Accelerate diemaking, reduce scrap
April 14, 2009
Punch tooling innovations help die builders and stampers improve lead times, reduce waste, and expand design options as well as eliminate slug pulling and minimize stripping pressures.
Hector Favela, a tool- and diemaker for Simpson Strong-Tie, Pleasanton, Calif., builds dies for the company's metal connectors and anchor systems. Punches were breaking two to three times per shift. He then started using punches made of M4 performance steel with Wilson Tool's Effex™ surface finish. The punches are ground parallel to the direction of the punch to achieve a smoother surface and less galling. Since then the punches no longer break and need sharpening only after the run is complete. Downtime has been halved.
Time is money. Whether a company is building dies or stamping parts, finding new ways to improve efficiency and reduce costs is crucial to overall success.
A number of tooling innovations have been developed to help die builders and stampers improve lead-times, reduce waste, and expand design options, as well as eliminate slug pulling and minimize stripping pressures. Some of these are:
Often as die builders strive to incorporate lean manufacturing, they find that special retainers are an obstacle to increasing turnaround times—a bottleneck.
Until recently die builders rarely attempted to build their own retainers in-house because producing the necessary blind, flat-bottomed, precision-angled hole was very difficult and time-consuming. The only other option was to purchase custom retainers from an outside vendor, adding cost and weeks to a job.
A new ball lock retainer insert simplifies making custom retainers in-house (see Figure 1). The insert is a mass-produced version of that blind, flat-bottomed, precision-angled hole. It has changed die design. Die builders and designers can redesign their tooling to accept the insert in place of a custom retainer.
Using the insert, the tool designer and die builder can manufacture the block and their own punch retainers with straight-line machining—without complicated jigs, inspection fixtures, or the know-how to machine ball holes in the punch retainer plate—on wire EDM, jig grinders, or machining or milling centers. No angles or tapers are required—just straight holes.
When machining is complete, there will be two intersecting holes: one fits the ball lock retainer insert, and the other fits the standard ball lock punch. The diemaker simply slip-fit installs the insert into the back of the block (the head of the insert secures the insert in place), and then installs the backing plate. Then he or she inserts the punch—any standard ball lock punch.
Proximity. This insert actually allows the punches to be close together; this is especially important when proximity is required in some applications.
One-day Lead-times. Because the insert is an off-the-shelf item, builders can actually reduce their lead-times from two or three weeks to one or two days. They can order the inserts in the morning, machine the plate that day, and install the inserts when they arrive the next day.
Existing Retainers. Die builders also can modify existing retainers using retainer inserts. Engineering changes often require moving an existing hole a few thousandths of an inch in one direction or another or adding additional features. Adding a retainer insert with the new specifications into the existing retainer eliminates the need to replace the entire block. The inserts can be reused for multiple stamping jobs, especially useful for short runs.
For many stampers, slug pulling is the cause of costly setbacks such as scrapped parts, downtime, and tooling and machinery damage. Slug pulling occurs when the slug does not separate from the skeleton and re-enters the die along with the work material. Then when the press begins the next cycle, the loose slug can damage the part as well as the tooling.
A new die/matrix design offers stampers the means to nearly eliminate slug pulling (see Figure 2). The die is designed to grasp the slug just below the cutting surface, preventing it from returning or springing back with the punch. This die design incorporates precision nibs, or interior protrusions, that extend the entire height of the die, helping to guide the slugs through the relief and eliminate marks or burrs on the finished part.
This die design effectively eliminates slug pulling and packing, which are especially problematic when stamping materials such as aluminum, stainless steel, foil, and plate. The design can be applied to all types of dies, including taper relief, stepped relief, and headed or headless.
If a punch fails quickly, the answer is not to buy two punches next time. A punch can be modified to make it last longer. Longer-lasting tooling means more parts between sharpening and less changeover time.
The stamping industry faces two divergent challenges. Increasingly, parts for the automotive, appliance, and furniture industry segments must be stamped of high-strength, low-alloy (HSLA), dual-phase (DP), and transformation-induced plasticity (TRIP) steels. So work materials are getting harder to work with. At the same time, environmental standards require stampers to use lubricants that don’t have the same viscosity they used to years ago.
What surface finishes and coatings do is bridge that gap in lubricity between tougher work materials and new lubricants. To make a punch last longer—strip more easily—its surface finish can be enhanced and a coating applied on top of that to give it a harder and slicker surface.
Surface Finishes. Newly developed surface finishes, from standard radial grinds to straight-line grinding methods, improve stripping pressures and help produce more accurate hole diameters, while also reducing galling and increasing tool life (see Figure 3).
Coatings. Chromium nitride (CrN), a relatively thick coating, has worked well in high-pressure forming as well as shave applications. Molydisulfide lubricant, sometimes referred to as dry lube, works very well where little to no lubrication oil can be used. This coating provides a very slick surface that increases wear, reduces galling, and decreases stripping pressure. Chemical vapor deposition (CVD) coatings, such as titanium carbide (TiC), are now becoming more common as the industry trends toward premium tool steels that allow for higher coating temperatures. CVD provides better adhesion and durability.
Coatings and surface finishes enhance lubricity, thereby reducing the friction coefficient of the punch so that the punch strips from the work material more freely. The result is greater wear resistance for a longer-lasting tool.
Tooling is an investment. High-quality, innovative tooling and performance-enhancing inserts, die designs, coatings, and finishes designed to increase efficiency, reduce waste, and lengthen tool life are likely to help stampers achieve a better return on investment.