Creating a hollow cavity

[partsman]

What is the recommended method for creating a hollow cavity in a part? The best example I can think of is a banjo fitting or like the picture below. I thought maybe lowering a keyslot cutter into the hole and then running a helix or something but not sure if that would work. Any suggestions?

Thanks in advance.

[fpworks]

Generally speaking, if some sort of undercut tool cannot be used, here is all I can think of:

3D printing
cast/mold
2 piece design, brazed together

[Rich Carlstedt]

1. In a mill, you could do a helical, but not with a key cutter, it would need a cutter with side relief.
2.In a lathe, you would use a offset boring bar.
3. For minimum cost, you would drill ,C'drill, then Silver solder a cap, on the bottom and drill/bore out. [2 pieces!]

[Wiseco]

If it must be done in a one piece block, go on a lathe with boring bar with long shoulder. Maybe you will have to put your boring bar at angle.

I have done several block like yours using my manual lathe. Ive put the boring bar at angle, make the first setup of the boring job but it was leaving an undercut to clear the boring bar shank that I turned on a second setup. What's dull with this job it's the chattering problem. If the boring job is deep, chattering will become a real pain issue.

If it can be done in 2 pieces, think using an insert like Rich said in the third suggestion.

Is it indiscreet to ask what is the job of that block? Maybe a different approach would do the job instead...