I am only making 1 or 2 of each shaft. These are CV axle shafts for custom AWD car build.
I am leaning very strongly towards the the heat treat and then machine. I am going to reach out to my local heat treater today and see how long of a bar they can do and if they can hang them vertically. But, my thought would be to by a 6 or 12ft piece and have the whole bar heat treated to 45 Rc. That way I would have plenty of stock on hand to make axles. After machining, they are ready to go.
Depends how much you have to remove. If you HT the whole bar before machining and then remove a bunch of material you're going to have a softer part. A quenched bar cools slower in the center, so the outside is the hardest and it will be softer toward the core. I don't think that they'll be able to vertical quench a 12' bar either. Maybe your best option if you want to go that route is to have a small sample bar hardened and then machine it afterwards and have the hardness checked. You may find you want the raw bar hardened to 50 HRc so that your core is closer to where you'd like it.
Keep in mind, this will raise your tooling costs; as Bob noted, it's usually a much better idea to rough out first and leave finishing stock for after heat treat so there's not a lot of machining of very hard material. It will be especially brutal on any HSS tooling, and even carbide will take a beating cutting splines from solid at 45- 50ish HRc.