A New Manual for Biscuit Joiners

Don’t waste your time measuring and marking the exact locations of the plates. Hold the parts together and make your marks by eye.

Don’t waste your time measuring and marking the exact locations of the plates. Hold the parts together and make your marks by eye.

If you need to make a joint in the middle of a piece, such as a fixed shelf in a cabinet, clamp a guide block in line with the location of the bottom of the shelf and make the cut by plunging vertically. The guide registers the cut, and lets you use the broad face of the tool to hold it steady while you make the cut. Mark your registration lines on this block if you will be making repetitive cuts.

The other useless, obsessive activity people engage in is measuring the exact location of each slot. Just make a series of pencil marks by eye (as shown in the photo at right) to locate the cuts. It won’t make any difference in the finished joint if there is some variation in the distance from plate to plate. And again, the variation in the locations will prevent you from putting parts together in a way you don’t want them.

Get a Grip
To get a good cut, the piece that gets the slot needs to be firmly clamped to the bench. If you try to hold the work with one hand and plunge the tool with the other, you’re giving away half the control you have. Put both hands on the machine, and put them in the right place. The handle on the top is there for a reason.

With both hands on the tool, you have more control. When you place the tool on the work, push down with the hand on the handle before pushing in with the hand on the barrel.

With both hands on the tool, you have more control. When you place the tool on the work, push down with the hand on the handle before pushing in with the hand on the barrel.

Most people ignore the handle and hold the fence down on the work with one hand. This tends to be self-defeating as you’re still likely to tilt the tool as you make the plunge. It also introduces some risk; you’re putting your hand in the path of a spinning saw blade. Accidents with biscuit joiners are rare, but if you put your hand on the handle you gain better control and you eliminate the risk of injuring yourself entirely.

Most biscuit joiners have pins or some other device to prevent the machine from shifting sideways as the saw blade moves into the work. When you put the tool in position, put it down, then move it in. This will keep t
he pins from engaging before you have the fence or base of the tool all the way down.

When you’re ready, with the work firmly clamped and your hands on the tool, lock your arms before you pull the trigger. Make the cut by shifting your weight forward from one leg to the other as seen in the photos above.  If you keep your arms immobilized while plunging, you won’t tilt the tool as you make the cut.

Apply downward pressure with the hand that’s on the handle, and forward pressure with the other hand. Take a moment to make sure the fence is flat on the work and the face of the tool is against the edge you want to cut before you turn on the motor.

Another effective way to make a cut without putting your hand at risk is to hook your thumb over the top of the handle, with your fingers at the top of the fence. From this position, you can plunge the cutter into the work by squeezing the fence and handle together.

When you make a cut vertically at the edge of a board, only a portion of the tool is in contact with the wood, and the weight of the machine is almost all beyond where the cut will be made.
I extend any registration marks so that I’m lining up the cut with the base of the machine, not the front. This keeps my head and shoulders above the tool. I push down with the hand on the barrel of the tool, and in with the hand on the handle as seen below.

To keep from tilting the machine when making a vertical cut, extend your pencil marks so you’re lining up with the center mark on the base, not on the fence.

To keep from tilting the machine when making a vertical cut, extend your pencil marks so you’re lining up with the center mark on the base, not on the fence.

The action of the saw blade plunging into the wood creates a lot of dust and wood chips. Most machines have a dust bag, but these tend to clog in the narrow chute between the blade and the bag. It’s still a good idea to use the bag, even though you’ll need to stop and clean it periodically.
If you’re plunging horizontally you might want to remove the bag, but going vertically the exhaust chute will be aimed at your face. Be mindful of the spinning blade as you put the machine down. Loose tools on the bench can be struck by the blade if you’re not careful setting it down.

Putting It All Together
One of the truly clever elements of biscuit joinery is the plates themselves. The textured surface is the result of the biscuit being compressed during manufacturing. When the plate gets wet, the wood swells up, resulting in a tighter joint, as seen above right.

Since most common woodworking glues are water based, this swelling occurs right after assembly. If you’re using a glue that doesn’t contain water, such as polyurethane, there won’t be any water present to swell the biscuit. A light mist of water sprayed on the plate, or a swipe with a damp rag just before inserting the biscuit in the slot will ensure that the plate swells.

The downside to this is that the plates can swell enough when exposed to damp humid air to make insertion difficult or impossible. Keep them in an airtight container or Ziploc bag and they won’t get fat on you.

Most of the weight of the tool is off the work, so it’s easy to tilt it in that direction as you plunge. Push in with your handle hand and down with the other hand.

Most of the weight of the tool is off the work, so it’s easy to tilt it in that direction as you plunge. Push in with your handle hand and down with the other hand.

If you’re just getting used to biscuit joining, or haven’t used your machine in a while, it pays to make a dry-run assembly to make certain that everything fits together the way you want it to. If you need to make a correction, you don’t want to try to do it with your glue drying and biscuit plates swelling.

If you have a miscut slot, you won’t be able to make an adjustment without making the slot too wide. Glue a biscuit in the slot; after the glue dries, trim it back to a flush surface. I look at the way the grain runs on the plate, and use a chisel pointed “uphill” to split it off just above the surface, then I trim it back to flush with a paring cut with the chisel. Once the hole is filled, the slot can be recut properly.

I’m careful about how much glue I use, and where I put it, as shown in the photos below. I put a bead of glue at the top of each side of the slot, staying about 1/2″ away from the ends. I put glue in all of the slots before inserting any of the plates. This lets the glue run down the sides of the slots, coating the sides of the slots.

When the biscuit goes in, I’m looking for the glue to coat the sides of the plate without squeezing out all over the place. If you put a plate in a glued slot and immediately pull it back out, you’ll be able to see if you are applying the right amount of glue.

The joining plates are usually compressed  by machine when they are made, and swell up as water from the yellow or white glue enters the biscuit. This is a dry plate.

The joining plates are usually compressed  by machine when they are made, and swell up as water from the yellow or white glue enters the biscuit. This is a dry plate.

I put the bead of glue for the other slot of the joint on the biscuit plate, as seen below right instead of in the empty slot. This keeps the glue from running out of the slots when the piece is turned over for assembly.

In addition to being messy, there is another negative consequence to using too much glue on the biscuits. The moisture in the glue has to go somewhere, and as the plate swells this moisture migrates into the surrounding wood. If it’s solid wood, and the slot is near the surface, the surface of the wood can swell directly above the biscuit. Because of the swelling of the plate, the joint will stay together after a short time in the clamps. Many people will proceed without waiting for the glue to dry.

This swelling will disappear when the moisture evaporates. If you plane, sand or scrape the surface above the biscuit without waiting for the glue to thoroughly dry and the excess moisture to evaporate, you can end up with biscuit-shaped depressions in a day or so. You can prevent this by avoiding excess glue and waiting at least overnight for any assembly to dry before further processing.

The biscuit joiner may not be the best choice for every situation, but it does deserve a place in almost any woodshop. When you know the secrets of using it, that place won’t be on a shelf, gathering dust. PW

COMMENT