Sharp corners, both internal and external, should be avoided whenever possible. Sharp external corners of punches or stamping dies tend to break down prematurely, causing more pull-down, larger burrs, or rougher edges of the blanked part in the area of the corner.
Correspondingly sharp interior corners of punches and stamping dies are a stress-concentration point and can lead to cracking and failure from heat treatment or in use. A general rule of thumb is to allow a minimum corner radius of one-half stock thickness and never less than 0.8mm. It should be remembered, however, that there will inevitably be a sharp corner wherever two edges produced by separate shearing, slotting, or blanking operations intersect at approximately a right angle or shearing, slotting, or blanking operations intersect at approximately a right angle or less. Such corners may have to be rounded by tumbling the part or performing some other secondary operation.
Whether a part remains flat or is eventually formed, the designer should consider its strength requirements relative to the grain direction of the material. If necessary, the desired grain direction should be indicated on the part drawing. (Grain direction usually runs lengthwise in the coil or strip.)
Many times it is possible to design a part so that it can be cut off from strip stock rather than being blanked by a more expensive die, which also leaves a web of unused material. Granted, this method applies only when the part can have two sides parallel, as in simple brackets, but if a specific shape or extra close width tolerances are not required, it is nonetheless an inexpensive and simple approach.
Long, narrow projections should be avoided because they are subject to distortion and require then, fragile punches. As a general rule, long sections should not be narrower than 1 1/2times stock thickness. If the projection or web is relatively short, however, this precaution can be relaxed.
If parts are shaved after blanking to provide a smooth edge, the recommended stock allowance for this operation.
When the formed section of a part requires extra resistance to flexing greater than that afforded by the strength, thickness, or temper of the material, stiffening ribs should be provided.
Sheet-metal stampings almost invariably are part of an assembly, and the methods for joining the parts into an assembly are many. One of the most widely employed and least expensive is the use of a screw fastener. An added advantage is the ease with which the components can be disassembled if necessary.
A disadvantage, in many cases, is the fact that the material thickness of a typical stamping does not permit an adequate number of threads for even minimum tightening ability. A rule of thumb for the minor thread diameter (tap-drill size) is that it nor exceed twice the stock thickness for steel and brass and 1 1/2 times the stock thickness for aluminum, copper, and zinc.
One of the simplest and least expensive solutions to the problem of getting sufficient thread length is by use of extruded or flanged holes. Generally, the height of the extrusion is limited to the stock thickness; taking into account the unavoidable lead-in or pull-down radius, one can expect a full thread length approximately 1 1/2 times the stock thickness. However, it is extremely difficult to define by formulation a standard for these extrusions. As a rule, most are developed in the stamping die room and are based on trial and error.
Set-outs can serve many purposes such a locators, rivets, cam followers, pins, etc. As such, they are economical in that separate components need not be purchased, handled, and assembled. Generally, to avoid fracture of the stock material, set-outs should be limited in height to one-half the stock thickness. The punch, of course, must be slightly larger in diameter than the set-out. If the set-out is made hollow, it is possible to obtain a height of approximately 1 1/2times stock thickness.
Designers should bear in mind the difference between the two sides of a blanked or sheared sheet and decide which face of the finished part should have the sharp edge and which side should have the rounded or pulled-down edge. When the stamping must move against another part, normally the rounded edge should be on the side in contact. It is also advisable to locate the sharp or burred side so that it cannot cause injury when the part is in use. This may be preferable to the inclusion of blanket notes like “Remove all burrs” or “Break sharp edges,” which are anti economical because burr removal is expensive. Curled, folded, or sharply bent edges should be designed so that the burr side is on the interior of the bend. In blueprints of stampings, unless otherwise specified, it is generally understood that the burr side is the top one as drawn.
For tooling economy, the designer should specify shapes that can be produced with standard existing, universal bending dies. This implies the following specific recommendations:
1. The inside bend angle preferably should be 90°.
2. In channel forming, the relationship between leg height and width (relationship between W and H) should allow the use of a single standard 90° bending tool, especially if production quantities are limited. Normally, a ratio of channel width to leg height of at least 2:1 is required, but the actual minimum ratio depends on the width of the forming punch.
3. To avoid twisting and distortion, the width of the formed portion of the part should be at least 3 times stock thickness.
Drawing operations are very seldom employed for small lots because drawing requires more sophisticated, more expensive tooling and more development work ans time than simply bent stampings. In short-run jobs, quite often drawn shells and boxes are changed to fabricated shells, which are easier to make and require simpler and less expensive tooling.
The only kind of drawing operation that is sometimes performed in small lots is very shallow drawing of round shells, which requires comparatively simple tools without blank-holding equipment. There are no hard-and-fact rules for the design of such workpieces. The limit of drawing without blank holders depend on the combination of comparatively thick stock, small shell height, and large shell diameter. Heights vary between 5 and 10 percent of shell diameter in thin and medium-gauge stock.
When it is imperative to make drawn shells of conventional design and shape (seamless shells), a few basic recommendations will help to keep down tooling and overall production costs:
1. Avoid tapered-wall shells and/or flanged shells. They are much more expensive than straight cylindrical shells.
2. Do not specify both inside and outside diameters; only one of these dimensions can be controlled because of variations in wall thickness.
3. Avoid sharp corners in the bottoms of drawn shapes. The recommended minimum radius is 4 times stock thickness.
4. For rectangular boxes, specify corner radii at least 0.25 times the depth of draw.
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