How to use a Biscuit Joiner
Woodworkers know that there are many approaches to joining two pieces of wood together. The key is to choose the appropriate joint for the application at hand. Sometimes a project might be subjected to heavy stress during its useful life (dining chair, step stool, etc.), and under such conditions you might call upon the industrial strength of mortise and tenon joinery, dadoes or other robust mechanical connections. On other occasions, you might choose to make the joinery itself a showcase portion of the project, in which case you might choose the timeless beauty and symmetry of dovetails or finger joints. But sometimes you are looking for a joinery method that is quick, simple, and strong enough for light to medium duty applications. In these situations, is when you want to turn to a biscuit joiner. Biscuit joiners are extremely helpful when it comes to making boxes, drawers, cabinet carcasses, face frames, miters, and edge banding. Believe it or not a biscuit joiner can even be used as a substitute for a mortise and tenon joinery as well.
It’s no secret that nothing beats biscuit joinery when it comes to quick and simple woodworking joints. Just mill slots in the mating pieces, apply glue, slip biscuits in place, assemble, clamp, and done. In about a minute, your joint is perfectly aligned, adequately strong for many applications, and the horizontal wiggle room provided in the joint makes it easy to make tuning adjustments at assembly time.
Let’s walk through some of the joinery applications where biscuits can be used but before we build, let’s talk about a few biscuit joiner tool best practices.
First thing first, you always want to check your depth of cut. Our Digital Fractional Caliper 3 Way/ 6” (MLCS item #424-2410) can help you determine your exact depth. Make a practice cut with your joiner to make sure it will give you the exact cut your looking for. The biscuit joiner should cut a slot 1/32” deeper than half the width of your biscuit when it’s adjusted properly. This provides the tiny bit of wiggle room, and the perfect area to place your glue. If your biscuit joiner is set to deep, the biscuits will be too far embedded on the one piece and won’t give the holding power that it should. At the same time, if it’s too shallow, the biscuit won’t fit properly. Most of the time this won’t be noticed until the joint fails.
The best way to test this is to mill a slot in a piece of scrap wood. Then, insert the biscuit into the slot, gently tapping it in until it bottoms out. Then, use a sharp pencil to draw a line across the biscuit where it meets the scrap board. Next, remove the biscuit, insert the opposite edge into the slot and draw another line.
Your first line should be hidden in the biscuit slot, and if it is not, you will need to adjust for a deeper cut. Remove the biscuit and measure the distance between the lines. If it is approximately 1/16″, you are ready to start milling biscuit slots. If not, refer to your owner’s manual to get instructions for adjusting the depth of cut on your specific machine. Also, be sure that your fence is square to the face of the machine when it is set at 90-degrees, as an out-of-square condition can cause sloppiness in the biscuit slot.
A word about gluing
It is important to get an ample amount of glue into the joint when using biscuits because biscuits absorb glue, which causes them to swell and provides some of their holding power and precision alignment. If you fail to get enough glue into the joint, you can end up with a joint that is prone to failure and poor alignment. There are a number of ways to get an adequate supply of glue into the joint. You can simply place the tip of your glue bottle, like the 3-in-1 Glue Bottle (MLCS item #9175) over the slot and squeeze the glue in, but that can be a bit messy and uneven. You can use our High Pressure Glue Injector (MLCS item #9305) which fits on the glue slot and delivers a perfectly portion shot of glue into the slot. Another way is to use one of our Glue Spreaders from the Glue Spreaders. 6 pack (MLCS item #9962) and a glue bottle. The flux brush spreads the glue evenly within the slot, getting it up onto the walls where it can coat the biscuit. You can also apply a bit of glue to each side of the biscuit to ensure good coverage. Here’s the trade-off: too much glue and you have a mess on your hands. Too little glue and you have a starved joint, and equally bad, the glue can set up so quickly during glue up that you lose the ability to make lateral adjustments. Generally, you want to opt for a bit more glue, which yields a solid joint, and provides more open time during assembly. It can be helpful to use a glue with a bit more open time for biscuit assembly, particularly for a complex assembly.
3-in-1 Glue Bottle High Pressure Glue Injector Glue Spreaders - 6 pk
MLCS item #9175 MLCS item #9305. MLCS item #9962
Choose the right biscuit size.
The rule of thumb here is simple; use the largest biscuit possible for your application. Most of the time use the largest common size; #20 (1″ x 2-3/8″) (1/4 shank - MLCS item #6065 ½ shank MLCS item # 8365).Occasionally if your working with thinner or narrower material, you want to scale back to a #10 (3/4″ x 2-1/8″) (Eagle America item #4466-1100) or #0 (5/8″ x 1-3/4″) For the smallest joints, FF (FaceFrame) biscuits (1/2″ x 1-11/64″) which are great for picture frames, small boxes, or face frames as we will use them in this article. Most joiners will accommodate the three most common sizes; #20, #10 and #0. Some will also cut a slot for FF biscuits, but a blade change will be necessary as the profile for FF biscuits is more round than football shaped, requiring a much smaller diameter blade. If a #20 biscuit will not fit fully within your joint, you may choose to trim off the portion of the biscuit that would protrude from the joint if that area will not be visible. As an example, this may be possible in smaller face frames if they will be painted (and therefore hidden using filler) or on a narrow carcass that will be covered by a face frame. This gives additional strength by providing a deeper mechanical connection within the joint.
Using the Biscuit Joiner
Make sure your work is firmly clamped to the bench, freeing up both hands for controlling the joiner. As you begin to setup for your cut, start by carefully positioning the biscuit joiner to align with your biscuit placement mark. Start with the handle high and the tip of the fence on the work piece. Slowly lower the handle until you feel the fence seated solidly on the work piece. Once you get solid placement of the biscuit joiner and fence, find the detent on your biscuit joiner’s fence (most have this), place your thumb firmly on the detent, and let go of the handle to allow the biscuit joiner to “bottom out” on the work piece. This extra step is one more check to ensure that you have proper positioning of the tool against the work piece. Then place your hand back on the handle, and slowly engage the power switch with barely enough pressure. Visually inspect the fence to verify that it is still solidly placed flat against the work piece. Then slowly plunge the cutter into the material with a steady motion, being careful to not rock the tool as it enters or exits the cut. The first couple times you do this, you will want to practice on scrap material, moving it in and out of this position before making cuts so that you can begin to feel the difference. After you have some experience with this it will become second nature. Remember, biscuit joinery is fast enough, so take your time during the plunge and your biscuit slots will be positioned correctly in the work piece. If you rush here, you will find yourself taking far more time to fix it later.
When to use biscuits.
This has been a highly-debated topic to say the least. The answers range from, “Don’t use them at all because they don’t add any strength in any situation” to “They are great in nearly every application. When considering biscuits relative to other types of joints, ask yourself whether glue and biscuits are strong enough for the application at hand. If you know that a joint will undergo a lot of torture, such as the leg/stretcher joint on a large table or chair, then use heavier duty joinery for that application. But for accent tables, small boxes, cabinet carcasses, etc., your fine to incorporate biscuit joinery into the project. Ultimately there is no clear-cut answer, just decide for yourself on a project to project basis.