Think like a machinist when creating solid models

Until the early 1990s, solid-modeling software was so expensive that only the largest companies could afford it. The software was difficult to use and its steep learning curve often deterred designers from making the switch from drafting boards or 2D programs to 3D. But today, programs are affordable and easier to use. Most designers and engineers learn solid modeling in colleges or technical schools and then launch into the business world to start designing machines and components.

But some newer (and even some more-experienced) designers and engineers never consider whether the 3D CAD component they have just created is actually manufacturable. The assumption is, “if I can create it on the computer, it can be made.” In most cases this is true, but at what cost?

The result: The machinist is often confronted with unnecessarily difficult or sometimes impossible machining scenarios. These can result in costly designs and are generally attributed to a lack of exposure to the machine-shop setting and those processes related to machining.

As an engineer with over 15 years of SolidWorks experience and an extensive machine shop background derived from more than 50 years in the business, I encounter machining challenges every day. I have made a business of reviewing outside designs and suggesting methods to make them compatible with general machining practices.

Some readers might claim that their designs are meant to be molded rather than machined, therefore this article doesn’t apply to them. However, a stable working prototype is often required before committing to production. Not all models can be satisfactorily duplicated using stereolithography or other additive-printing methods when the component is going to be used as a functioning part. For newly designed, one-of-a-kind items, or small quantities, molding is not an option.

Hopefully, pointing out just a few of the problems machinists face will offer insights into ways to make designs more economical to manufacture.

Is the stock size handy?
Often when researching materials, the required size appears on a vendor’s Web site or catalog but is unavailable when you try to order it. Whenever possible try to design around standard material size in the coarser fractional dimensions such as 1/8, ¼, ½, 5/8, and ¾ in. Try and avoid the 1/16, 7/16, and 9/16-in. sizes because most mills either don’t stock them, or require a special run. (This guideline applies less to sheet stock or round bar than it does to rectangular bar and plate.) Should your designs require metric dimensions, as many medical products do, consider that in the U. S., stock metric material is sometimes more difficult to obtain.

Whenever possible, try to design long, thin parts around more-readily available stock sizes to avoid or minimize machining long surfaces. Removing material from one side of a large surface usually causes the material to distort or bow, forcing machinists to keep removing material from alternate sides to bring back the straightness. A time-consuming operation, it often takes many passes to make the material flat again and, in some cases, the results are not predictable. As a rule, larger cross sections of material that need to be machined to thin profiles will likely distort.

As machinists review customer drawings, they evaluate features, the steps it will take to produce the part with the fewest setups, and workholding — a major consideration. Whenever possible, design your part to have at least two opposing parallel flat surfaces or a truly cylindrical surface, so it can be gripped by conventional vises and tooling. Otherwise, custom fixturing or additional material will be needed to anchor the part. This can boost manufacturing costs significantly.