It was never such easy to create mechanisms for hobby as it is now with 3D printers. But there is still a challenge to create 3D model of a mechanism.
There are many free and commercial software products which can build 3D models - some of them robust but complex or expensive, other are free and easy to use. PTC Creo Elements/Direct Modeling Express is one of such free and easy to start software product, however it has most features needed for hobbyist.
Let's assume we need to create a mechanism where a step-motor drives the pulley with a gear. The step-motor, a gear, bolts and a bearing exist but other parts need to be made on a 3D printer. Video-tutorial in this instruction consists of a few video-clips helping to start building 3D model for further printing.
Step 1: Introduction of the PTC Creo Elements/Direct Modeling Express
This is a "quick start" for this product. The way of making the 3D model is different than in polygonal modeling.
Step 2: Build 3D Model of the Step-motor
Measure the existing step-motor or use known dimensions taken from the drawing and use it to build the 3D model. This model will be a basis for next parts of the mechanism.
Step 3: Bracket for the Step-motor
This bracket holds the step-model on some base which carries the mechanism. The 3D model of this bracket can be saved as STL-file to be printed on a 3D printer.
Step 4: The Adapter Between Step-motor Shaft and Worm of the Gear
This adapter connect the shaft of the step-motor and the axis of the gear's worm. Diameters of a shaft and an axis could be different. The 3D model of this adapter can be saved as STL-file to be printed on a 3D printer.
Rest of the mechanism can be build same way.
This tutorial is another quick introduction of the PTC Creo Elements/Direct Modeling Express.
This instruction shows how to build 3D models of gears for 3D printing.