Recently I designed a low-cost electronic knob using Texas Instruments inductive sensing IC, LDC1312. For the Mechanical part of it I used ProtoDnD and got help from a 3D designer to convert my idea into a 3D model. Here is how a paper sketch was converted to a 3D model and 3D printed.
Step 1: Visit ProtoDnD.com and Start a New Project
ProtoDnD is a platform to get 3D Models made from real designers. Its very cheap to get a 3D model made, I paid $30 for this project.
Step 2: Upload Paper Sketch
Upload any relevant images/docs/description which you can share with a designer to get started on the design. You can also annotate images that you upload to be clear and specifc about your 3D model requirements.
Step 3: Work With Designer
Once the quote is accepted, designer starts working on your project. You can also send a message or share images or other documents with designer in your project dashboard on ProtoDnD.
ProtoDnD also has tools to annotate STL to specify a feature modification in 3D.
Payment is released to designer when the design is accepted as final.
Step 4: Finally 3D Print
This model was 3D printed on MakerBot.