Digitizing arm

Anyone out there built a 3D digitizing arm? That is a 3-axis arm with
stylus that is used to trace a 3D object and supply coordinates in 3-space
for creating a Cad model. I have an old profile mill in my shop that does
a similar thing - it mechanically feeds a milling head by tracing a 3D
shape.

Just wondering, people are paying big bucks in Cad magazines for what
appears to be a couple of metal arms, 3 encoders, a sylus and some
software....

Dick