figure 13 shows the inside cavity of a part being machined while the outside is being held by connecting tabs and reinforced from underneath. Sometimes connecting tabs are eliminated and special surface fixtures are designed to hold the part while machining the second side. Rapid fixturing is an art in itself to enable the accurate machining of all sides of a part without utilizing a costly five-axis CNC. Sometimes the nature of the design, as in the case of bar glasses, candle designs and certain bottles, requires the use of four-axis rotary machining. The programmable fourth rotary axis is an add-on to the three-axis CNC bed. Figures 14 and 15 show a screen shot of the toolpaths for cutting "V" grooves in a cylindrical bottle prototype and a photo of the actual machining on a desktop CNC mill on the fourth rotary axis. 

Commitment to CAM Education Utley's understands the need for investing in its employee's career growth in order for the company to stay competitive and as such has made a significant commitment to educating and retraining employees to advance their skills. On- and off-site CAM software training is provided for the CNC programmers to stay current. Employees also are encouraged to take advantage of local tech schools to upgrade their skills and stay on the cutting edge.

One of the leading New York schools, The Center for Advanced Manufacturing Studies (CAMS) - which specializes in CNC and CAD/CAM training for all levels - has trained several employees. This has allowed them in a short period of time to become more productive with the latest CNC technology and CAM software. In the machining department there is a CNC shop mill with a conversational controller. Figure 16 shows this machine tool with the author at the controller. This CNC machine tool can be programmed off-line with G-code translated into conversational language or programmed at the control in everyday machinists language instead of confusing G-code.

This particular conversational language replaces codes such as G00, G01 and G02 with such simple commands in English - Rapid, Line, Arc. A string of code such as G43, H, S, M03 and M08 are simply replaced by the word Tool#. The eventual goal is to allow the machinists to become more productive by adding basic CNC skills to their repertoire.

This contributes to the overall time savings by freeing up the off-line programmers to concentrate on more complex 3-D operations rather than simple 2-D operations such as facing and squaring up blocks, preparing simple fixtures, drilling and pocketing operations and so forth.