'Tis the season for all things winter-y (despite it still being November), which means snowflakes, ornaments, and CAD. Here's a basic tutorial for how to CAD a symmetrical snowflake as well as some ways to personalize your snowflake for making 3D printed ornaments (you could also use the files to make laser-cut ornaments, or print it out and cut them by hand).
All CAD shown here is done using Solidworks, but the same steps can be used in nearly any program such as Autodesk Inventor.
Step 1: Plan your pattern
Since this is for the ornament design contest, my snowflake had to be less than 3 inches in each dimension. I made a 3 inch bounding square to contain my design, then started adding elements for the "arm" of my snowflake. Only half of the arm needs to be drawn out since the design will be mirrored in the next step. It's helpful to know how many arms you will want (in my case, six) for figuring out the angles. I plan to put something in the center of my snowflake, which is why the arm doesn't go all the way to the center.
Once you are happy with your design, close the sketch.
Step 2: Mirror your design
Start a new sketch on the same plane and use the "Convert Entities" tool to transfer your previous design to the new sketch. Select all the bordering lines and use the "mirror" tool to flip it around the center line. This will give you one complete arm of the snowflake.
Step 3: Create a circular pattern
This is where the snowflake takes its final form. Using the "circular sketch pattern" tool, select all the lines in your design and the center point of the snowflake. Change the number of instances to the number of arms you want in your snowflake, and make sure to have "equal spacing" selected. Hit "Ok" and exit the sketch.
Step 4: Extrude the snowflake
Now that the design is done, simply extrude the sketch out to some desired thickness.
Step 5: Finishing touches
At this point you can go back and modify the original sketch to change the shape or modify the extruded snowflake. For 3D printing, I recommend adding fillets to all the edges to make them look nicer. I also added a loop for threading a ribbon through to hang it as an ornament.
For fun, I added a rotating centerpiece that is held in place with two pins. I won't go into detail on how I made them (that would be an entire other instructable) but theoretically you can print the entire assembly at once. The cut-out centerpiece can probably only be printed on a 3D printer using support material, but the solid centerpiece should print on a standard Makerbot-style consumer printer. You can also easily customize the centerpiece by adding a personalized message or design (the .stl file for a blank centerpiece is included in the zipfile).
Depending on the printer, you can even use different material for the center vs. the outside snowflake. My personal favorite is having the snowflake in white with a transparent center.