Glue and mortice and tenon joints.

[woodbrains]

Hello,

Recently there was a discussion about the strength of glue and joints in wood. I have just read an article in Fine Woodworking which confirms what I already know, and might just help some who are unsure of how best to prepare joints and how to glue them for maximum strength. The findings of the test only reiterate the conventional wisdom that joints need to be smooth and close fitting. There has been reference on these forums to a certain YouTube video that seems to contradict that convention, so I thought I would repeat some salient points from this article to redress the situation.

The test was conducted be a technician in the Wood Research Laboratory at Purdue University USA.

Quote ' The tests prove that tight fitting joinery is a must and that you can't rely on glue to bridge even small gaps. Ideally, the gap around the tenon (in the mortice) should be no more than 0.005 in. When the gap is doubled to 0.010 in the joint strength decreased by 21%.'

The article also stated that glue should be applied to both the mortice and the tenon for max strength. Something I always do myself. Funily enough, putting glue only into the mortice was actually stronger than putting it only on the tenon, so that is something to think about.

The walls of the mortice and the faces of the tenon were very smooth. The mortice made with , a slot morticer with a good surface finish and the tenons made accurately to size with a thickness sander (slip tenons obviously, to maintain consistency through the tests.

Also, the strength of the joint was tested with variation in dimensions. The upshot is that wider tenons give the greatest percentage increase in strength, length increases have the next best increase and thickness the least, though increasing any dimension added strength. So maximising one or all the dimensions appropriate to the work would-be a good plan.

This is a very condensed version, but I was keen to dispel the recent myth about glue having more strength in a greater thickness, when clearly it does not.

Make.

 

[meccarroll]

Mortices produced on a slot mortice machine and tenons that have been sanded and inserted as slip tenons are not the most common method of producing a mortice and tenon joint.

It may be more common for a bandsaw to cut the tenons and a chisel and auger to cut the mortices in a DIY situation, or perhaps cut using a router. I think these tests would probably be more closely related to a joint produced by a domino machine.

A mortice joint cut using a chisel and auger will be no where near as smooth as one cut with a slot morticer so the tests are only specific to the situation they were carried out in.

The tests will likely prove to be a good guide but probably not specific to a mortice and tenon joint produced by other machining techniques.

Mark

 

[LancsRick]

There have been many theoretical discussions on this topic, and many practical explorations of reality. What I think people often lose sight of what is actually required. The variations in strength of various approaches are often insignificant when you scale up the number of load bearing joints and the required load.

I'm not for one moment condoning poor joints, but would encourage people to consider what they're aiming to support before getting hugely concerned with precision joints or comparative merits of different joints.

 

[dzj]

0.005 in (0.127mm) is a bit too much. I aim for half that, or even a bit less, when using machines to cut the joints.
I don't measure hand-cut joints, but when dry fitting, it should come together without a mallet, and should be tight enough to
hold its weight, without the joint coming apart.

 

[woodbrains]

Hello,

The article was not about how the joint should be produced, but about what was the permissable tolerance on the joint. The tester used a slot morticer to allow consistency throughout the test and a thickness sanded produce measurable tenon thickness. It is not his joinery method that was at test, but the effect of the glue in a joint against the closeness of fit (or not) he also tested joint dimensions and strength as well.

My mentioning the article was not a recommendation of how anyone should make a joint, nor how common or not a slot mortice and slip tenon joint is. Just that the old advice of joinery to be smooth and close fitting is still the correct advice and any spurious claims to the contrary should be disregarded.

0.005 in was the maximum tolerance, I think I said so. But having said that, the tester did say that the joint should come together with hand power for most of the way and perhaps a clamp or hammer used to bring home the last 10%. So that again is pretty much the classic advice.

My main point was to reiterate that glue does not provide any strength in joinery in a thick resinous application, but requires close wood to wood contact.

Mike.

 

[Sheffield Tony]

My mentioning the article was not a recommendation of how anyone should make a joint, nor how common or not a slot mortice and slip tenon joint is. Just that the old advice of joinery to be smooth and close fitting is still the correct advice and any spurious claims to the contrary should be disregarded.

My "O"-level woodwork teacher said to aim for a joint to fit straight from the saw rather than adjusting with plane or chisel (a) so that you impress the examiners and (b) because the sawn finish offers a better surface key for the glue than a smooth planed one. It sounded logical to me, maybe he was wrong ?

My main point was to reiterate that glue does not provide any strength in joinery in a thick resinous application, but requires close wood to wood contact.

I presume the article related to a particular type of glue ? I wonder too about the effect of wood swelling on applying water based glues; occasionally this has caused me difficulty assembling tight joints, sometimes it makes poor joints look a bit less bad. Does it help by making the joint tighter during drying ?

 

[Phil Pascoe]

"The article also stated that glue should be applied to both the mortice and the tenon for max strength. Something I always do myself. Funnily enough, putting glue only into the mortice was actually stronger than putting it only on the tenon, so that is something to think about."

I put glue on both sides. Everyone I know puts glue on both sides. Nearly all glues have instructions that clearly say "Apply adhesive to one side only". Why one side only? This crops up perennially, and I live in hope that one day I will read an article by a manufacturer why this is recommended.

 

[meccarroll]

Hello,

The article was not about how the joint should be produced, but about what was the permissable tolerance on the joint. The tester used a slot morticer to allow consistency throughout the test and a thickness sanded produce measurable tenon thickness.
Mike.

Now I am confused Lol.

Mike the tests in this experiment were carried out by someone else, not you, there is no dispute as to your methods relating to how to produce a good joint. For the record I would tend to lean toward what you are implying:.......not to rely upon common wood glues such as a PVA glue to fill and bulk out a joint.

Mark

 

[woodbrains]

Now I am confused Lol.

Mike the tests in this experiment were carried out by someone else, not you, there is no dispute as to your methods relating to how to produce a good joint. For the record I would tend to lean toward what you are implying:.......not to rely upon common wood glues such as a PVA glue to fill and bulk out a joint.

Mark

Hello,

The reason I quoted the article was because the test were carried out by a research laboratory specifically set up for investigating the properties of wood. The tests did not tell me anything I did not know, but I thought an example of rigourous lab testing would back up what I and many other woodworkers already know. For glue to work at its most efficient, there needs to be wood to wood contact and no reliance on glue to make up any gaps.

I had said as such in another thread, where comments were made about a certain internet article/video, where the protagonist said the conventional wisdom was a myth and gave examples of wood being glued with extremely thick gluelines. When I said that he (the internet article writer) didn't understand how glue actually worked in woodwork, and it is not the glue in a thick resin layer that worked but close wood to wood contact and the glue bonding at a cellular level, (possibly acting on the weak molecular forces themselves) I was asked to justify my claims. Of course those who post YouTube videos have qualification to come up with ideas that seem to impress people! I guess my 30 odd years of woodwork have no validation here, so I thought I'd quote from the timely magazine article to back up what I said.

Regarding woodwork teacher urging the students to cut joinery accurately straight from the saw; it is always good practice to try and do so. But the saw should leave a fine enough finish for a good glue bond, i.e. within the 5 thou (or less) tolerance. And a good backsaw, properly sharpened will do that. If the saw teeth leave a rough surface greater than that, it may be worth thinking is a finer toothed/finer set saw. The roughness is not to provide a key, so in that regard the woodwork teacher was wrong, with regard to modern glues. However old fashioned hide glue will work with a key, so his info may have been just out of date. Unless of course you were using hide glue in school!

The idea of the wood joint being close fitting is to enable the slight swelling due to the moisture in the glue to apply the pressure needed for these types of glue. We do not clamp the cheeks of a mortice and tenon, so the pressure needs to come from within the joint. If the dry joint is a force fit, then 2 things can happen; the moisture swells the mating parts so the joint will not go together when the glue is applied, or too much glue is wiped from the surfaces of the joint. So we should aim for a fit in the dry joint, with firm hand pressure and the joint should hold under its own weight. Again, just conventional wisdom.

The glue in the magazine tests was Titebond PVA.

Mike.

 

[Sheffield Tony]

Regarding woodwork teacher urging the students to cut joinery accurately straight from the saw; it is always good practice to try and do so. But the saw should leave a fine enough finish for a good glue bond, i.e. within the 5 thou (or less) tolerance. And a good backsaw, properly sharpened will do that. If the saw teeth leave a rough surface greater than that, it may be worth thinking is a finer toothed/finer set saw. The roughness is not to provide a key, so in that regard the woodwork teacher was wrong, with regard to modern glues. However old fashioned hide glue will work with a key, so his info may have been just out of date. Unless of course you were using hide glue in school!

Wandering a bit, but our woodwork lessons were very traditional, and some things may be out of date. Since taking it back up again I have recalled learning many things at school that I had almost forgotten - we covered burnishing cabinet scrapers, which I had to re-learn. Sharpening and straightening backsaws. I did try veneering a small table top with hide glue, though the teacher thought it was a pretty quaint thing to do even then - PVA was the norm for regular joinery (I'm not that old :lol: ).

 

[MusicMan]

This is speculation, but applying the glue to the tenon means that it is scraped off (a tight fitting joint) during assembly. Applying it to the mortise means that it is pushed in and wiped over the joint. So maybe not so surprising after all.

 

[meccarroll]

Hello,

The article was not about how the joint should be produced, but about what was the permissable tolerance on the joint.

That's not what Dan spheroid said: "Most of the testing was done to determine............"

And who is Dan :?:

This is starting to look a bit suspect.

 

[woodbrains]

Hello,

Err, in a three page article one page and some overspill onto another was about proper application of glue, the tolerance of fit in the joints and what happens when the gap was increased. The rest was about optimising tenon strength by making the joint thicker, wider and longer and what had most effect, all of which I mentioned above and some about the testing methods. There was no prescribed method given for how craftsmen should make the joints. I did say I didn't have space to quote the whole article, but only the pertinent bit about glue and joint fit, since that was the bit I was interested most in sharing. What point are YOU trying to make precisely? If a university research lab is not the place to do such tests then where and by whom.

If you really don't believe that close fitting joints are the strongest way to achieve good joinery then what? Why do you seem to want to discredit the author or pick me up about quoting him unless you have knowledge of a better way and would like to share.

Mike.

 

[meccarroll]

Hello,

Err, in a three page article one page and some overspill onto another was about proper application of glue, the tolerance of fit in the joints and what happens when the gap was increased. The rest was about optimising tenon strength by making the joint thicker, wider and longer and what had most effect, all of which I mentioned above and some about the testing methods. There was no prescribed method given for how craftsmen should make the joints. I did say I didn't have space to quote the whole article, but only the pertinent bit about glue and joint fit, since that was the bit I was interested most in sharing. What point are YOU trying to make precisely? If a university research lab is not the place to do such tests then where and by whom.

If you really don't believe that close fitting joints are the strongest way to achieve good joinery then what? Why do you seem to want to discredit the author or pick me up about quoting him unless you have knowledge of a better way and would like to share.

Mike.

Here is a link to the article you cite Mike:
http://www.finewoodworking.com/membership/pdf/226630/011259060.pdf

This is you stating what the article is about:

Hello,

The article was not about how the joint should be produced, but about what was the permissable tolerance on the joint.

Mike, the word Gap is used but tolerance is not used at all. To be taken seriously you need to quote the article and not change the wording to suite your own purpose.

A good example of you misleading is this quote:

If you really don't believe that close fitting joints are the strongest way to achieve good joinery then what?

In this thread, I did agree with you in principle regarding joint fit in relation to PVA glue and that can be seen above. You have decided to change that, and you say that; I don't really believe that close fitting joints are the strongest way to achieve good joinery?
If I said that Mike quote me and if you can not quote me as saying that Mike, I'll take it as an admission from you that you have miss quoted me.

Regards Mark