Designing sheet-metal parts as 3D solid models sounds odd. The parts are punched, stamped, or burned out of flat material that’s easy to describe in 2D CAD. So why bother with solids?
It turns out there are plenty of good reasons. For instance, solids let engineers see the part in 3D rather than guessing what it might look like from a flat pattern or even top, front, and side views. Many sheet-metal programs now include knowledge bases that hold design and manufacturing information such as the tools available in the shop. Then if a designer calls for a hole that requires purchasing nonstandard tooling, the software quickly issues a warning to that effect, letting the designer reevaluate the request. What’s more, the software can calculate bend radii without having to pick up a handbook, flatten 3D designs for production, and automatically dimension drawings.
But all the capabilities provided by software can still get users into trouble if not used correctly. The experts we spoke to offered these guidelines and rules of thumb for designing with sheet-metal software.
The top ten
“My first suggestion doesn’t even involve software,” says James Gibney, a consulting engineer with Pro CAD Inc., West Caldwell, N.J. “Spend some time in the shop, if at all possible, to learn how parts are stamped, punched, and folded into products. How can you design something if you don’t know how it will be manufactured?”
The idea came to Gibney while working as the senior designer for a sheet-metal shop. When negotiations with clients got serious, the sales staff would bring him into the equation. It turned out the discussions would not revolve around costs, but on metal-bending technology, something the salespeople lacked. So why not let new salespeople and designers spend two weeks on the shop floor helping move products to the shipping dock before sending them out to sell? “They’ll learn the capability of the machines, available tools, and what experienced personnel can and cannot do. And they turn out to be better sales people than those who did not have the experience,” he says.
Training in the shop can be invaluable. When it’s not available, get proper training in the software and work through the tutorials, suggests Joseph Riden, a consulting engineer in San Diego. Working with 3D software requires a different mindset than 2D methods because it provides more downstream possibilities than 2D technology. For example, 3D sheet-metal models allow checking for interference in assemblies, or building tools from the part model.
In addition to learning the software, learn how work flows through your facility. “If you look at the design process from a high level,” says Riden, “you see a lot of activities and the people involved from idea conception through product release and manufacturing. It’s useful to know the detail in between because you’ll be making decisions everyday that feed into the flow — or work against it. And it’s easy to unconsciously make decisions that hinder the process flow,” he says.
For example, a design engineer might make a decision based on an assumption about a part rather than asking how it fits into an assembly. “I may design the part in a way that makes it more difficult to assemble,” says Riden, “but if I go to the manufacturing floor and look at the tools they have, I can get a better idea about how to manufacture the part.”
And don’t assume the part will be made of sheet metal, suggests Tony DiBari, who directs the technology team with Tyler Refrigeration, Niles, Mich. “Design the part for its function first and then decide whether or not to make it out of plastic or another material. Your volume requirements will play a factor as will durability requirements.”
When sheet metal is a good candidate for the part, don’t assign a thickness right off, suggests DiBari. The focus of the design should be on the part, not its thickness. “Use the power in the software to shape the part by starting with the allowable volume and coming to the right design. Then assign a thickness,” he says. Most systems can provide a default thickness that might not be good for the design.
Pro Cad’s Gibney suggests changing thickness only if you plan on thoroughly reexamining the part afterward. Assigning features and placing bends for one thickness may be grossly inappropriate for a thicker material. “Holes can turn into ovals or suddenly wind up in bends when you thought they’d be somewhere else,” he adds. “So don’t casually change the assigned thickness.”
Software training will probably not cover company design guidelines, so ask about the department’s design and shop manufacturing preferences. “You’ll have to take into consideration a lot of in-house developed bending calculations and allowances,” says Bruce Kniller, manager of CAD services with Utilimaster, Wakarusa, Ind. “Just about every design facility has a book or listing of their preferences gleaned from years of experience with the material and product.” Many of those preferences depend on the manufacturing process — how the material forms on the press brake, a machine tool that bends sheet metal. “We don’t use the numbers that come out of solid-modeling system because they are not appropriate for our product. We use our own bend and form angles that calculate the stretch of the material.”
A lot of companies have similar practices. Design software is productive, but it’s not all-knowing because its databases may hold only industry standards. “We’ve developed a book of company standards, site standards, and department preferences,” says Steve Peters, a manufacturing engineer with B/Line Inc., Modesto, Calif. “It tells users our mathematical preferences for bends and folds, and a list of frequently encountered terms. We want designs and drawings set up in the ways described because it minimizes confusion,” he says.
A corollary to observing a company’s best practices says don’t force your manufacturing assumptions on the shop. “For example, the most complex part of a sheet-metal model for our parts is often at corners where side folds come together and material distortions changes part thickness,” says Brent Thordarson, a designer with Hewlett-Packard Co., in Loveland, Colo. “We’re not too concerned about the exact details of those features. What we are concerned about is the dimension of an offset, or just that the corner is there. You want to agree with the shop that they are to use what works for them. You care about the overall dimension and that it will not cost a fortune to make.”
A variation to the rule is that if you don’t need a particular detail, don’t put it in the model. “Leave out details that makes the model unduly complex,” says Thordarson. “For example, if a corner is to be welded and ground, it’s probably not critical to model how the corner’s created before the welding and grinding. More depends on what the end function turns out to be,” he says.
Wisdom from the shop
The people on the shop floor have their own perspective on your work because they will find your mistakes, possibly after they’ve turned them into useless parts. To make their job, and yours, progress smoothly, they suggest several additional guidelines, particularly about accuracy and proper dimensions.
For instance, Tyler Refrigeration’s DiBari suggests you validate suspect dimensions. Since design is not manufacturing’s job, be smart enough to check that dimensions are correct. “On one occasion, a plastic part sent to production from the vendor wouldn’t properly fit in an assembly. Fortunately, we caught the problem with a prototype part. By comparing drawings, we found the dimension differences that would not allow the parts to fit. Prototypes are one way to catch such errors.”
Others expressed similar problems. “Parts get so complex with bends, tabs, and holes that users often overlook the dimensions for several details,” says Dawn Phillips, a manufacturing assistant with Georgia Hi-Tech in Vidalia, Ga. Filling in each missing dimension on the drawing can take up to an hour each. In the meantime, part production stops. Accuracy issues also surface with the mention of file transfers. Most manufacturers relate positive experiences. “But on occasion the electronic file, DXF or IGES, doesn’t match the drawing,” says Russell Page, CAD/CAM manager with Trident Precision Manufacturing Inc., Webster, N.Y. It’s usually a file-keeping problem, he surmises, where the designer translates the file and then changes the drawing. The mistake is easier to make in companies that keep every revision of a product. “At times we’ll find a feature that’s missing or an additional feature on the file that’s not on the drawing,” he says. “Our policy is to go with the electronic file, although it usually warrants a call to the designer. And that temporarily stops work,” he says.
Should your shop express no preference on file format, send them the model of the part you expect back, as opposed to a flat pattern, suggests Hewlett-Packard’s Thordarson. “Let the fab shop unfold what you have created,” he says. “We send them the 3D model and let them unfold it so they can do things their way.” When the shop is incapable of accepting CAD models, be flexible enough to supply them with a format most useful to them, such as an IGES file. The part that comes back must fit your assembly, so don’t make assumptions on what’s best for them.
If you’d like to see what your shop is receiving when using the Initial Graphic Exchange Standard, translate a model into an IGES file and then translate it back into your modeler. You find that not all IGES files are created equal. Some programs produce corrupted geometry such as disjointed entities, arcs that don’t meet lines, and lines that don’t intersect other lines. “You know the errors are coming from the sending system because the target system can translate only what it’s given,” says Page.
The most troublesome IGES translations usually come from older CAD systems. “It’s usually the complex models with lots of operations on which we find disjointed entities,” says Page. “And we might spend a half-day on file-repair work before the model can be unfolded.”
Some flawed IGES or DXF files come from CAD users simply following the brief instructions the software provides to produce a file. “We have to mend broken geometry before performing later operations such as unfolding or machining. Anything required downstream also takes more time.”
The solution is simple: Call your manufacturer and ask what file format they can receive most accurately, suggests Page. “We work with a modeler built on the ACIS kernel, so the SAT files come in with good accuracy,” says Page. “We also just started using STEP files with good success,” he says.
And since design changes are inevitable, always use the solid model and not the flat pattern when planning to improve or change a part. “A lot of designers coming from drawing boards or 2D CAD always made the flat pattern to drive the design,” says Tyler Refrigeration’s DiBari. “But when folded up, it didn’t make or equal the formed view they want.” DiBari recalls his shop had a case in which folded parts didn’t match the drawing views because someone modified the flat pattern instead of the folded design. “The shop worked through that ordeal. Fortunately, switching to solid modeling can eliminate the problem if process controls are in place and followed. These situations benefit from software that associates flat and folded models.”