The biscuit joint is a specific joint made by carving a small recess into two pieces of stock, then inserting a small piece of wood with glue to connect the pieces. The small piece of wood, called a biscuit, or a Lamello (trade name, and inventor of the biscuit joint) provides structural integrity to the joint.
Why Use A Biscuit Joint?
It provides numerous advantages to traditional joinery. For one thing, it is fairly quick to make, and accurate repetition is simple once the machine has been properly adjusted to the correct height setting. It's as easy as lining up the machine, and making the cuts which only takes a few seconds. As far as joint making is concerned, it is be the fastest kind that can be accomplished, short of using screws that is.
Learning how to use the tool takes only a few minutes, there is really only one setting to normally adjust. It is also relatively safe to use compared to most other woodworking tools. The blade retracts automatically within it's housing when there is no pressure being applied to both sides of the tool, so if it does slip, the blade is exposed for only part of a second before the tool can injure a hand for example.
Of course, a joint wouldn't be worth anything if it wasn't strong enough to hold itself together. Well, a biscuit joint is strong. O.k. fine, it's not as strong as a mortise and tenon, or dovetail joint for that matter as it only has the glue strength and not any mechanical strength holding it together. But that alone is sufficient. In most cases, the glue bonding strength is stronger than wood itself and breaking a joint will result in the wood cracking, and not the glue giving out.
How Does A Biscuit Joint Work?
It's all in the magic of the joiner. Well alright, it's not magic but it is really cool. The biscuit joiner (or plate joiner) is like a plunge router and table saw put together into one machine. The spinning blade is recessed into a compartment, and when you push forward on the handle, the blade comes out and rebates the wood. The cutting action can be applied at various angles to make many different kinds of joints.
You then apply some glue (usually PVA, white or yellow) onto the biscuit and insert them into the slots. You'll need to provide some clamping pressure, as with all standard woodworking joints. Below is a quick video of how one would create a simple 90 degree butt joint using the joiner.
What Exactly Are These Wood Biscuits Anyway?
Biscuits are traditionally made out of compressed beech wood, but they can be made from various woods (second most common is birch) or even synthetic materials for special hookups (more on that later). They are compressed to an exact thickness so that they fit properly in the slot. They also have the grain running diagonally within the biscuit so that there is no weak point if the joint is stressed. When glue is applied, the wood fibers of the biscuit absorbs the moisture and expands within the cavity that it sits in adhering the two pieces of materials together.
They come in three basic sizes that vary by width and thickness. Depending on the amount of space on the surfaces to be joined together, you will have the option to use different sizes such as:
#0 Size - 5/8" x 1-3/4"
#10 Size - 3/4" x 2-1/8"
#20 Size - 1" x 2-3/8"
When Should I Use A Biscuit Joint?
A biscuit joint can be used in place of many of the traditional joints. In the picture you will see the most common uses of biscuits.
Edge to Edge Joint: This is great for attaching two flat pieces of stock together in the same plane. It's great for use with MDF, plywood, or particle board which would not be able to be glued effectively together. As well, although it would not be needed structurally with a wood to wood glue up, sometimes biscuits are used to help with alignment, as it will stop the pieces from sliding apart from each other.
Butt Joint: Either the corner or Tee versions work well with biscuits.
End Grain Gluing: The end grain of stock does not glue well, but the addition of a biscuit or two will solve that problem.
Miter Joints: Provides great alignment and structural help.
Special Biscuit Joints
Instead of using a regular wooden biscuit, there have been a number of advancements off of this simple idea. There exists now detachable biscuits, basically a plastic biscuit that is separated into two parts and can be secured and tightened after the panels have been placed adjacent to each other.
No glue or clamping is necessary for this setup. It's a great system for reducing transportation costs, giving the ability to ship flat panels instead of pre-made boxes. The boxes can be built up at another location with minimal fuss. The video below of the Lamello Clamex system demonstrates one application of their use.
Problems With A Biscuit Joint
Ok, we all know the theory at this point, but what about the practice in a workshop setting. Is a biscuit joint really that easy to execute? There is really only one setting on the joiner, and that is of height. You'll usually want to put the biscuit in the center of the panel thickness, so once you get that properly set, what else can go wrong?
Mark Correctly Before Slotting
The key to making a good biscuit joint and avoiding problems is to double check the alignments. You will be making a slot on each of the two workpieces you will be joining together. The slots must be matched up on each component. There is some play however to allow for a slight misalignment. Just measure and mark your biscuit center in the right place on both pieces.
If you've really messed up and the glue is already applied and you don't want to cut another slot out, you can always not put a biscuit into the slots that are not lined up (if you have multiple biscuits in a row), or use a smaller biscuit if you have one that fits, or break the existing biscuit so that you still have some piece of it doing its job.
The Biscuit Shadow
Also, once in a while you have what I'm calling the shadow biscuit effect. As was mentioned, in order to do its work, the biscuit swells up as it absorbs the water from the glue. Well, if you can picture it, the wood (or fibers) in surrounding area also start to swell up as it absorbs some of the moisture too. So, temporarily before the glue is completely dry, you'll have a small space where the biscuit lays hidden within the joint, that is a bit expanded. On a flat panel, of say 3/4 or 5/8 inch thick, it may be more pronounced although you probably would not notice it.
This is all normal, and everything will go back to normal once the glue has dried properly. The problem lies in if you start to sand the area before the glue has dried. Since the area around the biscuit might still be raised a slight amount, the sanding will make it flat again. But now, the biscuit is still drying, and shrinking, which causes a depression in the panel where the biscuit lays. If you then put a finish on, especially a highly glossy one, you will notice the depressed shadow of where the biscuit lay hidden. Let me mention that this is a rare occurrence, but it does happen. The easy answer is just to let the glue dry completely before you work on the area.
Using the Wrong Glue
As mentioned, PVA glue is the go to adhesive in this joint. Some woodworkers might prefer some other glue to work with though depending on the final resting place of the finished product. In this case, if a polyurethane glue is being used for example, you will need to apply water to the joint and maybe soak the biscuit for a dew minutes to get a secure joint. You just need to know how your glue 'works' if it is different than PVA.
Ok, You've Had Enough . . .
For a simple joint, there sure is plenty to say about it. The only important thing that was not covered in this article about the biscuit joint was how to set the adjustments on the biscuit joiner tool itself. That information would be specific to each tool, so it's not covered here. So no matter how you call it, a biscuit, a Lamello, a plate joiner, it really is one of the easiest woodworking skills and tools to learn.