table tops - double or single row of biscuits?

[fobos8]

Hello all

I'm gonna join together some 190 x 45mm oak for a table top.

My question is whether I should have one or two rows of biscuits.

I know some people say you don't need any but I reckon at least one row is a good idea.

Any comments?

Best regards, Andrew

 

[Paul Chapman]

I made an oak table with a top of similar thickness and used two rows of biscuits, which seemed about right

Cheers :wink:

Paul

 

[fobos8]

does it matter what glue I use?

was just gonna get the PVA you get from B &Q

 

[Paul Chapman]

I always use PVA - I've standardised these days on the water-resistant type, usually EvoStik in the blue container.

Cheers :wink:

Paul

 

[Mr Ed]

Paul - out of curiosity how did you arrive at the number of biscuits to use there? If I was going down the biscuit route I'd have had about half that many in a piece of wood that size.

Ed

 

[Paul Chapman]

Hi Ed,

Nothing very scientific - just did what felt right. I was originally going to use a couple of continuous plywood loose tongues but couldn't find any decent plywood, so used biscuits. Here's a picture of the glued-up top, which was later cut into a circular top, 3 feet in diameter. You can see from the pencil marks where the biscuits were

Worked out fine and dead flat, which was the object of the exercise

Cheers :wink:

Paul

 

[dedee]

Andrew,
FWIW I made an oak table top from 45mm oak boards (4 of them) and used a single row of dowels (ramin) spaced IIRC about 6 inches apart. After 15 years there is no sign of movement.

If the mating surfaces are good and true the dowels or biscuits help more with alignment when you are gluing and clamping than with structural integrity.

Personally I would have thought that Paul's double row is a slight overkill.

Andy

 

[woodbloke]

I know some people say you don't need any...

[-( :-# - Rob

 

[OPJ]

I'd also go for a single row of biscuits, no closer than 6in. apart. Admittedly, you don't even need them that close, really...

A double row could make gluing up more difficult, if a couple of biscuits were thicker than the rest or, two mating boards were of unequeal thicknesser, meaning the slots didn't quite meet up.

I favour cascamite (Polymite, Extramite, etc.) as it won't creep over time (unlike PVA). But, in all honesty, if this is your first table then there isn't anything wrong in using PVA (it's not going to fall apart, or anything!).

Will you be gluing this up in your workshop or shed? It's bloody cold outside. PVA should dry clear but, in cold conditions, it usually dries white, which suggests it may not have cured properly... If at all possible, do the glue up in your 'shop, remove most of the excess glue and stick it indoors for a few hours until the glue has gone clear. :wink:

 

[Paul Chapman]

A double row could make gluing up more difficult, if a couple of biscuits were thicker than the rest or, two mating boards were of unequeal thicknesser, meaning the slots didn't quite meet up.

I favour cascamite (Polymite, Extramite, etc.) as it won't creep over time (unlike PVA).

Glueing up shouldn't be any more difficult with two rows. If the boards are planed up accurately and the biscuit jointer is referenced off the face side, then there's no reason why they shouldn't all line up. Any problems with biscuits varying in thickness should be sorted out at the trial cramp-up stage. If any have swelled because of damp storage, then a quick blast in the microwave will reduce the swelling and bring them back to proper thickness.

Surely creep ceases to be an issue if biscuits, Dominos or loose tongues are used :?

Cheers :wink:

Paul

 

[woodbloke]

I favour cascamite (Polymite, Extramite, etc.) as it won't creep over time (unlike PVA

There seems to be some confusion regarding glue 'creep' when discussing PVA adhesives. When two boards are joined ( as in say, in a table top) there is very little relative movement between each board as it's a long/long grain joint, so it doesn't matter what you use as the boards are going nowhere.

'Creep' in glue must be eliminated though if you're laminating a curved shape, where bent components under the stress of curvature may try to resume a straight form...the glue is the only thing stopping the curved laminations from trying to straighten themselves out, in which case you need a glue that sets glass hard (urea formaldehyde)

'Creep' is desirable though under certain circumstances, one of which is in chair work, where and element of joint flexibility is highly desirable (which is why Alan Peter's always favoured it) and this flexibility was used in a very novel way by John LLoyd to glue on breadboard ends (BW) using a cheap and nasty, highly flexible glue.

There's nothing wrong in using a perfectly normal, bog standard PVA glue for normal edge jointing of two boards with or without biscuits or doms, it's only when timbers are under certain types of stress does the maker need to consider suitable alternatives - Rob

 

[Paul Chapman]

Very well explained, Rob :wink: In my experience, creep is seldom an issue.

Cheers :wink:

Paul

 

[Mr Ed]

I can't say I have ever noticed anything I've made with PVA moving significantly enough to bother me. Is creep really an issue? I don't doubt that scientifically it does happen, but is it really of a magnitude that would bother most of us?

Ed

 

[Paul Chapman]

Is creep really an issue?

Not really in my view - except in the case of things like laminated curved shapes, such as Rob mentioned, where there are stresses acting on the glued joint. I take the view that the slight flexibility you get with PVA is a distinct advantage in view of wood movement. Many glues are too brittle, I think, for general woodworking.

Cheers :wink:

Paul

 

[head clansman]

hi
paul, table is coming together nicely , hope you finish a wip on the table now you started it .hc

 

[head clansman]

hi
paul, sorry didnt relise you had done this table some time a go . hc

 

[OPJ]

Fair enough, everyone has their own preference.

You're probably right, Paul, that, with the extra reinforcement of biscuits or splines, creep isn't an issue anyway.

Another thing I've noticed with PVA is that, in my experience, it swells biscuits more so than cascamite and can leave almost-invisible 'lumps' in the surface which require a bit of extra work to remove... I fear this is just me though, as no-one else I've ever mentioned this to seems to have experienced the same thing... :wink:

 

[Paul Chapman]

Another thing I've noticed with PVA is that, in my experience, it swells biscuits more so than cascamite and can leave almost-invisible 'lumps' in the surface which require a bit of extra work to remove... I fear this is just me though, as no-one else I've ever mentioned this to seems to have experienced the same thing... :wink:

This swelling of the biscuits telegraphing through to the surface depends on the material being used. In my experience, it's worse with chipboard, less of a problem with MDF and virtually not a problem with solid wood.

It's simply a product of the localised moisture and the biscuits swelling. The answer is to let the moisture fully dry out before sanding or planing the surface. If you don't, there's a danger of sanding or planing off the bumps, then the moisture dries out and you end up with depressions where the bumps were.

I think it was Fine Woodworking magazine that did a comprehensive test of all this several years ago. If the swelling is a problem, you really need to let it dry out for a few days.

Hope this helps.

Cheers :wink:

Paul

 

[Benchwayze]

Personally I would have thought that Paul's double row is a slight overkill.

Andy

Maybe Andy, but if you look at some of Brunel's designs, you could say the same thing. :wink:

Regards
John

 

[fobos8]

thanks for the replies.

will go with a double row but gonna stagger them.

gonna glue up in garage but will put a small fan heater on to help glue cure.

How long do I need to clamp them up for with PVA?

Andrew