1. Field of the Invention
The present invention relates generally to crank and slide presses for metal working. More particularly, the present invention relates to a crank and slide press for redrawing metallic cups into containers, and to a redraw carriage for crank and slide presses that includes improved apparatus for resiliently clamping the redraw cup between the redraw sleeve and a redraw die.
2. Description of the Prior Art
Crank and slide presses are used for punching, shearing, drawing, and redrawing operations in manufacturing articles from metallic sheets or rolls of metallic strip material.
In general, a crank and slide press includes a crank that is mounted for rotary motion, an electric motor that is connected to the crank and that imparts rotary motion thereto, a slide that is guided for reciprocating motion, and a connecting rod that cooperates with the crank and the slide to change rotary motion into reciprocating motion. A more detailed discussion of crank and slide presses is given by Maytag in U.S. Pat. No. 3,696,657.
One specific use for a crank and slide press is in the production of beverage cans. A redraw cup is drawn from a coiled strip by a first crank and slide press; and then the redraw cup is redrawn on a second crank and slide press by forcing the redraw cup through a tool pack that includes a redraw die and a plurality of ironing dies.
In the highly competitive container industry, the container must be made with the absolute minimum of material, which means that the finished container must have extremely thin walls. Further, because of the competitiveness of the container industry, the speed of the press must be maximized and downtime of the press must be minimized.
As should be apparent, maximizing speed and minimizing downtime are inherently opposite, because higher speeds impose higher stresses on the machinery and cause higher wear rates. Further, it should be apparent that the use of extremely thin walls in the finished containers places stringent accuracy and alignment requirements on such parts as the redraw punch and redraw die.
In the redrawing operation, the tool pack, with the redraw die thereof, is mounted circumferentially around the longitudinal axis along which the redraw punch is reciprocated; the redraw punch is reciprocated toward, into, and through the tool pack, including both the redraw die and the ironing dies; and the redraw punch is withdrawn from the tool pack and all of the parts thereof.
The redraw cup is positioned against the redraw die and is resiliently held against the redraw die by a redraw sleeve; the redraw sleeve is attached to a redraw carriage; and the redraw carriage is reciprocated toward and away from the redraw die.
The redraw sleeve serves two functions. One function of the redraw sleeve is to assure concentricity between the redraw punch and the redraw die. Of course, this concentricity is extremely important because of the thinness of the walls of the finished container and thus the small difference between the diameter of the redraw punch and the redraw die.
The other function of the redraw sleeve is to achieve resilient clamping action between a clamping face of the redraw sleeve, the redraw cup, and the redraw die.
In the prior art, such as typified by the aforesaid patent to Maytag, the redraw carriage has been actuated toward the redraw die by a cam that is attached to the crank of the press, and by a cam follower mechanism that receives motion from the cam and that transfers this motion to the carriage. It has been customary to use an air cylinder to maintain the cam follower in contact with the cam and to actuate the redraw carriage away from the redraw die.
It is important that the redraw sleeve resiliently hold the redraw cup against the redraw die with sufficient force to promote accurate redrawing as opposed to allowing the redraw cup to collapse and wrinkle as it enters the redraw die; but it is equally important to limit the clamping force of the redraw sleeve below a clamping force that will cause any of the redraw cups to rupture rather than being redrawn.
In the prior art, as exemplified in the aforesaid patent to Maytag, a spring mechanism has been interposed between the redraw cam and the redraw carriage in order to limit the clamping force of the redraw sleeve. While this construction has been, in general, satisfactory in the past, decreases in the thickness of the walls of beverage containers, and increases in press operating speeds have made the prior art apparatus inadequate for present needs.
More particularly, it is well-known that, at high compression velocities, a pressure wave is transmitted through a spring; so that the force actually exerted by the spring varies widely from the static force of the spring. Thus, in redraw operations, the clamping force of a spring will vary considerably from that which the spring would exert under static, or low speed, conditions.
Further, it can be appreciated that, with operating speeds of approximately two hundred and fifty redraw operations per minute, the compression wave may be oscillating back and forth in the clamping spring for a higher percentage of the time, or even the entire time, in which the redraw cup is held between the redraw sleeve and the redraw die; so the clamping force of the spring varies greatly during the clamping operation.
It can be appreciated that, if the actual clamping force is varying widely due to the compression waves in the clamping spring, then an excessively large portion of the satisfactory range in the clamping force is taken by the spring-caused variations in the clamping force; and the adjustment range for satisfactory operation becomes much more critical.
Further, it is important to make adjustments to the redraw press while the press is operating. It can be understood that, if the press is stopped, a small adjustment is made, a short run is made to observe the results of the change, the press is stopped again, etc., considerable production is lost. Because of the loss of production through stopping the press to make adjustments, there is the tendency to avoid making the number of minor adjustments that should be made to optimize the clamping force, and there is a tendency to delay making adjustments as long as possible, even though a more optimum clamping force would result in fewer rejects in the finished containers.
Thus, a principal object of the present invention is to provide apparatus and method for resiliently clamping a redraw cup between a redraw sleeve and a redraw die.
Another object of the present invention is to provide apparatus and method for resiliently clamping a redraw cup between a redraw sleeve and a redraw die that provides a uniform pressure during a high percentage of the time in which the redraw cup is clamped.
Still another object of the present invention is to provide apparatus and method for the clamping operation as described in which the effect of spring pressure-wave forces is eliminated.
Finally, it is an object of the present invention to provide apparatus and method as described in which the clamping force can be adjusted without stopping the crank and slide press.