2.2 Clamping knobs with threaded inserts (types of assembly)
Types of assembly that create optimum clamping conditions
The plastic base on the clamping knob should never rest on the clamping surface. In this way the metal inserts (threaded stud or tapped boss) are never subjected to abnormal twisting (“corkscrew” effect) when axial tensile stress is applied. Thus, the anchoring of the metal insert to the plastic material is stressed in the correct way, that's to say it is only subject to the torque applied to the knob for tightening it.
Incorrect types of assembly
When the plastic base of the clamping knob rests directly on the clamping surface, the threaded stud or tapped boss are also subject to an axial load (“corkscrew” effect), which could jeopardize its anchoring to the plastic material. The values of this force are always higher, with a broad safety margin, than those that may be applied by normal operations performed by hand, but designers who wish to take into account cases of improper use should avoid the situations illustrated in cases 5-6-7.