Workshop Wednesdays – Finger Joint Jig

Monday 9 July 2018

We recently discussed an ingenious jig for making finger joints on a router table. If you need to cut a lot of finger joints that are all the same size, it's an excellent solution. It may be, however, that you want to enhance the decorative aspects of finger joints by varying their size. Or it may be that you don't have a table for your router. If so, what I think of as the 'world's best' finger joint jig gives you that extra flexibility. This jig may take a little longer to make than the other jig, but I think you'll find its extra flexibility is worth the extra time and effort.

To demonstrate this jig, I'll joint two well-dimensioned boards that are each 90mm wide x 10mm thick. As with most jigs, the quality of the results will depend on the care and precision with which the jig is built.

Making the parts for the jig

We can start by making the handle. For this, I use MDF, but good quality plywood could also be used. The handle is composed of two equal sized pieces of MDF sandwiching a third piece that is the same width but rather taller. In the example opposite, the outer boards are 12mm MDF that is 225mm wide and 150mm tall, while the middle board is 25mm MDF that is 225mm wide but 250mm tall. These boards are simply glued together. You then need a small board to act as a reference board on the jig, which will be attached to one edge of this handle. This reference board needs to be the thickness of the handle plus about four times the thickness of the stock you propose jointing. In this case, the stock is 10mm thick, so the reference board needs to be about 100mm wide.

The jig fingers

We now need to turn our attention to the fingers. This is an important step in the making of this jig, as the fingers that you make for this jig will determine the quality of the final finger joint. With the 90mm boards we have lots of options. We could, for example, choose to have nine fingers that are each 10mm, we could have five fingers that are each 18mm, or we could have fingers of varying sizes.

The important thing to remember is that the total adds up to the width of the board – 90mm in this case. I think the joints look better if there is an odd number of them, but there is no hard and fast rule about this – as far as I know. In this case, we will divide the 90mm into seven fingers that are the following sizes: 10, 15, 10, 20, 10, 15, 10, as shown in the photo. If we were making a finger joint with all fingers of the same size – say nine 10mm fingers – then we could simply dimension one long finger and then cut it into the required lengths to provide the jig fingers needed. That way we would ensure that every finger was exactly the same size.

So we need four jig fingers that are each 10mm wide, two that are each 15mm wide, and one that is 20mm wide. As to length, I find that fingers that are around 80mm long work well, as they make for a slightly wider and more stable base for the jig. The thickness of the fingers, however, requires a bit more thought.

Affixing the jig fingers

The jig fingers provide a template of the desired finger joint, against which you can run a template cutter. That cutter needs to be thinner than the thinnest finger to be cut and long enough so that it can reach the shoulder line of the finger joint while keeping the bearing securely on the template. For this joint, the cutter I will use is 9.5mm wide x 20mm long. With 10mm boards, the shoulder line will be 10mm above the top of the jig finger, so the bearing will be some 10mm below. The bearings themselves are about 7mm wide, so I will want jig fingers that are about 18-20mm thick.

Once the jig fingers have been accurately cut, drill and countersink a pilot hole about 15mm from the end of each of them. These will be used to screw the jig fingers to the base of the jig handle.

Assembling the jig

Clamp the jig handle upside down in a bench vice – as shown in the photo. The reference board is centred on one end of the jig and sits proud of the base by something less than the thickness of the jig fingers. To attach it, I simply stick it on the jig with a bit of doubled-sided tape and then add a couple of screws. In this case, with 19mm jig fingers, it sits about 15mm above the jig base; this allows you to use the same reference board for attaching the jig fingers that will be used for the boards that will be finger jointed.

Begin by gluing the first jig finger on to the base, flush against the reference board and covering about two thirds of the width of the base. Using a square to ensure all the fingers are nice and square on the jig and they are snugly next to each other, attach the rest of the jig fingers, in the desired order, alternating them, as shown in the photo. Leave this now for a few hours to let the glue harden before carefully putting in the screws.

Because the same jig fingers provide the template for both the finger and the space into which the finger will go, if you simply attached all the jig fingers snugly next to each other, the resulting joint would be too tight; it would be very difficult, if not impossible, to get together. The answer is to relieve the spaces very slightly by inserting shims that are about a couple hundredths of a millimetre between each of the jig fingers. To do this, I find cigarette papers work well, with each being about 1/100mm thick. The jig is now ready to use.

Using the jig

First, stand the jig on the jig fingers with the handle pointing upward. Place the two boards to be finger jointed snugly up against the reference board, one on each side of the handle, then cover each of the boards with pieces of MDF that are roughly the same width as the boards and clamp everything together, being careful to ensure that the clamp is well away from the cutting area. These sacrificial boards, along with the base of the jig, help to ensure the cleanness of the cut, and they can be reused.

Set and lock the cutter so that it reaches the desired shoulder line. If you prefer the fingers of the joint to be slightly proud so that you can plane them flush, you would set the cutter just above the shoulder line accordingly.

It is best if you use a table, but if you don't have one, simply turn the jig upside down and clamp it in a vice. The jig can now be used with a hand-held router. Now rout between the jig fingers on both sides of the jig and job done!

If you were jointing narrow sections, say for a small box, you'd find that the jig would not be very stable – and safe to use – if it was built specifically for a narrow section. The answer to this problem is simply to make the jig handle nice and wide and add an outrigger finger at the opposite end of the jig to the reference plate; this will ensure the jig stays flat and stable, even though it's being used for only narrow sections of wood.

A word of advice: if you find that you don't need a particular jig any longer, you can unscrew and break off the jig fingers, remove the reference board and then cut off a small strip of the base of the handle with the bench saw. Reinstall the reference board and it's ready to receive your next set of jig fingers.

Angled finger joint jig

This finger joint jig has been shown to have several advantages: it is very accurate, you can get a wide variety of finger sizes from a single router cutter and it can be used on either a router table or not. That allows for quite a bit more flexibility for exploring the design possibilities of the finger joint. But this jig has another trick up its sleeve.

Most finger jointing applications involve jointing boards at a 90° angle to each other. But what if you want some other angle, as I did when constructing the trapezoidal case for a clock – in this case a frustum. This jig also gives you the option of finger jointing at a range of angles. You'll need to brush up on your geometry and mark the boards carefully, but the jig can be easily adapted to cut finger joints at an angle. Let's say you want to joint two boards at an angle of 30° from perpendicular. Simply cut the base of the jig handle on the tablesaw with the blade set to 30°. When you stand the jig up, this will leave you with the jig handle standing on the fingers at a slope of 30° to the perpendicular on one side and 150° on the other – the sum of the angles on the two sides of the jig will always equal 180°. This allows you to cut finger joints at the chosen angles. If you wish, you could cut the ends of the boards to the final angles before routing, but I find it quickest just to rout the finger joints and then to plane the ends after final fitting.

Two things that are important to keep in mind when cutting angled finger joints:

1. Clearly mark in pencil the edge of your boards with the required shoulder line so you can ensure that the shoulder line you will cut is parallel to the fingers. It is very easy to clamp your boards on the jig in the wrong direction, and to get shoulders sloping in the wrong direction.

2. You also need to ensure that your arrangement of fingers is symmetrical across the width of the board, because you will need to 'flip' the boards when cutting complementary angles.

Once you've grasped the principles behind this jig and you've tried it a few times, I'm sure you will appreciate the wide scope and flexibility it provides for reliably and accurately cutting almost any sort of finger joint you may want, and that it may well be 'the world's greatest' finger joint jig.