reible wrote:Well I don't know what to say but I disagree with Charlese post. I've been using glue line rip blades for some years now and they have worked fine for me. I no longer take the ripped boards to the jointer. What he said was true some years ago but no longer, well at least in my book....
Ed
Well, Well! With all due respect to Ed and Lover, I will have to back off the absolute condemnation of some special rip saws. But still, my hesitation to use saws for glue joints firmly remains.
My first exposure to glue joints in wood came as a three month college course titled "Wood gluing and physical properties". This was an elective course in my 5 year Forestry Education in Montana. Much of this course was aimed at the construction of wood laminated beams that were popular in the 50s and manufactured locally.
As said, this was in the 50s. Since that time much has changed in the field of glues, but wood is still the same.
Below is a few paragraphs from the USFS Forest Products Labratory's "Wood handbook". If you care to read it, the areas I highlighted are applicable to this discussion.
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Surface Properties of Wood for
Bonding
Because adhesives bond by surface attachment, the physical
and chemical conditions of the wood’s surface are extremely
important to satisfactory bond performance. The surface should be free
of burnishes, exudates, oils, dirt, and other debris that form
a weak boundary between the adhesive and the wood.
Both mechanical and chemical properties of a wood surface
influence the quality of adhesive bonds. Wood whose
surface is highly fractured or crushed cannot form a strong
bond even if the adhesive forms a strong bond with the
surface. The weak wood underneath the surface is the weak
link in the chain and the location of failure in the bonded assembly.
In other cases, poor bond strength is due to chemical
properties of the surface. Sometimes natural extractives,
overdrying, or chemicals added to modify the wood alter
the surface chemistry enough to harm adhesive bond performance.
Physical deterioration and chemical contamination
interfere with essential wetting, flow, and penetration of
adhesive, and contamination sometimes interferes with the
cure of the adhesive and resulting cohesive strength of the
bond.
Lumber Surfaces
Surfacing or resurfacing the wood within 24 h before bonding
removes extractives and provides a more wettable
surface. Surfacing also removes any unevenness that may
have occurred from changes in moisture content. Parallel
and flat surfaces allow the adhesive to flow freely and form
a uniformly thin layer that is essential to optimal adhesive
performance.
"Experience and testing have proven that a smooth, knife-cut
surface is best for bonding. Surfaces made using saws are
usually rougher than those made using planers and jointers.
However, surfaces sawn with special blades on properly set
straight-line ripsaws are satisfactory for both structural and
nonstructural joints. Furniture manufacturers commonly use
precision sawing of wood joints rather than two-step sawing
and jointing to reduce costs for labor, equipment, and
material. Unless the saws and feed works are well maintained,
however, joints made with sawed surfaces will be
weaker and less uniform in strength than those made with
sharp planer or jointer knives. Dull cutting edges of planer
or jointer knives crush and burnish the cells on the wood
surface. Not only are these cells weaker, they also inhibit
adhesive wetting and penetration. Damage to the surface can
be revealed by wiping a very wet rag over a portion of
the surface, waiting for a minute or more, removing any
remaining water with a dry paper towel, and comparing the
roughness of the wet and dry surfaces. If the wetted area
is much rougher than the dry area, then the machining has
damaged the surface. A weak joint results if the adhesive
does not completely penetrate crushed cells to restore their
original strength.
