Making it manufacturable

New tools help parts makers design for manufacturability, which reduces machining costs and improves part quality. 

Any veteran machinist has stories of laughable part designs. Parts with impossibly deep hole callouts, dead-sharp internal corners and grooves wider than the Grand Canyon. Design engineers love to dream up impossible-to-cut part geometries. It might sound harsh, but those engineers sometimes learn the hard way how not to design machined products. 

In his book “Design for Manufacturability and Concurrent Engineering,” David M. Anderson, fellow at the American Society of Mechanical Engineers, wrote, “Design for manufacturability is the process of proactively designing products to (1) optimize all the manufacturing functions: fabrication, assembly, test, procurement, shipping, delivery, service and repair, and to (2) assure the best cost, quality, reliability, regulatory compliance, safety, time to market and customer satisfaction.”

Courtesy of Boothroyd Dewhurst

This new air-duct butterfly-valve assembly, designed with DFM software tools, is manufactured from round aluminum stock with a near-net diameter and has a seal design with wide tolerances, reasonable surface finishes and an angled seating arrangement. These features allow for an easier insertion process than was used in the original valve.

That’s a mouthful. What it means depends on whom you ask. Engineering, cost accounting, logistics and quality departments each have their own version of the truth. To the people in the shop, it’s simple. Design for manufacturability (DFM) means making parts easier and faster to machine and, therefore, more profitable. 

Anderson suggests a number of no-brainer techniques: design parts that can be made in a single operation, avoid interrupted cuts and complex part geometries, specify reasonable tolerances and surface finishes, understand workholding principles and design features that can be cut with standard tools. Sounds great, but how does one learn how to design a more machinable mousetrap without first spending years in the shop? 

Courtesy of Boothroyd Dewhurst

Figure 1. DFM concurrent costing for an 8 " Torpedo carpenter’s level. All costs are relational.

DFM software is part of that answer. Brian Rapoza, R&D manager at Boothroyd Dewhurst Inc., Wakefield, R.I., explained how his company’s software helps designers estimate the costs associated with decisions made during the part design process and identify the potential for reduced cost and improved quality.

A typical example of this can be found in a large mechanical assembly. Boothroyd Dewhurst’s Design for Manufacturing and Assembly (DFMA) software analyzes the components used and looks for opportunities to combine them. “Rather than make a large number of simple, single-function parts, the software might recommend a multifunctional part in their place.”

But how is one big monster part better than a bunch of small ones? “It might be more difficult to machine than a single part, but in the end it could be less expensive because you gain higher throughput,” Rapoza said. “You have to evaluate the trade-offs, comparing production volumes and tooling costs, but this approach is usually cost-effective.”

Rapoza explained that the software works by presenting the user with a series of questions, such as potential part volume and projected surface area. “From this, we can estimate the time for each manufacturing process and, therefore, derive the cost to make the part as well as the tooling,” he said. “By identifying the major cost drivers in a part design, we can determine the most economical way to make that part.”

Courtesy of Boothroyd Dewhurst

A simplified CAD rendering of the level. Aztalan Engineering used DFM Concurrent Costing software to compare manufacturing processes and select the best one. 

Courtesy of Boothroyd Dewhurst

An extruded aluminum billet for the carpenter’s level is fixtured for machining. DFM analysis led to the elimination of this intermediate machining operation.

A recent case study by Boothroyd Dewhurst stated machine shop Aztalan Engineering Inc., Lake Mills, Wis., saved 25 percent when producing 8 " Torpedo-brand carpenter’s levels (see Figure 1 above, CAD rendering and photo above, and chart below). DFMA analyzed the customer’s CAD models for opportunities to improve part design, ultimately leading to more competitive prices and greater machinability. DFMA also plays an integral part of engineering design review at Aztalan, as well as procurement and process control. “It’s central to a lot of what we do,” said General Manager Jim Hale.

A 2011 report by Boothroyd Dewhurst about aerospace supplier ITT Aerospace Controls Inc., Valencia, Calif., outlines how DFMA revamped a 30-year-old air-exchange valve design. By reducing the number of components, simplifying the assembly process and creating a valve body design that could be machined in a single operation, ITT Aerospace reduced costs 76 percent.

Boothroyd Dewhurst boasts a large customer list, including Pratt and Whitney, GE and Whirlpool. During Boothroyd Dewhurst’s 2012 International Forum on Design for Manufacture and Assembly, Westinghouse Electric said it reduced the number of components in its Spider nuclear fuel assembly from 41 to two using DFMA and value engineering, a method of improving the value of manufactured products.

Find It Early

Another popular DFM tool is DFMPro from Geometric Ltd., Mumbai, India. Product Manager Rahul Rajadhyaksha stressed that his company’s software is different than other “pure-costing” DFM solutions. “DFMPro is built for design engineers, wherein they can identify and resolve problems right at the design stage,” he said. “These problems may be related not only to cost but also to quality, schedule, standardization and other issues.”

To this end, according Rajadhyaksha, DFMPro encourages the use of design best practices through a series of internal rules. “There are more than 100 built-in checks for manufacturability within the package,” he said. “This allows best practices and learning from downstream costing operations to be leveraged and then validated upstream during the design process. It makes life easy for design engineers; with just a click of a button, they can validate the design. DFMPro not only identifies potential problems but also suggests cost-effective alternatives.” 

Those best-practice checks guard against the same design mistakes that get designers kicked out of the shop, such as sharp internal corners, narrow slots and extreme length-to-diameter ratios. And because DFMPro integrates with popular CAD platforms, design flaws can be rooted out before they hit paper. 

  Teach Them Well

So, all a company needs to design good parts is DFM software, right? Not so fast. “While software tools are part of the solution, more is needed for an effective DFM approach,” said Ken Crow, president of DRM Associates, a product development and consulting firm in Palos Verdes, Calif. DRM offers a holistic approach to improved product design, one focused on training, organization and communication. DFM software is only one of many tools in that toolbox. 

Crow noted DFM is a hot button these days. With the poor economy and overseas competition, companies are under increased pressure to bring products to market quickly and at reduced cost. “We do a number of things to help our customers,” he said. “Some are only looking for DFM training, while others need more comprehensive help to put a DFM initiative in place. But all are looking to reduce costs.”

Courtesy of Boothroyd Dewhurst

This bar chart generated with DFM Concurrent Costing software compares what it would cost to manufacture the level by machining (A), forging (B) and investment casting (C). The investment casting would reduce costs by 25 percent. Aztalan and its customer have continued exploring cost reductions for the level, moving from an investment casting to a premachined extrusion or a premachined forging.

Regardless of the approach used, Crow feels education is key. “We don’t like to go in and simply do the work for them. It’s better to get everyone involved in the DFM process so they learn, rather than just pointing out problem areas in the part design,” he said. “Also, it’s difficult for an outsider to understand their requirements. Sometimes there are reasons why things have to be done a certain way. So they need to understand DFM methodology for themselves, learn the thought patterns of ‘how am I going to fabricate this part’ and look for design improvement opportunities.”

Crow said the key to success is getting design engineers to recognize the basic principles and objectives of DFM and establish a collaborative process that gets manufacturing involved early in the design process. “A lot of companies are still in the situation where drawings are produced in a vacuum,” he said. “The first time the supplier sees it is at quoting. If there’s still time in the procurement process, maybe they’ll supply feedback at that point, but it’s a lot more expensive to do ‘redesign for manufacturability’ than it is to collaborate on the product up front.

“By getting the supplier involved early on in the design process,” Crow continued, “you can review CAD models while they’re still in their infancy and talk about potential problems. So without spending a lot of time, you incorporate a lot of good feedback, leading to lower costs overall, a more manufacturable design and a smoother transition to manufacturing.”

Crow is also a firm believer in continuous improvement. “You need to review the actual manufacturing process after the initial design release, even while the parts are being made. Talk to the machinists and assemblers about what works and what doesn’t, and take those ideas back for future design improvements.”

Sounds like a lot of meetings. How do you balance the cost of product design committees and endless review sessions to churn out the perfect part against using a so-so design and getting the part to market quickly? “If you’re doing a lot of products, like in automotive, it’s imperative to go through a formal DFM review,” Crow said. “But it’s also true that many companies are focused on time to market, and don’t have the tools or the time for proper design review.”

When it comes to DFM, however, there’s always room for Jell-O. Crow said regardless of market pressures, you can still apply good DFM techniques to identify issues and reduce costs. There are a number of steps, or screens, that filter out potential design problems. One of the first and most important steps is to develop knowledge in design engineers about manufacturing process capabilities and sound design principles. A manufacturer must make DFM an integral part of everything it does.

A Good Feature

Even without special software or a formal DFM process, designers can still create very machinable part designs using traditional CAD tools. Jay Tedeschi, technical marketing specialist for Autodesk Inc., San Francisco, said his company offers an array of mechanical design products targeted at analysis, simulation and surface modeling.

An effective CAD system helps prevent mistakes by making part design easier and more consistent, and promotes a philosophy of adhering to best design practices, Tedeschi said. “It standardizes feature choices across the design team, which means mistakes can be avoided by applying standard policies in the form of rule-based feature templates. We call these iFeatures.”

Tedeschi described iFeatures as parameterized libraries, developed to match the available tools in a shop. A library of slots that can be cut with a supplier’s standard cutters and a shop’s machining capabilities is one example. Drag one of those slots onto the drawing and DFM is assured—for that particular feature, at least.

If a user doesn’t have the time or inclination to develop his own library of machinable iFeatures, there’s an enormous library of iFeatures available for free in the user community, along with software, design advice and downloadable tools, Tedeschi noted. 

Rubber Meets the Road

But what about those manufacturing companies that don’t have an engineering department—job shops that, like it or lump it, get stuck with their customer’s difficult designs? There isn’t a DFM technique or software package in the world that can help them, right?

Maybe not, but you can still pick up the phone. Johnson Matthey Inc., Westchester, Pa., produces microcomponents for the medical industry. Brian Woodward, general manager for medical components at its machining facility in San Diego, explained that even without special software or DFM techniques DFM is still possible. “Even though we’re basically a job shop, we still consult with our customers on improvements to manufacturability. We’ll take a customer drawing, for example, and often make suggestions on how to design a more cost-effective part.

Courtesy of CNC Software

Feed-rate optimization is a machining technique that reduces cycle time and machine wear and tear.

“A lot of times the design engineer specifies something that is overly stringent, not appreciating what it would take to make it in production,” he continued. “We work with them on how to make parts more manufacturable. Things like eliminating unnecessarily strict tolerances and surface finishes or suggesting a radius to prevent burrs. We can make five or 10 of anything, to any drawing, but when it comes to making 500 or 50,000, that’s when the customer gets more sensitive about the cost.”

Ben Mund, corporate marketing manager at CNC Software Inc., Tolland, Conn., agreed. “Shops are the workhorses in this situation and are limited on what, if anything, they can do on the design side. Despite this, there’s often room for collaboration on small design changes, making parts more machinable and, in some cases, even machinable at all.”

By increasing production efficiency, shops can make up for a loser job. “Techniques, such as con- stant cutter engagement and feed-rate optimization, can tighten cycle time and reduce machine wear and tear. And there are tricks that shops use on the production floor to help offset a bad design, such as cleaning out a tight corner radius with an equivalent-size cutter,” Mund said.

In any event, the next time you’re faced with a laughable part design, pick up the phone and call the design engineer. If nothing else, it’ll make you feel better. CTE

About the Author: Kip Hanson is a contributing editor for CTE. Contact him at (520) 548-7328 or khanson@jwr.com.

Contributors

David M. Anderson
(805) 924-0100
www.design4manufacturability.com

Autodesk Inc.
(800) 964-6432
usa.autodesk.com

Boothroyd Dewhurst Inc.
(401) 783-5840
www.dfma.com

CNC Software Inc.
(800) 228-2877
www.mastercam.com

DRM Associates
(310) 377-5569
www.npd-solutions.com

Geometric Ltd.
(480) 367-0132
www.geometricglobal.com

Johnson Matthey Inc.
(800) 442-1405
www.jmmedical.com