-calipers/precision steel rule
-3D design software(123D, Sketchup, Blender, Inventor)
ABS plastic is chosen over other types for its strength and degradation resistance. It is also very easy to machine or finish. PLA is not chosen because it is biodegradable(made from corn). PLA is useful for prototyping wherein multiple iterations yield a large number of obsolete parts, which can simply be composted. This becomes a problem for a functional part, however, if the part is exposed to water for an extended period of time, or if the part comes into contact with any strong chemicals. ABS is a fairly standard plastic that many consumer products already incorporate.
Ideally, one would print every iteration in PLA, switching to ABS only for the "final draft." Or better yet, if the service is available, print only in ABS and recycle any obsolete parts.