Wednesday, December 7, 2011

Rocker Table Disaster

Despite the workflow running smoothly, albeit slowly, the major failure that occurred was the deformation of the rocker table surface under the stiffness off the wood.

The table surface I've been using is 4mm ply with 4mm perspex over the top. The ply gives the stiffness required to get the nice parabolic concave profile that the perspex can't do on its own and in addition the perspex gives a brilliant finish on the surface right off the table.

For a foam core that has effectively zero stiffness the perspex was strong enough that the force required to bend the foam core was not enough to bend the clamped perspex. For a wood core with no rocker its also strong enough but as soon as you add rocker the lateral stiffness of the core cores through the roof even though the grain runs perpendicular to it. The net result is that the perspex lifted off the ply and through the clamping pressure of the jigs and so about 1 cm of rocker vanished in an instant!!!!!

Board at the lower side of image. Shadow shows the extent to which the perspex lifted off the surface

Rocker reduced. This gap should represent around 1/2 the rocker. which puts it in the 2.5 cm vicinity

Concave all but vanished!!!
 So not the board that I was aiming for but there are a number of great outcomes from this.

i) I've been very curious about how concave affects the performance (other than stiffness) so now I have an unexpected chance to see first hand. The board is quite similar to board #1 but without the concave. Rocker is about the same. So it will be possible to test by riding them back to back.
ii) Its increased my commitment to being able to put the entire table (or surface) is a vac bag so that I don't have the asymmetrical forces on the surface doing exactly what it did hear ( again another tick for their Royalty Highness's at Brokite)
iii) Lots of other things worked spectacularly well: The workflow setup, all other aspects of the rocker table, laying up both sides at once.

Not the outcome I was designing for but some great real experience !!!!!


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