Best Way to Edge-Join Boards?

[SeanS]

All,

I recently did a short woodwork course for beginners. Very good stuff for a rank novice like me! When we planed some boards to join them for a table top, we planed so that when the boards were laid against each other, there was a small gap in the middle of the boards. The ends were touching, with a gap increasing uniformally (as much as possible) to the point where it was appproximately 1mm wide at the middle for every 1m length of board (the rule of thumb given by the wood dude giving the course).

So for our boards of about 2m length, each joint was open about 2mm in the middle. We then used biscuits, glue and clamps to close the joints. The reason given was that if/when the boards expanded as they aged, they wouldn't crack (we had effectively pre-stressed them, and expansion would just lead to them relaxing back out). Has anyone else heard this? Is it a valid approach?

Cheers,

Sean

[Wood Borer]

Sean,

I always plane them so I can barely see any light between the boards but a slight gap in the middle, perhaps not nearly quite as much as you indicated. You must never leave a gap at the ends.

The information sounds OK, but I leave less of a gap.

- Wood Borer

[Bob Willson]

No it isn't.

[ozwinner]

I would have thought the straighter the better, if you are leaveing a gap you are putting the boards under tension on glue up.
If on the other hand there is no gap , there is no tension.
I would have thought a nice relaxed board would move less.

Al

[derekcohen]

Sean

When I edge plane boards - as you described, that is, with the two joining boards folded over and hand planed together - my aim is to get perfectly flat boards. Timber moves constantly and I don't see the point of adding another source on internal tension. I do recall articles that say to leave a slight gap in the centre, or more accuractely, to plane a slight bow. I think that this really was to obtain a more accurate joint since it is easier to clamp such a formation together and force the boards against one another. If this is so then it is poorer construction. Incidentally, it is certainly easier to plane for a gap than plane for flatness. I've done so many a time! Just put more pressure on the plane (such as a #7) at the middle section. The fun it getting rid of this!!

Regards from Perth

Derek

[Red neck]

Sean

Like Derek I have always planed the boards so that the edges were exactly parallel. But your suggestion has got me thinking. My work bench has 35mm hardwood planks splined and edge glued but has now 'gapped' at the ends. This was to be expected as when I built it years ago the timber was probably relatively green, but the point is over the years the ends have produced more shrinkage than the centres and I guess this would be expected since the ends are exposed to the atmosphre. "Wick effect?"

I wonder if your teacher lost something in the interpretation. Perhaps the original intent was to cater for the ultimate shrinkage of boards at the ends rather than a uniform shrinkage along the length of the board?

[hcbph]

Add another vote for exact match on the edges. In fact, if time permits on a job, I like to joint the board then leave them overnight so if there is any internal tension, it will show itself before I glue the boards up. Unually happens on a job when I don't have time to condition the wood well in my shop before using it.
I do furniture restoration/repair and I've had very few issues over the last 20 years doing it this way.

[journeyman Mick]

My approach is to check the moisture level before starting, making sure that the timber sold to me as seasoned really is (had a bad experience years ago), get an exact match on the edges, join using biscuits (for alignment) and epoxy (for permanence). I then sand and apply a good finish all around, same type and number of coats on all surfaces. If the timber you were gluing up still had some drying to do I can see that your teachers method might have some merit, but I'd prefer to use well seasoned timber and cut down on the stresses in the top.

Mick

[SeanS]

All,

Thanks for the anwers. I have also seen others go for a perfect flat or even surface between boards (Norm was one of them), thus my confusion! Being an engineer I liked the idea of pre-stressing (just kidding), but the weight of opinion here is towards an even match-up, so I think I have my answer!

Thanks again,

Sean

[soundman]

I have observed some discussion & differing points of view on this topic in the wood press over the last while.

Seems the concensis it that the prestressed method doesn't seem to have any benefit but still seems to be preached with vigor by some.

I can see that its better to have a gap in the ends if you can't make it flat but flat & straight makes more sence to me. Otherwise you would be intrudcing un necessary stress in the boards.

It as has been said before "wood is like a woman" extra stress is not rrequired.

[silkwood]

I have to disagree with pretty much everybody on this thread. My father, an engineer/pattern-maker always insisted on a slight concavity when edge-jointing boards, due to the tendency to gap or "end-part" of surfaces made of a number of edge jointed boards.

As he explained it (he is no longer with us, otherwise I would let him speak for himself) the slight (he worked in "thou", not mills, but same diff) gap in the centre allowed stresses to even out throughout the boards, as well as taking into account the inevitable negative stresses which occur at the ends due to the differential effect of drying out at the end-grain.

Funnilly enough I had not thought of this for years until, last year, a lecturer at one of the TAFE classes I attend gave the same reasons for the same technique. He also showed some classic examples of where this had not been used.

As an aside, if you do leave a small gap in the centre, you can almost guarantee the correct tension when clamping by putting a small amount of vertical pressure on the edge of two adjoining boards. When they emit a faint "creaking" noise, then are easily manipulated back into alignment you're spot on. No noise, too loose (Latrec ), no movement, too tight!

Perhaps straight isn't always the straightest answer?! Having said all of this, I usually simply join straight edge to straight edge (simpler and usually effective), unless I am making something in the classic style, in which case I follow Dad's advice.

Cheers,

[M-R]

I read way too many wood working books. I probably spend more time reading the books than I do with the wood.

The edge jointing technique where you leave a small gap in the middle of the joint is called a compression joint. This seems to be a pretty important thing over in the UK as all the UK books i've read mention it at one point or another, i assume this is because of the generally wetter climate... wood shifts more? It isnt brought up in the book, its just assumed you already know about it.

the reasons listed for using this method were something like better glue distribution, more evenly distrubted clamping pressure (better glue setting) wood movement is taken into account.

I've never actually bothered to use one of these joints before either... I have never had much trouble with wood movement, except for when i've used european beech, that stuff is like a sponge!

[silkwood]

M-R (strange middle name you got there!) at this point I should admit to being a pom, as was my father, hence the strong point of planing a slight "bow" might be more common over there (I consider here home now, but am happy to take abuse where-ever I can get it).

It would seem to me that somewhere like Sydney would have a greater differential air-moisture density than anywhere in the UK, so surely, IF this is an issue of movement (as it appears to be) it would be at least equally relevant over here. As I stated in my previous post I rarely use this technique.

I have just (tonight) discussed this with the lecturer who brought this up last year. His take is as follows: We see a lot of practices which used to be followed being ignored now. Often this is because of new machinery rather than changing technique (in this case, the introduction of surface planer [yes, yes, buzzers, planers, jointers, etc]). He asks- will the furniture built with more modern techniques last the hundreds of years much furniture of the past has?

Keeping it in balance, nowadays we see much more use of moveable mechanisms to connect table-tops to frames (buttons, figure-8s, etc), whereas in the late 19th/early 20th century we saw much use of pocket screws used for this purpose, resulting in many warped or cracked table-tops today. Perhaps this is an example of yesteryear's fashion, as mass produced, quality, inexpensive screws must have seemed to them like biscuits appear to us? Just an idea.

Cheers,

[echnidna]

This was the correct way to do it in the old times without power tools or machinery or KD timber. These days flat and no gaps is the normal method.

[SeanS]

Again, thanks to everyone for their insights and opinions, keep them coming! I am happy that what I thought was probably a stupid question turns out not to be!

Cheers,

Sean