Radii of curvature Rx and Ry in 3D

Hello I want to model the curvature in the optical fiber obtaining Rx and Ry. In this case X=cos(2piz), Y=y and Z=z. I tried with the definition of radii of curvature k=dT/ds, k was parameterized in function of z as k=dT/dz.dz/ds, where dz/ds=1./sqrt(x'2+y'2+z'2)or 1/(ds/dz). x'2, Y'2 are derivative of x and y. dT/dz=1./sqrt(x'2+y'2+z'2)*( x' y'2 z'), where dT/dz is the normalized tangent. However, i don't know if is correct it, because when i taste with a circle, the result is not good. Also, I do not know how to get Rx and Ry separately. I could say that Tx=dFx/dx and Ty=dGy/dy. Thank you very much.