Sheet metal forming processes shape components by plastic deformation rather than by removing material. Important processes include bending, deep drawing, redrawing, metal spinning and shear spinning or flow forming.
These processes are widely used in mechanical and manufacturing engineering to produce components ranging from simple brackets and enclosures to cups, cylindrical shells, vessels and rotationally symmetrical components.
Bending is one of the simplest and most widely used sheet metal forming operations. A load is applied to the sheet so that it plastically deforms around a defined bend line or tool profile.
Press tooling can use a punch and die to produce bends, channels and more complex formed sections. Unlike deep drawing, relatively little movement of material from distant regions of the blank is required for a simple bending operation.
When a sheet is bent, the material on the outside of the bend is placed predominantly in tension while material towards the inside is compressed. The resulting strain increases as the bend radius becomes smaller relative to the sheet thickness.
Important considerations include material condition, sheet thickness, minimum bend radius, bend direction relative to the material grain or rolling direction, springback and the required dimensional accuracy.
Deep drawing forms a flat sheet-metal blank into a hollow component by forcing the material into a die cavity with a punch. Unlike simple press forming, substantial material flows from the flange region of the blank towards the die cavity during the operation.
Circular cups are a common example, although deep drawing can also be used for non-circular components and considerably more complex shapes.
The blank holder bears on the flange region of the sheet and controls the manner in which material is drawn into the die. One of its important functions is to suppress wrinkling as circumferential compressive stresses develop in the flange.
Blank-holder force must be appropriate for the material, geometry and lubrication conditions. Insufficient restraint can permit wrinkling, while excessive restraint can impede material flow and contribute to excessive tensile stresses or tearing.
Deep drawing produces a complex combination of radial and circumferential stresses as material moves from the original blank into the wall of the formed component.
Wall thickness is therefore not necessarily uniform after drawing. Thickening can occur in some regions while thinning occurs in others, with the distribution depending on the material, blank geometry, punch and die radii, friction, blank-holder force and drawing conditions.
Control of material flow is one of the principal engineering challenges in the design of a deep drawing operation.
Typical problems which may occur during deep drawing include:
The required depth or diameter reduction cannot always be achieved satisfactorily in a single drawing operation. The component can therefore be produced using two or more successive drawing stages.
A previously drawn cup can be subjected to a redrawing operation to reduce its diameter and increase its depth.
Depending on the tooling arrangement, redrawing can be carried out using direct or reverse drawing methods. Reverse redrawing changes the direction in which the cup wall passes through the tooling and can alter the bending and unbending experienced by the material.
Successive cold-forming operations also produce work hardening. Where the accumulated deformation becomes excessive for the material, an appropriate intermediate annealing treatment may be required before further forming.
For a simple cylindrical cup, the relationship between the initial blank diameter and punch diameter provides a useful indication of the severity of a drawing operation.
A larger reduction requires greater material movement and generally makes the operation more demanding. The reduction which can safely be achieved in one operation depends on material properties, sheet thickness, tooling, lubrication and process conditions.
Metal spinning forms a rotationally symmetrical component from a circular blank. The blank is clamped against a shaped former or mandrel and rotated while a forming tool progressively forces the material towards the mandrel profile.
Typical spun shapes include cones, cups, hemispherical forms and other axisymmetric components.
The forming tool can be controlled manually or mechanically, hydraulically or numerically using specialised spinning equipment. The process is progressive, with the required shape developed through controlled movement of the forming tool.
Metal spinning can be particularly attractive for prototypes, development components and relatively low or medium production quantities because the tooling can be simpler than the matched punch-and-die tooling required for some press-forming processes.
Automated and CNC spinning machines also allow the process to be used for repeat production. The practical component size and production quantity depend on material, thickness, geometry, machine capacity, tooling and the economics of the particular application.
Shear spinning differs from conventional spinning because the process intentionally reduces the wall thickness of the material while forming it against the mandrel.
For a conical component, the original blank diameter can remain related to the maximum finished diameter while the material is progressively thinned and extended along the surface of the mandrel.
For ideal shear spinning of a conical component, the finished wall thickness can be related geometrically to the original blank thickness. Using the notation shown in the diagram:
t = T sin α
where:
This relationship is commonly referred to as the sine law for shear spinning. It represents the ideal geometrical relationship. Actual finished thickness can differ because of material behaviour, tooling, elastic deformation and process conditions.
A related process is used to reduce the wall thickness of cylindrical components while increasing their axial length. The tubular workpiece is supported on a mandrel while one or more rollers progressively displace the material.
As the wall is reduced, material is displaced predominantly in the axial direction and the component becomes longer. Conservation of material provides a useful first approximation to the relationship between thickness reduction and increase in length.
The earlier RoyMech description noted that, in an idealised constant diameter case, reducing wall thickness to approximately half its original value would correspond to approximately doubling the formed length. This should be regarded as a geometrical volume-conservation approximation, not as a universal production relationship.
Many ductile engineering metals can be formed using bending, deep drawing and spinning processes. Common examples include:
Formability cannot be determined from material name alone. Alloy, temper or heat-treatment condition, sheet thickness, grain structure and directional properties can all influence forming behaviour.
Friction has an important influence on sheet metal forming. In deep drawing it affects the movement of material beneath the blank holder and around the die and punch radii. In spinning it influences the interaction between the forming roller, workpiece and mandrel.
Suitable lubrication can reduce forming forces, limit surface damage and tool wear, and assist consistent material flow. Lubricant selection must also consider the workpiece material, subsequent cleaning or coating operations and the manufacturing environment.
Press-forming and deep-drawing operations can be carried out using mechanical or hydraulic presses, with the appropriate choice depending on the force, stroke, speed and control characteristics required by the operation.
Mechanical presses can provide high production rates for suitable operations. Hydraulic presses can provide controllable force through a long stroke and can be advantageous for some deep-drawing applications.
Metal spinning can be carried out on suitably configured lathes or, particularly for production work, on purpose-designed manual, hydraulic, automatic or CNC spinning machines.
The appropriate forming process depends on the geometry, material, production quantity and required component properties.
The following external resources were included on the original RoyMech page and provide further information relating to deep drawing and manufacturing processes.