Woodie G wrote: If installing bridges on new guitars, the expectation should be a near-perfect joint, so very low glue-line clamping pressure might be acceptable, provided the clamping system does not itself distort the bridge/top joint. For repair work, joints are never perfect without radical, often unacceptable modification to the top and/or bridge, so relatively high, evenly distributed glue-line clamping pressure becomes one of the two key factors in determining longevity of that joint.
Judging from the multitude of issues involving bridge failures and partial separations I saw at Greenridge and in the much slower-paced work since then, joint perfection, and therefore glue line pressure is an issue for both factory and custom builders. In other words, the expectation of joint perfection for a new instrument is often unrealistic, leading to those years-later service calls. The reality of what shops such as Mr. Breakstone's see in the door suggests that higher clamping pressure and more reliable adhesives should play a role in both new construction and repair work if bridge joint reliability is of interest to the builder and repairman. Further, the method by which clamping pressure is distributed across the glue line becomes of greater interest as joint quality deteriorates.
To pick on Mr. Breakstone's shop again, I suspect that Ann Arbor's experience with adhesives mirrors my own, with a experiential base that acknowledges that, while nearly any wood adhesive MAY work, the caveats associated with anything other than HHG or an aliphatic resin or modified PVA adhesive (AR/PVA) rule them out for bridge work. Further, the required very thin glue line needed for a reliable AR/PVA joint make either very high clamping pressure (minimum of 150 psi, with closer to 300 desirable on really poor joints) or joint perfection a functional necessity. As Mr. Breakstone mentioned above, HHG tends to have some characteristics which allow more deviation from joint perfection while still retaining an expectation of bridge-to-body joint longevity, but here too, clamping pressure and speed at which the joint is closed come into play, because ultimate glue line thickness is important here as well,
Other issues?
- Wood moves when wet, and any of the glues we use add water to the joint. The assumption that the new installation joint will remain perfect - and therefore suitable for a low clamping pressure glue-up - once the top and bridge are wetted out may be unwarranted. In general, either HHG or AR/PVA application will expand the mating surface of the bridge and top, leading to a reduction in bridge radius across the top and increase in radius on the body axis, while the top will increase radius across the width of the top.
- Additionally, one luthier's notion of joint perfection is another's barely adequate fitment. Experience matters, as Mr. Hall's oft-expressed aphorism applies here: you don't know what you know until you know it.
- Finally, clamps and cauls have some impact on success of the joint. A vacuum clamping system will never generate more than local atmospheric pressure-level clamping force minus a few PSI, so maximum clamping force over the roughly 7.5 square inches of a typical Martin-style belly bridge patch will be less than 100 pounds, or less than a tenth of the recommended MINIMUM glue line pressure for Titebond on a mediocre joint fitment. For the combination of two Ibex-style clamps and two cam-style Klemmsia clamps, that applied force might be closer to a minimum of 1000 pounds, or about 130 psi - still below the 150 psi Franklin suggests for mediocre joints, but sufficient to address much of what we see in everyday repair work. Cauls matter as well, with the lack of them occasionally being responsible for inducing top distortion or even damage in the form of cracks (e.g., the older-style Stewmac bridge clamp wing issues). Cauls ensure both efficient distribution of clamping pressure and support for often delicate surrounding structure.
Before this drags on too long, at Greenridge, our preferences mirrored those that Mr. Breakstone expressed: HHG (in a pinch or if the customer insists, Titebond) and a clamping and caul system capable of thinning the glue line to a uniform thickness of under a mil, and doing it quickly enough to avoid premature gelling of 315g HHG. Those lighter-weight clamps discussed earlier in the thread do not generate the 300 pound (min) force seen from an Ibex, nor the 200 pound or so force seen from a Klemmsia, so are a bit more of a gamble for repair work or where joint perfection is an issue on new bridge glue-ups.
There are other threads that cover the method to address HHG's gel time constraints. Suffice it to say that the combination of a warmed bridge, the right clamps and cauls, and adequate preparation allow a fairly leisurely, unrushed approach to the job.
Great post Woodie.
|
Hope this helps.
