This is my first attempt at an instructable. I hope it helps someone else.
Switches and Lights and fans, oh my...
Please read through the comments for more details.
I bought a CNC machine some time ago and I was never happy with it. It used a traditional parallel port controller and of course no modern computers have those. So it was always a point of frustration. I used an old Pentium based computer and ran LinuxCNC on it for a while but was still not happy with that. So I started looking for another solution. I decided that I would try to put together an Arduino based controller and try to use USB to communicate with it.
After studying that for a while I came to realize the Arduino was just not going to be able to do all that needed to be done on its own. The Arduino UNO just does not have enough memory or processing capacity to process a design file by itself. So I modified my solution and started working on using a Raspberry Pi as my host computer.
I created a Visio drawing of what I intended to do: It didn't turn out that way and the drawing continued to evolve as I learned more. It kept on changing until I finally got my project completed.
I have now built an Arduino based CNC Controller system that uses a Raspberry Pi as its host computer. The Raspberry Pi processes an "nc" file that contains a design that is described in GCode commands that can be understood by the Arduino. I have used a few tools on my Windows 8.1 Pro laptop to design a few simple things (like tutorials in makercam here: http://www.makercam.com/). This is a great introduction to designing things for CAD/CAM and it is really easy to use.
After creating a design that I wanted to use I wirelessly connected to the Raspberry Pi using WinSCP to transfer design files to the Raspberry Pi. You can get WinSCP here: http://www.soft-now.com/listing/123823/WinSCP?did=11055&pid=1&ppd=search,44532390848,winscp20download,e,,c,0,,,&gclid=CJuIkNHds74CFagWMgodpxMAJw. Its great, you can just drag a file from one window to the other to move files between systems. You will have to know the IP address of your Raspberry Pi to connect to it. If you have gone through the initial setup of the Raspberry Pi and setup a network connection you can get the IP address by using the ifconfig command on the Raspberry PI. A really helpful tool for getting your IP address and seeing that your Pi is online is a tool called the Advanced IP Scanner here: http://www.advanced-ip-scanner.com/. Another great tool is Wireshark: http://www.wireshark.org/
I then used Remote Desktop Connection (Remote Desktop connection is part of Windows and should be on your Windows machine. It can be run by entering mstsc in the Run Open: box) to connect to my Raspberry Pi. Otherwise you could use VNC or TightVNC: http://www.tightvnc.com/. TightVNC is what you need to put on your Raspberry Pi to allow you to use Remote Desktop Connection from another machine. I can now run the Grbl Controller on the Raspberry Pi which in turn connects to an Alamode (an Arduino like board that plugs directly into the GPIO connector on the Raspberry Pi and provides automatic voltage level translation. So that the Arduino can communicate with the Raspberry Pi directly. The Alamode also provides Arduino headers to allow for Arduino shields to be plugged directly into it. So I ended up with a stack of three boards that are tightly integrated due to being plugged directly into each other. The Alamode (Arduino) processes the GCode commands from the Raspberry Pi into signals that are sent to the CNC Controller shield to run the stepper motors and passes the various function signals on to the CNC Controller shield too. They can then be accessed from the external world. All is well so far... But now I have to connect these things to the external world.