This is a program made by ASMI and modified by Ronso in This Thread. This is a genius free LISP Program can be used in many trades, but has most usefulness in the HVAC, Mechanical, and Piping industry of 2D CAD Designs. It allows you to create Duct, Pipe, and Segmented Duct or Piping with automatic elbow or fittings to accurate dimensions via some initial input at the Command Prompt. Attached is a screenshot of the program in action:
DUCT.LSP