Finger joint jig

A jig for cutting a finger joint which eliminates the trial and error required to adjust the spacing between the key and the cutting blade. A key and an abutment are arranged on one side of the cutting blade. The key is connected to an alignment ledge on the opposite side of the cutting blade. The key, abutment, and alignment ledge can be aligned with the cutting blade so that the blade will trim all three when they pass over. When the cut is made, a gap is left which is the exact width of the blade. After the cut, the alignment ledge can be slide toward the abutment, closing the gap. The key, connected to the alignment ledge, moves a corresponding distance away from the blade. The key can then be fixed in position. A workpiece can then be placed flush against the trimmed face of the key and be cut, leaving a finger, between the key and the blade, which is the exact width of the adjacent kerf, formed by the blade.

FIELD OF INVENTION 
The invention relates generally to making finger joints in wood and 
particularly to making finger joints using a table saw or router. 
BACKGROUND 
The finger joint is a common joining technique used in woodworking to join 
two boards together, often at right angles. Its most common application is 
in the making of boxes. For this reason, the finger joint is sometimes 
referred to as a "box joint." 
In a finger joint, the ends of both boards are cut to form a series of 
fingers separated by spaces, or "kerfs." The boards are joined by 
interlocking the fingers of one board with the fingers of the other board 
and using an adhesive to maintain the joint. The key to a good finger 
joint is that the width of the fingers and the width of the kerfs be 
identical, providing a tight fit when the fingers are inserted into the 
kerfs. 
Outside of a production shop, finger joints are most often cut using either 
a router or a table saw. The router approach typically uses a template to 
guide the router. The template will have a separate slot for each kerf to 
be cut. the width of the finger is controlled by the spacing of the 
template and the width of the kerf is dictated by the width of the router 
bit. The table saw approach typically uses a fixture, or jig, to hold the 
board in position, on end, as it is passed over the table saw blade. After 
each pass the board is moved to a new position in the jig, often aligning 
the most recently cut kerf with a key to align the board for the next cut. 
The width of the fingers is controlled by the distance between the key and 
the blade and the width of the kerf is determined by the thickness of the 
blade. Dado blades are often used to provide wider cuts. 
The traditional approach to using a jig to cut finger joints on a table is 
described in Finger Joints, Woodsmith, v. 19, #110, April 1997 
(Woodsmith). The starting point for the jig is a fence, perpendicular to 
both the table and the blade which is fixed in position laterally relative 
to the blade but free to slide in the direction of the cut. This is 
usually accomplished by fixing a board to the table saw's miter gauge and 
setting the gauge for a 90 degree cut. The fence may be supplemented with 
a ledge, attached to the bottom of the fence, parallel to the table. The 
ledge supports the board being cut and provides support for the key. With 
the fence in place, the steps for setting up the jig are: 1) adjust the 
blade to the thickness of the stock; 2) cut a notch in the fence, and 
ledge; 3) make a key, sized to fit tightly in the notch and glue it in 
place; 4) using a spacer the same size as the key, adjust the fence so 
that the space between the blade and the key is the width of the spacer 
(and thus the key); 5) test the jig and make adjustments to the key 
location and blade height until a tight joint is achieved; 6) fix the 
fence in position with screws and/or glue. 
In the above process, steps 3 and 5 are critical to the setup of the jig 
and can be very time consuming. The key used in step 3 is quite small, 
often one to one and one half inches long and the thickness of the blade 
being used, often one eighth to one quarter inch. Because of this, it must 
be shaped by hand and must be an exact match for the width of the blade. 
Inaccuracies in the key may result in a loose fit in the notch and make 
the adjustments in step 5 more difficult. 
Step 5 is the final adjustment of the jig. A test joint is made by cutting 
two boards and fitting them together. If the joint is loose, the key (and 
fence) must be adjusted to move away from the blade. If the joint is 
tight, the key must be moved toward the blade. After the adjustment, a new 
test joint is cut, and the process is repeated. The adjustments in the key 
position may be in increments of 1/64ths of an inch (or less) and several 
iterations may be required before a tight joint is achieved. 
Finger joint jigs are also available commercially. One example is the 
"Aluminum Box Joint Jig" available from Woodsmith Shop (Summer 1995 
catalog, pg. 18). This jig provides a fence and ledge as described above 
but also provides a built-in adjustable key to alleviate the problems 
associated with step 3 above. It also provides a threaded adjuster for 
setting the gap between the key and the blade. However, the basic approach 
is as described above, and iteration is still required in step 5 to obtain 
a good joint. 
Once the jig is properly set, a large number of high quality joints can be 
cut it relatively short time. However, the trial and error approach to 
setting the jig can result in a high level of frustration for the 
woodworker. Where a single box is being made, the set-up time can easily 
exceed the time needed to actually cut the joint. Because of this, the 
finger joint may be avoided by amateur woodworkers and even professionals 
who work in very small quantity. 
There is need for a finger joint jig which eliminates the trial and error 
setup described above and which preferably does not require the hand 
fitting of the key piece to the slot cut by the blade. 
SUMMARY OF THE INVENTION 
The present invention is of a finger joint jig for use with a cutting 
device, such as a table saw or router table, which addresses the above, 
and other, needs. In the preferred embodiment, the jig comprises a support 
structure; a means for attaching the support structure to the cutting 
device whereby said support structure is free to move in one dimension, 
passing over the cutting blade, or bit; a key; an abutment means fixed to 
the support structure; an alignment means slidably attached to the support 
structure; a means for attaching the key to the alignment means; a means 
for fixing the key and the alignment means in position relative to the 
support structure and a means for adjusting the position of the key, the 
abutment means, and the alignment means relative to the cutting device 
whereby when the jig passes over the cutting means, a first side of the 
cutting means will trim the key means and the abutment means in the same 
plane and the opposite side of the cutting means will trim the facing 
surface of the alignment means. 
In a further embodiment of the invention, the key, abutment means, and 
alignment means are formed as a single unit which is removably attached to 
the support structure, and can be replaced for each new finger joint 
setup. 
In a still further embodiment of the invention, the support structure 
includes a fixed vertical panel which extends above the height of the 
cutting blade, or bit, and the means for attaching the key to the 
alignment means is a sliding vertical panel which also extends above the 
height of the cutting blade. 
Also disclosed is a method of forming a finger joint using the disclosed 
jig.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The following description is of the best presently contemplated modes of 
carrying out the present invention. This description is not to be taken in 
a limiting sense but is made merely for the purpose of describing the 
general principles of the invention. The scope of the invention should be 
determined by referencing the appended claims. 
The device of the present invention is generally applicable to the forming 
of finger joints to join together boards. This joint is commonly used to 
form either right-angle joints, as in making a box, or an end-to-end 
joint. Joining at other angles is also possible. While the preferred 
embodiment uses a table saw as the cutting means, other alternatives, such 
a router are anticipated. 
The following is a brief glossary of terms used herein. The supplied 
definitions are applicable throughout this specification and the claims 
unless the term is clearly used in another manner. 
Abutment--fixed portion of the jig which is trimmed by one side of the 
cutting means and against which the alignment portion will abut during set 
up. In the preferred embodiment this is the lower left portion of the 
ledge. 
Alignment stop--movable portion of the jig which is trimmed by the opposite 
side of the cutting means as the abutment and which is moved to be flush 
against the abutment during set up. In the preferred embodiment this is 
the right side of the ledge. 
Backplane--refers to the back, vertical portion of the jig consisting of 
one or more layers. 
Replaceable section--refers to that portion of the jig which is cut by the 
blade. In the preferred embodiment, this portion is replaceable so that a 
different piece may be used for each joint or each width of cut. In the 
alternative, the entire jig can be re-built for each joint. 
Front, Back--as used in reference to the jig, when the jig is in use, the 
front side is away from the user and toward the table saw blade and the 
back side is toward the user and away from the blade. 
Kerf--refers to the slot cut by the saw blade, specifically the gap between 
the fingers of the finger joint which is cut by the blade or the clot cut 
in the jig during set up. 
Key--that portion of the jig against which the workpiece is aligned. 
Lateral--describes movement parallel to the surface of the saw table and 
along the length of the jig. In a typical configuration this is at right 
angles to the blade 
Ledge--the horizontal portion of the jig extending outward from the 
backplane, parallel to the table of the table saw. 
Left, Right--unless clearly used otherwise, left and right are relative to 
a front view of the jig from the blade position looking back towards the 
user. Note that the jig is also fully functional as a mirror image, where 
all left and right references would be reversed. 
Support Structure--structural components of the jig, those sections which 
support the workpiece, hold the other parts in place, and slidably attach 
to the table saw. In the preferred embodiment this includes the backplane 
and portions of the ledge. 
The various drawing figures disclose the present invention in detail 
showing the preferred embodiment, and alternative embodiments. The 
following discussion is with reference to these figures. 
FIG. 1 shows the jig, 100, in position on a table saw, 102. The jig is 
attached to a conventional miter gauge, 104, which is set for a 90 degree 
cut. The miter gauge rides in one of the slots, 108, in the saw table. 
Either slot could be used, with the position of the jig relative to the 
gauge being adjusted accordingly. Alternatively, the jig could be attached 
directly to either one, or two, rails which ride directly in the table saw 
slots. Permanent attachment to two rails, riding in both slots, provides 
increased accuracy and ease of use. Either of the above alternatives 
results in the jig being fixed in position laterally relative to the 
blade, 106, while able to slide over the blade in the direction of the 
cut. While normally used as shown for a 90 degree cut, it would also be 
possible to alter the angle to form a more complex joint. 
FIG. 2 shows the preferred embodiment of the invention from the front side, 
looking from the blade of the table saw back toward the user. The 
backplane, 110, provides a vertical support for the workpiece as well as 
providing a significant portion of the support structure. The lower 
portion of the jig is a ledge, parallel to the table of the table saw, 
which comprises several sections. On the right side the ledge is a single 
piece, 118, which is a permanent part of the structure. On the left side, 
the ledge is bifurcated into upper, 114, and lower, 116, portions which 
are also permanent. The center of the ledge, 112, is replaceable, being 
consumed each time a new joint configuration is set up. The replaceable 
section, 112, will be made of an easily formed material which can be cut 
by the saw blade without excessive wear. Injection molded plastic is a 
good candidate. The replaceable section attaches to the permanent portions 
of the ledge at both ends. Many means of attachment could be used 
including a dovetail, as shown, a clamping mechanism, adhesive, and 
others. These elements are also shown in FIG. 20 from a perspective view. 
The replaceable portion, 112, is also bifurcated on the left side into 
upper, 120, and lower, 122, portions. These attach to the upper, 114, and 
lower, 116, portions of the ledge, respectively. The upper portion, in 
combination with section 114, serves the role of a support ledge, 
supporting that portion of the workpiece to the left of the key. The lower 
portion serves the role of an abutment, remaining in position and serving 
as a stop for the right side when the adjustment is made. The key, 124, 
protrudes above the upper portion of the ledge and is positioned above the 
bifurcated portion of the replaceable section. The right side of the 
replaceable portion, in combination with section 118, serves the role of 
the alignment stop. 
In an alternative embodiment, the support ledge could be a single piece, 
rather man composed of a permanent and a replaceable section. The same is 
true of the abutment and the alignment stop. In the following description 
support ledge, abutment, and alignment stop will be understood to include 
both the corresponding permanent and replaceable sections of the jig, 
unless clearly used otherwise. This use is intended to encompass 
alternative embodiments in which one or more of these sections is 
constructed as a single unit. 
FIG. 3 shows a cross section through the permanent portion of the jig. As 
this view shows, the backplane, 110, comprises two separate planes which 
can slide laterally relative to each other. The rear plane, 126, is 
attached to the abutment, 116, and is fixed in position laterally, after 
the initial set up. In use, the rear plane and the abutment remain fixed 
in position, laterally, relative to the saw blade. The front plane, 128, 
is attached to the support ledge, 114, and is also attached to alignment 
stop, 118. These three pieces are fixed in relation to each other, but can 
move laterally relative to the rear plane, and the saw blade. When the 
replaceable section, 112, is initially installed, it is a single piece and 
fixes all of the sections of the jig in position relative to each other. 
FIGS. 4 through 8 illustrate the set up steps for the jig. FIG. 4 shows a 
detailed view of the replaceable section, 112, of the jig and part of the 
permanent portions of the ledge. Area 130 shows the area which will be 
removed by the saw blade as the cut is made. For the jig to work properly, 
the cut must intersect the bifurcated portion of the replaceable section. 
The cut must also be flush with the right edge of, or preferably, remove 
part of the key, 124. The initial set up of the jig will align the rear 
plane and the key to establish this relationship. This can be achieved 
either by shifting the position of the support structure relative to the 
miter guage or by an additional adjustment which allows the abutment to be 
shifted laterally relative to the support structure. Prior to the cut, the 
two planes of the backplane will be fixed together to prevent movement 
during the cut, preferably by clamping, such as with a cam handle, 132, as 
shown in FIG. 21, or by means of a set screw. When the cut is made, the 
blade will pass completely through the replaceable section, from front to 
rear, as well as a small section of the backplane. 
FIG. 5 shows the same view after the cut has been made. The results of the 
cut are three-fold, first, the abutment, 122, is separated from the rest 
of the replaceable section. This will allow the two planes of the back 
plane, and their attached sections of the ledge, to move laterally once 
unclamped. Secondly, the cut face, 134, of the abutment and the cut face, 
132, of the support ledge, 120, and key, 124, are in exact alignment, 
having been cut by the same edge of the blade. Thirdly, the gap between 
cut face 134 and the cut face 136, of the alignment stop, 138, is the 
exact width of the blade and the kerf which it makes. The alignment of 
faces 132 and 134 and the gap between face 134 and face 136 are what 
result in the correct alignment of the jig. 
FIG. 6 shows the jig after being aligned. The abutment, 122, remains fixed 
in place. The alignment is achieved by unclamping the two planes of the 
backplane allowing the front plane, 128, to slide laterally. As discussed 
above, the support ledge, 120, and key, 124, are attached to the front 
plane of the backplane. Similarly, the alignment stop, 138, is attached to 
the front plane. As a result of this interconnection, the front plane, 
support ledge, key, and alignment stop will move in unison as they are 
slid laterally. These sections are slid laterally to the left until face 
136 of the alignment stop is flush against face 134 of the abutment and 
they are again clamped to the rear plane to fix them in position relative 
to the saw blade. The distance moved is exactly the width of the kerf. The 
effect of this is to shift the key, 124, to the left by the exact width of 
the kerf, leaving a gap, 140, between the right face of the key, and the 
left edge of the blade (which is aligned with face 134) which is exactly 
the width of the kerf. When the workpiece is cut, this gap corresponds to 
the thickness of the finger portion of the joint. The overall result is a 
finger which is the exact width of the kerf, with no fine tuning required. 
The height of the blade will still have to be adjusted for the thickness 
of the workpiece. 
As FIG. 7 shows, subsequent passes of the saw blade will again cut through 
the replaceable portion of the jig, removing section 142. This is why the 
sections of the jig must be fixed in place relative to each other prior 
making subsequent cuts. This can be achieved by again clamping together 
the front and read planes of the backplane, as described above. 
Alternatively, the bifurcated portions of the replaceable section can be 
glued together. This results in a more stable alignment suitable for 
long-term use. 
FIG. 8 shows the overall jig at the same point as the detailed view of FIG. 
5. The front plane, 128, alignment stop, 138 and 118, and the support 
ledge, 114 and 120 have been shifted to the left, offsetting the key, 124. 
The rear plane, 126, and the abutment, 116 and 122, have remained in 
position relative to the saw blade. 
FIGS. 9 through 11 show the jig in use cutting a finger joint in a 
workpiece. In FIG. 9 the workpiece, 150, is shown positioned for the 
initial cut. The end of the workpiece is placed against the ledge and one 
edge, 152, is placed flush against the face, 132, of the key. FIG. 10 
shows the workpiece after the first cut is made, forming kerf 154. 
FIG. 11 shows the workpiece after the second kerf is cut. To make the 
second, and subsequent cuts, the workpiece is shifted so that the left 
face, 156, of the previous kerf is flush against the right face, 132, of 
the key. Unlike the traditional approach, the key will not typically be 
the exact width of the kerf. It does not thus serve to hold the workpiece 
in position laterally and the workpiece must be held in position against 
the key either by the operator or by clamping or spring means attached to 
the jig. If desired, the jig can be positioned during set up such that the 
initial cut will trim the key to the exact width of the kerf. 
Alternatively, the key portion can be formed with a resilient core, or an 
imbedded spring, so that the left face of the key will be urged outwardly, 
urging the workpiece to the left, holding it in contact with the right 
face of the key. 
FIG. 12 shows an alternative, minimal, embodiment of the invention. The 
support ledge, 114 and 120, has been eliminated. The key, 124, attaches 
directly to the front plane, 128, of the backplane which in turn connects 
to the alignment stop, 138. Gap, 157, can occur between the abutment, 122, 
and the alignment stop, as long as it is narrower than the kerf cut by the 
blade. This embodiment functions in the same manner as the preferred 
embodiment, with the key sliding to the left after the initial cut. This 
approach is weaker than the preferred embodiment in that the key is not 
supported by support ledge. However, given a sufficiently strong material, 
or for light use, this design may be adequate. 
FIGS. 13 through 15 illustrate another alternative embodiment of the 
invention. As discussed above, the cuts which form the kerfs in the 
workpiece also cut through the backplane of the jig. This may be 
undesirable for long term use and requires that the backplane be made of 
material which will not damage the saw blade. As shown in FIG. 13, the 
replaceable section, 112, includes a vertical portion, 158, which serves 
as the front plane of the backplane. In this embodiment, it is inset into 
the rear plane. As the cross-section of FIG. 14 shows, the front plane, 
158, the key, 124, the support ledge, 120, and the abutment, 122 are 
formed as a single piece with a slight separation between the upper and 
lower portions. This approach has the further advantage that the permanent 
portion of the backplane can be formed as a single piece. As FIG 15 shows, 
the operation of the jig is similar to the preferred embodiment with the 
moveable portions of the replaceable section, 112, complete with front 
plane, 158, sliding within the rear plane, and the ledge moving as before. 
FIG. 16 shows another alternative embodiment of the invention in which the 
back plane is composed of three layers. The rear plane, 162, and front 
plane, 164, function as described for the preferred embodiment, above. The 
backing panel, 166, is removable. During set up of the jig, the backing 
panel is removed so that it will not be cut by the blade. After set up and 
before the first kerf is cut in the workpiece, the backing panel is 
inserted behind the key, 124, and optionally behind a portion of the 
ledge. As FIG. 17 shows, when the cut is made, the gap, 168, in the 
backing panel is exactly aligned with the kerf being cut in the workpiece. 
This allows the backing panel to support the edges of the kerf, minimizing 
splinters and tear-out in the workpiece at the edges of the kerf. 
FIGS. 18 and 19 illustrate another alternative embodiment of the invention. 
In this configuration the left, abutment, 122, the alignment stop, and the 
key, 124, are all positioned in contact with the surface of the table saw. 
As the top view of FIG. 19 shows, the key, 124, is positioned in front of 
the abutment, 122. The saw blade passes sequentially through the key and 
then the abutment, trimming them in the same plane. Otherwise, this 
embodiment functions as described for the preferred embodiment. 
While the preferred form of the invention and some alternative embodiments, 
have been disclosed above, further alternative methods of practicing the 
invention are readily apparent to the skilled practitioner. The above 
description of the preferred embodiment is intended to be illustrative 
only and not to limit the scope of the invention.