Wasn't it Marvin who wrote:
>
>I am modeling a screw and nut that are supposed to fit into each other
>without collision and stay without collision throughout the process of
>moving nut on screw  from top to end.
>
>However, I have used approximation formula that might not be exact:
>
>isosurface {
>  function { f_helix1(x,y,z, 1, 20/3*omega1, dy, inner_r+dy+dy*2/3, 1, 1.5,
>0) }
>         // distance_wind, thicknessm, r, shape, 0, cross-section type
>         // 20/3 is an empiric factor connecting number of windings of helix
>         // per unit of length with parameter P2 of f_helix1()
>         // ***********************************
>         // *** WHAT IS THE EXACT VALUE???? ***
>         // ***********************************
>  contained_by {
>     box { <-2, -l/2, -2>, <2, l/2, 2> }
>  }
>  max_gradient 1.5
>}

If you use the value "2*pi" where you've got "20/3*omega1" then you get
exactly one complete turn as the y value increases by 1 unit (e.g. from
-1/2 to +1/2)

-- 
Mike Williams
Gentleman of Leisure

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