Can I Rely on Glue Alone for a Strong Wood-to-Wood Joint?

      Modern adhesives are very strong, but smart joinery ó a mortise-and-tenon joint or the equivalent ó still makes all the difference. December 26, 2006

Question
With the new glues I don't need to make a mortise and tenon - I could use glue, right?

Forum Responses
(Finishing Forum)
From contributor A:
Wrong.



From the original questioner:
I thought the glue was stronger than the wood Ė isnít that right?


From contributor A:
In most instances even the weakest cheapest glue can be used to glue wood together and the wood will break first. But if that was true 100% of the time then I could just glue every newel post I install right to the floor without any other means of joinery. Try this - make a raised panel door and do not cope and stick it - just glue the butt ends of the stiles to the rails. It will stay together, but for how long? Drop it on the floor. What happens? Glue does not provide a good enough bond over time.

Dovetails, dados, rabbets, lock miters, box joints, mortise and tenon, dowels - they all have a place. Glue can and should be used with them all, but glue should never be used alone except to glue up panels, butcherblocks, tabletops etc.



From the original questioner:
OK, so thatís your opinion. The fact is if glue is stronger than wood then the joint should hold if it is just glued and the wood around it would give and break. I wonder if anyone has ever done a study on how strong a glued joint is a compared to a mortise and tenon joint.


From contributor B:
The end grain of wood bonds very poorly. The purpose of joinery is partly to provide a mechanical fastening, but even more so to provide sufficient long grain-to-long grain gluing surface. This is why you can edge glue wood all day, but a butt joint of end grain to long grain, where you would typically use a mortise and tenon, will fail rather easily.


From the original questioner :
I guess some one here at WOODWEB already did some testing. Thanks WOODWEB, I have found my answer to my question.
Knowledge Base information


From contributor B:
Beware of anecdotal information from other people whose credentials are unknown. A wood joint strength test with a sample size of 10, under unknown conditions, by a woodworker who you don't know, is not a scientific analysis of wood technology or how wood glues operate. Arm yourself with the best information available, and then you can make sound decisions based on your application and materials.


From Jeff Pitcher, technical advisor Adhesives Forum:
The short answer to your question is yes, there are adhesives available today that would allow you to not use mortise and tenon joinery. Should you use them? Probably not. The cost, ease of use, and chance of error would all be too great.


From contributor D:
One aspect I havenít seen mentioned yet is the issue of glue surface area. When you make a joint such as a mortise and tenon you are not only making a mechanical joint you are also greatly increasing the glue surface area. Doubling, tripling or even more the area that the mating surfaces are glued strengthens the joint considerably. Another benefit to mortise and tenon joinery is that the greatest glue surface is on side grain to side grain which is a much better bond than end grain to side grain.


From contributor E:
I've got 20 plus years of experience in this stuff and the majority of it was in yachts. There are several companies taking shortcuts similar to the ones you want to try on their joinery, particularly doors. It was great for me because even the cope and stick doors were falling apart within a few years under marine conditions (even the interior) and I made a lot of money doing mortise and tenon doors to replace them.


From the original questioner:
I was serious on my question and I have decided to use the glue and the pocket hole machine to make the whole thing work fast and cheap and be strong at the same time. Also I believe the mortise and tenon, if done right, is stronger. But just how strong does a joint have to be? If the glue is stronger than the wood then the joint should hold.


From contributor F:
Way back in 1980 I manufactured a complete louvered door kitchen for myself. It had 22 doors. I butt joined every joint edge grain and long grain. The glue that I used was white PVA (same as yellow glue of today). I had the house for a few years with an aggressive wife who would slam the doors now and again. Guess what - the joints held fantastically well. Would I do it now? No - I do think that joints add strength and look good too.


From contributor G:
Stiles, rails and slats, all butt-glued? How did you keep all those parts oriented during glue-up and clamping?


From contributor F:
It really wasnít hard to do; just a bunch of clamps and working on a large flat floor, (my young days). Remember "In order to be old and wise, you first have to be young and stupid."


From contributor C:
The simplest explanation seems to have been avoided here. Yes the glue is stronger than the wood but the wood is not all that strong, which is why we use mechanical fasteners or joints. Youíve never seen a house frame glued together have you? The glue would be fine but the wood itself would fail. No butt joint is as strong as a mortise and tenon. Even with pocket screws and/or biscuits. How strong does it have to be? How long do you plan on being in business? I give my clients quality work and charge accordingly. If your doors are not even as good as what they can buy for less at a home improvement center, why should they buy from you?


From contributor H:
Although you are going end grain to long grain the same idea applies - glue is as strong as the wood in most cases, but this refers to long grain gluing, not what you are describing.

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