Saturday, February 18, 2012

Rapid Prototyping Mill and FreeD

About 15 years ago I built a milling machine to cut a part directly from a 3D design. It did this without the intermediate steps of G-code generation, without tool and spindle speed specification, and without starting with a piece of material with a known shape and size. I called this machine a Rapid Prototyping Mill (RPM). Rod Davidson sent me a YouTube link of a similar device in motion called FreeD that a team at MIT put together.

What is common between RPM and FreeD is:
  • A three-dimensional CAD model is prepared beforehand that details the surface of the desired part.
  • The cutting tool motion is controlled by hand.
  • The position of the cutting tool with respect to the material to be cut is monitored by a computer.
  • When the operator attempts to move the cutting tool cutting tool inside the surface of the designed part, the system stops the cutting process.
What is different between them is:
  • The RPM monitored the position of the tool versus workpiece in 2D. The FreeD monitored six dimensions, 3D of position and 3D of angular orientation.
    • On the RPM, the workpiece moved in 2D on a table top stage. Its position was monitored by X and Y linear encoders.
    • The position and orientation of the FreeD is apparently detected by sensors on the hand piece monitoring an electromagnetic field sourced below the workpiece.
  • The RPM prevents cutting by retracting the cutter upward, vertically. The FreeD prevents cutting by retracting the cutter backward through the hand piece, whichever way the hand piece is oriented.
    • Cutting into the side with the RPM required remounting and reregistering the workpiece. Cutting into the side with the FreeD required no extra operations.

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