I have had the same situation many times myself, usually where we need to see a summary of how long an area will be in use, and don't like hammocks much myself either.

depending on how you want to show things exactly, this may or may not solve it for you.
Creat a WBS (summary) called "Development of screw". Put 2 tasks in it, being milestone a and milestone b. Link milestone a to your milestone 1 (start of development) and b to 2 (end of development). The summary bar will then give you the overall duration of the required activities.

Its not particularly elegant, but it does show the required info. Because you are essentially copying the original milestones (the links from a to 1 and b to 2 mean the start/finish always remain the same) you have visibility of the actual dates while hiding the detail. I usually end up with a whole section of these (lots of areas of work) and its easiest just to stick them under a summary task again so you can see everything at once.

Hope that's useful to you.