Seamless Deep Drawing Ideal for Big Runs
The deep drawing process combines some of the similarities and advantages of metal spinning with metal stamping. Like spinning, deep drawing produces seamless parts. Like stamping, it is very cost effective for large quantity runs.
One of deep drawing's similarities to metal spinning is the ability to produce circular parts, but deep drawing also produce parts with corners--rectangles and squares--as well as slender cylinders. And, like metal spinning, deep drawing produces parts from individual metal blanks. But the process of cutting and presenting the blanks is often automated--similar to stamping.
With such similarities, it is not coincidental that many shops that provide deep drawing also offer metal spinning and stamping. With expertise in all three operations, Shops are able to offer their customers the best process for each particular application.
Reel Deep Draw
Parts made by deep drawing range from tiny eyelets to suitcase-sized enclosures. The size of the part depends on the size and operation of the drawing press.
In the typical deep drawing process, the press pulls strip material from a reel through the first position on the machine. There, a tool specifically sized for the application punches a blank from the strip. This blank is automatically transferred to the second position on the deep drawing press as the metal strip is indexed forward to punch the next blank.
This differs from stamping, which typically carries the part along on the strip of material until the last operation of the press. It is not removed from the strip until just before ejection from the press. Early separation of the blank enables deep drawing to make parts with deeper shapes. Engineers would say deep drawing enables much greater length-to-diameter ratios than progressive die tooling.
It should be pointed out that some progressive die stamping operations can produce cups of very shallow depth without removing the part from the carrier strip until the final operation. The key determining factor is cup depth--or lack thereof.
At the second position on the drawing press, the blank is clamped around its edge, and another punch forces the blank a specified distance into a mold's cavity. The metal blank first bends, then straightens as it enters the mold.
The blank is transferred automatically to successive stations on the press, clamped around its edge, and pushed a little deeper into a new mold at each station, until the final part shape and depth are achieved.
The part is not completed with one push into a mold because the metal can only withstand certain limits of deformation at each position without being damaged. Although that limit will vary based on the material and thickness, it is often in the 50 percent range. Consequently, the number of positions depends on the type of material, along with the complexity of the part's design. The more slender a part design (greater length to diameter ratio), the more operations that are required to produce it. Certainly, some parts can be produced in one step.
As the working area of the metal blank enters the cavity, it is said to be 'drawn' over the edge of the mold cavity. The process gets part of its name from this action. The 'deep' in 'deep drawing' comes from the ability of the process to force the blank into consecutively deeper mold cavities until the final depth is created.
There are many materials suitable for deep drawing. The only major requirement is that they are malleable or 'flowable'. Among the most common are cold rolled steel, hot rolled pickled-and-oiled steel, stainless steel, brass, and copper. Somewhere between the production capabilities of metal spinning and those of high speed transfer presses lies the production volume that is best suited for the deep draw portion of metal stamping business.
Deep drawn stampings can be made from aluminum, brass, copper, and mild steel, along with stainless steel. Materials range in thickness from 0.010 to 0.200 inch. Parts can be drawn as large as fifteen inches in diameter and nine inches deep or as small as one quarter inch in diameter and one half inch deep.
Using conventional mechanical and hydraulic presses is very effective at production runs ranging from 4000 to 60,000 pieces per month of a typical drawn part. Shapes of drawn parts such as square, rectangular, or free-form are also common, including the ability to press trim these shapes.
Trim presses are used to trim the open end of the shell to 0.005 inch length, 0.003 inch flatness, and 0.003 inch perpendicular to centerline. This permits fusion welding of two shells to form a closed chamber. With pinch trimming and machining the end of the shell, the customer was experiencing a failure rate after welding as high as 30 percent. When this was replaced with press-trimmed shells, the failure rate dropped to less than one-half of one percent, and weld cycle times decreased by 50 percent.
Besides being highly accurate, press trimming requires less than one-third of the time it takes for conventional machine trimming. This press trimming process also produces a square, flat surface in contrast to the flared, rounded, and razor shape edge that is common with pinch-time operations.
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