In this article we will discuss about the general design consideration in forging of metals.
Factors Influencing the Design of a Forging:
The various factors which require due consideration and which influence the design of a forging are:
(i) Shape and size,
(ii) Grade and composition of metal,
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(iii) Quantity required,
(iv) Subsequent processing and heat-treatment, and
(v) End use.
The use of open-die forgings should be considered if only small quantities (around 25) are to the produced.
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The use of close die is recommended for:
(i) small repeat quantities if the shapes require considerable machining,
(ii) for almost any size and shape of pieces if required in large quantity (above 100) (by drop forging).
The drop forging is not practicable if parts have undercuts, deep straight holes, no draft on vertical sides (or very little draft) etc.
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In the case of parts subjected to high sustained or intermittent stresses, the direction of the major and important stresses should be at right angles to the direction of the flow lines. Heat treatment of parts before forging should be considered to eliminate the problems of warpage and change in size etc., though in introduces difficulty in matching.
Drop Forging Design:
The following points should be given due consideration in the design of drop forged products. As far as possible, the amount of metal in both the upper and lower dies should be equal and the parting plane should be flat. Draft angles, to permit removal of forging from dies should be about 7° for the vertical sides on the outside perimeter and 10° on inside pockets and holes.
This is the general rule but much depends on the experience. Corner radii and fillets should be as large as the design will permit. Large radii tend to improve the quality of the forging by permitting continuity of metal flow line and promoting natural flow of the metal during forging.
The allowance for machining of forging depends on its size and shape, relative length in proportion to width and thickness, number of holes and pockets, and grade of material being forged. It varies from 0.75 mm (for small forgings about 1.25 kg in weight) to about 1.4 mm minimum (for large forgings weighing 50—100 kg).
Press Forging Design:
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Press forgings to be made in impression dies are classified by material:
(i) Strong alloys such as steels, montel, metals, and aluminium alloys and
(ii) Various grades of forging brasses and bronzes.
It is desirable to forge symmetrical parts by press forging. In general, the design of steel press forgings is similar to the design of drop forgings. In case of press forgings in the softer metals, the draft angles and radii and machining allowances etc. may be reduced considerably. As the metal can be extruded as well as flowed in the case of soft metals, it is easier to fill all die- impression pockets without causing defects.
Upset Forging Design:
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The up setter machine grips, gathers, spreads the plastic metal which permits the forging design to be made without the draft consideration of the drop forging. Sides can be designed straight, where they are parallel to the axis of the bar. Most of the parts produced are symmetrical and may have holes at pockets also if they are parallel to the axis of the bar stock. Other considerations are same as for drop forging.