Biscuit Jointer

[Garno]

I have a biscuit jointer that I may have been using wrong.

I have always been wondering how to use the thing correctly and so today plunged into the deep pool known as youtube.

I have always cut the holes with the jointer, inserted the biscuit after gluing the holes,
So far so good but it is this next step that myself and those on youtube seem to have a difference of opinion.
The youtubers I have seen seem to only glue the holes (inside) and the biscuit and the join the wood together.
I on the other hand, glue the holes (inside) put more glue on the biscuit and then glue the full length of the edge before joining and clamping the wood.

What is the correct practice and how far apart should the biscuits be placed?

 

[spb]

Biscuits aren't there to join the pieces together, but to keep them aligned properly while the glue on the joined edges sets. The strength in the joint comes from the full length of the edge being glued; whether there's glue inside the biscuit slots doesn't really change much.

Dominoes or other loose tenon systems are a different matter; there, the piece of solid wood joining them does contribute to the joint strength and needs to be glued to gain maximum benefit.

 

[Inspector]

spb I disagree with you. Biscuits were invented to replace dados in sheet goods for making kitchen cases etc. So they are a structural joint. They made their way to solid wood and can still be used structurally, butt joints for example where the end grain would never hold otherwise. Their use in gluing long grain to long grain is just for alignment.

Garno put a little glue in the slots and on the mating surfaces but not the biscuit. Insert the biscuits and get the clamps on. When you put glue on the biscuit it starts to swell from the moisture in the glue to lock it tight in the slot. That can, on a large glue up make it hard if almost impossible to get the joints tight.

Pete

 

[Spectric]

Hi all

I glue the lot and clamp ASAP but don't rely on the biscuit for strength, it helps keep alignment while I clamp. For stronger joints I use dowels or loose tenons.

 

[Inspector]

Back in the olden days before Youtube Fine Woodworking Magazine had an article comparing different joints. Mortice and tenon, dowel, loose tenon, etc, joining short grain to long like you would in a cabinet door. They put them in a compression testing machine, testing the joints to destruction and the biscuits were only topped by the mortice and tenoned joint, equal to loose tenons and better than the doweled joint. Don't underestimate the strength of biscuit joinery. I used them on an oak screen door that I went through every day for 25 years without any sort of failure. I had the springs set so it slammed when shutting. I took it off the house when I sold it to bring here but haven't put it up because storm doors are more appropriate here. People have successfully used them to make chairs.

Pete

 

[Racers]

I have made bedside tables and big ply draws using biscuits as joints and they are holding up fine, they are quite strong.

Pete

 

[Eric The Viking]

If they weren't intended to make some contribution to joint strength, then there would be no reason to align the biscuit's grain so it runs across the narrow axis.

Mind you, I watched someone on youtube yesterday who plunged the jointer four times per hole. Yup, honestly. And he was relying on the fence for alignment, too, not the baseplate on the bench. I think the slots must have been 5 or 6mm wide by the time he was done.

 

[custard]

In 90% of applications I'm a proponent of gluing both the tenon and the mortice. Festool did some tests once and claimed 30% additional joint strength by doing this.

However, when it comes to biscuits I would only ever glue the mortice. The biscuit is designed to swell from the water in PVA, biscuits are also designed to give you some alignment flexibility. But if you glue the biscuit as well as the mortice the biscuit will swell up and make adjusting the joint very difficult if not impossible. Seems to me that gluing the biscuit runs counter to getting the best out of the machine's unique qualities.

Just my opinion.

 

[Phil Pascoe]

In 90% of applications I'm a proponent of gluing both the tenon and the mortice. Festool did some tests once and claimed 30% additional joint strength by doing this ...

So do I - but most glues specifically instruct you to apply to one side only. This has been raised over the years and no one to my knowledge has been able to explain why.
As an aside - the first time I used biscuits in serious joinery they were fitted tightly but pulled the surface of the wood (mahogany) slightly hollow above every biscuit. No one has ever explaned that either.

 

[Droogs]

Phil watch this and get your answer

 

[Phil Pascoe]

That makes sense - except biscuits swell with moisture, they don't shrink with it, and this showed before I'd even finished the piece. Odd.

 

[Sgian Dubh]

As an aside - the first time I used biscuits in serious joinery they were fitted tightly but pulled the surface of the wood (mahogany) slightly hollow above every biscuit. No one has ever explained that either.

The explanation for that phenomenon is straightforward. It's caused by prepping and finishing the assembled panel too soon after glue up; I'm assuming you're talking about panel assemblies such as table tops. Adding moisture (water based glue) to prepared edges, the biscuits, and the biscuit slots causes the wood to swell locally to a greater or lesser extent. The wood away from the glued up edge and slots doesn't get this added glue moisture, so the MC there remains pretty much unchanged. After glue up the panel needs time for that localised moisture induced swelling, including that taken on by the biscuits, to lose its moisture again and to shrink as the wood comes to a wood moisture content condition that pretty much matches the wood away from the glued edge.

It's best, in my experience, to set aside a glued up panel (stickered ideally) for two or three days prior to prepping and polishing, and a week is even better, to allow the wood MC at the joint to equalise throughout the panel: or as equal as is practically possible given the fact that all wood always has a moisture gradient.

It's not just rugby ball shaped depressions that can be seen when using biscuits for edge joint alignment, and/or in a perceived attempt to add strength to the joint, but similarly depressed furrows tracking the glued edges of standard plain edge jointed panels can show up too, as will the position of dowels and even loose splines whether they're solid wood or man-made board, etc.

The message is generally, 'Don't be in a hurry, if at all possible, to prep and polish a panel after glue up'. Slainte.

 

[MikeJhn]

I have made bedside tables and big ply draws using biscuits as joints and they are holding up fine, they are quite strong.

Pete

They must be very uncomfortable to wear.

 

[Phil Pascoe]

The explanation for that phenomenon is straightforward. It's caused by prepping and finishing the assembled panel too soon after glue up; I'm assuming you're talking about panel assemblies such as table tops. Adding moisture (water based glue) to prepared edges and the biscuit slots causes the wood to swell locally to a greater or lesser extent. The wood away from the glued up edge doesn't get this added glue moisture, so the MC there remains pretty much unchanged. After glue up the panel needs time for that localised moisture induced swelling to lose its moisture again and to shrink again as the wood comes to a wood moisture content condition that pretty much matches the wood away from the glued edge.

Yes. Of course. Thank you.

 

[Droogs]

To me, it also seems to be that kiln dried wood (a lot) more so than air dried is affected by this. It may be that it is due to it being at such a much more drastically lower MC. I also wonder if due to this, when using water based glues, that they are drawn much more deeply into the fibres which then sets and cures before the surrounding wood has a chance to equalise. thereby "locking in" the depression?

 

[Sgian Dubh]

To me, it also seems to be that kiln dried wood (a lot) more so than air dried is affected by this. It may be that it is due to it being at such a much more drastically lower MC. I also wonder if due to this, when using water based glues, that they are drawn much more deeply into the fibres which then sets and cures before the surrounding wood has a chance to equalise. thereby "locking in" the depression?

I don't believe the dryness you describe is relevant, and I can't see how that 'dryness' has a quantitative or measurable impact on how far any moisture is 'drawn' into the wood; the actual amount of water available through glue application to wood is really rather small in a glue up, whatever the wood's MC below 20% MC, the upper MC limit for use of wood adhesives (with the exception of polyurethane adhesives, which don't count as far as adhesive moisture content is concerned anyway).

As before, I'm sure the key to avoiding those visible oval depressions and furrows that track glue lines in panel glue ups is allowing enough time between glue up and surface prepping so that water adsorbed into the wood can later desorb, thus allowing the wood to shrink down to dimensions that match the rest of the board's thickness. If you prep and polish immediately (or too soon) after gluing up the panel, the area immediately either side of the joint and where the biscuits are will shrink later, and are likely to show, sometimes hardly at all admittedly, and sometimes the effect is rather pronounced, and most likely to be noticeable on horizontal surfaces where light is more likely to bounce off the surface into the viewer's eyes - so, best to let the wood shrink prior to the prepping and polishing. Slainte.