"""Captive panel screw envelope, M3 to M5, any length, as step.parts' published models draw it: a shank (drawn at the
thread's nominal diameter) and a round knob, two bodies.

The shank (diameter d, length `length`) runs along +Z from z = 0; the knob (diameter 2d, height d) is centred on the
shank's top end, so it overlaps the shank by d / 2 (two bodies, as in the models: the knob is a separate part of a
captive screw). No thread, retainer, spring or knurl.
"""
from steve import Model

thread = "M3"  # Thread
length = 20  # mm Shank length
material = "unspecified"  # Material

# mm, measured on step.parts' models captive_panel_screw_m<d>_l<length> (the cylinders of their two solids): shank
# diameter = the thread's nominal d; knob diameter 2d and height d, centred on the shank's top end (every model)
THREADS = {"M3": 3.0, "M4": 4.0, "M5": 5.0}
KNOB_DIAMETER = 2.0   # x d
KNOB_HEIGHT = 1.0     # x d
MATERIALS = {"unspecified": (None, None), "steel": ("steel-generic", "#9da2a8"),
             "stainless-304": ("stainless-304", "#c9ccd0")}

d = THREADS[thread]
material_id, colour = MATERIALS[material]

model = Model()
model.cylinder("shank", d / 2, length, name="Shank")
model.cylinder("knob", KNOB_DIAMETER * d / 2, KNOB_HEIGHT * d, at=(0, 0, length - KNOB_HEIGHT * d / 2), op="new",
               name="Knob")
if material_id:
    model.material(material_id, color=colour)
result = model.result()