The Ultimate Guide to Prototyping in Product Development Part 2: CNC Machining


This is the second in a 7 part series on prototyping in product development. In this article, we'll focus on CNC machining.

Intro to Prototyping

Creating prototypes is a critical step in any product design project. Prototypes reduce development costs over the life of a project, allow you to collect user feedback before launch, and ultimately result in a better end product.

A prototype will almost always result in design changes, so it’s best to get to this stage of product development as early as possible. Many potential issues can be sidestepped through good design, but you can never be sure just how a product will look or function until a high quality functional prototype is complete.

Very early on in the product development cycle, try to identify products or parts similar to what you’re trying to create. If something similar is available, think about modifying it to create an early prototype of your product. It won’t be perfect, but it will be much faster and cheaper than a custom made prototype. It will also allow you to learn a lot about your future product, and make any changes to your product design plan as early as possible.

Choosing the right prototyping method is important, and which method is best will depend on the part in question. The prototype should be as close to the production part as possible, so knowing which manufacturing methods will be used for full production runs is essential. Designing products with this in mind from the start will save time, money, and rework, so always have a solid plan in place.

When it’s time to create a proper prototype of your product, you’ll need to choose the right strategy. This series will attempt to cover a wide range of prototyping methods, but not all of them will be a good fit for your product development project - if you aren’t sure which is the right choice, consider consulting a product design professional.

Part 1: 3D Printing

Part 2: CNC Machining

Part 3: Sheet Metal

Part 4: Laser Cutting

Part 5: Waterjet

Part 6: Woodworking

Part 7: Finishing

CNC Machining

With CNC (Computer Numerical Control) machining, a computer controlled machine tool cuts away sections of a piece of material, leaving the desired part. Usually the machine is either a mill or a lathe, and there are many different varieties of each. To oversimplify, milling machines (vertical mills, horizontal mills, mill drills, gantry mills...) make prismatic parts (that is, rectangular parts with straight sides), and lathes (turning centers, vertical lathes, dual spindle lathes, swiss lathes…) make cylindrical parts. If you aren’t sure exactly which type of machine will handle your part, a machine shop or designer can point you in the right direction.

Unlike 3D printers, machine tools require careful setup; the work to be cut must be held securely, the tools used must be chosen, and the toolpaths planned to tell the machine what to do. This setup work goes into each new part design, whether 1, 10, 50, or 1,000 parts are made. For this reason you’ll find it’s often expensive to CNC machine a single prototype.

This method is most often used in production, but can still be used to prototype individual parts. The result can be extremely accurate with great surface finish quality. Mechanical parts can be assembled and tested just like the final product. If a part is to be CNC machined in production, a CNC machined prototype is an obvious choice.

Another advantage to machining is the wide material selection. Almost any material you’d typically find in a product can be machined, though you’ll most commonly see metals like aluminum, steel, or bronze. Other options include plastic, wood, foam, carbon fiber, and even ceramics.

CNC machining can produce accurate prototypes in a wide range of materials. Source: Machine Design

For complex parts, a 5 axis milling machine or mill-turn center might be required. These large, expensive machines are able to rotate the part and allow tools to access areas of the part on angles or sides without multiple setups. Not all machine shops have access to these machines, and you’ll pay more for the complex prototypes that require them. A good design team will avoid part designs that require this type of equipment, always keeping a design as simple as possible to save cost and time in development and manufacturing.

For CNC machined prototypes, some shops will work directly with a 3D model. Usually though, a drawing should be provided. Preparing a drawing for a part adds time to the prototyping process, but if the drawing can be edited and reused later for production, it will have been a worthwhile investment.

For most parts, the design starts with a detailed 3D model. Machine shops will need a technical drawing, which can be created once the model is complete. Some shops will agree to make a prototype directly from a 3D model, but a proper drawing includes details on tolerances, finishes, and other important information.

When designing parts for CNC machining, there are many factors to take into account. For CNC mills, these primarily relate to the cutter. Round end mills cannot cut sharp internal corners. Pockets must have rounded corners, and the larger the radii the larger the cutter that can be used, resulting in faster machining times. Side walls should be completely vertical where possible, as any angles will either require multiple setups or special tools. For a standard 3 axis mill, undercuts will require multiple setups, unless they are standard shapes like dovetails or t-slots. Some shapes are simply impossible to machine, and must be designed in multiple separate parts.

There are many potential issues when designing for CNC milling. The designer must have a thorough understanding of the process to produce an efficient part design. Source: Adam Bender

Want to try designing your own parts? Check out Adam Bender's guide, How to Design for CNC Milling here:

Designing parts for CNC lathes brings different challenges. Lathes typically create axisymmetric parts, which means they are symmetrical about an axis, like a cylinder or cone. Shapes that can be made by revolving a profile about an axis can be produced simply on the lathe, keeping in mind that the wide variety of lathe tooling all have different shapes and corner radii. For designs that incorporate other types of features, a CNC lathe with live tooling, or a mill-turn machine, may eliminate the need to set up the part in different machines.

Another challenge is finding a machine shop that will agree to prototype your parts. Because setup costs are high, machine shops make more money producing hundreds or thousands of parts using one setup than they do making one or two prototypes. Some shops may decline your project altogether, and the rest will charge a high price to cover their costs.

For this reason, many individual inventors or small businesses consider purchasing their own CNC machines to prototype their parts. With the introduction of small low cost CNC machines, this is a possible option, but keep in mind machining is a complex trade requiring experience, equipment, tooling, space, money, and lots of time. If you would like to be in the business of machining, it’s definitely doable. But if your goal is to create and launch products, you’ll save money and end up with a better prototype by paying professionals to do what they do best.

If your plan is to mass manufacture your product yourself, it’s true that you can save money and time by investing in a CNC machine rather than outsourcing. Just be sure to consider that it is similar to any other vertical integration strategy - you’ll be taking on more work to cut down on costs in the long run. Still, aim to outsource the prototypes and first production runs, so that if you decide not to continue with the product you are not stuck with equipment you don’t need. Only invest in your own CNC equipment if you are making consistent sales, have calculated that significant savings are possible, and are willing to take on the additional work and risk.

Investing in CNC equipment, like this vertical machining center, can pay off in the long run. But when you're just starting out, it makes more sense to outsource. Source: Hurco

Designing a part to be CNC machined properly requires as much knowledge and experience in the industry as machining the part. If you need a professional result, you’ll save significant time and money hiring an engineer or product designer to do it right the first time, just like the savings from hiring a machine shop to create the prototype. A full service firm will design the part to work well, prototype it quickly and accurately, and ensure the part can be easily manufactured.

In the next article, we'll discuss the differences between prototype and production sheet metal parts, and how to best design and prototype them.

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