Reply by Nicholas Hall

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Posted on Are Most Benchtops Woefully Underclamped during Glueup? (probably not)

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Nicholas Hall

352 posts in 2133 days

#1 posted 12-07-2012 05:25 PM

@Patcollins: It is a very over simplified readers digest version but essentially correct if you are trying to glue together a 1”wide by 3/4” thick by 12” long strip to a 6”wide x 3/4” thick x 12” long strip without using a wide caul, you will wind up with great joints where the clamps were and not great glue joints where they in between (like in the diagram in a prior post).

Here is the non readers digest version by the author of the Fine Woodworking Article:

RJ Rabiej, HD Behm (1992) The effect of clamping pressure and orthotropic wood structure on strength of glued boards, Wood and Fiber Science 24:260-273


Reference values for compression strength perpendicular to the grain were determined for radial and tangential sections of samples of sugar maple and ponderosa pine. Samples to be glued were matched according to specific gravity and orthotropic structure and bonded along the grain in tangential or radial sections. Magnitude of clamp pressure was controlled throughout a range of pressures commonly applied in industry, up to about 80% of the compression strength of the wood sample. Tests were conducted on the bonded samples to determine glueline shear strength and percent of wood failure at the bonded surfaces. Results were subjected to regression analysis to ascertain relationships. It was determined that clamping pressure had a different effect on both shear strength and percent of wood failure depending on species and orthotropic section. It is possible to maximize joint strength by applying proper clamping pressure. Results similar in direction but differing in magnitude were obtained with both PVAc and U-F adhesives. A generalized measure of clamping pressure was defined as the ratio of applied clamping pressure to the compression strength (CP/CS) of the wood section to be glued. Using this concept, the optimum clamping pressure for sugar maple was found to be 0.3 times compression strength using U-F glue and 0.5 times using PVAc glue. This approach to determining reliable clamping pressure data can lead to improved gluing practice and more precise testing procedures.

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