The problem of geometric curvature in manufacturing and assembling structures of CubeSat satellites

The very common problem in the manufacture of structures (chassis walls) for cubesat satellites is maintaining tolerances in 3d spatial geometry. According to our observations, more than 70% of teams who create such a body for the first time face this problem. And novice developers are often surprised, “where can there be errors in geometry if the parts are made on a cnc machine?”. They even measure the dimensions (x,y,z) - the dimensions are correct, but the body does not fit the launch adapter (deployer).

The reason for that is interesting and simple - internal residual streses in the material. Most of the walls are milled from thick sheet material, when the machine removes part of the material from one side - the part bends/twists. While it is in the machine, it is held by clamps, and its geometry is correct. But when you remove the clamps, its geometry changes. You will not notice changes in the linear dimensions along the main axes (they are too small), and the change in the size of the spatial diagonal of the structure (chassis) is usually 0.2-1.0 mm, and these dimensions is exactly what people usually forget to check.

The following illustration shows three basic situations: twist, shift and correct geometry. The scale of the curvatures in the illustration is hyperbalised.

This example with the twisting of the satellite body structures is the most difficult - the deflection angles are small, and when it is placed on a mounting table or rails, it rests on three points and small deformations can be overlooked. But this is the most insidious situation, it will not be installed in the deployer.

This problem is often disclosed only at the stage of fitting the satellite structure to the deployer, which creates many problems.

To solve this problem, various methods can be used, their choice depends on the geometry of the structure parts, the most common are: heat treatment (stress relief bake) of the workpiece and/or the finished part, design to minimize stress, the use of high – height transverse connector parts.