One for the engineers among you.

I can understand how you would form the splines at either end of a jackshaft to take sprocket or gears.

If the shaft required a centre sprocket or gear as part of a drive system, how would this part be mounted and how would the shaft be machined ?

Or would it be a two part shaft with splined flanges at the centre which would sandwich the sprocket or gear ?

Actually, I think I might just have answered my own question....

Move along, nothing to see here.

Keyways!

Got it.

id use an oversized shaft, cut the splines into the end and middle, and then turn it down to the correct size, leaving the splines the correct size for the sprocket.