// new example, using a low ratio harmonic drive floating
// splined belt with 60 teeth.
// s/b 30/1 when finished.
$aa=69.500000; // was 69.00000000 a blond one too big
$xx=9.750000000;
$dd1=111.40000000;
$dd0=117.80000000;
$fx = ([ 360.00000000 / 60.00000000 ]); // arc of circle per tooth, works but...
// overall span of armature plus bearings equals 96.6mm's
$fr = [ 0.00000000]; // establish its storage
echo ("10 ", $aa, $xx, $dd0, $dd1, $fr, $fx) ;  // echo? yes, once
difference() { // the outside smoother
    cylinder(d=$dd0, h=$xx, $fn=360); //outer is fussy
    cylinder(d=$dd1, h=$xx, $fn=180); //but the inner isn't
}

// for 6.00000 we got 60 teeth, for 62 teeth inc is 5.8064516129
// those 60 teeth rendered at 3.69mm peak to valley
// use this "for" for 62 teeth
//for($fr=[ 0.00000000: 5.8064516129  : 360.00000000 ]){ //can't pass $fd to this for, why?
//use this "for" for 60 teeth
for($fr=[ 0: 6.000000000  : 360 ]){  
//use this "for" for 58 teeth
//for($fr=[ 0 : 6.20689655172 : 360 ]){
//cylinder(h=11.75, d=8, center=true);

rotate([0.00000000, 0.00000000, $fr]) // works, finally
// this below works, but again, can't pass a var to translate
    // what the hell???????
    // 
translate([54.05000000, 0.00000000, 0.00000000])  // mv x only
scale(v= [.1000000, .1000000, 1.000000]) // might use a small xy
   // differential here to improve tooth fit.
rotate(180) // same here, using $var's doesn't work here or for
// cylinder. switch rotate on for outers and readjust sizes
cylinder(d=$aa, h=$xx, $fn=3); // make thick triangle
}; // switch h=9.500 for outer rings