A plate joiner, often referred to as a biscuit cutter, is a tool that has little use beyond the one task that it is designed to complete, but it performs that task so incredibly well, that it is one of the few single-task woodworking tools that I'd recommend for every workshop. This specialized mini-saw cuts thin slots in the edges of stock to hold a biscuit, which is used much like a dowel to hold two pieces of stock together.
What is a Biscuit?:
Before we can discuss the how to use a biscuit cutter or plate joiner, it helps to know what a biscuit is. Biscuits are thin, football-shaped slices of wood, typically made from compressed beech wood. A biscuit is glued into a slot in one edge of a board, and then into a corresponding slot in the adjoining board. Biscuits are particularly useful in cabinet construction or when glueing up a few individual boards to make a table top.
Why Use a Biscuit Cutter?:
I've spoken to many woodworkers who have used a router with a slot-cutting bit to cut grooves for biscuits, but I would strongly advise against it. First of all, it is difficult to consistently insert a slot-cutting bit on a router into the edge of the stock perfectly perpendicular to the edge of the material. Additionally, I'm of the opinion that this is a rather dangerous procedure with a router.
On the other hand, a biscuit cutter or plate joiner eliminates these concerns.
The blade, which spins at speeds up to 10,000 RPM, is completely housed within the guard of the saw until it is plunged into the stock. Secondly, the guide system of the biscuit cutter practically guarantees that it will be cutting perpendicular to the edge of the stock, which ensures a consistent fit.
Using a Biscuit Cutter:
To use a biscuit cutter or plate joiner to connect the edges of two boards, begin by checking the boards to make sure that they will line up properly. The two pieces of stock should be of the same thickness and should make consistent contact across the entire length of the joint. If the two edges don't match up, a pass through a jointer to machine-plane the two pieces of stock will ensure that you have two straight edges.
Once you have two straight-edged boards, place them on a work table in what will be their final positions. With a pencil, make a few small, evenly-spaced marks across the joint to denote the location of each biscuit on each board.
Now, set one board to the side. Using the biscuit cutter, place the guide fence flat on the top of the stock and line the cutting guide with one pencil mark. Start up the saw, and plunge the blade into the stock up to the stop. Remove the blade and repeat at each mark.
Once the cuts are completed in one board, switch to the other board and cut the corresponding slots.
When all cuts are finished, place some glue in the slots of one board and insert a biscuit in each slot. Then, put glue in the slots of the other board and slide the second board onto the biscuits. Use clamps to hold the joint while the glue dries, but take care that you don't tighten the clamps so much that you squeeze all of the glue out of the joint.
Biscuit Cutter Safety:
While the safety rules for woodworking tools are usually basic, common sense, they always bear repeating. First, be sure to read and follow all of the safety rules that come with the plate joiner's instructions. Second, always wear appropriate safety gear, such as safety glasses, when using your plate joiner. Also, be certain to use only sharp blades in your biscuit cutter, and never operate the tool with the blade guard removed. Be certain that the motor is at full speed before plunging, and never apply side pressure to slow down a spinning blade.
Most quality plate joiners have an adjustable depth scale, a sawdust port for dust collection and a bevel feature for cutting biscuit slots at angles of up to 45-degrees. Some plate joiners also have the ability to cut grooves in the edges of stock, but in my opinion, there are other tools that are better and safer for cutting for tongue-and-grooves joints.