Posted June 2, 2017
6 Common Mistakes in Designing for Manufacturability
Design for Manufacturing (DFM) is a mentality that should be ever-present in the minds of designers - it is a constant vigilance to design parts with their future manufacturing requirements in mind. By taking this into consideration early on, it’s less likely that you’ll have to return to your design for drastic changes later, potentially saving both time and money. Plethora’s CAD add-in gives you real-time DFM feedback for your machined part, but there are some design choices you can make to avoid encountering future manufacturing issues. Here are some common pitfalls we’ve seen in DFM that are easily avoidable:
#1: DESIGNING DIFFICULT-TO-MACHINE FEATURES
A part with very thin, deep walls may cause problems.
Some features are simply more difficult to machine than others, and may become impossible when using certain materials. Thin-walled sections, for example, may snap off while being milled due to excessive vibration. Small, but deep, threaded holes may require your manufacturing partner to go out and buy a long-reach tap. Complex 3D surfaces are time-consuming to machine, especially if you require a great surface finish. All of these are worth considering during the design phase, and should be avoided if possible.
#2: DESIGNING IMPOSSIBLE-TO-MACHINE FEATURES
Many features you design on your part may add complexity and cycle time, but there are others that will be simply impossible for certain processes. In milling, for example, curved holes cannot be milled and should be avoided. Also consider an object such as a whistle, which contains a small bean inside a housing - this would also be impossible to mill in a single part, as there’s no way for a tool to access the inside of the housing.
In this cutaway image, notice the curved hole, which is impossible to drill out.
While designing your parts, it’s always worth considering the process used to create it and the physical limitations it brings.
#3: BEING UNAWARE OF MACHINE CAPABILITIES
A common mistake we’ve seen in DFM is a general lack of knowledge about the machines used to create your part. In early conversations with potential manufacturing partners, it’s worth asking for a tour of their facility or a list of the machines they have on hand. This will immediately reveal a lot, including how they like to work, what their specialities are, and what sort of material they’re able to process. With this knowledge, you can immediately modify your design constraints.
An example of where this information may come in handy is in sheet metal design, where the maximum width of your bends is dependant on how large the sheet metal brake is at the manufacturer. For CNC material removal processes, knowing the machine may help with your tolerancing. It’s nice to be able to consider what tolerances come “for free”, or what tolerances are trivial to hit due to the quality of the machine. This changes dramatically between machines, and removing unnecessary tolerances from a drawing due to the quality of the machine being used can lead to a cheaper quote.
#4: ADDING UNNECESSARY AESTHETIC FEATURES
In a material removal process like milling, it’s important to consider how much material the manufacturer is going to remove and in what way they’ll go about doing so. Ask yourself questions like “is it necessary to be removing material for no reason other than looks?” and “is the single complex feature, which requires 5-axis machining, entirely necessary?” Avoiding both unnecessary material removal and aesthetic features that are difficult to mill will reduce machine runtime and programming time, decreasing your cost.
Notice the huge number of (potentially unnecessary) filleted edges in this image.
Also consider the way the customer is going to be using the product - there may be faces or surfaces that are never seen by the user in everyday use, so it’s not worth spending as much on a high-quality finish for that face. These potential cost-cutting routes will become clear as prototypes are handled and used by designers, too.
#5: LACK OF DETAIL IN PART SPECIFICATIONS
It’s always better to include too much information than not enough, especially with partners that you haven’t worked with before. Although engineering drawings typically appear on the tail-end of the design process for a part, it’s a huge part of DFM, as the numbers, text and images on a drawing are often necessary information for your manufacturer.
For example, in specifying the material to be used, be specific - “Aluminum” isn’t enough. Do you need 6061? 5052? And what temper? T4? T6? Do you need a material certification to ship with the part, to prove the material is exactly what you asked for? These decisions will affect both the function of your part, and the cost.
#6: USING NON-STANDARD MATERIAL AND FEATURE SIZES
It may seem minor, but it’s always worth considering how your finished part is going to start out its life. Does it come pre-cut and faced to the right stock size, so it can be thrown right into a machine? Or does your manufacturing partner need to receive an entire pallet of sheet stock, store it in inventory, and process each sheet before it can be machined? Perhaps your design could be changed so as to reduce the amount of prep work your manufacturer has to do.
This extends to other processes, too. Consider a 2D process like laser or waterjet cutting - the outer profile of your part determines how easily it can “nest” among its neighbors, and will directly affect its cost. If the profile shape in your design is arbitrary, see how many you can fit in a fixed sheet of stock, and then change the design to improve throughput.
Also consider the features in your design that require specialty tools to create, such as tapped or drilled holes. By adhering to common tap and drill sizes, it’s way more likely that your partner will already have the tools they need to make the part right away, instead of forcing them to hunt down a specialty size in order to complete the job.
While many of these tips only graze the surface of the wonderful world of manufacturing, it’s great practice to think about these as you go about designing a product. And remember, like any good relationship, communication is key! You and your manufacturer both want a successful partnership, so treat them with respect and let them provide advice in areas in which they’re an expert.