I’ve got a few designs in my head and I’d like to put them down on “paper”, or 3D
CAD file, I should say.
Can you recommend a good 3D CAD program for the beginner?
Thanks a lot.
I’m a mechanical design consultant here and think you need to ask yourself (and tell us 
) a few questions. Are you looking for something that you can model ideas up in and then send them out for quotes/evaluation in the manner of a budding inventor? Or are you just wanting to be able to take something you thought up and throw it on the computer screen and spin it around?
Are you talking about houseplans, landscape designs, consumer goods?
-rainy
It really depends if you want to spend actual money or not. Given your beginner’s requirements, I’ll assume not. If you’re not too bothered about precise dimensioning, and are more after a quick 3D prototyping sort of thing, you could do a lot worse than Sketchup, a sort of freehand 3D modelling package acquired by Google a year or so ago. There’s a free version available for download. Run through the tutorials, it’s really quite good fun to use. It is also possible to dimension things accurately, it’s just a bit of a pain in the butt. Unfortunately you’ll need the full version to export models to common formats such as VRML and AutoCAD, and to produce layout-style drawings, but for a free package it’s certainly not a bad starting place. The full version is about 500 bucks, but that’s still cheap for CAD software.
Professional packages run into the thousands upon thousands of dollars, so for further input I guess we really need to know what your budget is, if indeed you have one at all at this point. The most basic version of Pro/Engineer, for example, runs to about $5000 for a commercial version. If you’re a student you can get a copy for a few hundred bucks, but Pro/E (in common with most fully-featured CAD packages) is certainly not what I’d describe as a beginner’s tool. I worked on it for a summer, and it took me 3 months of full time work before I was even vaguely familiar with it, but I still hadn’t scratched the surface.
So, yeah: what’re you doing, and how much are you willing to spend?
Thanks for the answers.
The files I create will be for personal use and I won’t be exporting them.
Basically, I’d like to design a custom PC case I’ll be constructing.
I’d like to be able to design individual components and then merge them. I’d like to be able to go back and alter specific dimensions of a given component.
I’d like to design everything in metric and be able to say, “This section needs to be 40mm instead” and see the change updated immediately.
I’d like to be able to alter the location and degree of bends in tubular sections, etc…
That sort of thing.
You’re going to want something with parametric capability and probably able to handle sheet metal forming and layout and/or plastic extrusion. I’d recommend SolidWorks or Solid Edge for this, which are both mid-level parametric solid modeling packages as being very capable with modest-sized assemblies and fairly easy to learn from documentation and tutorials. Be advised, though, that these will run in the $5k range not including training, and will require a relatively good video card and memory. The low-end or free solid modelers I’ve used just haven’t been worth the hassle of trying to get them to actually do anything, and high-end packages like Pro/ENGINEER and UGS NX are radical overkill and way more expensive than you’d need to spend. (Don’t even speak of Catia…)
Stranger
Hmm. Sketchup is probably not be for you, then, as its assembly capabilities (i.e. merging components) are limited at best, and I’m not even sure that bends in tubular sections are possible (curved surfaces in general are a bit of a weakness). It rather sounds like the capabilities you’re after are of at the level of professional CAD packages.
If it’s solely for personal use, then maybe Pro/Engineer’s personal version might be for you at $249, and doesn’t seem to require proof of student status (despite the product name). There’s also a 30-day tryout available, although again I’d be kind of surprised if you managed to get to the stage of creating complex assemblies within a month.
I should point out that I don’t have extensive experience here; I’ve had a couple of summer jobs using Pro/E, and have been searching recently for cheap CAD packages for a tangential project to do with my research, without any luck. Professionals like rainy may have some much better suggestions, but certainly in my experience CAD software just doesn’t come cheap, and nor does it come with an easy learning curve. Best of luck, though. 
Dead Badger is absolutely right about CAD software not coming cheap. I do contract work out of my house part-time so I can keep my two little ones, and the sting of ponying up $5k is still clearly felt. A student version or personal version is probably the best option for you (if you’re a student) considering the things you want to do with it. I haven’t seen a cheap piece of CAD software that was very capable at all.
My bias leans toward SolidWorks, which is a very capable program, and it holds a large market share. So getting a student version of it (which I believe requires a copy of a current transcript) and learning to use it would be a great little something to add to a resume. It is also very easy to learn. Pro-E has a much, much harder learning curve. AutoCAD Lite is similarily cheap, but I can’t remember if it had any 3D capabilities in the Lite version or not, since I always had a full-version seat of the software.
-rainy
I use TurboCad. The base version (which I use) is US$150. It does 2D and a bit of 3D. Looking at your requirements, TurboCad won’t do them. I however find TurboCad great for me. It’s best with a scroll mouse (to zoom), and learn the keyboard short cuts.
Here’s a bracket drawing
That about sums it up. If you want anything beyond the most basic functions (and parametric modelling is NOT basic) then you’ll have to pony up big bucks. My own experience is with structural and architectural CAD and industry standard programs like AutoCAD, Softplan, Chief Architect, etc., all range in price from $3,500 to $5,000.
I use AutoCAD full version in the office and AutoCAD Lite at home. Lite’s 3d modelling capabilities are significantly crippled compared to the full version. There are some capabilities to extrude solids but its just not worth the effort to work around its many limitations. You can’t even really navigate very well in 3d space, instead you’re limited to the standard isometric views.
ACAD Lite is a top-notch 2d CAD drafting program but I’d go elsewhere if you need 3d capabilities.