For every stretch forming part, aPriori attempts to evaluate two different routings:

These routings correspond to two different types of stretch forming machines. The transverse routing involves the use of a transverse stretch forming machine, and the longitudinal routing involves the use of a longitudinal stretch forming machine.

With a transverse machine, the jaws of the machine grip along the stock edges that are perpendicular to the direction of the part’s greatest curvature.

With a longitudinal machine, the jaws grip along the stock edges that are parallel to the part’s direction of greatest curvature.

The gripping jaws of a longitudinal machine (unlike those of a transverse machine) have a special rotational capability that aids in wrapping the blank around the tool. This capability is typically unnecessary with transverse stretching.

For a given part, transverse and longitudinal stretch forming differ with regard to required pull force, and the required stock addenda, among other factors that affect cost.

Transverse stretching is generally preferred, but in some cases it is not practical. For example, transverse stretch forming is sometimes infeasible when the blank edges perpendicular to the direction of greatest curvature are very long. The length of these edges may exceed the greatest length along which the jaws can grip, for any stretch forming machine. In such a case, the solution is to use a machine that can grip the shorter edges, which in this case are the edges parallel to the curvature direction. In other words, the solution in this case is to use a longitudinal machine.

Transverse stretch forming is also sometimes infeasible when the part’s concavity is not sufficiently consistent along the direction of greatest curvature. The part shown below requires longitudinal stretch forming.

See Process Feasibility for Stretch Forming and Stretch Form Machine Selection for more information about process and machine feasibility.

When both transverse and longitudinal stretching are feasible for a given part, aPriori costs both approaches and determines which is less expensive. (The cost model does not support scenarios that require both transverse and longitudinal stretching on the same part.)

It is important to note that the Stretch Forming cost model distinguishes between direction of greatest curvature and direction of sharpest bend (smallest radius of curvature). The direction of sharpest bend is often the direction of greatest curvature, but there are cases in which it is not. This is because the cost model considers degree of curvature in a given direction to have two components:

Consider for example a saddle-shaped part. Such a part might have bends of similar sharpness along two, orthogonal directions; and the bend in one direction might be significantly longer than the bend in the other direction. In such a case, the cost model evaluates both the direction of sharpest bend and the direction orthogonal to it, and combines considerations of sharpness and length. (The cost model uses curve height as a reflection of a combination of sharpness and length. See Xsection GCDs and Xsection Curve Height for Stretch Forming for more information.)

For the part below, the direction of greatest curvature, in this sense, is along the part’s length, even though the direction of sharpest bend is along its width. So a transverse machine would grip the edges along the stock’s width, and a longitudinal machine would grip the edges along the stock’s length.