Wasn't it Reiner Jung who wrote:
>
>Hello,
>
>I've searched the povray site and googled, but either I am too stupid to
>formulate my question correctly or I'm blind or overlooking something.
>
>I am working on a larger hobby project and therefore I need screws, 
>nuts, bolts,
>etc. Therefore I created a function which generates a screw thread. Also I
>constructed a hexagon nut. For the hole in nut I tried to subtract the screw
>thread from the nut like this:
>
>difference {
>      object {nut_body}
>      union {
>  isosurface {
>    function{ f_helix1( x,y,z,
>   1,    // number of helixes, (1 = single helix, 2 = double helix etc.)
>   (2*pi)/incline,   // period,      turns on the length of 2*pi
>   incline*2, // minor radius,
>   nominal, // major radius,
>   1,    // shape parameter,
>   2.0,  // cross section type,
>         // (0.0 to 1.0 = square ... rounded to circle
>         // over 2.0 to 3.0 = rounded to diamond and concave diamond
>   0 )   // cross section rotation angle
>    }
>    contained_by {box { -length/2 , length/2 }}
>    threshold 0
>    max_gradient 3.168
>    accuracy 0.0001
>    all_intersections
>  }
>  cylinder {<0,-length/2,0>,<0,length/2,0>,inner}
> }
>}
>
>As a result I get a nut with a squared hole in the middle instead of a hole
>drilled into the nut_body by the isosurface.
>
>The povray documentation states that isosurface can be combined with solid
>objects, however this does not work in this example. So what am I missing? Or
>do I have to remodel the nut_body with a function as well and subtract both
>functions?

That works fine for me. I had to guess suitable values for length, 
incline, nominal, inner and nut_body. Perhaps you're using unsuitable 
values for some of those. Here are the values that I used:

#declare length = 2.5;
#declare incline = 0.5;
#declare nominal = 0.1;
#declare inner = 0.5;

#declare nut_body = box {<-1.5,-1,-1.5><1.5,1,1.5>}

#include "functions.inc"

-- 
Mike Williams
Gentleman of Leisure

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