Manufacturing Tip: Improve Aerospace Tooling Part 4

Tooling for composite aerospace parts is in the form of molds and winding mandrels. Mold making remains something of an art, but digital metrology is turning it more in the direction of science, minimizing trial and error. Let’s take a closer look…

When a toolmaker receives the model of the structure, such as a wing or tail part, they create a virtual tool that includes the molding surfaces. The art comes in knowing how to construct the overall tool so that the mold surface is supported rigidly and will not deflect under the weight of the mass of polymer. Today, mold makers rely on Invar, an alloy with a very low coefficient of thermal expansion. Its thermal stability makes it ideal for B-stage resins that will be cured in an autoclave. The downside of Invar is that it is expensive and difficult to machine, so it is important to avoid machining mistakes.

Molds are comprised of two regions, the solid surface that is in contact with and shapes the composite, and a support structure. Both are crafted from Invar. Absolute rigidity is essential. Once the mold surface is formed, the support structure must be machined so that the off-side of the mold surface perfectly “nests” in and is supported by the structure. Each vertical column in the support structure is like a bracket beneath the molding surface. To get the nesting shape correct, toolmakers use a laser tracker to measure cuts in the structure until the nest exactly matches the underside of the mold.

Few designs ever make it all the way to assembly untouched, but with a laser tracker, modifications do not have to slow down tool development; the laser tracker can be used to position running modifications. By locating a modification such as an extra support bracket or hole with the laser tracker, engineers only have to do it once because positioning is so precise.

Surfaces of molds and winding mandrels are checked quickly with a laser tracker. By sliding the target sphere over critical sections of the tool surface, a technician generates a line of points in the instrument’s computer, not just a single point. By covering the region on the tool in a grid pattern, the technician captures the shape of its surface, as a 3-D point cloud. Comparing this to the surface CAD file instantly tells tool developers where and how much the surface needs to be modified.

Read more about Laser Trackers
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