This would be a good excuse to use the "Show working versions" folder permission. Basically you'd turn that off for the products folder for the machine shop group and turn it on for SolidCAM. Then you'd need to give them read rights to WIP and released. That's all assuming you're rev'ing the file when it gets to released because the show working versions permission is based on the revisioning functionality.
Are they moving files to different folders to work on them? We have a 'Products' folder and a 'Projects' folder. Files will generally move to 'Products' when it gets ready to release but after that it stays there even through future engineering change orders.
We have two workflows.....a project 'conceptual' workflow with a workflow link to the 'Products' workflow when its ready to go to production. Future engineering changes occur in the 'Products' workflow where most people only see the 'Release' state.....none of the WIP states.