Very simple and easy to make with the things you have got . looks very shiny and can be used to play around though with sharp points .

Step 1: Getting Things Ready

To make this simple project you need these..
1. A old hard disk.
2. A permanent marker pen.
3. Metal ruler.
4. A rotary tool to cut out the metal . I suggest you to use a Dremel 3000 or 4000 model as it will be easy to use and has a great performance to cut very small areas. I have used Dremel 3000 with flexible shaft and metal cutting carbide wheel.

Step 2: Drawing the Ninja Blade Design on to Hard Disk

Open up hard disk and carefully take out the metallic shiny mirror like disk and using the marker and ruler sketch the design as in above image. The shaded part in the image are the blades of ninja wheel.

Step 3: Cutting Things Out

once you done with drawing all the sections of the design you need to cut out the thing with lots of patience ad the hard disk is a strong alloy metal it won't let you do the job so easily . so take your time and cut out the things carefully using dremel or any rotary tool of your choice that can do this job perfect.

Step 4: Finishing the Blade

once you cut out all the unwanted material from the disk , it might have left with sharp edges and jagged edges. So I suggest you to use a grinding tool with dremel to finish the Blade. later you can use a polishing buffing pads to make it shine even more.