Cabinet Joinery Failure Modes

[spacelabstudio]

Hey, I've seen the marketing of some cabinet makers touting their dovetail joints as better because they are stronger than a box/finger joint.  Which I'm sure they are.  The question on my mind lately, though, is does it matter?  I've been around a lot of bands and a lot of music gear and I've just never seen a wooden speaker or combo cabinet fail at a joint, regardless of the type of joinery used.  Have any of you guys seen a cabinet fail structurally?  Would a different type of joint have made a difference?  Just looking to see if there's any anecdotal evidence beyond my experience out there.

[spacelabstudio]

Thanks, that was an interesting thread!  Just adding a cross reference here to make it easier to find later:

http://www.el34world.com/Forum/index.php?topic=13490.0

[burt_toast]

corner construction probably matters less with a closed-back cab, since the baffle/back won't let it parallelogram on ya. good finger joints are hard to beat. glued-in corner blocks help if you don't have dovetail/fingerjoint equipment.

scott


ps, before i got into pipe organs, i was a cabinetmaker for about 15 yrs.

[HotBluePlates]

... dovetail joints as better because they are stronger than a box/finger joint. ...

Generally, the glue is stronger than the wood. When you try to break apart the joint, the glue hold and the wood near it shears apart.

And since the dovetail shape provides more surface area, there is more glue holding it together. Of course, the shape also leads to the two pieces interlocking.

... I've just never seen a wooden speaker or combo cabinet fail at a joint, regardless of the type of joinery used.  Have any of you guys seen a cabinet fail structurally?  ...

I once saw a bigger Fender amp (don't recall the model) where it seemed the glue just disintegrated in the finger-joints. The chassis and the tolex were the only things keeping it from falling over ("parallelograming" as Burt said) into a flat pile of panels. But that amp had been seriously abused. I remember wondering at the time if something had seeped in and dissolved the glue in the joints, as aside from other damage the amp looked like it might have been left outside for 5-8 years.

In fact, it reminds me of the definition I once heard for "poor condition": 'It's either an amp or a ham sandwich, but it's not in good enough condition to tell...'

[G._Hoffman]

Generally, the glue is stronger than the wood. When you try to break apart the joint, the glue hold and the wood near it shears apart.

That statement is extremely over simplified.  Not wrong, per say, but only accurate under very particular conditions.  Granted, they are the conditions under which the glue joint will be spending most of it's time, but it is not at all hard to make it not true.  Leave your glue joint in the back of a car on a hot day and the glue joint starts to soften in less than 10 minutes, and if it is under tension it will be ruined in less than 15 minutes.  Also, it depends on the kind of strain the glue is under, and what direction the force is coming from.  Most glues are great under tension, and of course under compression their strength doesn't much matter, but any sheer force can be a big problem.  Also, glue does a lot better under a steady force than a shock.

In all honesty, the main reason I always use dovetail joints for cabinets is that - with a modern, inexpensive dovetail jig - they are MUCH easier than finger joints for a one off joint.  Now, part of that is that I have a lot of experience with routers, and I have a very nice router that makes fine adjustments very straight forward, but even if I just use one of my very basic Porter Cable 690's, I have never had a problem getting a very tight fitting joint in just a few test passes.


Gabriel

[G._Hoffman]

Dovetail joints are a mechanical joint. Without glue, will the joints rattle? Something that you might not want in an amp. 

Dovetail joints are mechanical joints which are reenforced by glue.  I don't think anyone has ever used a dovetail joint without glue, and you certainly wouldn't in an amp.

But to answer your question, it would depend on how well they are cut.  I tend to cut mine tight enough that it probably wouldn't rattle at all.  But then, when I make a bolt on neck, I make the joints tight enough that I don't need screws for it to handle string tension.


Gabriel

[Willabe]

I watch a lot of PBS, one of the woodworkers (out of South Carolina), does just about all dovetails without glue.  (All hand cut).

Dovetail joints are mechanical joints which are reenforced by glue.  I don't think anyone has ever used a dovetail joint without glue, and you certainly wouldn't in an amp.

I think Gabes right.

Certainly with a guitar amp/speaker cab where it's moved around and with all the vibration from the speaker you definitely want to glue the corner joints even if you use self locking full dove tail joints.

But I also think there's more to consider that goes with it.

The wood is going to dry out and shrink and when it does not if, the joints will loosen. If you don't glue them up they won't last long, they can't. The glue helps seal the wood so it can't dry out and shrink away from each others touching face.

Only takes a few .001's of an inch and now your once nice and tight dovetails that you took the time to make sure were as perfect as you could get them are not so perfect any more. Even if you were to seal them with oil or some kind of finish over time they will still shrink enough to loosen up the joints. Might take longer but it will still happen.

Think about your guitars neck. If you live some where that has 4 full seasons, because of the seasonal change of humidity even though the neck has a finish on it, it still moves enough for you to have to adjust it twice a year.

Even if you where to make the dove tails joints perfect, fit the cab/box/carcass together and then put a finish on all of the exposed wood inside and out, all edges and ends, it will still dry out enough to loosen the corner joints.  

Dove tail joints have a structural advantage over finger joints because of the angel that locks them together. It gives them an extra shoulder to have to work against to get them selves free from each other. Finger joints don't have that extra shoulder/angel to work against. But their still not perfect, they can still come apart with out glue or some kind of mechanical fasteners to lock them in place. And/or as Gabe said to reinforce them with glue.

But you still need to fasten them permanently. Wood workers use glue, carpenters use nails or screws/bolts. Although there is some cross over with each at times where wood workers use some type of mechanical fasteners along with glue and carpenters use some type of glue along with mechanical fasteners.



              Brad      

[Willabe]

I remember that violins, guitars used a glue that was kind of reddish, that was designed to be almost as strong as the wood.  This would allow the repairman to open the instrument and then re glue. 

That's animal hide glue. That's the only glue they had in the old days. There are many, many examples of old instruments, furniture, etc. from what 2,3,4 centuries ago that were glued up with it that are still in use to this day. Modern synthetic glues only became available and popular, what 40, 50 years ago? (Maybe not even quite that long ago.)


                    Brad       

[Willabe]

I like Gorilla glue. 100% water proof, non toxic, wont dull your saw blades and router bits. Pushes into the woods grain because it expands as it cures.


                 Brad      

[G._Hoffman]

Well made violins are all made with hide glue.  

The first synthetic glue I know of was cyanoacrylate, which was developed during the Korean War as a quick alternative for stitches during surgery.  It didn't start getting used for industrial an eventually woodworking purposes for quite a while.  My guess is that it was probably during the 1960's "space race."  

The first man made glue to become popular for woodworking was Franklin's Tightbond, which is a aliphatic resin glue.  Tightbond came out in the late `50's.  All yellow wood glues are of this type, and in most situations it is still very good.  Tightbond or similar is usually the best choice for woodworking, as it has very good strength, a fairly long working time (you've got about 5-8 minutes to get all your clamps in place), is chemically pretty neutral, so it isn't at all dangerous to work with, and it isn't exothermic.  Shelf life is less than a year.  It can be taken apart with heat and/or moisture, but you need to completely clean the joint if you are going to reglue it.  Clean up is easy, and can be accomplished either before it drys, or after.  

Hide glue is gelatin.  Basically, it's jello without the sugar and the coloring.  There are some other technical differences, defined as the "gram strength," but in a pinch (and if you are REALLY quick with your clamps) you can use any flavorless commercial food grade gelatin as hide glue (Knox and the like).  Food grade gelatin is actually a higher gram strength than you use for most woodworking, but it comes in a large range of strengths.  Hide glue comes dry, and to use it you mix it with water and heat it up to around 150 degrees Fahrenheit.  If it gets too hot, it will weaken, so controlling your heat is critical.  Most people use some form of a electrically controlled glue pot.  The strength is very dependent on both the gram strength of the glue, and the proper ratio of flakes to water (which, sadly, isn't consistent from batch to batch.  Fortunately, when dry it will last your lifetime, so if you are using a lot of it, you can just buy a big bag of it and it should all require the same ratio).  Once mixed, it has a shelf life of a couple weeks, maybe a month.  Working time is the big drawback for most people, because unless you heat your work pieces you have less than a minute to get all your clamps in place.  It isn't exothermic, and has very good dry heat resistance.  It is also completely reversible with hot water, and can be reactivated by heat and water; so if you need to reglue a joint, you don't need to clean it out - just add a bit more hot hide glue, and clamp.  Done  properly, a remade hide glue joint will maintain 100% of its strength.  No other glue will retain ANY of its strength.  Clean up is very similar to Tightbond.  The reversibility and the tone is why violin makers still use it, along with a growing number of guitar builders.  Hide glue, in a tight fitting joint, drys much harder than any other glue on the market, and so the theory is that it transfers sound better than other glues.  At the very least, it seems to make instruments sound better, but there is not way to determine this conclusively.  For most things that I do, it is my favorite glue.  


Gabriel

[G._Hoffman]

Polyurethane glues, such as Gorilla glue, tend to foam up a bit, and consequently are good if your joints aren't as tight as they should be.  It is not any stronger than other glues, and drys fairly soft and flexible, so it is not a particularly good choice from an acoustic point of view, but has its purposes.  Speaker makers tend to like it because it helps to ensure their joints are airtight, which is important if you are making an infinite baffle or ported cabinet.  I tend to think they should be more careful making their joints, but then I'm used to a different kind of woodworking.  Open time is pretty average, but clean up sucks.  It MUST be cleaned up completely before it dries, or you are screwed.  Well, you can sand it off, but if it sticks to a finished surface you'll remove the finish trying to get it off. 

The real key to the strength of glues, though, is that their strength is very directional.  Most glues have great tensile strength (i.e., if you try to pull them apart at a right angle to the glue joint), and of course compression isn't really an issue for most glue joints, but most of them have very poor shear strength, and do even worse under heat; which more than anything is why no glue joint is ever "stronger than the wood."  As I said earlier, they can be stronger in some directions, and in some conditions than the wood, but wood will win out in other situations.  This is why it is best to have a combination of the strength of wood and the strength of the glue; hence my preference for dovetails. 

There is, to my knowledge, no traditional use of full or half blind dovetails which does not use glue.  There are occasions where it makes sense to use a sliding dovetail without glue, and they are never glued the full length of the dovetail because they are always cross grain.  Wood tends to expand and contract in one direction (cross wise), but not in the other (length wise).  Gluing a cross grain dovetail completely will almost always cause one piece to crack. 


Gabriel

[G._Hoffman]

 

Dovetail joints are mechanical joints which are reenforced by glue.  I don't think anyone has ever used a dovetail joint without glue, and you certainly wouldn't in an amp.

I watch a lot of PBS, one of the woodworkers (out of South Carolina), does just about all dovetails without glue.  (All hand cut).

Oh, and if you are thinking of Roy Underhill, don't take his word for anything.  He gives very bad instructions, at times, including the using some tools in manners that are very dangerous.  

My guess, however, is that what you are referring to is not gluing the bottom of dovetailed drawers, which is common to all drawer bottoms for solid wood drawers.  This is much the same thing as for sliding dovetails - they need some play in order to avoid splitting the drawer bottom, or sometimes the drawer front sides or back.  You still need to glue the dovetails themselves.


Gabriel

[G._Hoffman]

GH, I've seen lot's of stitches on Roy Underhill hands.  

I'm not surprised.  

I've got all my fingers, and have never had any serious injury on my hands!  (Well, not from wood working.  Genetics and repetitive stress are a whole other issue!)


Gabriel

[BlackCloud]

Make your joint as Marshall did in the 60's it is a strong joint, I just finished building a 67 model Marshall 4 x 12"s cab and it came out great, don't have all the speakers yet but when the back is closed it sounds real tight and I'm happy. Look around, research and do what you want, I didn't ask only because I've worked with wood for ever and no I don't know everything but I build acoustic guitars so I know a bit about wood and good acoustic glue which I use.

[Ritchie200]

Gab,

Great information!  As always, thanks for sharing your expertise!

Jim

[hesamadman]

Im just gonna say that dovetail and finger joint are both pointless unless youre planning on seeing the bare wood. Its a beautiful look but if youre planning on covering with tolex or duratex there are a million ways to get a cab substantial. ESPECIALLY if its a sealed back/fixed baffle. Pull the tolex off an old marshall...youll see finger joints. Which is almost necessary for those cabinets because the back plate they used were PARTICLE board. And I think they still are. (corner cutters).

But a lot of your custom manufacturers today use anything from biscuit joints with pocket screws, half lap and pocket screws, tongue and groove with pocket screws, even locking miters. Ive thrown a cabinet off a roof that was built using the "drill and plug" method. Which is pre drilling the wood. Drilling a countersink. Glue the but joints. Screw in screw. And fill the countersunk hole with a 3/8" wooden plug. (you can get a tool to drill these plugs. Makes the build look like an old piece of furniture or old hardwood floor. But that 15 foot drop didnt phase that cabinet one bit.

There is a fine line you cross in joinery. Yes. take two pieces of wood and join them with dovetail and the 2 more and join with another method. Then try breaking with your hands....the dovetail will prevail. But build a speaker cab.....you wont see a difference. As long as whatever you decide to do...is done right.

[G._Hoffman]

As long as whatever you decide to do...is done right.

That is the key to most works of craftsmanship, yes.


Gabriel