Hi and welcome to the forum. What is a CNC shield? Its hard to answer questions without links to what you are trying to do. We don't tend to use Arduino's to run cnc machines.

Hi,
New to the forum, came from the 3D Printing world having conquered it and have to say... the open source software, firmware and hardware for 3D Printing is WAY more advanced than for CNC machines.
Not sure I understand this statement have a look at LinuxCnc and see if you change your mind about advanced open source.

There are many people on here running 3D printers as well as cnc machines.