I was thinking about graphite in the way to work today

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dshaker
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I was thinking about graphite in the way to work today

Post by dshaker » Fri Apr 29, 2016 5:14 am

I was thinking about bracing and the way the Gore-Gilet method uses graphite while I drove to work today.
It isn't a long drive, so I don't have any deep thoughts but here is what I came up with:

1) An objection I have heard from repairmen to the graphite/hardwood bridge plate is that using epoxy to glue the graphite/hardwood bridge plate to the soundboard means that it is harder to replace the bridge plate. This objection is irrelevant with a falcate design since the falcate braces are even more of an issue than the bridge plate. With an X-braced soundboard, though, the objection has some substance. But why not make the bridge plate out of a hardwood/graphite/hardwood sandwich (like the bridge, but MUCH thinner) and then use HHG to install the bridge plate?

2) The prescribed method for using graphite with the braces themselves is a layer of graphite under the brace and then a layer of graphite on top of the brace. The rationale is that we are creating an I-beam with the graphite and so we need graphite on both sides of the spruce to make the I-beam. But isn't the stress on the brace all downward stress from the soundboard? If that is the case, then isn't the graphite between the brace and the soundboard only in compression and therefore superfluous? Couldn't we trim weight AND make the braces easier to repair by limiting the graphite to the upper surface of the brace (that is, the surface of the brace furthest from the soundboard) and eliminating the graphite between the brace and the soundboard?

Any reaction?
-Doug Shaker

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Trevor Gore
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Re: I was thinking about graphite in the way to work today

Post by Trevor Gore » Fri Apr 29, 2016 9:20 am

Regarding the bridge plate, I think you have to look at reasons why bridge plates get replaced. Among the major reasons (and I don't do many repairs, so the repair folk might want to weigh in on this) are 1) incorrect design/construction where the bridge plate is too small, too large, the bridge pin holes miss the bridge plate etc. 2) bridge plate warping/lifting/splitting/cracking etc. 3) excessive wear around the bridge pin holes.

Sensible design (i.e follow instructions in the book) should guard against 1). 2) The chances of 2 are minimal, given the CF cloth beneath the bridge plate. If anyone manages to break a bridge plate due to external trauma, I would think a new top is likely in order. 3) is potentially an issue, but the repair community have numerous ways of managing that from filling with epoxy/CA and sawdust then re-drilling to using the StewMac repair cone system. So if the system is used as intended, I don't see any problems with it. Making a wood/CF/wood sandwich as a bridge plate and gluing on with hide glue is probably making more potential trouble. In my limited repair experience, most failed glue joints I've seen have been hide ones. I'm not sure why it might be beneficial to use CF in a bridge plate on a X-braced design anyway. Done right, there's no shortage of cross-grain stiffness in the X-brace structure.

Regarding your second point, CF used in composite structures works in compression just as well as in tension. The typically epoxy matrix that the CF fibers are embedded in prevents them buckling in compression, so the majority of the CF's compression modulus is seen in the structure. That's a fundamental of all composite design, be it GRP, reinforced concrete or CF/epoxy. Also remember that in front of the bridge the exposed side of the brace is in tension and behind the bridge the exposed side is in compression.

dshaker
Myrtle
Posts: 77
Joined: Sat Aug 31, 2013 6:38 am
Location: Palo Alto, California

Re: I was thinking about graphite in the way to work today

Post by dshaker » Fri Apr 29, 2016 2:15 pm

Obviously, I need to find a route to work that allows for deeper and more thorough thought. Perhaps via Peru.
-Doug Shaker

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Trevor Gore
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Re: I was thinking about graphite in the way to work today

Post by Trevor Gore » Fri Apr 29, 2016 5:59 pm

I find engaging in anything is far too risky unless I've ingested a sufficient amount of caffeine. The necessary dose seems to get higher every year.

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