“Research suggests that manufacturers spend 30% to 50% of their time fixing errors and almost 24% of those errors are related to manufacturability,” according to Kashyap Vyas in Machine Design. That’s a lot of wasted material, time and money due to mistakes. The good news is by following principles of design for manufacturing (DFM) and design for assembly (DFA) you can spot these errors and even avoid them early in the game.
There’s more than one way to design a part and several factors at play: the part’s intended function, the raw material best suited to that function, available equipment, and skilled technicians, and the timeframe for production.
Beyond these, DFM and DFA emphasize ways to make parts more efficiently (i.e. faster and with less scrap), stronger and more durable, with less risk of damage during fabrication, and as error-proof as possible during assembly. The benefit to you? Money saved and faster production.
Below are 5 considerations to improve DFM and DFA.
1. CAD vs. Reality
There are big differences between the ideal, mathematical world of CAD and engineer’s drawings and the real world on the shop floor. Drawings are great for seeing how parts fit together and where bends are located, but they don’t always take into account how sheet metal behaves in the real world. For example, bends can distort holes if they’re not located correctly. Also, consider what the available equipment can and cannot do (e.g. what tonnages can the press brake machine achieve?). A visit to the shop floor lets you see first-hand the capabilities and limitations of each machine and gives you a chance to talk with the fabricators who have expertise in turning drawings into finished pieces.
2. How Does Your Metal Behave?
As Art Hedrick says in Stamping Journal advises, “think like your metal.” Metals have properties that vary by the type of metal and by grades within a single type. Things like luster, heat and electrical conductivity, malleability, ductility, weldability, and corrosion resistance directly impact how a given metal will react to fabrication. For example, if your punch tool and your raw material are the same grade of steel, they’re likely to stick (i.e. galling), which can distort the hole and damage the punch. Consider how metals behave alone and when mixed and try to anticipate what various grades will do when bent, punched, stamped, cut or welded.
3. Best Practices for Part Features
How wide should flanges be? Can you keep a tab from cracking or tearing? Part features like bends, holes, tabs and welded joints work with or against your intended design (see Vyas’ article in Machine Design for a solid introduction).
- Bend theory includes guidelines for when to bend vs. when to weld, how to anticipate and prevent tears with bend relief cuts, knowing which forming method is best for the inside bend radius and how wide a flange can be depending on the thickness of the sheet. Find more tips in The Fabricator on bending at the press brake.
- Like paper and fabric, metal has a grain. While it may be easier to bend or fold parallel to the grain, in metals this risks cracking at the bend, which means a weaker, less durable part. “The recommended practice is to form lugs perpendicular or at an angle less than 45 degrees towards the grain direction.”
- Holes are used for fasteners, for aligning components and in some cases for providing access. When the placement or dimensions of a hole are critical, make sure you’ve thought about where it is relative to bends, edges and other features. For example, if a hole is too close to the edge it will stretch or tear if the metal surrounding it is not strong or thick enough. Similarly, “when the punch contacts the material, it naturally draws some of that material inward as it shears and penetrates the material,” which distorts the area around the hole. This effect is magnified if your sheet is perforated with many small holes.
4. How Low (or High) Can You Go?
Tolerances are the total amount a feature’s dimensions are permitted to vary, given as a range between maximum and minimum numerical values (e.g. a hole must be placed between 0.7500 and 0.7549 cm from the edge of a sheet). Specifying tolerance values with your dimensions helps the designer and the manufacturer: the designer uses tolerances to remove ambiguity in fabrication and to signal the technician that the feature’s dimension matters to the finished part. The technician relies on tolerances to ensure the part is made correctly. See Stefan Menin’s Working with Dimensional Tolerances for more.
5. Putting It All Together
Even the best-looking streamlined part with perfectly-positioned holes is a waste if it’s tricky to assemble. One way to simplify assembly is to reduce part count. Fewer parts means, “fewer drawings to keep straight, fewer tolerance issues, fewer assembly stations/equipment (and thus less assembly labor required), fewer tools, jigs, fixtures, etc., necessary and a simpler supply chain overall,” according to Jessica Irons in Appliance Design.
Another technique for error-proofing is to build in notches, holes, tabs and other visual clues. These help the technician line up pieces and, in the case of tabs and physical markers, ensure there is only one way the part can come together. Tabs can further reduce the need for fixturing before welding if they are placed so the part holds together temporarily. Including features like these in your part design makes assembly easier for the technician and faster for your schedule.
The old saying goes, “if it ain’t broke, don’t fix it”; but, DFM and DFA caution that it might not be broken, but there could still be a better design. Grant Hagedorn, in The Power of Sheet Metal Design, says, “good sheet metal design should reduce, simplify, and mistake-proof shop floor processes to ensure greater efficiency and, ultimately, dramatic cost reductions. [If, for example,] a new design eliminates welding but makes the bending process incredibly complex, the process is moving backward.”