Biscuit Jointers - An Overview

Biscuit jointers (flat-plate dowelling machines) may seem to be a fairly recent invention, but they've been around for fifty years. In 1955, Hermann Steiner, a Swiss cabinetmaker and engineer, came up with the idea for a machine to cut a slot for use with a type of spline for a joint - he developed his idea to completion and then went on to found the Lamello Corporation.

Biscuit jointers have come to prominence in recent times due to their frequent appearance in DIY and woodworking television programmes and this publicity raised awareness, then demand, swiftly - resulting in a wide choice of makes (and qualities) of machines becoming available. This page is intended to explain how to get the best out of these useful tools.

What are they?

They are simply a 100mm x 4mm slot-cutting blade which is protected by a metal housing. The blade is spun (clockwise when viewed from above) at approximately 10,000rpm by an electric motor and the blade protection housing is kept safely in its forward position by a spring-loaded mechanism. The spring is overcome by pushing against it and the tool is designed to allow the spinning blade to protrude when operated (plunged). The amount that the blade protrudes is adjustable and it is this adjustment that determines the depth of the resulting slot in the workpiece. The slots produced are arcs of the same radius as the cutting blade, so the more the blade protrudes, the deeper and wider the slot becomes.

The blade protection housing is removed here, to show the cutting blade. Many have four teeth, some have six (preferred). The most durable blades of all have scoring teeth and replaceable carbide cutters.
Smaller diameter blades are also available, especially useful for small face frame work.

This picture is also a tribute to effective dust clerance (see later).

Here is the blade in the "at rest" position when viewed from the front. Also seen in this picture are the measuring scales for the horizontal fence and the two oval rubberised pads at the lower outer corners of the blade housing. These are designed to help avoid any sideways motion caused by the spinning blade coming into sudden contact with the workpiece and being deflected to the left (again, when viewed from above). Note also the indented index marks for the centre of the blade (and the maximum cut size). The center marks are provided on the top, bottom and one side of the housing on this model.
This picture shows the adjustable horizontal fence fitted and set to place the center of the slot 10mm below the face of the workpiece. Index marks for the measuring scales are provided on both sides of the fence to remove the possibility of skew. The fence is tightened by a dovetailed cam lock (top left as viewed). The fence also has the center of cut and maximum cut size index marks on its leading edge and a viewport at the center.
Here, the vertical fence is shown at one of its five pre-set stops (45), but it can also be locked at any desired angle. Whilst this feature is generally available, it's quite rare to find it on "budget" versions.

Whilst the vast majority of biscuit joinery can be achieved with 90 and 45 angles, making hexangonal, octagonal or triangular boxes, for example, definitely needs the "any angle" feature.

What do they use?

Most biscuit jointers are designed to use three popular sizes of 4mm biscuit:

Other sizes may be available (as they are in Europe and the UK), for example: "S6" - 85mm x 30mm and "H9" - 38mm x 12mm (the H9 is only 3mm deep, so is used with a different blade). Here are the different sizes compared visually and then the three common sizes shown, first separately and then laid one over the other:

What determines the depth of cut is how far the blade protrudes through the face of the housing on its cutting stroke (or plunge). Most machines are fitted with a simple mechanism to control this, the commonest type being a dial, marked with the biscuit size. The dial controls three (or more) depth stops, pegs or recesses, which act as the end stop for a datum pin as the machine is plunged against the spring. Some datum pins are also adjustable (with an Allen key on this machine), either for fine tuning, or to accommodate other sizes of cutting blade and biscuit (note that this model also has a pre-set for the S6 biscuit, plus a 'manual' setting).
You can see the difference in setting for size "0" and "20" by comparing these two pictures (ignore the dust - this is a hard-working machine!):
The datum pin will meet resistance earlier when set to "0" than it will when set for "20," hence the setting for "20" will produce a deeper cut than when set for "0."
Most would recognise this picture as showing the "normal" way to use a biscuit jointer - with the fence on top of the workpiece to set the distance of the cut from the top. There will be no problems in using the machine this way if it is used correctly, but the slightest amount of lean, as shown here, will mean that the biscuit will not sit parallel to the face and the boards will not line up correctly. It would appear to be comparatively simple to keep the fence flat to the face by downward pressure on it, but on some "budget" machines, the fence itself can flex due to the weight of the main part of the machine.
Depending on the size of the stock, it is perfectly possible to remove the horizontal fence entirely and use a flat work surface to remove any possibility of lean. In this picture, the machine is sitting on a bare flat surface, but it could equally well be shimmed up on a thin piece of flat material, such as masonite. Referencing the slot upwards from the bottom of the piece is not only as easy as more conventional methods, but once you get used to it, it's far more reliable. For a double row of biscuits in thicker workpieces, lay out, plunge one row, turn the workpiece through 180 then plunge the second row - it's a breeze!
The exhaust end of the operation, fitted with the straight outlet (this model also comes with a cranked outlet). I attach this to a shop vac via a small adapter and it works very well. The reason for the shop vac is to save any clear-up, as the chippings certainly fly a long way if you use the tool without one. Some machines come with dust bags, but these clog swiftly, which affects the finished cut, often leaving chippings or shavings in the slot, which then need to be removed manually. I would rather clear up the mess than attach a dust bag! All pictures on this page were taken immediately after cutting a couple of dozen slots, testament to the good working of the chip clearance when fitted with a shop vac.

Laying out is fairly straightforward and two common options are shown here:

With the butt joint, remember that it is also possible to mark each piece separately by measuring from the relevant face and edge. This is useful if the stock is large and difficult to hold whilst marking. With any panel-making project the same is true, but as you will want to lay the boards out to match grain, true up edges, alternate growth rings, etc. it's a simple matter to run a straight-edge across and mark up while you're at it. Remember to allow for any other milling operations you are planning - such as panel-raising - and avoid placing a biscuit where it will embarrassingly appear in the otherwise beautiful curve of the raised panel, or as an unwanted "decoration" around the perimeter of your circular table top! This is easily done, so please take layout seriously, unless you've got an endless supply of wood.

Cutting slots into knot-free softwoods, MDF or plywood is an easy task. Hardwood needs a little more care to avoid a kick from a blade plunged too swiftly. For this reason, I recommend that you treat all slot cutting with the same careful process, in order to remain safe and trouble-free.

  1. Make absolutely certain that the slot will be parellel or perpendicular (as required) by carefully positioning the tool
  2. Switch on the motor and allow the tool to come up to full speed
  3. Push the tool forward against the spring slowly but firmly - you're cutting wood, not administering an injection with a horse syringe!
  4. Once the cut is complete, release the tool in the same manner - it's a power tool, so treat it with respect.

You can just whack the blade into the stock as fast as you like and most times it will cut OK, but then you risk getting complacent and trying the same method on an expensive piece of hardwood, only to wonder why the tool kicked back or sideways, so the slot is now off position. Re-cutting a poor slot only produces a very sloppy fit - if you wanted that, why not just chisel one out in the first place? Easing the tool into the stock is also kinder on both the blade and the motor - it's best practice all round.

When cutting parallel slots, working on a flat surface and looking down on the layout marks is the best method. Perpendicular slots are more of a challenge. The natural tendency will be to plunge the machine with the tool held horizontally, as in figure "A" below. The weight of the tool will work against you because you are only balancing the fence on a narrow area of stock. If you try to counter-balance by downward pressure on the horizontal fence, as soon as you plunge inwards, the "see-saw" situation becomes unbalanced - an unstable situation.

Laying the workpiece down, then positioning the tool vertically, as in figure "B" above, overcomes both disadvantages. Now the face of the tool is resting on the stock and the weight of the tool is working with you. Holding the fence inwards and flat against the thickness of the stock becomes simple. Even the plunge is now "gravity-assisted." All you need to do is to run your layout marks around so that you can see them against the index marking(s) on the underside of the tool.

Here's a picture of how it looks in practice:
It's when you start using the tool regularly that you may want to try to extend the ways in which you can use it and some accurate measurements taken beforehand, plus a couple of test pieces on scrap will help. I'll try to explain what I mean, starting with the taking of accurate measurements.
First, remove the power cord from the outlet - safety first! Next, remove the horizontal fence and take a look at the faceplate of your machine. Take as accurate a measurement as you can of the distances marked as "A" "B" "C" and "D" in the diagram opposite. You may find that "A" and "B" are equal, also that "C" and "D" are equal too, but it is not always the case and you need to know, as you'll see.

Now we're ready to use the machine. Clamp a straight-edge across a piece of ply or MDF scrap. Draw a fine pencil line along it, then, with the horizontal fence still removed, place the underside of the machine along the straight-edge (and the pencil line) and carefully plunge a cut. Then turn the machine through 180 and place the top of the housing against the same side of the straight-edge then plunge again in another spot. Remove the straight-edge, find the centre of the cut and confirm the measurements you took previously. The cuts will line up if "A" equals "B," but if they do not, then this is important information for the future. Make a note of these confirmed offset measurements (I did my tests twice to be certain) and keep them safe as they will become very useful.

Next, re-assemble the horizontal fence and find a large box or use the inside of a cupboard if you can. Again, remove the power cord from the outlet (don't want any accidental cuts!) and place the tool into a left-hand corner, see if you could complete a plunge by looking along the left-hand side, then mark the position of the center of the blade. Measure this from the left-hand side of the box or cupboard. Now repeat the process in a right-hand corner, mark and measure again. Because of the way that fences are assembled and locked onto the jointers, it is rare for these measurements to be the same. So what? - well, when you're using biscuits to fit face frames to an already-assembled carcass, you'll now see why it becomes important to do the cutting on the carcass before assembling it, as you now know accurately whether or not you can get your machine in there. Also, if you're marking out using dead-measurement, you now know how close to a right-hand or a left-hand vertical side you can cut a slot after assembly of the carcass.

So far we've only looked at working on the edges of stock, but what about a butt joint located away from the edge - like a central cupboard divider, for example? Usually, this type of joint is made with a shallow dado, but that doesn't always look right and biscuits can offer a suitable alternative.

This application of the tool does require spot-on accuracy in the marking out and that's where the measurements taken earlier come into play, as can be seen in the examples below.

First, lightly mark the position of the vertical board on the horizontal piece (shown here as dotted lines). Mark both surfaces for biscuit slots, extending the marks on the horizontal board as shown. Measure, calculate and accurately mark the center line of the vertical board (shown in blue).

Now transfer the relevant offset measurement ("A" or "B") and clamp a straight-edge at that setting. Now you should be able to remove the horizontal fence, confidently set the machine (faceplate shown dotted) against the straightedge, align with your slot marks and plunge the slots.

Having cut the horizontal stock, refit the fence, find the center of the thickness of your vertical stock and cut the mating slots.

It is also possible to use biscuit joinery in combination with other joints, such as a rebate (rabbet) or housing joint (dado). The way this is done varies with the thickness of the stock and therefore the width of the rabbet. If using thick boards, then depending on your machine's offset measurement, it will only be possible to cut the slot in a rabbet if the rabbet itself is wide enough. If not, then just as with dado joints, the slots will need to be cut before the rabbet or dado is milled. In order to do the required calculations, we'll need to refer again to the overall sizes of the three commonly-used biscuits:

As you can see, the 20 is 4mm taller than the 10 and the 10 is 4mm taller than the 0. We can use this! Look at these examples:

Here, a conventional joint, using a size 20 biscuit. The slot is simple to cut (using methods described previously) but the strength of the joint is limited to the glue line plus the biscuits. A dado would add both glue surface and strength. Here is the combination joint. A size 20 slot is cut in the horizontal board only. Next, a 4mm deep dado is cut, centered on the slot. The vertical board now has a size 0 slot cut and the fit will be perfect. This method also works with rabbet joints.

The reason that this works may not be obvious, so here's the math:

Size 20 overall height = 23mm. Therefore, 11.5mm is the depth of the slot (half of the biscuit).
Size 0 overall height = 15mm. Therefore, 7.5mm is the depth of the slot.
A 4mm dado will inset the vertical timber so that it is 4mm closer to the bottom of the size 20 slot.
This makes the depth of the remaining slot 7.5mm (11.5 minus 4) which is a size 0 biscuit slot.

A size 10 biscuit could have been used if the depth of the dado was kept to 2mm rather than 4mm.

Joints made this way are very strong and take hardly any extra time or effort once the technique is mastered. Try it with some off-cuts and see for yourself.

The UK uses the metric system of measurement and I have become so used to working with it that I think (and write) metrically now. I hope that this does not prevent you from trying out some of these ideas. Please accurately measure your biscuits in order to perform some of the later calculations in a different measuring system.

Regarding the purchase of a biscuit jointer, please remember that you mostly get what you pay for. Machining moving parts to fine tolerances costs money and nowhere is this more obvious than in fence mechanisms. My first machine did what I bought it for, but I could feel that it had been cheaply made, both in the adjustable parts and in the plunge action. For everyday tasks, a budget machine will do the job, but be prepared for some disappointment if you expect absolutely perfect panels or a rock-steady fence. My current machine is about as good as they get and I'm delighted with it. I know that it will outlast me if I look after it - and because it's such a pleasure to use, I most certainly will!

N.B: I have no connection or affiliation with the Lamello Corporation, other than being a very satisfied customer.

Ray Girling 1998 - 2019