DESIGN FOR MANUFACTURING:DESIGN FOR BASIC PROCESSES—METAL

DESIGN FOR BASIC PROCESSES—METAL

Liquid State

Early in the planning of the functional design, one must decide whether to start with a basic process that uses material in the liquid state, such as a casting, or in the solid state, such as a forging. If the engineer decides a part should be cast, he or she will have to decide simultaneously which casting alloy and process can most nearly meet the required dimensional tolerance, mechanical properties, and production rate at the least cost.

Casting has several distinct assets: the ability to fill a complex shape, economy when a number of similar pieces are required, and a wide choice of alloys suitable for use in highly stressed parts, where light weight is important or where corrosion may be a problem. There are inherent problems, too, including internal porosity, dimensional variations caused by shrinkage, and solid or gaseous inclusions stemming from the molding operation. However, most of these problems can be minimized by sound design for manufacturing.

Casting processes are basically similar in that the metal being formed is in a liquid or highly viscous state and is poured or injected into a cavity of a desired shape.

The following design guidelines will prove helpful in reducing casting defects, improving their reliability and assisting in their producibility:

1. When changes in sections are required, use smoothly tapered sections to reduce stress con- centration. Where sections join, use generous fillets and blending radii.

2. Machining allowances should be detailed on the part drawing so as to ensure adequate stock and avoid excessive differences in casting thickness.

3. Remember that when design castings to be produced in a metal mold or die, convex forms are easy to mill but concave notches are both difficult and expensive.

4. Raised lettering is simple to cut into a metal mold or die; depressed lettering will cost con- siderably more.

5. Avoid the design of thin sections since they will be difficult to fill.

6. To facilitate the secondary operations of drilling and tapping, cored-through holes should have countersinking on both ends of the holes.

7. Avoid large, plain surfaces. Break up these areas with ribs or serration to avoid warpage and distortion.

8. For maximum strength, keep material away from the neutral axis. Endeavor to keep plates in tension and ribs in compression.

Table 1 identifies the important design parameters associated with the various casting processes and provides those limitations that should be incorporated by the functional designer to ensure prod- ucibility.

Solid State

A forging, as opposed to a casting, is usually used because of improved mechanical properties, which are a result of working metals into a desired configuration under impact or pressure loading. A refinement of the grain structure is another characteristic of the forging process. Hot forging breaks up the large dendritic grain structure characteristic of castings and gives the metal a refined structure, with all inclusions stretched out in the direction in which plastic flow occurs. A metal has greater load-carrying ability in the direction of its flow lines than it does across the flow lines. Consequently, a hot-formed part should be designed so that the flow lines run in the direction of the greatest load during service.

An extension of conventional forging known as precision forging can be used to acquire geometric configurations very close to the final desired shape, thus minimizing secondary machining operations.

Guidelines that should be observed in the design of forging in order to simplify its manufacturing and help ensure its reliability are as follows:

1. The maximum length of bar that can be upset in a single stroke is limited by possible buckling of the unsupported portion. The unsupported length should not be longer than three times the diameter of the bar or distance across the flats.

2. Recesses in depth up to their diameter can be easily incorporated in either or both sides of a section. Secondary piercing operations to remove the residual web should be utilized on through-hole designs.

3. Draft angle should be added to all surfaces perpendicular to the forging plane so as to permit easy removal of the forged part. Remember that outside draft angles can be smaller than inside angles since the outside surfaces will shrink away from the die walls and the inside surfaces will shrink toward bosses in the die.

4. Deeper die cavities require more draft than shallow cavities. Draft angles for hard-to-forge materials, such as titanium and nickel-base alloys, should be larger than when forging easy- to-forge materials.

5. Uniform draft results in lower-cost dies, so endeavor to specify one uniform draft on all outside surfaces and one larger draft on all inside surfaces.

6. Corner and fillet radii should be as large as possible to facilitate metal flow and minimize die wear. Usually 6 mm (0.24 in.) is the minimum radius for parts forged from high-temperature alloys, stainless steels, and titanium alloys.

7. Endeavor to keep the parting line in one plane since this will result in simpler and lower-cost dies.

8. Locate the parting lines along a central element of the part. This practice avoids deep impres- sions, reduces die wear, and helps ensure easy removal of the forged part from the dies.

Other Basic Processes

In addition to casting and forging, several other processes that may be considered basic since they impart the approximate finished geometry to material that is in the powdered, sheet, or rod-shape form. Notable among these are powder metallurgy, cold heading, extrusion, roll forming, press form- ing, spinning, electroforming, and automatic screw machine work.

In powdered metallurgy, powdered metal is placed in a die and compressed under high pressure. The resulting cold-formed part is then sintered in a furnace to a point below the melting point of its major constituent.

Cold heading involves striking a segment of cold material up to 25 mm (1 in.) in diameter in a die so that it plastically deformed to the die configuration.

Extrusion is performed by forcing heated metal through a die having an aperture of the desired shape. The extruded lengths are then cut into the desired length. From the standpoint of producibility, the following design features should be observed:

1. Very thin sections with large circumscribing area should be avoided.

2. Any thick wedge section that tapers to a thin edge should be avoided.

3. Thin sections that have close space tolerance should be avoided.

4. Sharp corners should be avoided.

5. Semiclosed shapes that necessitate dies with long, thin projections should be avoided.

6. When a thin member is attached to a heavy section, the length of the thin member should not exceed 10 times its thickness.

In roll forming, strip metal is permanently deformed by stretching it beyond its yield point. The series of rolls progressively changes the shape of the metal to the desired shape. In design, the extent of the bends in the rolls, allowance must be made for springback.

In press forming, as in roll forming, metal is stretched beyond its yield point. The original material remains about the same thickness or diameter, although it will be reduced slightly by drawing or ironing. Forming is based upon two principles:

1. Stretching and compressing material beyond the elastic limit on the outside and inside of a bend.

2. Stretching the material beyond the elastic limit without compressing the material beyond the elastic limit without stretching.

Spinning is a metal-forming process in which the work is formed over a pattern, usually made of hard wood or metal. As the mold and material are spun, a tool (resting on a steady rest) is forced against the material until the material contacts the mold. Only symmetrical shapes can be spun. The manufacturing engineer associated with this process is concerned primarily with blank development and proper feed pressure.

In electroforming, a mandrel having the desired inside geometry of the part is placed in an electroplating bath. After the desired thickness of the part is achieved, the mandrel pattern is removed, leaving the formed piece.

Automatic screw machine forming involves the use of bar stock, which is fed and cut to the desired shape.

Table 3 provides important design for manufacturing information for these basic processes.