One way to make the yellow slot on the tube is to create a sketch of the tube (Sketch in the attached file) and Part WB > Extrude it. Then make a sketch (Sketch001 in the attached file) of the desired path and Part WB > Extrude that too. From there make a Part WB > Section of the two Extrudes to get the slot (black geometry in the image below). You can use the black geometry to Part WB > Sweep a profile along it to create your slot, but you'll still have to model the curved ends.
FreeCAD 0.16 didn't show the result of where the arc in Sketch001 intersected with the tube so I tried with 0.18 but I think 0.17 will work too.
I named some of my Constraints and used Expressions to link to them in the other Sketch. There is also a Redundant Constraint in Sketch; change either the 30 degree Angular Constraint or the 15 mm Horizontal Distance Constraint from driving (red) to driven (blue).
- 20181116a.png (16.19 KiB) Viewed 177 times
OS: Ubuntu 18.04.1 LTS
Word size of OS: 32-bit
Word size of FreeCAD: 32-bit
Version: 0.18.15162 (Git)
Build type: Release
Branch: master
Hash: 1d72ee43435fa099ae97a2f6e3d15a225d4f32d3
Python version: 2.7.15rc1
Qt version: 4.8.7
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: English/UnitedStates (en_US)