How tight should they be? Joints that is!

[will1983]

So a question came to mind the other day when I was putting together a screen I have been working on recently.

Just how tight should a joint be? I have been making them "push together with firm hand pressure tight" as I've seen recommended elsewhere. However this usually has the effect of scraping almost all the glue off a tenon which then requires cleaning up.

In this recent example I tried to counteract the scraping it by spreading glue in the mortise and on the tenons but it still concerned me that I may end up starving the joints of glue.

So what do all the experts here think?

 

[MikeG.]

Scratch up the face of the tenon if you're worried, following the same principle as dowels which are grooved long their length. Personally, I make the joints tight. Loose joints are relying too much on the glue anyway, and (personal anecdote alert) I haven't had a joint fail in nearly 40 years.

 

[Racers]

That's what I aim for.

For M&Ts I coat the sides of mortice and lightly brush the tenon faces with glue, not the sides or the shoulders to minimise squeeze out.

Pete

 

[custard]

A mortice and tenon should be a firm hand push fit when glued with PVA.

There's really not much latitude. Using accurately settable machinery with typical furniture hardwoods I've found little more than 0.1mm separates a tenon that's "a bit too tight" from one that's "a bit too loose".

PVA has almost zero gap filling properties, if you cut your M&T sloppier than this to give yourself an easy life during assembly, then you're just setting your furniture up for future joint failure.

 

[Ttrees]

How long you have the glue on could be a factor, due to swelling whatever glue it may be...
Apart from epoxy, It apparently doesn't swell the wood, or maybe not as quickly?
Correct me if I'm wrong on above though!

If you really need the glue, its probably the wrong joint ....
Says me, a laminating scavenger :roll:
Tom

 

[thetyreman]

sounds like your joinery is a bit too tight, you want it tight but not insanely tight where it's compressing or bruising the wood, custard is right, when I first started out I didn't spend long enough getting a perfect fit on tenons, a bookshelf I made has a slightly loose tenon and it makes it a bit weaker than it should be overall, so it's worth spending the time getting a good fit.

 

[MattRoberts]

I glued some work up the other day and a tiny bit of squeeze out dried on the piece as it was touching another bit of wood. The next day, I had to use a mallet to separate them! Don't be worried about the strength of a thin layer of PVA - especially with a strong joint like a mortise and tenon.

 

[skipdiver]

If joints are too tight, you can't get a good drag out of them. So i heard anyway.

 

[Sheffield Tony]

Using accurately settable machinery with typical furniture hardwoods I've found little more than 0.1mm separates a tenon that's "a bit too tight" from one that's "a bit too loose".

On which basis, you might think that getting a hand cut joint right from the saw would be next to impossible. I remember the teacher in my "O" level woodwork class explaining that you should aim to get it right first time, not only to impress the examiner but because the finish from the saw made a better joint than a pared surface. The slightly fluffy, torn surface provides a bit of squishiness, better purchase for the glue, and swells on soaking up the moisture from the glue.

 

[custard]

I remember the teacher in my "O" level woodwork class explaining that you should aim to get it right first time, not only to impress the examiner but because the finish from the saw made a better joint than a pared surface. The slightly fluffy, torn surface provides a bit of squishiness, better purchase for the glue, and swells on soaking up the moisture from the glue.

I'm glad you mentioned O Level Woodwork. The phrase is sometimes used in a pejorative sense, like it was pretty crude and basic. In fact O Level Woodwork was an order of magnitude more complex than many of the projects you see recommended in the woodworking magazines.

It illustrates the decline in craft skills to consider that, just a few years ago, it was thought entirely reasonable for a 16 year old to tackle a practical test like this; cutting all the joinery within the circle, against the clock, and using only hand tools. And yes, this is an actual O Level practical exam,

Today making something like that with Kreg screws qualifies as a serious woodworking challenge.

Ho hum.

 

[skipdiver]

I remember the woodwork examiner with dread but i passed somehow.

 

[Phil Pascoe]

I'll go along with Custard's post - I did "A" level. I had to prepare the wood for the practical in the week before the exam, so had a week to look at a pile of bits and work out what the joints were to be. I couldn't. It transpired that it was the corner of a desk - the dovetailed front of the drawer was sloping, which of course was why it wasn't apparent.

 

[Jacob]

I remember the teacher in my "O" level woodwork class explaining that you should aim to get it right first time, not only to impress the examiner but because the finish from the saw made a better joint than a pared surface. The slightly fluffy, torn surface provides a bit of squishiness, better purchase for the glue, and swells on soaking up the moisture from the glue.

I'm glad you mentioned O Level Woodwork. The phrase is sometimes used in a pejorative sense, like it was pretty crude and basic. In fact O Level Woodwork was an order of magnitude more complex than many of the projects you see recommended in the woodworking magazines.

It illustrates the decline in craft skills to consider that, just a few years ago, it was thought entirely reasonable for a 16 year old to tackle a practical test like this; cutting all the joinery within the circle, against the clock, and using only hand tools. And yes, this is an actual O Level practical exam,



Today making something like that with Kreg screws qualifies as a serious woodworking challenge.

Ho hum.

Not even O level - the woodwork we did in the 2nd form age 12 was more sophisticated than 90% of what goes on on here. We learned about face/edge marks, proper work sequence, freehand sharpening, etc; absolute essential basics. I didn't appreciate how essential it was until many years later when I got more into proper woodwork - as distinct from general bodging.

 

[ED65]

The slightly fluffy, torn surface provides a bit of squishiness, better purchase for the glue, and swells on soaking up the moisture from the glue.

Worth mentioning that a rough (or roughened) surface bonding better is old thinking. Occasionally still a misconception even today which is surprising given it has been more than a century since the contrary was proved and first published :shock:

 

[LBCarpentry]

I actually think that if you have to use a "firm" hand to fit your joints together then they are too tight and any movement within the joint can result in twisting / over strain. Your right, the difference between tight and loose is a fraction of a millimetre.

I know (or believe!) I have a joint just right when I need to firmly push them together to start with, but after the piece has been dry fitted 2 or 3 times then they go together a lot easier, but there is still no lateral movement. Whether I apply glue to the mortice, or the tenon is determined by how the joint is to be used.

 

[bourbon]

I'll probably get slated for this, but wouldn't a 'foaming' glue fill any gaps and 'firm up' a loose joint? I don't know from experience. I'm not that great at woodwork and usually use screws to join things together

 

[MikeG.]

It would, bourbon, but that's not the point. Gaps filled with glue will show up badly when a finish is applied, for a start, but just as important is the thought that all glues give up in the end, and you don't really want the joint to depend on something with such a finite life.

Reminds me of a cricket bat Duncan Fearnley made way back when, which was returned to him after 6 months use because the owner thought that something was wrong with the handle. It turned out that it had gone out of the factory with a dry joint: there was no glue holding the handle to the blade. The guy had batted all season with it before it started to go wrong. That wouldn't have happened without a very precise joint.

 

[Jacob]

.... all glues give up in the end, and you don't really want the joint to depend on something with such a finite life.
......

Yebbut you are talking hundreds of years - I've never known glue to fail. Glue joints can fail if badly done - not sufficient coverage and weak spots, or if subject to environmental extremes temperature/humidity etc but otherwise they go on and on.

 

[Racers]

I'll probably get slated for this, but wouldn't a 'foaming' glue fill any gaps and 'firm up' a loose joint? I don't know from experience. I'm not that great at woodwork and usually use screws to join things together

It does foam, but foam isn't very strong, epoxy will fill gaps.

Pete

 

[MikeG.]

.... all glues give up in the end, and you don't really want the joint to depend on something with such a finite life.
......

Yebbut you are talking hundreds of years........

We're never going to agree on that.