September 2011 Machining Tip

Firstcut Machining: When, Why & How

Computer Numerical Control (CNC) machining uses programmed toolpaths to automate the motion of a machine tool. Once the required motions are programmed, the process produces (almost) identical parts quickly and efficiently. The key hurdle is in programming the toolpaths. Even with today’s Computer-Aided Manufacturing (CAM) software tools, programming the toolpaths has been time-consuming and costly, requiring a skilled human programmer to select the correct machining strategy for each area of the design. The cost of programming the toolpaths has meant that producing only a single part by CNC machining can be very expensive. Normally it takes a fair number of pieces to bring the per-part cost down to a reasonable level. It can also sometimes be difficult to find machine shops that will accept an order for very small numbers of machined parts because it ties up a scarce resource: the CNC programmer.

Our Firstcut CNC machining service replaces the costly human programmer with about a million lines of sophisticated code running on a massive parallel compute cluster (see Figure 1). Since the non-recurring “engineering” requires no human labour, a single machined part costs far less, and short runs become fast and affordable. It is so affordable that it can compare favourably with additive RP processes like stereolithography (SLA) or three-dimensional printing (3DP) while delivering a solid, rather than layered, product. For more in-depth information on CNC machining and additional prototyping processes, download our Prototyping Processes white paper.

Figure 1: The Firstcut process uses proprietary software running on a large-scale computing centre that translates your 3D CAD designs into programs that run on state-of the-art CNC milling equipment.

Figure 1: The Firstcut process uses proprietary software running on a large-scale computing centre that translates your 3D CAD designs into programs that run on state-of the-art CNC milling equipment.

Faster lead times and low cost at low volumes are features which differentiate Firstcut’s automated machining and Protomold’s rapid injection moulding. Many designers choose to machine prototype parts in the early phases of product development before moulding prototypes for larger scale functional testing in later phases. But Firstcut’s automated machining is more than a low volume stand-in for rapid injection moulding. The process has some capabilities significantly different than those of injection moulding that make it an even more valuable addition to the developer’s tool kit.

One major area of differentiation is available materials. We work with a variety of stock materials, many of which cannot be used for rapid injection moulding. Those currently include metals like aluminium and brass. We can machine plastics that are too corrosive for rapid injection moulding, such as PVC and CPVC, and resins that need to be injected at temperatures that are too high for use with the aluminium moulds used in rapid injection moulding. We can also machine plastics that cannot be injection moulded at all, such as PTFE. Of course, the machining process can be used on a wide range of resins that can also be injection moulded, making it ideal for initial testing of parts that will later be injection moulded. There are some exceptions; for obvious reasons, Firstcut cannot currently machine rubbery materials like TPE or TPU.

Firstcut machining can hold tolerances of +/- 0.1mm and can be used to form shapes that may be difficult or impossible using rapid injection moulding, such as threaded holes. Parts can be machined with less concern for relative thickness of walls; for example, thick areas that could cause sink are no problem when being machined. And because resin doesn’t have to flow through a mould, thin areas that could impede the flow of injected resin aren’t a problem either, although extremely thin areas—typically below 0.5mm thick—cannot be reliably machined. And, machined parts have no knit lines.

Since machined parts don’t have to be ejected from moulds, there is no need to draft features. The exception is certain deep, narrow cuts. These must be drafted for the same reason that rapid-injection moulded parts must have draft on some tall narrow features: long thin cutters can chatter and gouge the side walls of a deep cut, and draft allows the use of more stable tapered cutters.

Finally, Firstcut can machine undercut features that would be produced by side actions in a moulding process. This is because our process does 3-axis milling from up to six (orthogonal) sides of a work piece.

If you need any of the incremental capabilities machining can offer—thick and thin walls, no draft, and undercuts—and if your volume is low enough, machining can be your solution. If you are using Firstcut to produce prototypes that will later be produced by rapid injection moulding, these same capabilities can mislead you into incorporating features that are difficult or impossible to mould. There is, however, a simple way to ensure that your machined design will also be mouldable. When you submit your 3D CAD model for a free FirstQuote® and design analysis, submit it for a free ProtoQuote® as well. The ProtoQuote design analysis will identify any un-mouldable features, allowing you to address any problem features up front.

As outlined above, Firstcut machining can duplicate most features that can be produced by rapid injection moulding, with one significant limitation: size. Size limitations are a complex function of available sheet stock, materials, and the number and type of side setup. The best way to check if your part meets our dimensional limitations is to upload a part for a quote, or check our Design Guidelines. Best of all, we can make parts in as little as one day, at prices as low as £50.

Read past Machining Tips.