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Designing some vacuum jigs and wondering how deep I should rout for 5.5mm diameter cord? It's probably some imperial size, but I work in metric.

I recall reading some time ago that you need room for the cord to compress into, so I was thinking a 5mm wide square shoulder trench and perhaps 4mm deep.

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Allen R. McFarlen
Barron River Guitars & Ukuleles
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Cairns, Australia

Allen, Yes, you need room for the O-ring to compress into. You want the work piece making contact with the fixture not setting up on the cord. FWIW, I use 1/8" cord and use a 1/8 wide X .085 groove. Also, it helps to use a low durometer cord stock for this application and use some CA glue to bond the ends of the cord stock together to prevent leaks there.
You might try searching for the Parker O-ring hand book for more information on O-ring grooves.

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Jim Watts
http://jameswattsguitars.com

From an older post by Bob Garrish in this thread:
http://www.luthiersforum.com/forum/viewtopic.php?f=10106&t=38998

So far as parameters for the cord go, always use a flat bottomed channel at full gasket diameter and cut ~60-70% of diameter for depth and you won't have any issues with the gasket sinking in perfectly. I use 3/16" gasket and cut channels at 0.110", and no problems have ever been the gasket's fault.

I've found that round cord in a square channel at 70% of gasket diameter in depth has worked well for me

Scott

Thanks. I'll get out the calculator and work out the depth.

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Allen R. McFarlen
Barron River Guitars & Ukuleles
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Cairns, Australia

What Scott said. And what Jim said.

Get the mushy neoprene foam stuff (Allstar Adhesives and McMaster both have it) and the round stuff works better. I used to glue the ends together, but now I just cut the cord a little long so the ends are compressed together in the groove a bit. Both ways work fine.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services