11 August 2009

World?s largest grasshopper escapement clock - indicates time through laser cut slits

The Corpus Clock that now graces the new library building at Corpus Christi College in Cambridge was invented and designed by Dr. John Taylor in tribute to John Harrison, arguably the world’s greatest clockmaker. One of his many innovations was the grasshopper escapement that effectively turned rotational motion into a pendulum motion for timekeeping.

Dr. Taylor’s chronophage or time-eating escapement on the Corpus Clock is an inside out version of this mechanism but with neither hands nor digital numbers to indicate the time. Instead it has a series of slits cut into its face, each a tenth of a degree across. When the escape wheel moves, blue lights behind the slits illuminate in concentric circles to mark passing seconds and pause at the correct hour and minute. The company responsible for the 5 axis laser cutting of the slits into the clock face was GF Laser in Birmingham, a task that was performed on a Trumpf TLC Cut 5.

As the 1.5m diameter of the stainless steel clock face was close to the axis limit of the machine the company used the laser fixturing software that is part of the TruTops Laser CAD/CAM system. This automatically designs an interlocked egg-box type fixture based on the shape and form of the 3D workpiece. The sections of the fixture are then laser cut from flat sheets and slotted together. The laser also marks a datum point on the fixture which is then referenced by the machine to ensure cutting accuracy.

To assess the rigidity of the fixture and confirm the accuracy of the laser, a reject clock face was test cut. This part had failed a quality check following explosion forming to create wave-like mouldings. The test run was vital as the position of the slits needed to correspond exactly with the LED lights that illuminate the face.

“With just a few minor alterations, the final go-ahead was given to us cutting the final part,” concluded John Hickman of GF Laser. “The skills of the programming team, our laser operators and the repeatability of the Trumpf TLC Cut 5 ensured a first rate job.”

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