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The digital ‘evolution’

Bridging the CAD/CAM gap

CNC’s many roles

Here are plenty of factors that can slow the process for developing medical devices. Depending on the device and its function, verification may be required to meet regulatory requirements. In some cases simultaneous development of subsystems can speed up development, but as with many technical products there are steps in the process that must take place sequentially. For example, shell design can rarely be finalized until the final shape, size, and layout of internal components have been determined. Software development often cannot begin until chip selection or board design has been completed. In other words, many factors add time and cost to the process, which is why developers must seize every opportunity to speed up development and ensure precision and consistency while also controlling costs.

There are many ways to produce parts quickly, consistently, and cost-effectively at mass-production volumes. These methods include injection molding, die casting, and die stamping. At lower volumes (and for producing parts that are too complex for other production methods), computer numeric controlled (CNC) machining is suitable. CNC machining has become an increasingly viable production method because once the toolpaths that control the machine tools have been developed and programmed, a computer manages the entire process turning out virtually identical parts in any of hundreds of metal or plastic materials. And because the work is done robotically rather than by humans, ongoing labor costs can be kept to a minimum.

Machining goes digital

Like all computerized applications, CNC has developed over time. Programs that once ran on paper tape are now digital, and the job of the machinist has, to a large extent, changed from that of machine operator to programmer. Machining cells combine multiple tools, further reducing the need for human intervention. As a result, traditional CNC machining has evolved from being a heavily “front-end-loaded” process to today’s labor-saving, digital process.  Toolpath creation was once a complex process, taking many man-hours of programming before the first part could be produced. Cost-per-part dropped sharply as the computer and machining cells churned out unlimited quantities of identical parts. Production volume was large enough, the high setup cost could be amortized over a large number of parts, making machining a cost-competitive production method. However, machining small numbers of prototypes or small numbers of production parts quickly and cost-effectively remained a challenge.

Digital prototyping

Manually machining parts, though slow and costly, has long been used as a prototyping method. The first viable alternative was 3D printing, invented in the 1980s and made commercially available in the 1990s. 3D printing took the output of computer-aided design (CAD) programs such as SolidWorks (solidworks.com) and ProEngineer (proengineer.com) and sliced these representations into layers.

Specialized equipment laid these layers down in sequence and solidified them using a variety of technologies—stereolithography, fused deposition modeling, and selective laser sintering—to produce a solid form in the shape of the 3D CAD model. The process was quick, automated, and inexpensive, but had several shortcomings when compared to machining.

First, because the parts were created in layers, they typically had rough surfaces that had to be either tolerated or corrected using a secondary process. Layered parts didn’t have the strength possessed by parts that either began with solid stock, as machining did, or were solidly fused by molding. More importantly, each of the additive processes supported a very limited palette of plastic resins. This not only precluded the possibility of making metal parts, it also presented challenges for plastic parts. Unless the process’s material happened to match the intended production resin, it ruled out the possibility of doing serious functional testing for durability, chemical resistance, electrical and insulating characteristics, and temperature tolerance, all of which depend on the material from which a part is made.

Some of these characteristics can be particularly important in medical applications, and considering the stakes for both developers and users of medical devices, not to mention regulatory demands, rigorous testing using actual production materials makes sense.

Bridging the CAD/CAM gap

Recent developments have made CNC machining a competitive method for making metal or plastic parts in prototype quantities. In the past the challenge was the time and labor cost of creating first-part toolpaths. Fortunately, sophisticated software is now replacing the human programmer, cutting costs and delays.

For example, software used by Proto Labs can convert a complex 3D CAD model to toolpaths in only hours. This automation of toolpath development not only slashes the cost of that first part, making CNC machined parts cost-competitive with layered parts, it also eliminates the possibility of error being introduced by the human involvement between computerized-aided design (CAD) and computer-aided manufacturing (CAM).

CNC machining in the product development continuum

Machined prototype parts are functionally equivalent to metal and plastic parts produced by a variety of industrial processes; layered parts are not. Further, machining offers these advantages: fast production, low costs, and smooth surfaces. As for the parts, machining makes possible production using virtually any machinable metal or plastic and because the parts are made from solid stock, machining eliminates weaknesses associated with layering processes.

Both processes, however, have a place in product development. Typically, the beginning phases of product design produce no physical prototypes at all. The process begins with 3D CAD modeling, which allows parts to be designed, assembled, and even tested using finite element analysis (FEA) in the virtual world. The first step into the physical realm may be 3D printing, especially in early stages of product development when rough surfaces and“stand-i” materials may not make much of a difference.

Furthermore, the availability of on-site 3D printing equipment is hard to beat for speed.

As development proceeds and functional testing begins, material and structure matter more, making rapid machining a better choice. Quick turnaround and moderate cost of automated CNC machining let developers begin functional testing early in the development cycle and avoid the need for costly, time-consuming backtracking later. This may entail several iterations of machined prototyping as development proceeds and testing identifies potential improvements in the design.

Finally, as designs approach completion, other methods—rapid injection molding for plastic or casting and stamping for metal—can produce larger numbers of parts for volume testing or test marketing.

CNC machining for production

Even at its most efficient, machining may not be practical when large numbers of parts are needed. It can, however, be cost-effective for shorter production runs, which may be all that is needed for some specialized medical devices. CNC machining can also be used to produce jigs, forms, or fixtures used in various production processes.

Finally, the concept of “lean startup” is leading to the early introduction of devices to markets while development continues and products are fine-tuned based on market feedback. In such cases, the line between prototyping and production becomes blurred, and products may go to market in a series of evolving, low-volume production runs. This approach may or may not be practical for medical devices that are subject to heavy regulation.

CNC machining options

CNC machining can be done using 3-axis, 4-axis, or 5-axis milling equipment. In 3-axis milling, the workpiece moves along the x and y axes (side-to-side and forward-and-back) and the cutting tool moves up and down along the z axis. The simplest method for milling is to position (or fixture) the workpiece in 90° setups. More complex fixturing can be used, but this will add time and cost depending on complexity of the setup(s). Four-axis equipment adds a turntable to move the workpiece around a rotational axis, eliminating the need to reposition the piece to mill various sides while adding some cost due to potentially higher machine rates and engineering time. Five-axis equipment adds a second rotational axis, allowing the cutting tool to address the piece at upward and downward angles, again, adding cost due to higher machine rates and engineering time.

Three-axis milling can match most (though not all) of the capabilities of the 4- and 5-axis equipment by repositioning the workpiece using specially designed fixtures. It offers several advantages including lower cost of equipment and, hence, lower production costs. And because it offers just one way to reach each point to be milled, automatic generation of toolpaths is simpler, faster, and less costly. With fewer components taking up workspace, 3-axis milling allows machining of larger parts. Finally (and perhaps most importantly), it provides less opportunity for error due to equipment wear and hardware or software problems. Error can be a particular problem in the rotational axes of 4- and 5-axis systems, where inaccuracy is magnified by distance from the axis. In short, the simplest machining option that will accurately produce the design may be the best, most cost-effective choice.

Summary

While no one method of prototyping meets all needs, CNC machining has grown in capabilities and is a powerful tool for part prototyping and low-volume production. From the early stages of product development through product introduction and full-scale manufacturing it can generate parts quickly, accurately, and affordably. And with the development of faster programs for toolpath generation, wider selection of materials, and service providers specializing in short-run prototyping and production, this is a technology worth exploring to help reduce costs and time to market.