Spru bushing

Create a spru bushing and spru bushing hole(s) through the relevant plates according to the "Oversize" values.

Parameters
D2	Spru bushing head diameter.
D1	Spru bushing diameter.
D3	Runner diameter at the nozzle contact point.
A	Runner cone angle.
R	Radius of nozzle touching the surface.
T	Depth of nozzle touching the surface.
S1	Length of Spru bushing head.
S2	Length of Spru bushing thin part.