What's the point of dovetail joints?

[Student]

A friend of mine made the following comment

"Why dovetails? Carpenter’s vanity? They look pretty (unless they’re less than perfect) but are invariably hidden away. If you compare the glue area between a dovetail and a box joint, there’s nothing in it so strength isn’t an issue. Even a plain mitre can be strengthened with a couple of splines, especially angled splines."

Lights blue touchpaper and retires.

 

[Phil Pascoe]

(unless they’re less than perfect) shome mishtake shurely?

 

[DiscoStu]

I'm no expert but having made my first one the other day it seems immediately obvious that in one direction you can pull a dovetail apart but not in the other so for a drawer for example you'd make the dovetail so it couldn't pull the drawer apart from the front. With a box joint you can pull the joint apart in both directions so for a drawer you be constantly trying to putting the joint at risk when opening the drawer.

That's only my observations


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[Phil Pascoe]

And with glues that are stronger than the wood that matters how much ? ...

 

[thetyreman]

I'd make him a dovetailed box then one made from festool dowels and power tools, get him to try and break them and he'll see, you show him!

 

[Jelly]

And with glues that are stronger than the wood that matters how much ? ...

Wood is stronger in compression than tension, and it's shear strength in a plane parallel to the annular rings is its weakest strength characteristic... Yet timber which is quarter sawn (generally accepted as best quality, and of superior dimensional stability), then unavoidably results in the common casework joints exerting their loads in the weakest direction.

It's feasible to impose a load via the glue in a box joint that would exceed the shear strength of the wood, through its weakest plane. With a dovetail you have to do that whilst applying a compressive force which acts to push the wood together at that plane.

Thus, if you use flat sawn timber there is unlikely to be much gain from dovetailing... But using quarter or rift sawn timber dovetailing gives a theoretical advantage, from a mechanical properties point of view.

 

[Fitzroy]

No knowledge, but is it only modern glue that is stronger than the wood? Were historic glues weaker, if so then the joint structure becomes much more important. Could it just be historic precedent?

F.

 

[MattRoberts]

Simple - dovetails are there to make other woodworkers jealous!

 

[DiscoStu]

And with glues that are stronger than the wood that matters how much ? ...

I never said it mattered only that when pulling a join in one direction the dovetail is stronger than a box joint.

Why bother with either why not just but them together and use glue?


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[Sheffield Tony]

Should we really believe that modern glues are stronger than the wood ? I mean not just initially, but after a few years of the wood moving with the seasons and repeated loading. Part of my pole lathe failed at the weekend, and it was a glue joint that pulled apart neatly under shear force (glue was titebond 3, about 3 years ago).

Whatever, it can do no harm to add mecanical strength to the joint rather than relying solely on glue.

 

[novocaine]

your friend is a philistine and should be beaten with 2 compound sticks, one dovetailed and one box jointed together, he may choose which one first.

nothing to add above what has been said above.

 

[Rorschach]

It was probably a combination of insurance against older glues/the ravages of time, as well as being an opportunity for craftsmen to show off their skill. In a time when everything was done by hand I am sure that dovetails were no slower than most other joints of similar strength.

 

[dzj]

Carpenter’s vanity?
These days carpenters are more into bolts, brackets and nail guns.
Unless they're Japanese.

 

[Setch]

Older glues were less reliable than modern, and prone to failure due to variations in humidity, and insects insects literally eating them or of the joints.

As such a mechanically strong joint for drawer fronts meant it would pull off if the glow failed. Also, pre tablesaw jigs and the like, cutting an accurate finger joint would have been far more time consuming than a tradition dovetail joint.

 

[Eric The Viking]

I've seen a lot of old, rustic (or very old) furniture and other construction, with dovetails and no glue, usually held together with handmade nails or pegs. Made properly and used properly, they seem to take the intended loading as long as the wood lasts.

So presumably the dovetail developed first, taking advantage of compression, as Jelly said.

Nowadays, most heavy use drawers in modern furniture are actually joined in some other way - Ikea must sell thousands of drawers for every one made by craftsmen in this country. Modern drawers do fail, obviously, but they seem to last well enough for people not to care, given the cost of manufacture is so low.

Me? I think through dovetails look nice, and they are traditional, and I aspire to make nice ones. For quicker stuff in volume there are mitre-lock joints (or other locking profiles), or even biscuits or dowels -- dominoes haven't yet cleared the field of the competition entirely!

E.

PS: Last week I replaced the metal runners on our (very overloaded) cutlery drawer. It must be 30 years old and made of uPVC-wrapped chipboard, with a hardboard slide-in bottom (the front is oak though). Really horrid, but it's stayed together, and, although I didn't break it down, that can only be 8mm dowels, with very little glue. And its been banged shut many times a day.

 

[Jacob]

Dovetails basically a simple way to make a board corner joint, with simple tools, and more long lasting than the alternatives. Works without glue. Better with glue needs no clamps. They were originally just practical but later became a fashion thing.

 

[Sawyer]

No knowledge, but is it only modern glue that is stronger than the wood? Were historic glues weaker, if so then the joint structure becomes much more important. Could it just be historic precedent?

F.

That's a good point. In fact, some of the traditional glues are very strong; probably as strong as modern ones under perfect conditions, but they also had disadvantages. Animal glue for example, besides being susceptible to microbe attack, has minimal resistance to moisture or heat and becomes brittle in very dry conditions. Also, until the advent of modern adhesives, a reliable exterior glue was simply non-existent. Joiners of old had therefore to rely on the mechanical strength of their joints and techniques evolved accordingly.

Taking the example of a dovetailed drawer front, due to continual timber movement and stress loading, the hold of the glue - any glue - can weaken over time. How often we see on low class, non-dovetailed work, drawer fronts come loose as soon as the glue fails. Yet in restoration work, it is common to dismantle drawers for repair and I often find that an old but still functioning drawer will come apart very easily when tapped sideways. In other words, the drawer has been continuing to function for years on joint mechanics in the absence of a firm glue hold.

 

[Sawyer]

"Why dovetails? Carpenter’s vanity?

Yes, thank goodness!!! Although there are many other good reasons too. But for 'vanity', let's read a highly-developed level of skill, which any good worker should be proud to celebrate and taking a genuine pride in one's work. If wanting to aspire to something better than a deluge of mass-produced mediocrity is 'vanity', then so be it!

 

[Jacob]

A friend of mine made the following comment

"Why dovetails? Carpenter’s vanity? They look pretty (unless they’re less than perfect) but are invariably hidden away. If you compare the glue area between a dovetail and a box joint, there’s nothing in it so strength isn’t an issue. Even a plain mitre can be strengthened with a couple of splines, especially angled splines."

Lights blue touchpaper and retires.

With simple hand tools DT is easiest and most effective. Also self locks. Box joint is strictly a machine joint and mitres/splines (by hand) much more difficult than a DT.
NB DTs were almost always done very quickly freehand - hence very practical - non of the cautious and meticulous fiddling about that goes on nowadays - vanity didn't come into it at all!
Given just a plane, a saw and a chisel what better joint could you come up with for a box corner?

 

[Eric The Viking]

Box joint is strictly a machine joint

Strictly a glue joint, surely?
Even then you do see them, unglued, on the finest Chippendale card tables, where they're known as "hinges." ;-)

Given just a plane, a saw and a chisel what better joint could you come up with for a box corner?

Quite agree on that .
:wink: