Break free from biscuit blunders
Thinking ahead pays off when joining workpieces with biscuits. By properly positioning your slots you'll enjoy perfect-fitting joints every time. Regardless, remember the best tip for all biscuit applications: Make test cuts on scrap pieces before cutting the real thing.
Mistake #1: Exposed biscuit slots
Cutting through the face of a workpiece's beveled end, as shown in the opening photo, causes damage nearly impossible to repair. This goof happens when you cut a slot for a large (#20) biscuit at the midpoint of the bevel in 3⁄4 "-thick stock.
Solution: Avoid this mistake by cutting the slot closer to the inside corner of the beveled end. To do this, adjust your joiner's fence, as shown above, to move the slot closer to the inside corner. This allows you to still use a #20 biscuit without cutting through the face. (You should always use the largest biscuit possible for maximum holding power.)
Mistake #2: Mating surfaces misaligned
If the mating biscuit slots you cut don't match up perfectly, the workpiece surfaces will not be flush, as shown above.
Solution: First, reference your cuts from the same workpiece surface (the top face). Second, don't use your joiner's base and fence together for alignment when cutting the slots; the two might not make parallel contact, resulting in uneven slots. Instead, reference your cuts either with the base resting on the benchtop or worksurface as shown above right, or with the fence resting on the face of the workpiece.
Mistake #3: Crooked or unsquare slots
The start-up torque of a joiner can cause it to lurch when powered up. Resulting slots might be unsquare, as shown above.
Solution: Never make field cuts (in the interior of a workpiece) freehand. Instead, clamp a stop to your workpiece—right on the line for the bottom of the mating board—and use that as a reference point for the joiner, as shown above right. To counteract the lurching, start the motor and then line up the joiner with the mark before plunging to make the cut.
Mistake #4: Exposed biscuits
It's easy to forget where you positioned the biscuits in an edge-glued workpiece. This can lead to cutting into them when machining a profile, as shown.
Solution: Simply put, plan ahead. If you know you're going to machine the edges and ends of a panel, be sure to locate the biscuits far enough from the edges and ends so they won't be exposed.
Mistake #5: Glue-line depressions
Even though biscuit joints are strong enough to unclamp after a couple of hours, planing or sanding these workpieces right away could result in scooped recesses over the biscuits. Why? The glue around the biscuit causes the wood to swell slightly. Machining off these temporary "humps" results in shallow depressions once the glue fully dries, as shown.
Solution: Allow your glued-up workpieces to dry for 24 hours before machining. The humps will have shrunk back to normal size by then.
Biscuit Joinery Basics
A biscuit joiner (also known as a plate joiner) cuts half-oval slots in mating workpieces; then you glue in a football-shaped "biscuit" and clamp the joint tightly. (Common biscuit sizes are shown at above.) Biscuits add strength to joints and assist you in aligning workpieces. Here's how to set up a joiner to cut a typical joint.