Material-dependent constants for heat set insert clearance

[martukas]

In our CAD, clearance holes for heat-set inserts are parametrized, and we have a single constant value defined in global_params.ipt. This same constant is used for all materials, acrylic or 3d printed.

For ideally suited materials, heat-set insert manufacturers define a tapered and stepped clearance hole. Such a hole could be defined by multiple diameter and depth parameters. However, such a hole is not feasible in the materials we use:

• cannot cut tapered shapes with straight laser into acrylic sheets
• resin-printed parts do not melt from heat, but instead become brittle, and a tapered hole would be too tight for direct insertion

It can be said that our 3d-printed parts are actually meant to be injection molded, and current design is just an approximation that need not be perfect. The currently used "major diameter" value works well for these.

Acrylic, however, does melt, and the holes may be a little too wide for it. It has been observed in practice that they are a bit loose and may pull through if too much force is applied. It may be wise to at least add a lower diameter. This diameter should be arrived at experimentally.

This problem identified in #1318

Additional reading:
https://www.cnckitchen.com/blog/tipps-amp-tricks-fr-gewindeeinstze-im-3d-druck-3awey
https://hackaday.com/2019/02/28/threading-3d-printed-parts-how-to-use-heat-set-inserts/