First failing miserably creating any kinda of paths with 'face' or even '3d pocket'. After more research i started sketching a round channel and then I was 'usable' by '3d pocket' to create a path. Big step forward!

though one question here: why can't a path be created from a horizontal cylinder using the part workbench. (from a design perspective is so much more time consuming doing sketch and pad)

Milling the newly created path of the round channel, I quickly came to realize that the finish is not that great using square endmill. (unless you run step down rate of thousands)

Tried to use a ball end mill instead, but was not successful to configure it properly in FreeCad 0.18. The dialog offers ball endmill, but it seems not to work for me (and be used still as a square end mill). Does anybody how to setup round endmilsl a path workbench? Is it fully supported?

Not giving up I finally came up with the idea to create my own gcode. The math is really not that bad. Using a round endmill it is pretty much up to defining the cut radius, calc the coordinates for the position and then running the endmill back and forth. So I created a small c program to calculate the needed coordinates.

you can easily create a 'custom' path operation, and cut and paste the above easily into your g-code. For a better finish I used 5 degrees steps, though 15 degrees already beat the the square end mill approach by miles.$

./halfpipe -b 12.5 -l 30 -x 0 -y 80 -z 4

(--- halfpipe D=32.00 L=30.00 zcut=4.00 center x/y/z= 0.00 80.00 12.00 hpR=16.00 bemR=6.25 cutR=9.75 )

G0 Z10.000

G0 X0.000 Y86.894

G0 Z5.000

(halfpipe path 45 degrees: ysin/zcos=0.707/0.707 hpcoff_yz= 6.89 6.89 bemx/y/z= 0.00 86.89 -1.14)

G1 X0.000 Y86.894 Z-1.144 F50.000

G1 X30.000 Y86.894 Z-1.144 F200.000

G1 X30.000 Y73.106 Z-1.144 F200.000

G1 X0.000 Y73.106 Z-1.144 F200.000

G1 X0.000 Y86.894 Z-1.144 F200.000

(halfpipe path 30 degrees: ysin/zcos=0.500/0.866 hpcoff_yz= 4.87 8.44 bemx/y/z= 0.00 84.88 -2.69)

G1 X0.000 Y84.875 Z-2.694 F50.000

G1 X30.000 Y84.875 Z-2.694 F200.000

G1 X30.000 Y75.125 Z-2.694 F200.000

G1 X0.000 Y75.125 Z-2.694 F200.000

G1 X0.000 Y84.875 Z-2.694 F200.000

(halfpipe path 15 degrees: ysin/zcos=0.259/0.966 hpcoff_yz= 2.52 9.42 bemx/y/z= 0.00 82.52 -3.67)

G1 X0.000 Y82.523 Z-3.668 F50.000

G1 X30.000 Y82.523 Z-3.668 F200.000

G1 X30.000 Y77.477 Z-3.668 F200.000

G1 X0.000 Y77.477 Z-3.668 F200.000

G1 X0.000 Y82.523 Z-3.668 F200.000

(halfpipe path 00 degrees: ysin/zcos=0.000/1.000 hpcoff_yz= 0.00 9.75 bemx/y/z= 0.00 80.00 -4.00)

G1 X0.000 Y80.000 Z-4.000 F50.000

G1 X30.000 Y80.000 Z-4.000 F200.000

G1 X30.000 Y80.000 Z-4.000 F200.000

G1 X0.000 Y80.000 Z-4.000 F200.000

G1 X0.000 Y80.000 Z-4.000 F200.000

G0 Z10.000

Hope that helps other to achieve their goals.

For me it saved me spending $$$ on a special sized (probably custom) ball end mill.