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Design of a mechanical stripper advances production technology.

In metalforming and piercing applications, effectively stripping the workpiece is a key element for a successful operation. It's what makes the tools last longer and ensures there are no double hits or parts being stacked up in the die that can cause catastrophic damage.

Today, there are basically three different types of strippers that are used. One is a die-mounted stationary stripper that is attached to the bottom of the die for flat parts; another is a spring-loaded or moving steel stripper pad type that is mounted to the top of the die for use with parts that have contours; and the third is a pressure type. Moving steel stripper pad-types have permanently attached stripping plates that are custom-built with stripping pads riding on pillars against mechanical die springs. Although both stationary or moving steel strippers are effective, they are also very costly because of the additional design engineering that is required to integrate the stripper plates or pads into the tooling design.

As a result, a more common and simpler approach is to use a pressure-type stripper. These are generally a cylindrical-shaped urethane design (see Fig. 1) that use the compression capability of varying urethane durometers to achieve the required compression springback.

Plymouth, MI--based Moeller Manufacturing Company, a designer and manufacturer of standard and special die components, recently introduced a mechanical stripper design called the True-Strip (patent-pending) that provides a new level of stripping technology (see Fig. 2). The True-Strip is a precision-machined device that is compact, convenient, interchangeable, and fits directly onto industrial standard retainers with one screw and complies with all NAAMS specifications. The screw is provided with an anti-loosening, thread-locking patch to reduce the chance of loosening during operation.

Jason Burton, Moeller Manufacturing engineering manager, says: "What we have accomplished with the True-Strip is to provide manufacturers and tool and die makers with a high-performance, fully self-contained, off-the-shelf, standardized machine tool-like stripping device. True-Strip allows the user to get away from urethane stripping devices that have very unpredictable wear life. True-Strip is extremely durable, very accurate, and has the versatility and flexibility to pierce and strip material thickness up to 2mm for a wide variety of material and production requirements. This design allows all of the stripping forces to be completely isolated in the stripper unit body, so no forces are being transferred to any outside component. In addition, by merely changing the spring, the stripping force is adjustable. An added benefit is the fact that the stripping unit attaches to all NAAMS type retainers without any retainer alterations."

Burton explains that the basic design features what is called a flanged steel "can" (see Fig. 3), which is precision-machined and burnished from pre-hardened steel and then black-oxided for corrosion resistance. A bronze or hardened and coated steel stripper plunger is inserted through a precision hole in the front part of the can. The plunger has an ID that matches the hole-piercing punch with clearance that provides the stripping action. The plungers are available in standard lengths for flat-piercing operations and an extended length version that allows for stripper contouring. These plungers can also be supplied with finished angles or 3-D contours on the face of the plunger for specific stripping applications.

"A significant design feature that differentiates True-Strip from competitive brands is the fact that the stripper is guided, not only by the land of the steel can, but also by the head on the stripper. The hole in the can is precisely machined and micro-finished to provide a true and very smooth surface to accommodate stripper movement. This feature creates a dual-guiding condition, adding stability for stripping side loads." Burton says.

"The stripping unit is designed in a "D" shape, which guarantees proper orientation and ensures the center of the "D" is on the same center line as the punch and the pierced hole, simplifying the design and enhancing the accuracy and consistency of manufacture. Behind the plunger is a steel mechanical die spring, which provides the stripping force. By using a spring, the unpredictability of the life expectancy of urethane designs has been eliminated. The spring is pre-loaded 5 percent and is retained in place by a threaded retaining cap that screws in and bottoms out in its mating pocket to compress the spring to its preloaded condition," Burton continues. "These retaining caps are designed with a hexagonal hole that functions to accurately locate the entire assembly to the punch shank and provides for six arc points of contact, conveniently locating the unit and allowing disassembly with a simple hex wrench (see Fig. 4). In position, the spring-retaining cap sits adjacent to the top of the punch retainer. This is where the strongest attachment point exists, and provides a concentric relationship between the punch point and the stripping plunger."

The combination of guiding the stripper plunger in two places and the precision characteristics of the spring cap allows True-Strip to be mounted up to a 15deg angle for mechanical cam applications, resulting in a 30 percent improvement over other conventional mechanical and urethane type devices.

True-Strip is currently available in 7" and metric, heavy-duty, ball-lock shank diameters. CAD data and solid models are available for download from Moeller Manufacturing's website. Custom designs are available on request, including multihole, light-duty, and shoulder type punch configurations. Each unit is accompanied by a specific size mechanical spring to provide the optimum stripping force. A variety of spring pressures are available, allowing the user to adjust the stripping force for specific applications, including thin metal and pre-painted materials where marring can be an issue. Moeller Manufacturing Company,
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Title Annotation:metalforming
Publication:Tooling & Production
Geographic Code:1USA
Date:May 1, 2006
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