Hi Jim. Very observant, not too many people pick up on that aspect of the test. I suppose it’s kind of unusual, but all our tests are carried out in the most honest way possible. That one was a shock, it was carried out in the office and didn’t just fail, it exploded. Luckily my wife was out at the time.
On your website you show a photo of two joint failures.
Since the wood broke on the tenon joint, the limit is the wood, not the tenon. It would be interesting to repeat this test with a tenon designed to leave as much wood cross section as the smaller dowels. It looks like the glue failed on the dowel test. Perhaps more glue surface area would improve the dowel joint result.
Most people think that the multiple dowel joint has more surface area than the mortise and tenon, but I recall doing a calculation which indicated that comparatively the m&t was slightly more. Again you are on the right track when you bring up glue application. Virtually every test we’ve done over the past 15 years has shown the dowel to be the stronger. Other tests done by others showed the dowel joint to be slightly weaker. This puzzled me for some time until the light dawned. It was the improper application of glue. It’s easier to apply glue evenly to a m&t joint than a dowel joint. When I do the failure tests I apply glue evenly to all surfaces, and the inner periphery of all bores using an artist’s paintbrush. If there’s 9 sq. ins. of surface area there’s not much point to applying glue to 4 sq. ins.
The topic which drives me crazy is when people make statements like a biscuit, or a pocket hole is stronger than a dowel joint. This is completely false.
The mortise & tenon is a fabulous system, but for me it’s too labour intensive and time consuming. I don’t work full time but it takes me 2 to 4 months to complete a project, some projects include about 70 joints and I’m not about to tackle those using the m&t system.