Hi Mayoayo,
Myself I am interested in adapting different eyepieces in my scopes,and since is harder and harder to find machine shops to have these adapters crafted,i thought 3d printing would be interesting.
Sounds like a great idea!
One thing to be aware of is that the tolerances of printed parts and the minimum size of structures are not really up to the standards of traditional metal working techniques.
I haven't had anyting printed in metal yet (and for your purpose, plastics might be fine), but apparently it's a standard technique (also applicable to plastics) to design the part with undersized holes, drill them to a precise size, and cut the threads with traditional metal working tools to ensure they are fit for their purpose.
While that's not quite as easy getting a ready-to-use part fresh off the 3D printer, I figure it still simplifies the metal working part quite a bit (and you might be able to use plastics, which is another simplification).
With regard to part design, I'm using Freecad 0.16, but I can't really recommend it whole-heartedly. It's a great program, but the low version number shows that it is not expected to work perfectly, and the issues it has have quite some impact. I've seen that OpenCAD is quite popular, and I've installed it, had a look at the demos, and really liked what I saw, but I haven't actually designed anything with it yet, and it's basically a command-driven interface, so you don't really draw anything in the traditional sense.
In the past, there used to be some cost-free software packages from Autodesk, but they seem to have dropped these recently for a fairly expensive subscription scheme that's probably more aimed at the full-time professional.
The 3D printing service Shapeways promotes some software packages aimed at the amateur which might be worth checking out, but I haven't really tried these. A lot of the Shapeways business seems to be with jewelry and accessories, and I'm not sure how well the software they are promoting is suited to technical applications (because I committed to Freecad, which didn't leave much time for anything else).
If you're interested in Freecad, these video tutorials might be of interest:
https://www.youtube.com/watch?v=_HEvhclR4-o
I watched like the first 10 of them, which gave me a good start. The problem is, not everything that should work actually works, and it's quite easy to break a model in Freecad, and difficult to repair it - which is not covered by the tutorials.
Regards,
Henning