Hmmm,

Just been playing speeds & feeds for my single flute 5mm tool...

Manufacturer specs...
12k Rpm
2.5m DOC
1800mm/min feed - insane i think?

My previous settings...
12k rpm
2.5mm DOC
900-1000mm/min feed

F&S Calculator recomendations...
12k rpm
1.8mm DOC
280mm/min feed - much lower ?

Maybe these lighter settings will be better?

Its only the bigger tool that has issues so does seem to be a stress problem, I will also attach a bungee cord to the head to try and remove any rotational slack in the quill.