123D

Autodesk 123D was a suite of hobbyist CAD and 3D modelling tools created by Autodesk. It is similar in scope to Trimble SketchUp and is based on Autodesk Inventor. As well as the more basic drawing and modelling capabilities it also has assembly and constraint support and STL export. Available for the software is also a library of ready-made blocks and objects.(by wikipedia)

In this i made the the bevel gear

In this i first take the circle then i give to them dimention then i use the extrude command and i click on its dimension part and i give the -25 degree to top of that part.

In this i use the substractive command to subtract the middle part of gear for that i take the cylinder and put it on its top then i use extrude but at inside that below part and then subtract then i get the hollow part.then i use one rectangle part for the Gear notches I reduce its size to the height of that hollow part then i attach this part to the hollow part

then i rotate this part around the whole system and my bevel gear is ready

conclusion

this is open source software and easily understandable.some fetures are good.