What characteristic of the English wheel allows it to create double curves?

The ability of the English wheel to create double curves is primarily attributed to the tension of the wheels. The mechanism relies on the interaction between a lower wheel, which is typically larger in diameter, and an upper wheel, which is smaller and can be adjusted in tension. By varying the tension on the wheels, artisans can manipulate the forming process while the metal is passed between them.

This process allows for the control of the curvature and contour being formed, enabling the creation of complex shapes that have both convex and concave characteristics. The adjustability of tension creates pressure that alters the shape of the workpiece, allowing for precision in the development of double curves, which are essential in aircraft materials that often require aerodynamic shapes and structural integrity.

While the shape of the wheels, the pressure applied during use, and the material being worked on play roles in the forming process, they do not specifically account for the mechanism by which double curves are achieved. The critical factor in manipulating the shape effectively is the tension adjustment between the wheels.