This worked for me. Spline was bald on alfa 159 wiper arm. Took the chance and it worked. 5 minutes. The metal is purposely soft on the wiper arm. Make sure saw is straight and thin. I made about 10 grooves.
Worked fine. Make sure spline on wiper linkage is clear also. All it wants is something to grip onto.
Not a mechanic. Just gave it a go.