I encountered what I consider to be the equivalent of an old wives tail on the weekend. It happened in our local college shop. A student saw me jointing curly maple boards with my #8 plane and asked the old question “Isn’t a hand planed surface too slick to give a proper glue joint?”
My opinion is that wood glue easily permeates the smooth wood surface and bonds at the molecular level, kind of like “chemical welding” and therefore the smoothest, truest surfaces are far superior due to the increased contact area in the joint. The best possible joint is produced by hand planing and not the floor mounted jointer.
The opposing view was that the machine marked surface from the jointer was better because it gives the glue something to hook on to. I think of this theory as the “velcro ” theory. They even suggested roughing a surface with sandpaper for that reason.
I don’t think I was able to successfully convince anybody so before I spout off too much, I thought I’d better get an expert opinion. Which is better, the “welding” theory or the “velcro” theory?
Replies
Yes, you have encountered a factoid, also know these days as an urban legend. Your theory is just right and the rest of the misguided soles in the shop need some education. If you look at any veneering manual written before 1950, there will be instruction on the use of a toothing plane to rough up veneer substrate. No one uses these planes anymore and you could not have much slicker surface than MDF which is an excellent veneer substrate, especially in the water resistant grades.
Solder, braze and glue joints are all sheer stress problems. With all of these joints there is some optimum gap for the solder, braze or glue. A gap less than this will cause problems and well as a greater gap. As you surmise the interaction of the bonding material, in our case glue, is at the molecular level. As you increase a gap the strength of the bond drops by a power of three. If an optimum gap (and I believe that varies with the type of glue) were .002" then increasing it to .004" would be a doubling of the gap--a factor of 2 times. 2 to the 3rd power gives 8 and your joint is 8 times weaker. Again, as you surmise, the fit of the joint is what is important and all the scuffing and fiddling probably does not do anything for this.
I am not sure a hand planed joint give a vastly stronger joint that a jointer but it certainly gives a more concealed joint that hides from the eye better in well matched grains. I would bet it is slightly stronger as well, but you have so many variables that it would be hard to test. Glue gap which is dependent on clamping pressure would come into play, etc.
In Fine Woodworking Magazine we will be running a very interesting article, actually testing the power of different glues and glue gaps with actuall laboratory stress testing. The research was conduct by my friend Dave Mathiessen at Case Western University here in Cleveland.
With best regards,
Ernie Conover
Thanks Ernie. I look forward to magazine article.
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