I replaced all of those nylon inserts with rivnuts epoxied in. The original design was poor & the assembly poorly executed , there is insufficient material around those inserts to adequately resist the turning moment of a self tapping screw when initially assembled, thus the frame is weakened from the start.
Fitting rivnuts reenforces the framework & allows you to reassemble without the original fitting dictating the torque required to tighten; that remains firmly in your hands. The machine screw you are now using is totally under control.
M

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