Machining a double D

[Dean jeffery]

Internal was guessing rounded corners, but ideally with out.
Not sure what the smallest ended end mill would be but any smaller than 4mm this would prob not be possible.
What would be the smallest end mill poss, or max over cut corners.

Cheers

[m_c]

You can get sub mm micro endmills, but none that would manage to go 20mm deep.
Having had a quick look at a catalogue, the smallest diameter that will go 20mm deep is 4mm, so you'd be looking at a 2mm radius on every corner, although you may need to increase that to avoid chatter in the corners.
.
Do you really need to use a double D?