The aggregation, arrangement and bonding together of a set of components to form the radial spoke, a protein complex that links the outer microtubule doublet of the ciliary or flagellum axoneme with the sheath that surrounds the central pair of microtubules. [ http://www.ncbi.nlm.nih.gov/pubmed/24124175 http://www.ncbi.nlm.nih.gov/pubmed/27940518 http://www.ncbi.nlm.nih.gov/pubmed/21692193 http://www.ncbi.nlm.nih.gov/pubmed/8408197 http://www.ncbi.nlm.nih.gov/pubmed/21613541 ]