Tooling Article

Amongst specialty deep-drawing applications, a recent requirement at custom motorcycle- and snowmobile-exhaust manufacturer Factory Pipe, Ukiah, CA, brought a special challenge. The fabricator needed a die cushion with unique features, since the bed of its stamping press had a rib in the middle. This required the development of two separate die cushions, synchronized to ensure uniform stripping action across the entire bed. It also required the ability to run the two cushions with the same tonnage and stroke; at different tonnages and different strokes and in any combination, plus the ability to run the cushions one at a time.

Deep drawing sheetmetal for exhaust systems requires a wide range of forming

To achieve synchronization in changing pneumatic and hydraulic tonnage in the die cushions recently installed on a press at Factory Pipe, hydraulic oil is directed through a double gear-headed drive. Valves placed between the two heads allow synchronization or dissociation between the oil and the air of the two sides. By splitting these two sides, Factory Pipe can operate the cushions with individual tonnage or stroke.

and stripping tonnages—from 2 to 100 tons. The hydraulic press employed for this job at Factory Pipe needed a holddown with controlled return to avoid back-driving the ram. Additionally, the press has no pit, so installation height and the means to guide the cushion presented a difficult situation. And the press installation left no room available under the bed for the typical plumbing associated with hydro-pneumatic cushions.

The application also required one adjustment to the cushion that would simultaneously, and in proportion, change the air tonnage and the hydraulic tonnage. And, the company wanted a “down” command that could be used to set pins or to drop the cushion during die changeovers.

The Cushion Solution

Factory Pipe’s die-cushion supplier, Dayton Die Cushions, Eden Prairie, MN, provided a die cushion to meet all of those needs. It achieved synchronization by directing the hydraulic oil through a double gear-headed drive that forces equal volumes of oil for each cushion. Valves placed between the two heads allow synchronization or dissociation between the oil and the air of the two sides. By splitting these two sides, Factory Pipe can operate the cushions with individual tonnage or stroke. And by splitting and dumping one hydraulic cylinder to a tank and the air cushion to atmosphere, the press also can run in single-cushion mode.

The wide tonnage range required normally would be limited when using hydraulic valves—with the typical pressure drop/control ratio ranging from 4:1 to 5:1. For example, if the valve maximum pressure drop is 2000 psi, its minimum reliably controllable range is 400 to 500 psi. This was all the more true in Factory Pipe’s application, since the additional requirement of a single adjustment for pneumatic and hydraulic pressure demanded an air-piloted pressure-relief valve for tonnage control. Static friction would typically become very noticeable in a 20:1 pilot circuit running in the low end of its range. To prevent this, Dayton Die Cushion dropped the hydraulics altogether on the downstroke and instead just uses air pressure. The high degree of air compressibility brings ability to reach a very light tonnage. Hydraulics still control upward movement of the cushions through valving and pressure-compensated flow controls. This wide adjustability allows the press, with its new cushion, to form a wide range of workpiece thicknesses using the same clamping apparatus. A precision air regulator allows simple addition or subtraction of pressure as needed to adjust for variations in the properties of supplier-delivered materials, drawing fluids or surface treatments.

Accurately locating the cushions’ base-plate holes relative to the press-bed wall holes required a new way to plumb. Here, a pipe with no threads, socket or flanges was used to connect through the press wall. The pipe was given only radfial, not axial, alignment. This tube could float as much as 1⁄8 in. axially and still seal. This design resulted in reduction of under-press plumbing to a 5-in-long, 2¼-in.-dia. tube, easily accessible from outside of the press wall.

Jumping Through Installation Hoops

Installation challenges proved formidable. The required system stiffness excluded the use of hoses; yet, hard plumbing from the bottom of the cushions to

Die-cushion installation challenges proved formidable on the press at Factory Pipe. Through the drilling of holes in the press bed and other measures, crews were able to install hard plumbing as the use of hoses was not an option.

the outside of the bed seemed impossible once the cushion was in place.

Factory Pipe agreed to cut four small 3-in. holes through the back of the press-bed wall, directly adjacent to the bottom of the cushion base plate and away from the edges of the wall web, to avoid the creation of stress-concentration points. The base plate was appropriately cross-drilled to create what amounts to a simple manifold. Accurately locating the cushions’ base-plate holes relative to the press-bed wall holes required a new way to plumb. Here, a pipe with no threads, socket or flanges was used to connect through the press wall. The pipe was not given axial alignment, only radial alignment. The cushion-base holes and press-wall manifold were counter-drilled with seal-finish surfaces and a high-pressure heavy-walled tube with a radial seal on both ends was inserted. This tube could float as much as 1⁄8 in. axially and still seal. And due to equal pressure at each end of the tube, no net forces are exacted on the tube.

This design ultimately resulted in reduction of under-press plumbing to a 5-in-long, 2¼-in.-dia. tube, easily accessible from outside of the press wall.

Dayton Die also arranged for the “down” command without employing a hydraulic pump, even though considerable force is needed to break the hydraulic check valves in the piston-regeneration circuit. The cushion design takes advantage of two cushions, each with an air-over-oil surge tank. One tank is held pressurized at 100 psi while the other drops to atmospheric pressure. Pressurized oil flows through valve lines to the top, or rod-side, of the hydraulic cylinder. The bottom, or blindside, is directed to the atmospheric tank. Thus a simple ball valve allows for relatively fine manual control of the “down” function. MF

Information for this article provided by Dayton Die Cushions, Eden Prairie, MN; Tel. 952/937-0702, daytondiecushions.com.