September 24, 2009
There are several ways to shorten the die design-build-die tryout process to keep diemaking from becoming a bottleneck.
Pressure to shorten the time from order to manufacture has never been greater. Drastic changes in the automotive industry—new design concepts, new power sources, stringent safety requirements, higher fuel efficiency standards, and a higher degree of customization—are all driving stamping on a speed course.
Because of its process complexities, metal forming's bottleneck long has been diemaking. Industry sources offer some helpful suggestions, tips, and tricks to abbreviate the die design-build-tryout process.
Software, die design strategies, modular dies and components, faster cutting methods, and special speed-facilitating components all represent time-cutting potential.
Solid Modeling. Although designing a stamping die using solid modeling may take longer initially, the added functionality and visualization have the potential to save time and rework later in the build process, many designers contend.
"I think solid-modeling software has been key to a number of innovations that are helping to shorten the lead-times for a tooling project," said Scott Christensen, owner, Summit Tool Design, Rockford, Ill.
"A solid model of the stamping die assembly, or virtual die, perfectly represents the design intent (see Figure 1). A good solid model does not allow for the fudging and approximations that a 2-D design does." As a result, costly interpretation errors and miscommunications that cause project delays and rework during die tryout can be avoided, Christensen explained.
Forming Simulation. For example, with forming simulation, a part and process can be analyzed for problems such as wrinkles, folds, and splits that may occur during die tryout.
In addition, simulation allows designers to calculate the initial profile for developed blank shapes and accurately calculate the die forces needed, Christensen said. By reducing the number of trial blanks that need to be produced, this approach reduces die tryout time.
Force calculation saves die design time and also helps prevent building a die with insufficient spring forces, or overbuilding a die—both of which can be time-consuming to correct after the die is built, according to Christensen.
Simulation also helps to validate the appropriate number of operations. Too few stations in a progressive die to form the part successfully may require an expensive secondary operation to complete it. Conversely, too many stations will increase die size, extend build time, and add cost with no benefit. A larger die may also create the need to run the part in a bigger press with a higher overhead rate.
Solid modeling enables diemakers to check a design for interferences in the design office where they can be corrected quickly, saving production time, Christensen asserted.
Die designer Rob Driver, engineering manager, Midwest Tool & Die, Fort Wayne, Ind., relays a process-shortening practice he says has been used by mold shops for years: Take advantage of the data relayed in the CAD data to minimize the level of drawing detailing and, therefore, die design time.
"We reduce design time considerably by not fully dimensioning (adding contour notes) anything that is machined from CAD data. You would be surprised how many dimensions of detail drawings are never used," he said. Driver warns that this approach can be applied only when the 2-D geometry or solid model used to manufacture the detail can be relied on to be very accurate.
When applicable, modular dies enable diemakers to go from design to production within a day, said Brendan Whistler, sales and marketing manager, S.B. Whistler & Sons Inc. "They can go from a thought, a design, to manufacturing production in a day or so using off-the-shelf, standardized components (see lead image). So rather than die cutting or EDM-working components for a custom die, they just pull these standardized components, put them into a template, and they're off and running," Whistler said. All of the components that comprise a die can be modular, he said.
With standardized tooling, retainers are universal. "You just take out the punch, drop in a new punch, and you're off and running again. So die design is minimal. And generally you're down for just a minute or so," Whistler said.
Family of Parts. Modular dies are especially suitable for manufacturing a family of parts. "Generally, you see them used in meter boxes, panel boxes, farm implements, and office furniture that have the same features but are different sizes; for example, 12-in., 14-in., 20-in. shelving. So you're reusing the same tools to punch holes in different locations," Whistler said.
Medium Runs. The modular dies are most often used for volumes that are too high for turret punching and too low to justify the expense for custom hard tooling. High-volume parts, above 500,000, should be hard-tooled, and quantity runs under 1,600 parts might be better-suited to a turret or a laser, Whistler said. "That in-between area is where our tooling fits it."
The dies are Class A, Whistler said. Stampers buy only one reusable die set of a standard size and revise it for each particular job. Then standardized templates are machined to specific part prints.
Some features that are unique to a product may have to be customized, but diemakers can use standardized retainer holders, punch blanks, and bushing blanks, Whistler said.
Limitations. Shallow forming—lances, piercings, knockouts, louvers, and extrudes—can be performed on the modular dies. However, profile parts with a large radius or large notches are not good candidates for the modular dies, Whistler said, because of stroke and shut height limitations. Nor are preformed parts good candidates, such as profile parts deeper than 3/8 in. or parts needing 90-degree bending and wiping.
Design Considerations. "I always tell customers to try to standardize their designs; to have the forethought to think about tool build while they have their design hats on," Whistler said (see Modular Dies Case Example).
The entire die build process can be sped up simply by using faster cutting processes, including waterjet cutting and high-speed machining.
Waterjets. Waterjet technology advancements and accuracy improvements now allow diemakers to use a waterjet to either precut, or "rough-cut," a die or to finish-cut a die, depending on the application, said Tim Fabian, market manager, Flow International Corp., Kent, Wash. (see Figure 2). "In the past, waterjets typically could only be relied on for maybe 0.010-in. accuracy. Now they're accurate to 0.001 in.," he said.
The time savings can be significant. "You can cut a large stripper plate in an hour and a half that would take perhaps three days to do conventionally," Fabian said.
"With most dies, you can cut the components to the finished size without having to perform any secondary machining. Examples are stripper plates, shim packs, backup plates, clearances, and guide rails," Fabian said.
Waterjets cut up to 8-in.-thick material, and a taper control head can automatically cut draft angles, making them suitable to cut slug clearances, Fabian said.
Another way that stampers are using waterjets is to produce the part before the die build, Fabian said. "Often they might build two dies—one to do the blanking and one to do the forming. And they want to develop those dies concurrently, but it's very difficult to build a forming die until you have the blanking die done for the same part. They use the waterjet to develop the strip or the prototype of the part so that they can build both the blanking and the forming die concurrently," Fabian said.
High-speed Machining. High-speed machining helped Competition Engineering, Marne, Mich., accelerate the diemaking process, said Gene Hughes, project manager.
"Using our Makino V55 with shrink-fit collets and high-end cutting tools, we are able to reduce the amount of time required to machine details before heat treatment," Hughes said. "Because we can produce fine finishes, we are also more often than not able to assemble those details without the need for additional hand work, such as breaking down and stoning."
Hughes said that the rigidity and accuracy of the machining center enable him to "hard-cut" details post-heat-treatment as well. "This is a significant time-saver for manufacturing details that require a high degree of accuracy and are prone to expansion or contraction through the heat-treating process."
The capabilities of today's advanced equipment allow die builders to eliminate postmachining heat-treat on many components altogether, according to Midwest Tool & Die's Driver.
"Very high-accuracy tooling usually requires precise heat treating," he said. "But the turnaround time on outsourcing heat treating is considerable," he said. "Instead, we pre-heat-treat many plates of varying thicknesses, and then use equipment that can process it in the hardened state, thus eliminating the postmanufacture heat-treat time."
Today's advanced machining capabilities allow die designers to design complex shapes that reduce the number of die components needed (see Figure 3). "An example might be a wire-EDM blanking punch with retainer and backup plate. Advancements in hard milling technology enable accurately finishing a single-piece punch after heat treatment," Summit's Christensen said. "This eliminates the need for the retainer and backup plates. By eliminating the two plates, all the setup and machining operations for those two plates are eliminated also," he said.
Punch Retainer Inserts. Builders can cut special retainer lead-times down to two working days using ball-lock retainer inserts, said Jerry Dwyer, regional manager, Wilson Tool International®, White Bear Lake, Minn. Die shops can make special retainers in-house using simple, straight-line cutting equipment. The retainer inserts replace the ball hole; punches are held in place with a straight-line-machined hole that can be cut in-house. No angles or tapers are needed.
The retainer inserts eliminate the need for complicated jigs, inspection fixtures, and specialized knowledge to machine ball holes in the punch plate or the need to send them to an outside vendor.
After machining, there are two intersecting holes—one fits the retainer insert, and the other fits the standard ball-lock punch.
Surface-mounted Guide Pins. Traditionally, die builders had to bore holes in a die set to insert pins and bushings to align the top with the bottom, said Art Hedrick, STAMPING Journal columnist and president of Dieology, Greenville, Mich. "Surface-mounted guide pins are screwed and doweled right to the surface of the plates. They're a lot easier and faster to mount, so they really reduce the die set build time, as well as cost. And they're just as accurate as some of the bored guide pins," he said.
Die Cushions. Die cushions accelerate the diemaking process in several ways, according to Tom Pedersen, president of Dayton Die Cushions in Eden Prairie, Minn. "Die cushion pins use simple, small-diameter holes drilled through with twist drills. They are simpler and quicker to make than specially designed milled pockets. The pin locations are standardized, allowing standardized die construction. This means modularity in the CAD database for die creation," Pedersen said.
Pressure can be adjusted by turning a thumbscrew or handle on the regulator valve, he added.
Diemakers may be able to shorten die-building time using urethane or aluminum, which can be cut more quickly, for short runs and service parts, suggested Hedrick.Urethane can be used as an alternative to a spring- or pressure-loaded stripper pad, or as a substitute for coil and gas springs. The key is to avoid using urethane when a great deal of compression is required. Using urethane also typically allows a part to be made in fewer steps, Hedrick added.
Don't overlook time-saving techniques that shorten the stages of diemaking that are indirectly related to die build, but that affect the process time nonetheless, such as the die component procurement.
"Our technology is an e-commerce platform that lets buyers and sellers do business instantly on the Internet," said Tim Stephens, founder and CEO, Speedraft. "Unlike simple RFQ Web portals, our patent-pending technology automates the selling price calculations for build-to-order products.
"We eliminate RFQs," Stephens said. "With our technology, buyers can buy without waiting, and sellers can sell without quoting."
Perhaps as important as expediting diemaking is that dies be constructed for engineering changes, which are occurring with increasing frequency, and that they are engineered for interchangeability and serviceability.
Too, Midwest Tool's Driver warns that designing a die in a way that makes die building faster but that slows part manufacture will result in a net zero-sum game—or even a net loss.
"In producing a die, the die designer often has a very different perspective than the die builder or the troubleshooter," Driver said. "It is important that everyone involved in producing the stamping die has a basic understanding of the overall impact of their decisions."
One modular dieset customer wearing his tool builder hat while designing is Gerald Sparks, manufacturing engineer, DeVilBiss Air Power Co. (division of Black & Decker), in Jackson, Tenn. The company's primary product lines are air compressors, generators, and pressure washers.
"Some of our product lines are small, hand-carried compressors. The tanks are what we call hot dogs. They are long and slender, 6 or 8 inches in diameter and 15 to 32 inches long, have a tank head welded on each end, and threaded fittings. The tanks usually go into welded tank assemblies that have handles and frames," Sparks said.
"In the past, we bought 24-ft.-long sections of welded tubing. We cut them to length and punched holes in the round—in the cylinder. All of the holes have welded fittings for the input and output lines that run between the pump and the compressor and to the air outlet hose. That way was slow and difficult to maintain accuracy. And of course, any kind of design change just kills you," Sparks said."As my boss and I were walking through the plant he said, 'You know, we'd really like to pierce these in the flat, and then roll and weld them like we do our horizontal compressor tanks. How do you think we could tool that?'
"I said, 'Oh man, a Whistler die would be perfect for that.' So we looked into it, and that became part of an overall project. Of course we had to buy rolling equipment, and the seam, head, and stud welding equipment," Sparks said.
"Now, we can change over from part to part in 15 minutes. The die set mounts in our T-slots just like any other die set would. All you would have to buy is the template sets, the sheet metal that holds the die and punch retainers and you can just change the parts—the retainers and the punches and the strippers—from template to template," Sparks said. "We have a dedicated press that we just leave the die in, and all we do is change the templates."
Cost Savings Too. Converting the operation from a welded tube to a flat blank saved money too, Sparks said. "We were able to realize a substantial cost savings in this segment of our product line, and implement a process that is efficient and relatively trouble-free.
"We can tool a new part from the ground up, depending on the number of holes, for about 10 to 15 percent of a traditional blanking die," Sparks said. "If one of our design engineers wants to make a change—for example, to move a fitting—it costs us less than $1,000 to make that change. So it's very economical to tool up parts on the front end, and to make those changes that always happen.
"It has been really good for our application. This was just what we needed," Sparks said.
STAMPING Journal® is the only industrial publication dedicated solely to serving the needs of the metal stamping market. In 1987 the American Metal Stamping Association broadened its horizons and renamed itself and its publication, known then as Metal Stamping. Print subscriptions are free to qualified stamping professionals in North America.