Machine Design

DIY DFM makes designers part of manufacturing

Minimizing the variety of machine tools and putting best-design practices online are a few ideas from one company's design-for-manufacturing program.

Paul Dvorak
Senior Editor

The paper version of Insight shows designers what tools are available and what they do.

Artifacts are a block of anodized aluminum cut with frequently used and easily measured features. Every Renishaw Ramtic machine periodically measures its artifact block to check its cutting accuracy.

When U.K. manufacturer Renishaw ran into stiffer competition for their bread-and-butter products, touch-probe-inspection systems, the company initiated a designformanufacturing program as a way to cut costs. The program grew from a lecture series for designers to eventually include machines specialized for company products. How the company got their program up and running is instructive for others looking to make similar adjustments.

Manufacturers in the company were first to suggest to the engineering department that there were faster ways to design. "For example, company products often took three to four years to get to market, partly because of the two to three iterations needed," says Mark Buckingham, principal manufacturing engineer at Renishaw. Now the cycle is shorter because the product is closer to correct the first time.

To prepare the engineering department for the transition, Buckingham conducted many in-house seminars on design dos-and-don'ts. These eventually turned into the written DFM guidelines. Later, the growing collection of guidelines was put online for more rapid updates.

Although the DFM program started on the manufacturing side, managers could see that designers would not benefit from it unless they were considered a part of manufacturing. One early problem stemmed from the craftsmen who built prototypes to closer tolerances than general manufacturing would allow. "About a decade ago, a 10-m m tolerance was seen frequently because craftsmen made prototypes. And they worked. But production tolerances are generally greater than on prototype models. So when engineering asked for 10m m, they got it. Although costs at the time were high, they were not too much of an issue," says Buckingham.

To pull engineering into the program, he promised company designers that if they designed to the guidebook, products could be produced in less time than previously possible. "A product called MIP was the first to take full advantage of the program. It went from concept to market in nine months. Before the DFM program, the product would have taken two years, in my estimation," says Buckingham. Still, he admits that DFM has been a major cultural upheaval for both design and manufacturing departments.

The company's manufacturing system is now based on four principles. Each lets the company standardize some aspect of design and manufacturing. For instance:

Standardize when possible on material, machines, tools, and sizes. At one time the company has about eight different machine tools, each with a different inspection and calibration process. But as the DFM program grew, the company was able to eliminate machines. Today the company has only three different machine tools, but more of them. The same thinking was applied to materials. The company now uses only aluminum and steel.

Even off-the-shelf software can be modified. For example, the company developed a method for conveying design intent in a CAD language that directly translates process changes without analysis or interpretation. This involves providing design intent or specs together with critical features that need inspection.

The company also identified shortcomings in the existing machine tools and so built their own. Ramtic (Renishaw automatic milling turning and inspecting center) is based on a vertical-milling machine but it can do turning operations and then inspect the work with a touch probe. The machine uses a 25,000-rpm spindle for good surface finish, particularly in aluminum.

In addition, Ramtic's 50-toolholder is large enough to produces six different parts. "The machine tool is where we hide our work in progress," says Buckingham. Parts come off one fixture and go on another for the next set of tools. Twenty such machines are in use at the company.

To operate, a forklift driver need only position a pallet of parts at the input station and push a Go button. The machine reads a bar code on the pallet telling what it holds. It then downloads machining instructions, loads the parts, machines them, and unloads them onto an output pallet. Each machine runs about 140 hr/week unmanned.

Standardize the tool setup, holders, tool projections, cutting parameters, CNC programming, and techniques. The selected — and only — CAD system includes NC-toolpath generation so it has no difficulty reading CAD geometry. The main effort is to take variability out of the machine tool.

The guide book initially standardized the tooling because the company began building families of parts for which there were standard tools. The book made suggestions such as "If you design a roller plate, the rollers will be this size while other features are that size, and so on," he says. The company decided to leave legacy items in their original format. Boothroyd Dewhurst ideas for DFMA (the Providence, R.I., company develops software for simplifying assemblies) are used but mostly for trimming part counts in assemblies.

Another observation led the company to make its own milling cutters. Accountants noted that the company was spending about $540,000/year on cutters. Commercially available tools have tolerances of +0, 50m m. But making their own gives company tools tolerances of +0, -20m m. "This lets us hold 6-m m tolerance on machined parts, better than the machine-tool manufacturers said the equipment was possible of holding. This also lets us bring the machine-tool accuracy figure close to its repeatability," says Buckingham. In addition, the effort fine tunes the cutter flutes for a better finish on aluminum.

Determine the performance through SPC and document machine capabilities. For example, a calibration technique called artifact comparison lets the Ramtic tools check their own work and make corrections when necessary. It works like this: An aluminum block cut with easy-to-measure features such as bores, bosses, and slots stays with each Ramtic machine. After so many hours of operation, the machine pulls up the inspection block, its touch probe measures the features, and compares them to stored values. Then it adjusts itself for accurate operations. One major inspection technique is to check only critical features. Designers select these features and put touch points on the solid model for a touch probe. These become probe points in the NC program. "By controlling the

critical features, we assume, and fairly so, that other dimensions are OK. We have proved this to ourselves over the years," says Buckingham. The 3D solid model is the master document for critical inspection features, so no drawing is needed for production functions.

Educate designers and manufacturers to the benefits of DFM. Buckingham says he expected a lot of resistance from designers and quick acceptance from manufacturing people. "Actually, the opposite happened," he says. "Because the design book suggested how a part should be machined, the manufacturing guys thought the guidelines would take their jobs." To quell that fear, Buckingham suggested that manufacturing guys sit with the people in the design department, look over their shoulders, and make suggestions that would simplify manufacturing operations. The action let the manufacturing personnel know their experience would not be easily replaced.

Renishaw had about 11 different types of machine tools on their shop floor over a decade ago. More recently, they are down to three.

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