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What is Metal Stamping? -Part Five

Nonferrous Metals

Aluminum and copper alloys are the two principal nonferrous materials used for stampings. Other stampable nonferrous metals are nickel alloys, zinc, magnesium (when heated), titanium, and many less common metals.

Copper and many of the brasses have excellent stamping characteristics. Characteristics. Cartridge brass is particularly suited for deep-drawing and sever forming applications. Yellow brass has similar stampability. Phosphor bronze and beryllium copper, even though less workable, are frequently used for electrical contacts and other applications for which spring action is required. The preferred alloys for coining are those with very high copper content: electrolytic-tough-pitch (ETP) copper, gilding, commercial bronze, and red brass.

Aluminum alloys 1100 (A91100) and 3003 (A93003) have excellent forming characteristics and are low in cost. If greater strength is required, 3004 (A93004), 5052 (A95052), 5154 (A95054), and 5036 (A95035) are suitable. If the finished stamping requires a smooth or polished surface, 5053 (A95053), 5252 (A95252), or 5457 (A95457) is recommended. Types 1100 (A91100), 3003 (A93003), and 5005 (A95005) are the aluminum alloys most easily deep-dawn.

Nickel 200 (NO2000) and Monel 400 (NO4400) are the most easily press-worked nickel alloys. If the amount of forming is not severe, they are often processed in the one-eighth-hard or quarter-hard tempers to promote better blanking and piercing.

Magnesium alloys are normally formed at temperatures between 120 and 430 degree. However, large-radius bends can be made in most alloys at room temperature. Alloys AZ31B-0 (M11311-0) and LAI41A-0 (M14141-0) are the most easily formable alloys at room temperature.

Gold, silver, and platinum are all highly suitable for press working, including coining.

Nonmetallic Material

Many nonmetallic materials in sheet form are processed with punch presses and stamping die of the type used for sheet metal. Operations are normally limited to blanking and piercing, although with some materials bending, light forming, and embossing are possible. Another exception is ABS are made by deep-drawing on metal-working presses.

Nonmetallic materials commonly blanked and pierced include fiberboard, paper, leather, rubber, cork, wood and wood-based composition board, and various plastics, especially laminated thermosets. Except for glass-reinforced laminates, these materials are ideally suited for blanking with steel-rule dies and other short-run tooling.

Stock Utilization

Stampings should be designed for the economical use of material. Shapes that can be nested close together are better than those which must be more widely spaced on the stock material. An L-shaped part will nest better than a T shape. Such improved stock utilization also requires close liaison between the designer and the die maker or, at the very least, the ability of the designer to visualize a strip layout such as the die maker would make.

Consideration also should be given to utilizing scrap portions to produce additional parts. In the case of large projects, many parts no doubt will require the same thickness and type of material. By designing a small part of such a shape that it can be made from a piece of stock left over from the blanking of a large part, the designer will save material. The designer should make a drawing notation in such cases to carry the information to the manufacturing personnel. Another common example is the typical motor lamination die in which the field an armature parts are made from the same strip with very little waste of material.

Holes

The diameter of pierced holes should be not less than stock thickness. Special intermeshed punch-support sleeves or the fine-blanking process permits smaller holes, but with conventional stamping tooling, punch breakage becomes excessive if pierced holes below the prescribed minimum are attempted.

The spacing between holes should be a minimum of 2 times stock thickness; 3 times is preferable from a die-strength standpoint; i.e., if the wall thickness is too little, the dies ability to resist the pressure of piercing is seriously impaired.

The writers have employed the following method to eliminate or minimize distortion when the design requires the lowest edge of the hole to be closer than the recommended minimum. A nonfunctional window, either square or rectangular, is pierced directly beneath the desired hole or holes. (A minimum wall thickness between the holes should, of course, be observed). Thus, during forming, no (or at least very little) stress or distortion is transmitted to the hole. Trial and error and experience are necessary to determine stress.

Often it is desired to include two aligned holes in opposite legs of a U-bent part for holding a shaft or for some other purpose. Designers should realize that it is difficult to form such a part from a prepierced blank and have the holes aligned precisely. Several alternatives can be considered: (1) Pierce or drill the holes after forming. This is more expensive but provides excellent alignment. (2) Use broad tolerances on the holes, or make one a slot, i.e., allow for misalignment of the function of the part permits. (3) Include a pilot hole in the bottom of the U bend. This hole is located over a pin in the pad of the forming die that will consistently position the blank. Another requirement, if truly close alignment is to be achieved with this method, is the use of stock of close thickness control. Although material of close thickness tolerance commands a premium price, the extra cost may be more than offset by the savings realized by not having to perform the second operation. The designer of stampings should always try to specify round holes instead of holes of square, rectangular, or other shapes. Tooling costs for round-hole punches and dies far below those for holes of other than round shapes.

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