The whole _axis object is shared, isn't it? The QExtSerialPort variable is also shared and it can't be protected with a mutex as it's a QObject so you don't have total control over it.
Its slots will be executed in the main thread, by the way - I don't know if this is something that you intended. You can read its data only from the main thread.

Also get rid of the connect and disconnect statements. I'm not entirely sure what you meant them to do but I'm pretty sure it's not a good design.