Using Domino Spacers for Narrow Material

Using the Festool Domino with Narrow Material
by
Cary Swoveland
(cary@swoveland.com)
May 2008
1. A Truly Marvelous Invention
Anyone who has used Festool’s Domino cannot help but be impressed with the concept,
design and construction. In the world of woodworking tools, I know of no other machine that
represents such an engineering tour de force. It holds down the #1 spot on my “cool tool” list.
But is it perfect? Could the design be improved?
There is one area where I think the tool falls short, and that is cutting mortises in relatively
narrow material (say, material less than about 4” wide). The problem relates to the positions
of the retractable registration pins on each side of the cutter. To cut a mortise in each of two
boards to be joined, one often snugs-up one of these pins to an edge of each board. This
ensures that the center of the mortise will be 37mm from the edge of the board. The problem
is that when cutting a mortise in the end of a relatively narrow board, registering the pin
against one side can result in part of the mortise being cut from air. With slightly wider
boards, there can be a similar problem when cutting the second of two mortises.
There is a simple solution to this problem, and I hope the Festool designers are listening.
(OK, I’m sure they’ve thought of this). I would like the Mark II version of the tool to have
side-to-side-adjustable registration pins (with detents). In the meantime, we must make do,
which is what this paper is about.
2. Spacers
Domino owners have all dealt with the “narrow board” problem in one way or another. One
approach is to simply mark mortise centerlines on the boards and then carefully align the
Domino to the centerlines before cutting. This works pretty well, but sometimes it results in
the fit of the joint being a tad off. A variant of this approach is to cut one of the two mortises
oversize in width, but then we cannot depend on the domino to line up the boards at the joint.
Another approach is to place a spacer between the pin and the edge of the board. This ensures
that the joint will fit properly, but it requires that a spacer be cut to the proper width, and the
latter sometimes requires a bit of basic math. Also, a third hand may be needed to hold the
spacer in position while making the cut.
Festool sells an accessory for the Domino called the “Trim Stop”. This is used to hold narrow
pieces in place while mortises are being cut. It is designed to cut at marked centerlines, not to
cut both pieces being joined from reference points.
This paper is about systemizing the spacer approach. I have constructed a set of eight spacers,
and prepared tables that show which spacer to use, depending on the type of joint (90-degree
butt joint or miter joint), the width of the board, the number of dominos used (one or two),
and, where two dominos are used, the spacing between them (more on that later).
The eight spacers are shown in
Photo 1. The 13mm spacer is
shown in use in Photo 2. I made
mine from phenolic, but any
durable material could be used,
such as hardwood, brass,
aluminum or steel.
The dimensions of the eight
spacers are given in Table 1.
The “size” of each spacer—
which I’ve written on the
spacer—is the distance between
the edge of the board abutting
the spacer and the near edge of
the Domino’s registration pin. In
Photo 2, the spacer offsets the
Domino’s registration pin 13mm
from the edge of the board. The
spacers I’ve selected range in
size from 4mm (about 5/32”) to
25mm (about 1”).
When I suggest using the 0mm
spacer, I mean that no spacer
should be used (i.e., one of the
Domino’s registration pins
should be placed against the
edge of the board in the
conventional way).
Photo 1: Domino spacers.
Photo 2: Domino spacer in place for cut.
Table 1. Spacer Sizes and Dimensions (mm)
Size (Pin Offset from edge)
Overall Width
Edge to Mortise Centerline
25
55
12
22
49
15
19
43
18
16
37
21
13
31
24
10
25
27
7
19
30
4
13
33
0
N/A
37
1. Spacers are 5mm thick.
2. Holes in spacers are 5mm in diameter, centered horizontally, with center 6mm from the top of the spacer.
The diameter of the hole in each spacer is the same as the diameter of the Domino’s
registration pins, 5mm. The overall width of each spacer is therefore twice the spacer size,
plus 5mm.
The height and thickness of each spacer is arbitrary. Notice that the spacers have a rabbet at
the top. This is so that the lip will slide under the Domino’s fence, holding the spacer snugly
in place. This can be seen in Photo 2. I made the spacers 5mm thick at the top
(approximately how far the Domino’s registration pins extend from the face of the machine)
and 9mm thick at the bottom. Each spacer should be tall enough to permit a line to be scribed
2
on it that shows the offset from the edge of the board to the middle of the mortise to be cut
(explained below). I made my spacers about 50mm tall.
The center of each hole should be no more than 8mm from the top of the spacer. Anything
more can restrict the position of the height of the Domino’s adjustable fence. I suggest 6mm.
The distance from the center of the hole to the edge of the lip is 10mm. Therefore, if you drill
the hole centered 6mm from the top of the spacer, the rabbet should be 16mm (6mm + 10mm)
wide. You may need to sneak up to the width of the rabbet, to achieve a good fit.
If you examine Photo 1 carefully, you will
see that there is a horizontal line scribed on
each spacer. (There are two such lines on
the 25mm spacer—I’ll get to that
momentarily.) The distance from this line
to the bottom of the spacer is the same as
the distance from the registration edge of
the board to the middle of the mortise
being cut. Those distances are shown in
the last line of Table 1. These scribed lines
are helpful when making “T-type” joints,
as shown in Photo 3. To cut a mortise in
the face grain of the board lying flat, first
place the vertical board in the desired
position, and mark the registration edge
with a pencil, as shown. (The checkmarks
provide optional registration information.
See the inset on the next page for an
explanation.)
Photo 3: Marking position of T-type joint.
The next step, shown in Photo 4, is to mark
the centerline of the mortise on the end of
the board. This is done by lining up the
line drawn in Photo 3 with the scribed line
on the spacer that will be used to mortise
the vertical piece. The Domino can now
be positioned for the cut by lining up its
centerline with the mortise centerline just
scribed, as shown in Photo 5.
Photo 4: Marking centerline of mortise.
The second line scribed on the 25mm
spacer—the one nearer the top—is for use
in making a T-type joint when no spacer is
used. The distance from this line to the
bottom of the spacer is 37mm.
Photo 5: Aligning Domino to centerline
marked on edge of board.
In Photo 2, you can see that the material being cut contacts three solid surfaces. This makes it
easy to make fast, accurate cuts to very narrow boards—just clamp the board, hold it against
the three surfaces and plunge. My Mark I version of the spaces did not have the lip, so care
3
had to be taken to keep them upright and keep them from falling off the registration pins.
Adding the lip overcame both problems.1
Tip: Use Registration Marks
After deciding how one or more mortises are to be cut, it is easy to become confused
and cut a mortise in the wrong location. This problem can be easily avoided. The
key is to observe that the Domino is always positioned using two registration
surfaces, or one surface and one line. Ambiguity can be avoided by simply marking
the two registration points on each of the boards being joined, as I have done in
Photos 3 and 6. (You may wish to use a soft pencil, rather than the large-tip
permanent marker I used for these photos.) If presented with a board with two such
registration marks, there is only one way to position the Domino for the cut that is
consistent with those marks.
Photo 6: Marking Domino registration faces.
I picked up this suggestion from the Dowelmax user’s guide (that product being #2
on my “cool tool” list).
I have considered two types of joints—90-degree butt joints and miter joints—and for each
I’ve considered using one or two dominos.
Recall that the Domino allows the choice of what may be regarded as a snug, loose or veryloose side-to-side fit for dominos. A snug fit is appropriate when a single domino is used, and
is also appropriate for the first mortise cut when two dominos are used. Any of the three fits
can be used for the second of two closely-spaced mortises, but I’ve only considered the case
1
I posted a draft of this paper in September, 2007. Charles Wilson and Les Spenser suggested I use ring
magnets to hold the spacers to the pins. That worked well, but I found the magnets weren’t needed after I added
the lip, which also kept the spacer vertical. I wish to thank Charles and Les for their suggestions, and others who
contributed as well.
4
where the second mortise is also a snug fit. That provides the strongest joints, and, because
there are no cross-grain, wood-movement issues, there seems to be no reason to not use a
snug fit for the second domino.
3. Spacers for 90-Degree Butt Joints
Consider first 90-degree butt joints, where one or two dominos are used.
Definitions
It is convenience to define four terms:
-
-
Width of board: the width of the board measured across the face in which mortise(s)
are to be cut.
Narrower board: of the two boards to be joined, the board having the narrower width
(if one is narrower than the other) or the width of each board (if they are of the same
width). The narrower board is often one in which the mortise(s) are to be cut into end
grain. In determining spacer sizes, I have attempted to center the mortise(s) in the
narrower board.
Registration edge: the edge of a board that abuts a spacer, or, if no spacer is used,
abuts one of the Domino’s registration pins.
Mortise indents: for the narrower board, the distances from each edge of the board to
the closest end of the mortise (if one domino is used) or to the closest end of the
closest mortise (if two dominos are used). The mortise indents indicate how close
each mortise is to the edge or edges of the board.
One Domino
I recommend spacer sizes for the use of a single domino in Table 2. I have considered eight
(narrower) board widths, ranging from 1” (25mm) to 2.75” (70mm) If, for example, the board
is 1.75” (44mm) wide, I suggest you use the 16mm spacer (regardless of the size of the
domino). As I mentioned earlier, the zero spacer width indicates that no spacer should be
used (e.g., for material 2.75” wide).
Table 2. Spacer Widths for One Nearly-Centered Domino, 90-DegreeButt Joints
Inches
mm
Spacer
(mm)
Domino
(mm)
5
6
8
10
1
25
1.25
32
1.5
38
25
22
19
3/4
2/3
N/A
N/A
Material Width (inches/mm)
1.75
2
2.25
44
51
57
2.5
64
2.75
70
4
0
Mortise Indents from Edges of Material (mm/mm)
6/8
9/11
12/14
15/18
21/18
24/22
5/7
8/10
11/13
14/17
20/17
23/21
4/5
7/8
10/11
13/15
19/15
22/19
3/3
6/6
9/9
12/13
18/13
21/17
28/24
27/23
26/21
25/19
16
13
5
7
The mortise indents are given in the last four rows of the table. To continue with the example
of using the 16mm spacer with a 1.75” wide board, if the mortise is cut for an 8mm domino,
the indents would be 10mm and 11mm, 10mm being the distance from the registration edge
to the closest end of the mortise, 11mm being the distance from the opposite edge of the board
to the other end of the mortise.
This table shows that 8mm and 10mm dominos should not be used for boards 1” wide. With
8mm dominos, both indents would be only 1mm; with 10mm dominos, the indents would be
virtually zero.
Two Dominos
I recommend spacer sizes when using two dominos in Table 3. In addition to the width of the
board, the spacer size also depends on the spacing between the two dominos.
Table 3. Spacer Widths for Two Nearly-Centered Dominos, 90-Degree Butt Joints
Narrow Domino Spacing
Wide Domino Spacing
Material Width (inches/mm)
Material Width (inches/mm)
Inches
2.5
2.75
3
3.25
3.5
3.75
4
3.25
3.5
3.75
4
4.25
4.5
4.75
Mm
64
70
76
83
89
95
102
83
89
95
102
108
114
121
Spacer (mm)
Domino
(mm)
22
19
13
10
7
4
0
19
16
13
10
7
4
0
5
6/7
9/10
15/10
18/14
21/17
24/20
28/23
9/9
12/12
15/16
18/19
21/22
24/26
28/28
6
5/7
8/10
14/10
17/14
20/17
23/20
27/23
8/8
11/11
14/15
17/18
20/21
23/25
27/27
8
4/7
7/10
13/10
16/14
19/17
22/20
26/23
7/6
10/9
13/13
16/16
19/19
22/23
26/25
10
3/7
6/10
12/10
15/14
18/17
21/20
25/23
6/4
9/7
12/11
15/14
18/17
21/21
25/23
1st/2nd Mortise Indents from Edges of Material (mm/mm)
There are two possibilities for domino
spacing: narrow and wide. These are
illustrated in Photo 7. The registration edge
is on the left side of each board, as indicated
by the registration checkmarks. Note the
arrows pointing to one end of the first mortise
cut in each board. Those arrows indicate the
placement of the Domino’s registration pin
Photo 7: Wide and narrow domino spacings.
for cutting the second mortise. For wide
spacing, the pin is held against the right end
of the mortise (i.e., farthest from the registration edge); for narrow spacing, it is held against
the left end of the mortise.
Suppose, for example, the board is 3.5” wide. Table 3 shows that if narrow spacing is used,
the 7mm spacer should be selected, whereas the 16mm spacer should be chosen for use with
wide spacing.
Mortise indents are given for each combination of board width, wide or narrow spacing and
domino size. When using 8mm dominos with 3.5”-wide boards, for example, with narrow
spacing the indents would be 19mm (from the registration edge) and 17mm (from the
opposite edge); with wide spacing they would be 10mm and 9mm, respectively.
6
4. Spacers for Miter Joints
I recommend spacer sizes for miter joints in Table 4. As in the case of 90-degree butt joints, a
zero spacer width indicates that no spacer should be used. As in Table 2, “N/A” means that a
domino of the size indicated cannot be used for the given material width (one domino) or
combination of material width and domino spacing (two mortises).
Table 4. Spacer Widths for Miter Joints (mm)
Material Width (inch/mm)
Inches
1
1.25
1.5
1.75
2
2
2.25
2.5
2.5
2.75
3
Mm
25
32
38
44
51
51
57
64
64
70
76
Mortise size (mm)
Height
Depth
Spacer Widths for Two Dominos (mm)
1
Spacer Widths for One Domino (mm)
Narrow Spacing
Wide Spacing
5
12
13
5
15
N/A
6
12
N/A
10
6
15
N/A
N/A
6
20
N/A
4
8
12
N/A
10
10
10
8
15
N/A
7
N/A
7
8
20
N/A
N/A
0
0
0
N/A
0
0
N/A
0
0
8
25
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
10
12
N/A
10
10
7
0
10
10
7
10
10
7
10
15
N/A
7
7
7
0
N/A
7
7
N/A
7
7
10
20
N/A
N/A
0
0
0
N/A
10
7
N/A
0
0
7
7
4
7
4
0
0
10
N/A
10
7
10
10
7
4
4
10
7
4
7
N/A
10
25
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
1. The depth of the mortise nearest the outside corner of the miter joint. There is no restriction on the depth of a second
mortise, nearer the inside corner (i.e., it can be as much as 25mm in all cases).
Recall that dominos can be purchased for mortises with heights and depths (mm/mm) of 5/15,
6/20, 8/20, 8/25 and 10/25. The entries in the greyed-out rows of Table 4 correspond to
situations where dominos must be shortened to be used.
One Domino
Consider first the use of a single domino in a miter joint. By way of example, assume that the
material is 1.5” wide. Table 4 shows that one should use the 7mm spacer with 5mm x 30mm
dominos (15mm mortises), the 4mm spacer with 6mm x 40mm dominos (20mm mortises) and
the Domino’s registration pins with 8mm x 40mm dominos (20mm mortises). The 8mm x
50mm and 10mm x 50mm dominos cannot be used (because they are too long). These longer
dominos could be used, however, if they were shortened, to fit in mortises shallower than
25mm. The table shows that 10mm dominos can be shortened to 40mm (to fit 20mm deep
mortises), in which case no spacer would be used. Alternatively, they could be shortened to
30mm (for use with the 7mm spacer) or 24mm (for use with the 10mm spacer).
For those interested, I will now take a moment to explain how the “one domino” entries in
Table 4 are derived. If this is of no interest, you may want to skip to the “Two Domino”
subsection.
7
To determine which spacer should be used for each combination of domino size and material
width, I computed the minimum and maximum width spacers, and the spacer width that
would center the domino. I then selected a spacer among the set available (0mm, 4mm,
7mm,…, 25mm) that was between the minimum and maximum widths and closest to the
spacer width that would center the mortise.
The drawing below shows how these calculations were made. Of interest are the distances i1
(“indent 1”) and i2 (“indent 2”). i1 is the shortest distance from the bottom of the mortise to
the surface of the board. If the spacer is too large, the Domino’s bit will cut through the
board. The value of i1 depends on the width of the spacer (s), and the depth (d) and width (w)
of the mortise. I arbitrarily set i1 = 2mm (about 0.08”), and solved for the corresponding
spacer size, s. This is the maximum spacer size for this mortise. Notice that this does not
depend on the width of the board, m.
i2 is the shortest distance from
the mortise to the inside corner
of the joint. If the spacer is too
small, the Domino’s bit may
cut through the material at the
inside corner of the joint. Of
course, this is only potential
problem for quite narrow
boards. The value of i2
depends on the size of the
spacer, width of the board (m),
and the depth and width of the
mortise. I arbitrarily set i2 =
2mm, and solved for the
spacer size. This is the
minimum spacer size for this
board width and mortise.
Let me illustrate this calculation with an example. Suppose a 5mm x 30 mm domino is
chosen for use with material 1-1/4” wide. Table 4 suggests that the 7mm spacer should be
used. The calculations I just described show that the spacer should be between 3.4mm and
9.8mm wide, and that a 14.5mm spacer would center the mortise (but with the last spacing the
Domino would cut though the outside surface of the board). Of the set of spacers I made,
either the 4mm or 7mm spacer could be used. I suggest using the 7mm spacer because it
would place the mortise closer to the center of the miter face than would the 4mm spacer.
If 8mm x 25mm or 10mm x 25mm mortises are cut without using a spacer, the minimum
distance between the bottom of the mortise and the outside surface of the board (i1 in the
drawing) is less than 1mm. In some tests I did with 10mm x 25mm mortises, the Domino’s
bit sometimes left a pinhole in the outside surface of the board.
Two Dominos
When two dominos are used in a miter joint, the choice of spacer again depends on the board
width and the mortise depth and width, but it also depends on the whether wide or narrow
domino spacing is elected.
8
If you treat the two mortises in each board as one very large mortise, extending from the left
end of the left mortise to the right end of the right mortise, it is evident that the calculation of
spacer sizes for two dominos is very similar to that for one domino.
Consider the example of using 8mm x 40mm dominos with 2.5” wide boards. If narrow
domino spacing is chosen, no spacer should be used. To use wide spacing, the mortise nearer
the outside corner of the joint can be 12mm or 15mm deep, but not 20mm deep. Therefore,
the domino placed in that mortise must be shortened accordingly. If the mortise is cut 15mm
deep, for example, the 7mm spacer should be used. The second mortise, nearer the inside
corner of the joint, can be the full 20mm deep.
9