OpenSCAD Screw Holes

This provides a module to drill holes for screws.

The following standards are currently understood:

DIN912, DIN963, DIN965, DIN34811, ISO4762, ISO2009, ISO7046, ISO14581 and ISO10642.

#### Usage

```openscad

include

difference()
{
solid_object();
// Drill a hole for a DIN965 M3 screw of length 20 mm,
// with 10 mm thread.
screw_hole(DIN965, M3, 20, 10);

// Drill a M4 hole through the 30 mm thick object, with a countersunk
// head with a 90 degree angle that is 8 mm wide at the top and 4 mm
// high.
translate([-10, 10, 0])
screw_hole([8, 4, 90], M4, 30);
}
```

The top of the head of the screw will be in the xy-plane, with the screw extending into the positive z direction.

Note that angles larger than 90 degrees might only be printable if the

top of the screw is at the top, or you'd need support while printing it,

in which case you might as well drill the hole afterwards.

The following global variables can optionally be changed after the include:

```open