6. ASSEMBLY MEASURES
Plastic is a poor conductor of heat and has a different thermal expansion coefficient from that of the metal inserts so measures must be taken, while remachining the hole, to stop the hubs and bosses from overheating: in fact, the heat produced is not dissipated and the metal parts expand and create stress inside the body of the plastic which has a damaging effect on the strength of the assembly (Duroplasts). In addition, for thermoplastics (Technopolymers), temperatures close to their softening point could be reached with the risk of the metal insert coming loose.
It is therefore always necessary to adopt cutting and feed rates that do not produce marked localized heating and to cool intensively when the holes have a large diameter and depth with respect to the size of the bushing.
To conserve maximum gloss of the surfaces, we recommend, once machining has been completed, to avoid leaving the plastic wet for a long time, by removing all residual emulsified water from the surfaces; use oil only, if possible.
The machining processes commonly required for the assembly of handwheels or knobs are:
- remachining of axial hole in the bosses (blind hole)
When remachining the hole of a built-in metal boss, always avoid operating as shown in Fig.4, because both during the drilling operation and during the insertion of the small shaft, an area of the plastic covering may be subjected to stress, with the risk of cracking or detaching the part indicated with cross shading. The operation shown in Fig.5 is the most rational.
Note that in the ELESA parts, remachining of the axial hole may be performed under the correct conditions indicated above in that the length of the built-in bosses is always indicated in the table of each article so, for the depth of the hole, reference should simply be made to the basic plan.
- remachining of the axial hole in the bosses (case of a pass-through hole)
If the drilling operation affects not only the metal boss but also a layer of the covering material, the handwheel must be centred carefully and drilling should be started from the plastic side otherwise, chipping may occur when the tool is removed.
- transversal threading in the boss for grub-screw
To be performed in accordance with the instructions given above. Avoid threading both the metal and the plastic: it is better to drill the hole in the plastic part and thread the metal part only.
Drilling or threading operations to be performed entirely in the plastic are exceptional. Bear in mind that the difficulty with which the heat produced locally is dissipated, also through the abrasive action of the plastic on the tool, worsens considerably the latter’s working conditions, resulting in a rapid wear of the cutting edges (use hard metal tools).