A manufacturing drawing showing one of the parts which Pioneer Aero has recently remanufactured. This particular drawing describes part numbers 5062769-8 & -9, stiffeners belonging to the spar web assembly described in a recent update. (image via Pioneer Aero Ltd.)

The Old School Way:

Aircraft manufacture during WWII relied heavily upon manual labor. Many parts were formed by hand - coerced into their desired shape by the hard-won skills of master craftsmen. This is the same process which warbird restorers have used for generations as well, until more recent times when numerically controlled machines began to hold sway. While, admittedly, such modern tools are often the only economical way to reproduce some of the complex forgings and castings which many wartime aircraft featured, shaping sheet metal parts for warbirds still relies heavily upon hand-forming techniques - as shown in this article which illustrates the process used in forming a stiffener for the wing leading edge spar web described in a previous update.

The x-y coordinates describing the shape of the curve in this stiffener are shown at the top left of this original wartime manufacturing drawing for the part. (image via Pioneer Aero Ltd.)
Using the curve information from the above drawing, Pioneer's technicians cut out a cardboard template of the shape to ensure that any errors would show up early in the process. This also created a template to lay out both the left and right handed forming blocks for shaping the metal. (image via Pioneer Aero Ltd.)

Forming Blocks:

There are a variety of ways to manufacture small sheet metal parts featuring compound curves. One of the more common methods is employs a 'forming block', typically made of wood or another firm, but easily cut material. Using rubber mallets or other planishing tools, an engineer will shape the sheet metal blank over the forming block. The outer dimensions of the forming block usually conform to the inner shape of the part being created (as is the case here). Sometimes a single forming block can be used to manufacture both left and right-handed versions of the part. However, in this case, Pioneer had to create individual forming blocks for each stiffener, due to the flange angle relative to the shape of the curve.

One of the forming blocks part way through its fabrication process. (image via Pioneer Aero Ltd.)
The completed wooden forming block ready for use. Note how the edges for the completed component have been marked on the block as well. (image via Pioneer Aero Ltd.)

Preparing the Sheetmetal blanks:

The next step in this process involves marking out and cutting appropriately sized 'blanks' for each part from a sheet of appropriately-gauged metal. In this case, the part is made from 2024-T0 aluminum, where "2024" refers to the alloy type, while "T0" indicates that the material is untempered and in its softest state. Even though aluminum manufacturers can (and do) provide this alloy in its far stronger, tempered state (e.g. 2024-T3), such material is much stiffer and significantly more prone to cracking from work-hardening during the shaping process. Fabricators, therefore, tend to shape 2024 aluminum sheet in its T0 state, then heat treat the finished product into its T3 temper, which has a yield strength nearly three times as strong.

Marking the rough outlines of the part in the aluminum sheet, placing them in a way to minimize wastage. The blanks are then cut from the sheet, allowing a little extra material for final trimming to its ultimate dimensions following the forming process. (image via Pioneer Aero Ltd.)

Forming the Sheetmetal:

Shaping the sheetmetal blank over the forming block can proceed in a variety of ways. For instance, you can sandwich the blank between the forming block and a backing plate (to prevent surface buckling) and then use a planishing hammer or a mallet to bend the overlapping material into shape. Or, one could use a hydraulic press to do something similar, slowly squeezing the forming block into the blank which then presses up against a die or simply a block of heavy duty rubber to create the desired shape. In this case, Pioneer's technicians used both methods. They applied the hydraulic press to start shaping the part, and then finished the process with nylon mallets (while the aluminum sheet was clamped against the forming block in a vice). It is a multi-stage endeavor, which requires removing the part from the vice periodically to either shrink or stretch the overlapping material so it can eventually conform to the desired shape without wrinkling or tearing.

Clecoing the finished part in place to test its fit, prior to painting and riveting. (image via Pioneer Aero Ltd.)