flank drive nut recess

smidge=0.001;
w = 5;          // width of rectangle
h = smidge;     // height of rectangle
dh = -1.0 ;     // delta height of the curve
l = 5+h/2+dh;   // length of chord of the curve
weird=0.206;
module curve(width, height, length, dh) {
    r = (((length+weird)/2)^2+dh^2)/(2*dh);
    a = 2*asin((length/2)/r);
    translate([dh-r,0,0])    rotate([0,0,-a/2])  rotate_extrude(angle=a) translate([r,0,0])  square(size=[height,width],center=true);
}
for(a=[0:60:359])   rotate(a)   translate([+l,0,0])    { $fn=32;  curve(w,h,l-dh/2,dh); }