AAahh, I’m soo slow! It seems that this extension rod goes into the existing shaft, so the total length endmill shaft would be much longer? I tried to figure out if I could dissasemble the router, and pull out the original shaft, to insert the ER11 rod. It seemed impossible.
If this extension extends the lengt of the router mill - wouldn’t a simple 1/8" collet do better? (Granted I would only need 6mm and 1/8" bits) For example this one: https://www.amazon.com/Makita-763626-6-Collet-Cone-8-Inch/dp/B00018ABSW or the one from Elaire Makita Router collet adapter
Wouldn’t these options provide a better stability and less length to the mill shaft? I don’t know your precise needs, I’m thinking of my own setup here.