Design for Manufacturability

Collaboration on design leads to serious press room savings

Originally published in Stamping Journal- January 14, 2010- Republished with permission from Bachman Machine Co.  
It's the little things that snowball into so much trouble. It could be a bend angle, hole location, or tolerance callout. A drawing comes in the door, and after one look, tooling personnel know the part won't make it through the stamping press at the specified tolerance. That's the best-case scenario. The worst-case happens after the tool is in the press. At that point, discovering a part can't be manufactured to spec is expensive, not to mention a major headache. 

No wonder collaboration has stepped to the footlights. With product development time squeezed, not many in the metal forming supply chain can afford not to collaborate—and this includes part design. After all, despite advances in software, simulation, die design, tooling components, and all the rest, you can't change the physics of metal forming. Some parts just can't be formed, no matter how advanced the available manufacturing technology is. 

Fortunately, part design isn't always immutable.

Designing, Tolerance and Manufacturability

"It has become more routine for us to have early involvement with product designers," said Glenn Sadowski, engineering and design manager at Bachman Machine Co. "Sometimes we come in during the infancy of a program, or later for an existing program. We're very much in favor of it, of course, because it benefits us." It also benefits all parties involved. 

Sadowski boiled down the issue to three areas: first, manufacturability; second, the part drawing, documentation, and dimensioning; third, other part costing information, including material usage, press requirements, and production rates. 

Bachman deals with all three areas. The contract manufacturer offers both toolmaking and production stamping and often gets involved early in the design phase, shepherding a project all the way through production. This way the tool designer, production manager, quality assurance personnel, and various customer representatives all can be on the same page, able to see a project from conception through completion—no "throwing a part print over the wall" to see if a tooling guy can whip up a die that will work, then over the wall again to the pressroom. 

Complete Preparation, Smooth Production

Bachman Machine deals with all the acronyms pervading the stamping lexicon. Customers request advanced product quality planning (APQP), value analysis and value engineering (VAVE), feasibility reviews, and more to ensure that the part can be produced efficiently and cost-effectively, with few if any surprises at the stamping press. The company's engineers have enough metal forming experience to pinpoint many problems, but to ensure nothing gets missed, they send the 3-D part file through simulation software. The software, from Forming Technologies (FTI), shows engineers percentages of thinning, thickening, as well as problem areas. The module, BlankWorks®, works inside the company's SolidWorks® software. 

At times customers get Bachman involved during a program's infancy, and may even send entire teams to the stamper's St. Louis facility to discuss potential avenues for production. That's the ideal scenario, but such collaboration doesn't always happen. Like most stamping and tooling shops, Bachman bids on part prints. But for a growing number of jobs, the company uses its tooling and production stamping expertise during quoting. A Bachman engineer often makes subtle suggestions, sometimes stipulating it in the written quotation and adding that the suggestions are based on further review of the manufacturability and tolerancing. 

More drawings coming in Bachman's door now have process capability elements. Some have significant characteristics and critical characteristics (SCs and CCs), the latter usually safety-related, that must hold to specific tolerances and capability. Bachman conducts process capability studies on those CCs and SCs to ensure they can meet minimum CpK (predicted process capability based on a short run of early samples) and PpK (actual process performance during the run) requirements. This dovetails into Six Sigma work. All this is weaved into Bachman's front-end collaboration with the customer, and much of it relates back to part design and tolerancing. Change this bend angle or that radius, or widen a certain tolerance, and the ultimate CpK and required sigma ratios (so many parts per million) become much easier to attain. 

Material is the primary expense at Bachman, like anywhere else, so it's no surprise that blank size alternatives enter the picture. Here's where the advantages of having tooling design and production under one roof really shine. Tooling engineers establish a blank width and progression (most Bachman tooling consists of prog dies), and the production manager works with tooling personnel to determine press requirements. With the blank size and number of stations established, engineers determine the die size, which in turn determines the press size and tonnage. Also, if engineers notice a particularly difficult form in a part, they may make room in the prog die for a few extra stations, just in case. 

Learning to Juggle

Part design, documentation and traceability, quality requirements, press size and tonnage—all of it is determined from the get-go. This is the ideal, but sources conceded that it doesn't always happen this way. Like anywhere else, Bachman works with some incredibly truncated lead-times, which means a lot happens at once. 

Bachman's customers, many of whom are in the automotive arena, often forward a project while the part itself still is in the design stages. "As they are designing these parts in 3-D, customers will send parts on to us to get a jump on the costing of the tooling of the parts," Sadowski explained. "They may not even be finished with the design, so they're continually refining that as they go through their crash tests, prototyping, and computer analysis." 

In other words, Bachman's juggling a lot of things at once. If a critical feature is changed midway through tooling development, the costs will change as well. This is where that creative thinking comes into play. For instance, Bachman recently worked on a job during which the customer doubled the part thickness, from 2 to 4 mm, and changed the shape as well. Engineers were about two-thirds the way through the tool design when they had to put on the brakes. Some significant rework simply was unavoidable. But thanks to some part design tweaks, engineers were able to keep the same progression and, hence, save the customer money. The part had a few corners with flanges so long that they couldn't be formed within the available stations in the prog die. 

"We asked them if we could shorten the flange just a little bit," Sadowski explained. "They said it could be. Because of this, we were able to save part of the tooling design." 

Getting involved early allows engineers to catch part design and manufacturability issues immediately. For instance, if Bachman engineers see a design with a pierced hole on a bent flange, one that would require a mechanical or hydraulically driven cam to pierce it, they know such an operation would increase tooling costs as well as die maintenance costs. That hole might also have a tight tolerance because it must match precisely with another hole for a bolt to thread through during assembly. 

But what if that hole was changed to a slot, giving the assembly a wider tolerance in one direction? The slot could be a few thousandths of an inch or so in two directions, and the bolt still would thread through. Also, that simple slotted hole "could be punched in the flat, instead of cam-piercing it," said Harold Thomas, lead tool estimator. 

"That's an example of us taking into account the inherent variability in the process that the part designer might not realize the significance of," added Ed Ide, quality assurance manager. 

This kind of job could be a candidate for in-die operations as well, such as in-die assembly, including staking and nut insertion. If the estimator thinks a secondary operation could reduce production costs, he gives the customer a call, before tool design begins.  

One part design recently sent to Bachman had a corner flange that raised a few red flags. Analyzing that geometry requirement, engineers considered the length of the flange and radii involved. Usually these flanges were designed into this part to add strength. The high-strength, low-alloy material thickness was 2 mm. It had about a 1.5- to 2-mm inside corner radius, and the flange was about 8 mm. But engineers noticed that this part didn't have a particularly high strength requirement. Did the flange need to be that long? After talking with the customer, they found the part didn't need as much strength as the drawing showed. After running the part through simulation software, they discovered some significant savings. By increasing the inside corner radius to about 3 mm and reducing the flange to about 6 mm, Bachman made the part a lot simpler to form, reducing the number of hits from four to one. 

If two parts are of identical thickness and material—two parts to be assembled together, for instance—Bachman may be able to nest a smaller part inside the scrap area of the larger part, Thomas explained. "It's like a free part, really. You don't have the cost of the material or running it. You just have a little additional cost in the design and build of the tool. But it's much cheaper than having to design and build two separate tools." 

Recently Thomas quoted a job so that one part would be nested in a certain way inside a larger part not only for material savings, but also because the material was 6 mm thick and high strength. Regarding the small, narrow part, "It's very difficult to run narrow coils through," Thomas said. Nesting the part inside the scrap of a larger part on a wider coil strip "makes it a lot easier for the pressroom people to run the job." 

Collaborate to Compete

The recession did force Bachman to downsize, as it did many other stampers. The shop employs 80, down from 104 in early 2008—not good, though not incredibly bad considering how tied Bachman is to the automotive sector. During these tight times, the stamper has been using this collaborative approach to bring more work in the door, and it's been integral during the economic recovery. By the third quarter of 2009, the stamper had a significant uptick in business. 

"Collaboration has become an ever-increasing part of our business," Sadowski concluded. "And as far as we see, it will become more common yet."

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