In sheet metal forming, once the tools are released, there is an elastic recovery in the sheet metal called spring back. During spring back, the sheet metal relaxes away from the shape of the tool. Thus the shape of the tool or the extent of the stamping must compensate for spring back in order to achieve the desired shape. Spring back can be measured or predicted with FEA. The resulting data can be used as input to api_spring_back.
Spring Back Mapping
The action of api_spring_back can be thought of as a mapping from a subspace of R3 ? R3. The action of the mapping on an input body creates a new body with a desired new shape. The spring back mapping is specified by giving the before and after positions on a collection of discrete positions and curves (wires). Also, the spring back mapping can simultaneously fix another collection of discrete positions, curves (wires), and surfaces (faces). Collections of these mapping specification objects make up constraints, discussed below. In between constraints, the spring back mapping interpolates smoothly.
The spring back mapping domain is an xyz box over which the mapping is defined. The bodies subject to api_spring_back must lie within this box. For a single call to api_spring_back, the user need not ever consider the domain of the spring back mapping – it will be automatically calculated. However, advanced users may wish to apply the same spring back mapping to several bodies. In that case, the domain of the spring back mapping must be enlarged to include all of the candidate bodies, before the first call to api_spring_back. The SPA_spring_back_def holds the domain of the spring back mapping, and its method, expand_def_domain, carries the domain enlargement.
A fixed constraint is a user specified collection of faces, wire bodies, and positions that define sub-manifolds of the domain that are to remain fixed under the action of the spring back mapping.
A shaping constraint is a pair of corresponding collections of wire bodies or positions that specify the before and after placement of sub-domains under the action of the spring back mapping.
Restriction on Wire Constraints
Note that constraint wire bodies must be connected, unbranched, with adjacent edges tangent at the connecting end points. Any desired wire body can be broken up into a collection of wire bodies such that each wire satisfies these requirements.
Also note that the API does not do any orientation matching on the edges. As a result, if a "before" wire is reversed relative to its "after" wire, the spring back will very likely give bad results. Similarly, one should note that the start point of a "before" wire goes to the start point of the "after" wire
Two constraints which require the same region of space to end up in two different places are called conflicting constraints. In general, constraints are also said to be conflicting when their mutual satisfaction causes distortions in the spring back mapping. The result is that constraints will not be satisfied (large gaps), or the resulting surfaces will have bad shapes or bad parameterization. This will be discussed more in the constraint guidelines section. Note: measured or simulated spring back data from a single stamping is unlikely to generate conflicting constraints. However, it is easy to generate conflicting constraints with contrived data.
The Springback API attempts to meet all user specified constraints, but may only approximately meet the constraints. The gap associated with a constraint is the distance between the desired final entity position and the actual position of the entity. The actual gaps produced by the spring back mapping can be queried on a per constraint basis.
A boundary plane divides the spring back mapping domain into a fixed region and a movable region. A boundary plane may offer a convenient workflow when the region affected by a stamping can be isolated by a plane. For example, multiple stampings on opposite sides of a part.