I know this thread is old but I have just found out about
Onshape. It was founded by Jon Hirschtick who founded
Solidworks which is the number one CAD software for product development and mechanical engineering. That and Pro Engineer. Anyway, Hirschtick's new software is cloud-based and you can Cad off any device like an iPad. More importantly the basic version is free whereas Solidworks costs around $7K/yr for a license. And the basic version is quite good, it's only if you want more storage in the cloud that they start charging more.
To talk to CNC machines though, most machinists (I know) use
MasterCAM. However if you just want nice scaleable mechanical drawings then Solidworks or Onshape is great and you can send those drawings straight to 3D Printers.