July 2010 Machining Tip

CNC Machining of Prototype Parts

Welcome to the first issue of our First Cut Machining Tips. For years, Proto Labs has been publishing our popular monthly Protomold Design Tips, so we thought it might be useful to do something similar in support of our First Cut service. This initial issue provides some background about the rise of CNC machining as an effective prototyping process, and contains some introductory information about our First Cut capabilities.

It’s a fact of modern life: as computers get smarter, the tasks they perform become cheaper. At the same time, work done by highly skilled humans remains relatively expensive. With the introduction of computer-controlled machining—first using paper tape and eventually digital technology—the cost of producing machined parts, once a strictly human task, fell sharply. But even after manufacturing was automated, the initial setup, done by skilled engineers, could be a painstaking process of turning computer-aided design (CAD) into computer-aided manufacturing (CAM). Once the toolpaths had been created and the computers took over, however, computer numerical controlled (CNC) machine tools buzzed merrily along churning out parts by the dozens, hundreds, or thousands.

Machining equipment could combine multiple tools into a single “cell,” allowing complex parts to be created with no human intervention at all. As a result, production was very inexpensive. The high cost of setup could be amortised over the number of parts produced and was quickly made up by the low cost of machining.

The parts themselves were strong, precise, and could be cut from virtually any machinable material, but because setup was a fixed cost the first few parts could be crushingly expensive. In other words, this powerful technology was not very cost-effective for short-run production. And in most cases it was far too pricey for use in prototyping, especially since prototyping could easily involve multiple iterations of the same part.

Technology—computers and inventors—came to the designer’s rescue with a partial solution. If machining small numbers of parts couldn’t be done cost-effectively, perhaps parts could be built up instead. The result was software that took a CAD design and sliced it into thin, virtual layers. These were then turned over to hardware that duplicated the slices in resin and laid them down, one upon another, in an additive process that resulted in a three-dimensional solid approximating the original design. The materials and processes for creating the solid varied. Among the successful technologies were stereolithography (SLA), selective laser sintering (SLS), and fused deposition modeling (FDM).

Because these processes worked directly from CAD models with virtually no human intervention, the first piece produced was less expensive than the first piece produced by CNC machining. However, each subsequent piece cost about the same, so machining eventually caught up, but for a handful of parts the cost advantage clearly favored the additive processes.

Unfortunately, for all their innovative engineering and low cost, additive processes had their drawbacks. The duplication of form was reasonable, but layering typically left “steps” on the surface of the part. And because bonds between the layers weren’t as solid as those in either injection moulded parts or the solid stock used in machining, the parts were not as strong as production parts would be, making them unsuitable for rigorous functional testing. Finally, because each of the additive processes was standardised on just one or a handful of resins, the resulting parts were often made of different material than that specified for full-scale production, again precluding functional testing. Simply put, except for their lower prices, additive processes were still inferior to machining for the production of high quality ultra-short-run and prototype parts.

One major obstacle standing in the way of CNC machining as a method for prototyping was fixturing, an essentially manual, and therefore costly, process. Another was the engineering cost of creating toolpaths from the original design, selecting tools, and loading g-code—the codes that position machine tools—onto the milling equipment. This was a far more complex task than slicing a 3D CAD model for use in any of the additive methods. Automation of those steps would require enormous amounts of processing power and hundreds of thousands of lines of software code. Fortunately, both of those had already been developed for another purpose.

The code was being used to mill injection moulds directly from 3D CAD models, and the processing power resided on the industry’s biggest compute cluster located at the headquarters of Protomold (now Proto Labs). Once founder Larry Lukis realised that the same software that had automated mould milling direct from 3D CAD models could be transformed for direct machining of parts, the conversion for direct machining was relatively straightforward. By eliminating the need for an engineer to run the CAM software residing between the original CAD design and the CNC machining equipment, Proto Labs slashed the cost of setup and drove down the fixed costs of CNC machining. Suddenly, the total cost of machining one or several parts was competitive with that of the additive “rapid prototyping” methods, and the First Cut service was formed to develop and deliver automated CNC machining services.

The benefits of automated machining as a prototyping methodology are clear. The process produces parts from solid stock without the inherent weaknesses of layered parts. Surfaces can be smooth with none of the stepping found in layered prototypes. The process can begin with any of dozens of plastic resins and aluminium, and the list of available materials continues to grow.

Parts can be made and shipped within one to three business days of order placement, and quoting is equally speedy. First Cut’s online quoting engine provides free FirstQuote® quotes for production of parts from uploaded 3D CAD models, made possible by the same massive compute cluster that manages parts production.

Obviously, automated CNC machining is limited to shapes that can be cut from solid stock using CNC milling machines. The First Cut process can currently produce parts that fit within a 254mm x 177mm x 95mm envelope. The maximum depth that can be milled from any side of the part is 50mm. Fixturing limitations currently require a minimum part size of 6mm x 6mm x 6mm.

The First Cut process is currently limited to 3-axis milling from six sides. Your quote will highlight any features that cannot be machined within these limits. Sharp inside corners on a part will be radiused as a natural result of the CNC milling process, and the quote will highlight areas where these radii will occur.

Some very small features may be difficult to machine effectively and will be identified in the quote. Recessed text, for example, should have a minimum stroke width of 0.5mm, and the spacing between characters on raised text should be 0.5mm or greater.

Unlike injection moulding, CNC machining can produce thick walls, and wall thickness need not be uniform. Walls should generally be no thinner than 0.5mm. Overall, tolerances of +/- 0.1mm are typically achievable.

A list of available materials for First Cut automated machining can be found here.

For a detailed comparison of prototyping processes, download our white paper.

Upload a 3D CAD model for a FirstQuote® automated quote.