I tried to take a lot of pictures while making the nozzle for this latest motor and thought I would show the progression and talk about each of the steps a bit. I'm not a machinist and this isn't meant to be a tutorial, just a bit of insight into my thought process. Feel free to comment.
In this image I have the rough stock in the lathe. It's about 0.75" longer than the nozzle needs to be but I will cut it to length later when I turn it to work on the other side. I begin by facing the end, if nothing else I take this as an opportunity to ensue that the cutting tool is at the correct height.
After using a center bit to start drilling out the throat I progress through larger drill bits until I get close to the final dimension.
I then switch to a boring bar and cut the throat to the final dimension using several light cuts. Although the piece is pretty securely held by the chuck I use light cuts are to minimize any deflection of the piece.
Once the throat is to the final dimension, I use the tail stock live center to help keep the piece as rigid as possible and I begin taking the piece down to the correct diameter. I am working on the convergent side of the nozzle at this point.
Once to the final OD, I roughly mark the end of the "shoulder" where the retaining screws will eventually be added and cut the o-ring gland using a cut off or parting tool. This action demands that the piece be very rigid and often I find that this is an operation that is easier if done closer to the chuck. In this instance it was fine though.
Back to the boring bar and then we began working on cutting the convergent angle.
This is looking right down the length of the boring bar.
This a close-up showing the inlet to the throat area, it's currently a pretty sharp angle. Ultimately you want a smooth transition from the convergent angle to the throat rather than a sharp edge.
I will spend some time later sanding this area to get a smoother transition. One thing that I like to do is knock the edge off a bit with the boring bar. The convergent angle is 30, so I reset the tool to 15 and take just a bit off as seen in the above picture. The rest will be smoothed with sandpaper later.
The final step before flipping the piece around is to index the holes for the retaining screws. This particular lathe as an indexing locations on the main gear that allow me to do this. I use a Dremel and center drill to mark out the locations on the piece. I then removed it from the chuck and rough cut it to length before re-chucking it and working on the other side.
It's critical to take your time when re-chucking the piece to ensure that you get it centered correctly. Here I am facing it to the final length needed.
I took one quick cut with a0.625" bit to remove some excess material from within divergent side.
Then I brought the live center back in and roughed out the excess material from the outside of the divergent cone.
Here I have it cut to the max diameter of the divergent section.
I like to mark out some rough locations so I know what I'm working towards.
I then set the tool to 10 degrees and begin removing the rest of the material from the outside of the divergent cone. Sometimes cutting towards the right...
and sometimes cutting towards the left.
All of the excess material around the divergent cone is now removed.
I then add the steady rest for rigidity and bore out the divergent cone.
The finish looks a bit rough in this image but it actually cleans up nicely with a bit of sand paper and time. I do most of that finish detail on my smaller lathe as it's variable speed control makes it a bit easier.
The last step is to remove some excess material from the shoulder area. This is tricky on a piece this small but with a little patience and the right tool it isn't too bad.
Nearly completed nozzle. All that remains is to drill and tap the four screw holes and sand/polish the inside smooth.