Sheet Metal Forming – Counter-Drawing Style

Today part quality and piece price cost are the decisive parameters when executing purchase orders. The use of energy and productivity in part production has a major impact on price calculations regarding bid submissions. Manufacturing processes and line concepts must be tailored to the particular requirement of the customer. Counter-drawing is a suitable forming method which takes into account costs and productivity factors which become vital to the success of any stamping plant.

Counter-drawing integrates all the advantages of time, metal flow, forming ease and reduced tooling wear. Time plays a key role in the metal forming process due to the various tonnage values applied for the types of material used. For example, deeper draws and hardened steels required more tonnage and time for the metal to flow. The flowing of metal occurs upon specific applications of the right tonnage value employed from the press at the right time on the right surfaces. When all is complete the part is formed and takes on the new shape for utilization in its specific application. Along with the flowing process of a metal a certain amount of heat and wear breakdown is experienced by the tooling. Heat and wear can make the tooling dull and produce inferior parts. The act of counter-drawing reduces the heat and wear breakdown incurred by the tooling and displaces the forces to a variety of areas of the tooling and forming blank.

Figure A shows the counter-drawing tool whose design corresponds to a conventional tool. The press used in the illustration is a hydraulic double action press with a powered bed cushion. Press bed cushions are powered by pneumatics, hydraulics or mechanical operations to create resistance force.

Figure A (1) shows the initial sequence of functions consisting of four steps. (1) The upper tool is clamped to the slide, commonly known as the ram. The lower tool together with the blankholder rests on the press bed or bolster (term used referring to moving bolster quick die change operations) while the punch which is guided through a clearance in the bolster plate (moves a bolster pin plate hidden in the moving bolster itself). The bolster plate rests on the bed cushion. (2) The slide moves down, the tools are closed to begin the metal forming process. The blank is gripped between the upper and lower blank holder by the slide force. (3) Subsequently the press bed cushion is powered and the forming punch, with the assistance of the cushion, move upward while the part is being formed. (4) After finishing the forming operation the slide returns to Top Dead Center and the press bed cushion returns to its starting position to begin with another stroke of the press.

Figure A

The force of the press bed cushion cylinder is only 60% or less of the rated press tonnage. As a result, the bed cushion is pushed up by the press cushion at a maximum of 1.6 times of the normal press speed. The shortening of the press cycle results in an increase of the output rate of a given press. Energy is transferred to the forming operation instead of being dissipated as waste heat into the cooling water system.

If the slide (ram) force remains unchanged, the bed cushion force has to be increased for the forming operation. The slide stroke is reduced to the part height plus transfer clearance. The slide stroke is reduced to the part height plus transfer clearance. Likewise, the size of the die space and construction height of the presses is reduced. Cost investment is expected to be balanced out.

Counter-drawing used in transfer presses

Transfer presses offer a number of beneficial features which a pressline is lacking. For example, less space requirement, lower cost investment and shorter change-over times are realized with the transfer press use. With these advantages the transfer press usually employs mechanical operation and hydraulic die cushion powered for smooth performance and speed. While the transfer press has come of age, the press hydraulic die cushion during operation does encounter variable load values displaced over the surface of the tooling which must be compensated by the press.

Here in figure B, the counter-drawing process provides a good alternative to traditional metal forming which so far was the exclusive domain of the mechanical presses. Tools set specifically designed for transfer presses are employed which have limited uses for other presses. With a conventional tool design, the parts are formed by the drawing stations against the blankholding force (sometimes referred to as binder rings) of the bed cushions while there is no material contact in the following tools. The load value pressures thus created are so inconsistent that the functionality of the system as a whole is questioned. A variety of displacement cylinders are attached to the cushion to provide a more even pressure build up phase.

Figure B

Here, a component is formed ready-to-be-mounted by two drawing and three follow-up tools. While the forming paths in the first two tools are 6 inches and 7 inches respectively, the last ½ inch before bottom dead center (BDC) is used for cutting, edge forming and calibrating by the following tools. The design of the drawing tools for the counter-drawing process implies that the forming process implies that the forming punches are mounted on the bed cushions. Separate short-stroking blankholder pins are designed to upper or lower tooling. After closing of the slide, the part is formed by the progressive tools and the blankholder pins are displaced. Here the slide forming path is shorts and balanced. The slide is hydraulically blacked in the BDC by a fixed stroke limiter. The bed cushions are powered by individual drive units. During the slide opening, the bed cushions return to their basic position.

The slide with its large piston area to apply nominal force operates only over a short stroke. The low forming speed does not affect cycle times. More important is the bed cushion speed as the cushions have to travel the long forming paths. The advantages of the system certainly will pay off in terms of output rates for drawing depths starting from approximately four inches.

For applications where the counter-drawing method is useful, a press such as a transfer press with hydraulic parallelism cushion control may provide a suitable solution to metal forming requirements.