SHEET-METAL FORMING

 PROCESS DESCRIPTION

Various processes are used to form cold rolled sheet metal using die sets, formers, rollers, etc. The most common processes are: deep drawing, bending, stretch forming and roll forming.

MATERIALS

·         All ductile metals available in cold rolled sheet form, supplied as blanks, flat or coiled.

·         Most commonly used metals are: carbon steels, low alloy steels, stainless steels, aluminum alloys and copper alloys. Also, nickel, titanium, zinc and magnesium alloys are processed to a lesser degree.

·         Also coated materials, such as galvanized sheet steel.

PROCESS VARIATIONS

·         Mechanical drives: faster action and more positive displacement control.

·         Hydraulic drives: greater forces and more flexibility.

·         Deep drawing: forming of a blank into a closed cylindrical or rectangular shaped die. Incorporating an ironing operation improves dimensional tolerances.

·         Bending: deformation about a linear axis to form an angled or contoured profile.

·         Stretch forming: sheet metal is clamped and stretched over a simple form tool.

·         Roll forming: forming of long lengths of sheet metal into complex profiles using a series of rolls.

·         Beading: edge of sheet bent into cavity of die. May be used to remove sharp edges.

·         Hemming: edge of sheet folded over. May be used to remove sharp edges.

·         Can incorporate initial sheet metal shearing operations.

ECONOMIC CONSIDERATIONS

·         Production rates vary, up to 3000/h for small components using automated processes.

·         Deep drawing punch speeds a function of material; high to low – brass, aluminum, copper, zinc, steel, stainless steel (typically 800/h).

·         High degree of automation is possible.

·         Cycle time is usually determined by loading and unloading times for the stock material.

·         Lead times vary, up to several weeks for deep drawing and stretch forming; could be less than an hour for bending.

·         Material utilization is moderate to high (10–25 per cent scrap generated). Bending and roll forming do not produce scrap directly. Deep drawing and stretch forming may require a trimming operation.

·         Production quantities should be high for dedicated tooling, 10 000+. Minimum economical quantities range from 1 for bending to 1000 for deep drawing.

·         Tooling cost is moderate to high, depending on component complexity.

·         Equipment costs vary greatly; low for simple bending machines, moderate for roll forming machines and high for automated deep drawing, sheet metal presses and stretch forming.

·         Labor costs are low to moderate, depending on degree of automation.

·         Finishing costs are low. Trimming and cleaning may be required.

TYPICAL APPLICATIONS

·         Cabinets

·         Mounting brackets

·         Electrical fittings

·         Cans

·         Machine frames

·         Automotive body panels

·         Aircraft fuselage panels

·         Light structural sections

·         Domestic appliances

·         Kitchen utensils

DESIGN ASPECTS

·         Complex forms possible: several processes may be combined to produce one component, or a series of operations used to progressively form the part.

·         Working envelope of machine and uniform thickness of sheet can restrict design options.

·         No inserts or re-entrant angles.

·         Draft angles maybe required (0.25°).

·         Minimum bend radii are a function of material and sheet thickness, but typically four times the sheet thickness.

·         Radii kept as large as possible, particularly if parallel with grain of material.

·         Square or rectangular boxes limited by sharpness of corner detail required.

·         Minimum sheet thickness =0.1 mm.

·         Maximum sheet thickness: deep drawing =12 mm, bending =25 mm, roll/stretch forming =6 mm.

·         Sizes ranging Ø2–Ø600mm for deep drawing; 10 mm–1.5m width for roll forming; 2 mm–3.6m width for bending.

QUALITY ISSUES

·         Bending and stretch forming are limited by the onset of necking.

·         The limiting drawing ratio (blank diameter/punch diameter) is between 1.6 and 2.2 for most materials. This should be observed where drawing takes place without progressive dies, otherwise excessive thinning and tearing could occur.

·         Variations in stock material thickness and flatness should be controlled.

·         Other problems include: spring-back (metal returns to original form) and wrinkling during drawing (comparable with forcing a circular piece of paper into a drinking glass), eliminated by adjustment of blank holder force.

·         Spring-back can also be compensated for by over-bending, coining and stretch-bending operations.

·         High residual stresses can be generated. Subsequent heat treatment may be necessary.

·         Surface detail is good.

·         Surface roughness is approximately that of the sheet-material used.Process capability charts showing the achievable dimensional tolerances are provided.