Tipster Levi Connolly explains how to trim all lines on one side of a cutting edge in AutoCAD.
"The old Command line–only ExTrim is a very useful and forgotten command. When you execute the command, it prompts for a cutting edge just like the normal trim operation. Next, you are asked to select a side to trim on, and it will trim all polylines, lines, circles, arcs, and ellipses that cross its path."
Notes from Cadalyst tip reviewer Brian Benton: The ExTrim command is an Express Tools command. You used to have to pay extra for the Express Tools add-on package, but now it is standard with AutoCAD. It does not come with AutoCAD LT or AutoCAD for the Mac.
It's simple enough to use, and fast. Start the command, pick the cutting edge, then choose which side to cut away — and you are done. It trims everything on that side of the cutting edge, so you don't have to pick each object to trim them.