Sure is an old thread ya found!
Some standard ones may be cold formed (particularly the V-type splines found on fine spline Torana diffs as opposed to the squaretooth splines found on coarse spline Torana diffs), I've only ever seen them machined but I haven't worked in a factory that mass-produces them.
One of the problems with the typical production car splined arrangement is that the outer diameter of the splined area is the same as the shaft diameter. This means that you get effectively a smaller diameter axle under where the splines are situated. So you're using excessive material for the same torsional rigidity if that makes sense. Basically you end up with a spline diameter that is undersized and a shaft diameter that is nominally oversized (or both undersized in the case of a Torana LOL) and isn't giving you any strength anyway because that isn't the weakest point
The best way to do a splined shaft is to have a larger diameter at either end of the shaft, with a radius leading into that diameter so that there are no stress concentrations (another problem with the typical production car spline arrangement). If the smaller diameter matches the depth of the splines, then you just machine the splines into that larger diameter, back and forth with an indexing head of some description, until you reach the nominal shaft diameter. This also means that the axle is allowed to 'wind up' a little bit so that the peak destructive forces are spread out over a matter of a very small timeframe which seems to make a difference, and when you have a shaft of this design it ends up being lighter too, which in a diff is bloody important if you actually want to go around corners and get the car to handle as well as possible.
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