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A 3D Cutting Method for Integral 1DOF Multiple-Tab-and-Slot Joints for Timber Plates, using 5-axis CNC Cutting Technology


Abstract and Figures

Integral Mechanical Attachment (IMA) uses features in the form of components for their connection. In addition to the transfer of forces, locator features are used as integral assembly guides. Prismatic, single-degree-of-freedom (1DOF) joints only allow for a single assembly motion and therefore a simple, rapid and precise assembly. In modern timber construction, such CNC-fabricated 1DOF joints are commonly used in frame structures. Recent research is investigating the application of similar techniques for the joining of timber plate components, inspired by traditional handcrafted joints from cabinetmaking. The method presented in this paper builds upon previous research, allowing for new geometric variations such as non-orthogonal 1DOF plate joints and a simplified cutting process using a 5-axis simultaneous cutting technique. In addition to the use of milling tools, the method is compatible with 5-axis laser cutting and 5-axis waterjet cutting. Advantages and disadvantages of the different methods are being discussed.
Content may be subject to copyright.
Christopher Robeller1, Yves Weinand2
ABSTRACT: Integral Mechanical Attachment (IMA) uses features in the form of components for their connection. In
addition to the transfer of forces, locator features are used as integral assembly guides. Prismatic, single-degree-of-
freedom (1DOF) joints only allow for a single assembly motion and therefore a simple, rapid and precise assembly. In
modern timber construction, such CNC-fabricated 1DOF joints are commonly used in frame structures. Recent research
is investigating the application of similar techniques for the joining of timber plate components, inspired by traditional
handcrafted joints from cabinetmaking. The method presented in this paper builds upon previous research, allowing for
new geometric variations such as non-orthogonal 1DOF plate joints and a simplified cutting process using a 5-axis
simultaneous cutting technique. In addition to the use of milling tools, the method is compatible with 5-axis laser
cutting and 5-axis waterjet cutting. Advantages and disadvantages of the different methods are being discussed.
KEYWORDS: Timber plate joints, 5-axis CNC fabrication, Dovetail joints, 5-axis laser cutting, 5-axis waterjet cutting
Integral Mechanical Attachment (IMA) is known as the
oldest method of joining. It uses features in the form of
parts for the connection, instead of additional fasteners
or adhesives. Connector Features are used to transfer
forces, and Locator Features are used for a rapid and
precise assembly. [1]
IMA used to be common in traditional timber
construction, but was widely replaced by mass produced
mechanical connectors during the industrialization. A
Renaissance of IMA has begun with the introduction of
numerical controlled machine technology. With the
proliferation of automatic joinery machines, IMA was
repatriated to timber frame structures, bringing back
joints such as mortise and tenons. These joints are so-
called prismatic, or 1DOF-joints, where the form of the
joint constrains relative movements between the parts to
a single remaining motion path. In addition to the
mechanical features, such 1DOF joints are used as
guides for a simple, rapid and precise assembly. Other
joints with multiple DOF, such as 3DOF finger joints are
the subject of related research. [5]
For plate-shaped wood components, 1DOF integral
connectors such as dovetails have been used in
traditional cabinetmaking rather than carpentry. Instead
of single tabs and slots, plate joints are using multiple
1 Christopher Robeller, EPFL,
2 Yves Weinand, EPFL,
tabs and slots (MTSJ). These joints were used for the
joining of solid wood boards, for furniture such as
cabinets or drawers and their use was limited to plate
edges which are oriented perpendicular to the wood
fibers (Figure 1, bottom right). Analog to the use of
CNC-fabricated 1DOF joints in timber frame structures,
recent research is investigating the application of 1DOF-
MTSJ for the assembly of cross-laminated timber plates.
Due to the quasi-orthotropic behavior of cross-laminated
panels, joints can be applied to both sides of the plates.
Figure 2 shows how this allows for various new
applications, such as box girders or hollow wall or roof
Figure 1: 1DOF-MTSJ, top left: CNC fabricated on cross-
laminated wood veneer plate, bottom right: Handcrafted on
solid wood
In 2010, the application of 3-axis CNC cut dovetail
joints on plywood, for the production of furniture has
been examined by Simek and Sebera [2]. A first
application of dovetail-jointed CLT panels in an
experimental building structure was demonstrated in the
Curved Folded Wood Pavilion in CH-Mendrisio in 2013,
where the joints were combined with adhesive bonding.
[3]. A self-interlocking folded-plate structure, made of
1DOF-MTSJ jointed laminated veneer lumber (LVL)
plates, without adhesive bonding, has been examined by
Robeller et al. in 2014 [4], followed by a study of the
semi-rigid behavior of 1DOF-MTSJ on LVL composite
box girders (Fig. 2) by Roche et al. in 2015 [6], and the
examination of the rotational stiffness of 1DOF-MTSJ
on LVL by Roche et al. in 2015 [7].
Figure 2: Application of 1DOF-MTSJ connectors for the
construction of LVL composite beams.[6]
State-of-the-art methods for the CNC fabrication of
dovetail joints [2] use CNC routers with three
translational axes. Figure 3 illustrates this method, where
the tail part of the joint is using side-cutting, with the
milling tool positioned normal to the plate surface, while
the pin part is cut using spot-facing, with the tool
positioned normal to the side face of the plate.
Figure 3: Fabrication of dovetail joints with 3-axis CNC
technology, limited to orthogonal joints between plates.
This method allows for the cutting of the typically 10-
20° inclined faces on the pin part with a 3-axis machine,
but it also results in several constraints, such as the
vertical clamping of the pin-part, which is difficult with
flatbed routers. Furthermore the method requires the re-
clamping of parts where multiple edges are to be jointed
and it allows only for the joining of plates at a dihedral
angle of 90°:
Vertical clamping: The vertical clamping
(YZ-plane in figure 3) of parts is time
consuming and it is more difficult to properly
clamp the cantilevering work pieces, avoiding
vibrations which reduce the cut quality and feed
rates. The vertical space (z-axis height) of CNC
flatbed routers is limited for plate cutting; larger
parts cannot be fixed vertically.
Re-clamping: When applying joints to multiple
edges on one plate, the part must be released,
rotated and fixed again. This requires a new
referencing of the work piece and causes
imprecision. This problem may be solved with
an additional rotational table, synchronized with
the machine [2], however it only works well
with circular or quadratic shaped plates, and a
sufficient clamping is difficult to achieve.
Dihedral Angle φ: Traditional dovetail joints
were used for the joining of plates where the
dihedral angle is =90°, such as the drawer in
figure 1. Recent research projects have
demonstrated the use of 1DOF MTSJ with non-
orthogonal dihedral angles [3][4][7][8]. This is
not possible with the previous method.
The cutting method presented in this paper takes
advantage of 5-axis flatbed CNC routers (Figure 4),
which are already used by many larger wood processing
companies. Figure 5 shows that in addition to the usual
three translational axes X, Y and Z, 5-axis enabled
machines are equipped with two additional, cardan
rotational axes (here A and B), which allow to orient the
tool along directions  which are not perpendicular to
the machining table (XY-plane). With such rotations of
the tool, we can fabricate integral 1DOF MTSJ joints,
while the work piece is simply clamped on the
machining table.
Figure 4: 5-axis CNC router with automatic tool changer
Figure 5: 5-axis CNC router schematic with axis notations
The maximal tool inclination  that can be achieve
with our method depends on the plate thickness  ,
the geometry of the cutting tool (cutting length,
protrusion) and the geometry of the tool holder and
spindle, as shown in figure 6. Larger inclinations can be
achieved with longer cutters and tool extensions, but the
feed velocity
 needs to be reduced accordingly. In our
experiments, we have cut with cutter inclinations of up
to  =60°.
The right side of figure 6 shows how the minimum and
maximum possible dihedral angle  and  result
from the maximum tool inclination . With an
inclination of  =50° , we can fabricate dovetail
joints for folds with dihedral angles ranging from
 =40° to  =140°.
Figure 6: relationship between maximum tool inclination and
minimum / maximum producible, variable joint angles.
Polytonally shaped plates with multiple 1DOF MTSJ
joined edges can have both positively and negatively
inclined joints on the same work piece. We want to
fabricate these parts without re-clamping or reversing of
work pieces in order to achieve precisely fitting joints.
Figure 7 shows a cross-section schematic drawing of the
tool position on a positive (regular) and negative
(undercut) inclination. The tool center point  lies
above the flatbed router XY-plane for regular cuts, and
below it for undercuts. This is taken into account for the
maximum possible inclination, which we adjust
Figure 7: vertical shift of the tool center point during inclined
cutting, positive for regular cuts and negative for undercuts.
The manual programing of 5-axis CNC cutting with
standard computer aided manufacturing software (CAM)
is not adequate for the fabrication of MTSJ. The manual
programing of hundreds or thousands of tabs and slots
with various 3D rotations and custom details would be
too time consuming. We have therefore developed a
custom algorithm for the generation of the G-Code
(ISO6983) machining instructions, which are sent to the
CNC router.
The fabrication of MTSJ requires the cutting of
polygonal shapes, for which we use tungsten steel shank-
type cutters with a diameter of 10-20mm. The
polygons include various concave corners or slots (cut-
outs within parts). In contrast to convex corners, such
sharp, concave corners cannot be cut with a shank-type
cutter, as its radius will remain as a fillet. We solve this
problem through additional notches (sometimes referred
to “Mickey Mouse Ears”), as illustr ated in figure 8.
Figure 8: FSS made from LVL using open-slot MTSJ [2]
The figure shows a schematic tool path offset around a
polygon , at a distance equal to the milling tool
Diameter . Notches must be added at the concave
corners and . Such notches are required for the
assembly of the MTSJ; also they reduce the notch
stresses compared to sharp corners. However, the
notches also reduce the important contact surfaces of the
joints. We therefore minimize the size of the notches
through the use of tangential circles, see figure 9, option
c. Also our algorithm will cut the notches in a final pass,
using a smaller diameter tool than for the nesting and
cutting of the joints.
Figure 9: Different types of notches possible with the side-
cutting method with cylindrical milling tools. Depending on the
type of notch, different contacts of the joint are reduced in size.
Our 3D method uses tangential bisector notches (c.), similar to
[2], adapted to the aligned 3D cutting process.
Figure 9 illustrates how different types of notches reduce
the contact surfaces (a) of the joints in size. We have
chosen the tangential bisector notch (9c) to minimize this
problem. Generally, the ratio between notch size and
plate thickness decreases with thicker plates. Let the
ratio between cutter diameter and cutter length
(protrusion) be 1:7.5, and we require a protrusion of
90mm to achieve a 3D tool inclination of =50°, the
cutter diameter must be =12. In consequence the
notch radii, and therefore the loss of contact surfaces is
critical for thin plates (such as in the fabrication of scale
models with plate thicknesses of 8-15mm) but greatly
reduced for thicker plates (21-39mm) in building
construction applications.
For the 3D cutting, we define the shape of plates through
pairs of polygons, as shown in figure 10a. There is a
lower polyline and a top polyline
in every pair,
each consisting of a list of points. The 3D geometry of
the part is described through a loft surface between these
pairs of polylines. A plate with cut-outs (e.g. slots)
within the outer contour is described through additional
pairs of polylines for each cut-out, where the orientation
is clockwise, while outside contours are oriented counter
clockwise. Figure 10a shows a pair of polylines,
describing the pin part of a dovetail-type MTSJ, with a
single slot. As explained in figure 7, the tool center point
must be offset three-dimensionally, in contrast to a 2D
offset, as shown in the schematic figure 8. We therefore
process the pair of polygons in segments, where each
segment (representing a joint face) is described by 2
points on the lower polygon ,  and 2 points on the
top polygon , . The line, on which the tool center
point  of a cylindrical tool with the radius
 will
translate, is defined through a point , which is
offset from along the vector , which is the cross
product of =  
and  =
 
, multiplied by the tool radius
 =/2.
 = + (×) 
The tool center point must pass through this point, in the
direction . The start and end points of the tool
path, which both lie on this line, are found through the
intersection of the line with two bisecting planes. The
planes are bisecting between the plane of the current
joint face, and the neighboring ones before and after.
The figure 10b shows how the tool center point transits
on these planes, between the tool path lines of joint faces
with different, three-dimensional inclinations.
Figure 10: new 5-axis milling algorithm using 3D offset
On concave corners, which can be identified through a
negative cross product × , we add a tangential
notch, as described in section 5.1. In 3D, we find the
center axis of this cylinder (shown as a blue line in
figure 10b) defined through the point  =+
((×)/2)  
 and the vector  at this point.
Figure 10c shows the simultaneous translation and
rotation of the tool, in segments where the tool
orientation  at the start point of the segment is
different from the one at the end point of the segment.
A pseudo-code algorithm for the generation of such a 3D
tool path is given in Figure 11:
Figure 11: Pseudo-Code Algorithm for 3D cutting
We have implemented the previously described
automatic G-Code generator algorithm as a plug-in for a
visual programing environment in a commonly used 3D
CAD system. Figure 12 shows that the plug-in consists
of individual components for functions such as 3d
cutting or drilling, which we have used for fixation holes
for the clamping of parts. Input parameters include a
hatch-selected and automatically sorted and processed
list of closed polyline pairs defining the polygonal plate
contours, as described in section 3.2., as well as
adjustable values for the tool radius, security and retreat
planes, the number of vertical passes and separate tool
feed rates for horizontal and vertical movements. The
automatic cutting of tangential notches in the final pass
can be activated and de-activated. On the right, output
values side, the figure shows that we instantly obtain the
G-Code file, including various automatically generated
comments, which are skipped by the CNC control
system, but allow for simplified reading and checking of
CNC files. Changes in the input parameters will appear
directly in the G-Code display.
A third algorithm, visible on the output parameter side in
figure 12, is used for the real-time visualization of the
tool paths in the CAD software. Figures 14 and 15 show
the display the tool paths, and how the motions of the
machine can be simulated to check for collisions with
the tool, tool holder or spindle.
Figure 12: Automatic G-Code Generator CAD Plugin. Left:
input / cutting parameters, middle: separate components
(functions) for predrilling (clamping) and 1DOF MTSJ side-
cutting, right: live G-Code display with comments.
Figure 13: Detailed view of 1DOF MTSJ side- cutting function
inputs top to bottom, 1. Curves: hatch-selected list of closed
polyline plate contours, which will be automatically sorted into
pairs. 2. Zret: retreat plane, 3. Zsec: security plane, 4. Header:
Editable G-Code header, 4. Tool radius, 5. Number of infeeds,
6. Horizontal feed rate, 7. Vertical feed rate, 8. Notches on last
infeed on/off, 9. Rotational axis letters
Figure 14: Display of tool paths and tool orientation for 3D
cutting of 1DOF MTSJ in the CAD software [5]
Figure 15: Integrated Machine Simulation to check for
collision points (Tool path display in this figure is without tool
length offset G-Code function G49). [4]
While the LVL composite box girder in Figure 2 shows
the application of the 1DOF-MTSJ for a simple
orthogonal plate assembly, the methods allows
producing non-orthogonal joints for the fabrication of
freeform space structures such as single-layered Folded
Surface Structures [4] (Figure 14), curved-folded surface
structures [3], double-layered Folded Surface Structures
[8] (Figure 15) or segmented Curved Shell Structures [9]
(Figure 16). The figures show both photos of the final
prototype structures and schematic drawings of the
plates, which are defined through polygon pairs, as
explained in section 3.2.
Figure 14: Single-layer Antiprismatic Folded Surface
Structure made from LVL using open-slot MTSJ, 2014 [4]
These experimental structure prototypes combine the
generally advantageous properties of wood, such as its
sustainability and favorable weight-to-strength-ratio,
with the particular easy machining of the material with
the dimensional stability and quasi-orthotropic behavior
of cross-laminated veneer lumber, and the widely
available 5-axis enabled CNC technology in the wood
processing industry, which allows for the efficient
production of large series of individually shaped parts.
This enables the efficient design and fabrication of self-
supporting thin shell structures, where the structurally
beneficial global shape of the structures is achieved
through incremental changes in the shape of the
individual plates. In addition to the load-bearing
performance of the 1DOF MTSJ joints through integral
connector features, the integral assembly guides in the
form of locator features are essential for the rapid and
precise assembly of freeform structures.
Figure 15: Double-layer Miura-Ori Folded Surface Structure
made from LVL using closed-slot MTSJ, 2015 [8]
Figure 16: Double-layer segmented curved shell structure
made from LVL using closed-slot MTSJ, 2016 [9]
The fabrication of integral 1DOF MTSJ requires the
cutting of polygonal contours with concave corners and
slots. Such cuts are not possible with saw blades.
Instead, cylindrical shank-type cutters must be used,
which results in multiple drawbacks, such as the need for
notches and increased waste and emissions such as dust.
The dust contains particles from both wood and the
adhesives used for the lamination of the veneer layers.
On 3D cutting systems, such as the 5-axis machine for
our tests, the extraction of the dust is particularly
problematic due to the large motion space of the cutting
spindle. The motion space is extended even more
through long cutting tools, which are required for the
cutting with a large tool inclination , in order to avoid
collisions between parts of the machine and the
Another challenge is presented by the clamping of work
pieces on the table of the flatbed CNC routers. The
milling with shank type cutters creates vibrations, which
are influenced by the machine feed rate, tool geometry
and the clamping of the work piece, requiring
compromises. Standard cutting tools are typically
designed for perpendicular, not inclined cutting, and
nesting cutters also create traction forces normal to the
plate for the dust extraction, requiring rigid clamping of
the work piece. Rigid clamping is particularly
challenging in large batches of individually shaped
plates such as in the prototype structure examples
=    
60 1000
   
   
1000 /
From the above equations, we see that a high tool
rotational speed  is required to achieve a sufficient
cutting speed
. It results from this that also a high
machine feed rate
 is required, but these required feed
rates cannot be realised with standard cutting equipment
and tools, due to the polygonal shape of the plates with
MTSJ. In consequence, cutting too slowly, cut quality is
reduced while tool wear and emissions are increased. We
have cut our parts with a feed rate = 6 m/min and
multiple vertical passes (infeeds). The cutting parameters
are provided in table 1:
Table 1: Effective cutting speed using 3D milling
The 3D side cutting method that we have used with the
5-axis enabled CNC milling machine is compatible with
our 5-axis enabled flatbed cutting systems, which we
have investigated as alternative cutting technologies. The
first alternative cutting technology which can be used
with the method and algorithm are 5-axis laser cutting
systems, such as the one illustrated in Figure 4, which is
commonly used in the automotive industry.
In state of the art literature, the laser beam for
woodworking purposes such as separating parts is
considered as a possibility, but infeasible due to a lack of
efficiency, caused by its excessive energy use [10].
However for our application, the cutting of polygonal 3D
contours with concave corners, the laser system provides
advantages. The cut width of the laser system is only 0.6
mm, which allows for the cutting of LVL plates with a
thickness of up to 39 mm. In contrast to the milling
system, this low cut width is greatly reducing waste and
cut offs, and the previously introduced notches on
concave corners are not required (see Figure 19).
Figure 17: 5-axis Laser Cutting System
The system peak power consumption of the 5-axis
milling system used for our tests was 19.5 kW; the one
of the 5-axis laser system used for our tests was 96 kW.
The high power consumption on the laser system is
largely due to its external cooling system. However, the
cutting of up to 39mm LVL with the laser system was
performed at
, = 11 m/min in one pass. With the
milling system, we have cut 21mm LVL with
5 m/min in two passes, resulting in an effective feed
rate of , = 1.5 m/min. The comparison shows that
the laser system allowed for precise 3D cuts at a feed
rate which was more than four times faster than the one
on the 5-axis CNC milling system.
Figure 18: 5-axis 6kW CO2 laser system setup for LVL cutting
Table 1 shows that on a milling system, the number of
vertical passes  increases with an increasing
thickness  of the LVL plates. Therefore the
effective feed rate
, is reduced. Table 2 shows our
tests using a 3D laser cutting system, where cutting is
always performed in one infeed. We have cut spruce
LVL plates of up to 39mm with a feed rate of =
11 m/min.
Table 2: Effective cutting speed using 5-axis laser cutting
While the effective feed rate is greatly improved with the
laser system, the system peak power consumption for
our two testing systems is 19.5kW for the milling and
96kW for the laser system. The 4.9 times higher energy
consumption of the laser system is balanced by the 3.6-
7.5 times faster effective machine feed velocity we
observed in our test cuts with 21-39mm cross-laminated
spruce LVL plates. On the 5-axis laser-system, cut
quality was independent from the cut inclination ,
which was tested up to  =45°.
Figure 19: 5-axis 30° inclined laser cut of a convex (left) and
concave (right) corner on 13-layer 39mm spruce LVL plates.
With its cut width of 0.6mm, a major advantage of the
laser system is presented by its ability to cut sharp
concave corners (or with very small radii), allowing for
cutting MTSJ without notches or “Mickey Mouse ears”.
(Figure 19). As previously discussed this is particularly
relevant for the cutting of thin plate thicknesses, where
the notches would be relatively large and reduce the
contact surfaces considerably.
3.1 Drawbacks of 5-axis Laser Cutting
Two major drawbacks of the laser cutting system are the
burning or charring of the cut edges (Figure 19) due to
the high temperature cutting process, which is mainly a
visual, aesthetic problem, as well as the burnt odor of the
final work pieces. Hazardous emissions are presented by
the fumes generated during the laser cutting, as well as
the laser light. Provided a class 4 visible-light,
continuous-wave laser system, which was used for our
tests, even scattered light can cause eye or skin damage.
Therefore, a complete enclosure of the machine is
required. For the hazardous fumes, extraction and
filtering systems are used. For the cutting of LVL, the
fume emissions are higher than for the laser cutting of
metals. Particularly powerful extraction systems and
measures for fire protection are required for such
The second alternative cutting technology compatible
with the previously presented side-cutting method are
abrasive waterjet cutting machines that are equipped
with an additional tilt axis (see figure 20). We have
performed our tests with a system where the tilt axis can
be rotated up to a maximum of  =59°, which
allows for the fabrication of non-orthogonal 1DOF
MTSJ with a dihedral angle ranging from  =41°
to  =139°.
Figure 20: Abrasive waterjet cutting system equipped with an
additional tilt-axis
The cut width of our water jet cutting test setup was
0.6mm, for spruce and beech LVL plates with a
thickness of up to 39mm, which is identical to the 5-axis
laser cutting system (Table 3). The same applies for the
number of vertical passes, only a single cut is needed to
separate the pieces. With the values provided in table 3,
high edge quality was achieved, as shown in figure 21.
Similar to the laser system, the cutting speed is not
greatly affected by the thickness of the plates.
Table 3: Effective feed rates using 5-axis waterjet cutting
Identical to the laser cutting system, the low cut width of
0.6mm allows for the cutting of concave corners without
or with very small additional notches. As explained in
section 3.5, this is particularly important for thin plate
thicknesses. Similar to the laser cutting system, the
clamping of workpieces is simple. Only small clamps
(figure 18) or even only weights (figure 20) provide for
sufficient clamping. There are no hazardous emissions
such as dust and fumes, only noise protection is required.
Figure 21: 15mm thick plates for a CSS [7] scale prototype,
cut with a 5-axis equipped abrasive waterjet cutting system.
Due to the cut width of only 0.6mm, no notches are needed.
Waterjet (3)
Table 4: Peak power consumption and effective feed rates for
the cutting of 1DOF MTSJ with our side-cutting method and
our systems used for testing: 1. Maka mm7s, 2. Trumpf
TruLaserCell 7040/TruFlow6000, and 3. OMAX 5555/30HP.
4.1 Drawbacks of 5-axis Abrasive Waterjet Cutting
Waterjet cutting is performed with the work pieces over
water basin. The work pieces are either completely
submerged (10mm under the water surface), or just
above the water surface, for splash and noise protection.
For the second case of cutting just above the water level,
we have still observed the work pieces getting wet due to
water splashing.
The 3D side cutting method introduced in this paper
allows for the rapid and precise fabrication of single-
degree-of-freedom (1DOF) integral timber plate joints,
such as dovetail joints or through-tenon-joints. In
contrast to time consuming manual programing with
CAM software, the automatic geometry processing
allows for a rapid generation of machine code for CNC
milling, laser or waterjet cutting. Unlike in previous
methods, it is not necessary to position the cutting tool
normal to the side face of the plate. The method
therefore allows for the use of standard flatbed routers
without any technical modifications, and it allows for the
processing of parts of any size that fits on the flatbed
table of the CNC machine. Instead of positioning the tool
normal to the side face of the plate, inclined faces are cut
with the tool inclined at an angle , which is possible
with modern 5-axis enabled CNC routers. For the
milling with shank type cutters, the maximum tool
inclination  is determined through possible
collision points with the tool, tool holder or spindle and
the work piece. With standard CNC cutting tools, we
have found this limit at 60°. The 5-axis abrasive waterjet
system used for our tests is constrained to a maximum
rotation  =59°. With the laser system, we have
successfully cut inclinations of up to  =45°.
Unlike previous methods, the variable inclination of the
tool allows for the fabrication of 1DOF MTSJ (such as
dovetails), where the dihedral angle between two
joined plates is not constrained to =90°. Instead we
can fabricate joints with a large range of dihedral angles.
While a fixed rotation of  =15° would be
sufficient for the fabrication of orthogonal assemblies
with inclined dovetails, such as the box girder presented
in section 1, a large variable range of angles is required
for the fabrication of freeform shell and spatial structures
built from timber plates, where the global curved shape
is achieved through incremental modifications in the
shape of a series of individually shaped plate elements.
In the last two sections of the paper we have discussed
the drawbacks of 5-axis CNC milling, and presented two
alternative side cutting technologies, which are
compatible with the method and algorithm presented.
While the mechanical behavior of 1DOF MTSJ
produced with 5-axis milling machines has already been
studied [5][6], further research is required to investigate
the behavior of 5-axis laser and 5-axis waterjet cut joints.
6 Acknowledgements
This research was supported by the Swiss National
Centre of Competence (NCCR) in Digital Fabrication.
The Authors would like to thank TRUMPF Laser
Technology, OMAX Waterjet Cutting Systems, Eric
Vassalli and Francois Perrin.
[1] R. Messler, Integral Mechanical Atta chment , 1st Editi on,
Butterworth-Heinemann, 2006
[2] V Sebera and M Simek: Finite element analysis of dovetail joint
made with the u se of CNC technology. Acta univ. agric. et silvic.
Mendel. Brun., 2010, LVIII, No. 5, pp. 321328
[3] C. Robeller, B. Hahn, P. Mayencourt and Y. Weinand. CNC-
gefräste Schwalbenschwanzzinken für die Verbindung von
vorgefertigten Bauteilen aus Brettsperrholz, in Bauingenieur, vol.
89, p. 487-490, 2014.
[4] C. Robeller and Y. Weinand. Interlocking Folded Plate Integra l
Mech. Attachment for Structural Wood Panels, in Int. Journal of
Space Structures, 30/ 2, p. 111-122, 2015.
[5] J. Li and J. Knippers, Segmental Timber Plate Shell for the
Landesgarten sch au Exhibition Hall in Schwä bisch Gmü ndthe
Application of Fi nger Joints in Plate St ructures, in Int. Journal of
Space Structures, 30/ 2, p. 132-140, 2015.
[6] S. Roche, C. Robeller, L. Humbert and Y. Weinand. On the
Semi-rigidity of Dovetail Joints for the Joinery of LVL Panels, in
Europ. Journal of Wood .Prod., 73/ 5, p. 667-675, 2015.
[7] S. Roche, G. Mattoni and Y. Weinand. Rotational Stiffness at
Ridges of Timber Folded-plate Structures, in Int. Journal of
Space Structures, 30/2, p. 153-168, 2015.
[8] C. Robeller and Y. Weinand. Fabrication-aware Design of
Timber Folded Plate Shells with Double Through Tenon Joints.
In Rob otic Fabric ation in Architecture 2016, Springer 2016.
[9] C. Robeller, M. Konakovic, M. Dedjier, M. Pauly and Y.
Weinand. A double-curved Vault Structure built from Timber
Plates - Multi-constraint optimization for Assembly, Prefab-
rication and Structural Design, accepted in Advances in
Architectural Geometry 2016, Springer Verlag.
[10] E. Csanady and E. Magoss, Mechanics of Wood Machining,
Springer Verlag 2013.
... This joint type allows a degree of freedom for self-locking mechanism and builds rigid structures without excessive reinforcements. Still, machining interlocking joints requires more acute angles, requiring machines with 5-axis instead of current conventional 3-axis machines [6]. Nevertheless, using ...
... This joint type allows a degree of freedom for self-locking mechanism and builds rigid structures without excessive reinforcements. Still, machining interlocking joints requires more acute angles, requiring machines with 5-axis instead of current conventional 3-axis machines [6]. Nevertheless, using 5-axis machines for CLT generates some problems: cutting spindle needs to move through large acute spaces, making dust extraction challenging; even with the additional axis, custom tools are required to achieve the prismatic geometry, and the clamping systems is difficult for large batches of individual plated, reducing the cutting velocity and quality [6]. ...
... Still, machining interlocking joints requires more acute angles, requiring machines with 5-axis instead of current conventional 3-axis machines [6]. Nevertheless, using 5-axis machines for CLT generates some problems: cutting spindle needs to move through large acute spaces, making dust extraction challenging; even with the additional axis, custom tools are required to achieve the prismatic geometry, and the clamping systems is difficult for large batches of individual plated, reducing the cutting velocity and quality [6]. Quality and productivity are selling points for offsite construction, and improved systems are needed to overcome the costumer bias against prefabricated components [7,8] . ...
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Cross-laminated timber (CLT) is an innovative construction material that has brought advantages over traditional wood structures, reducing cost and lead time of buildings in recent years; yet, CLT benefits primarily from offsite construction methods instead of automation or safety, while keeping the human onsite. The few advancements in automation for CLT panels have been in the implementation of dedicated CNC machines. Nevertheless, using CNC machines for machining CLT panels have disadvantages like clamping batches of massive panels with individual profiles, lacking the flexibility to access all acute machining angles, and struggling with the extraction of dust while the cutting spindle moves through large tight spaces. These disadvantages can be overcome with industrial robots' help, which the construction industry has not been traditionally favorable on their application, giving then the research gap in this study. This paper explores the introduction of a robotic cell for the machining of cross-laminated timber panels. The robotic cell is designed using 3D modeling and validated through motion simulation in a virtual environment. The proposed cell design is based on a minimum viable product and compared against a minimum throughput benchmarked on the Canadian market. This study aims to research the feasibility of CLT's automated machining by providing clear production characteristics of the designed robotic cell, such as material and tool utilization rates, lead time, or production efficiency.
... Such connections are an integral part of the panels and require a customized automated prefabrication. A large number of connections with various geometries can be easily designed with CAD and manufactured with CAM in a single operation using CNC machines [125]. The single degree-of-freedom (DOF) of MTSJ and TT allows for their fast and precise assembly since only one insertion vector is possible for their positioning. ...
... Automatic design-to-production workflow was developed. The underlying algorithm has already been used in previous projects and was further improved and adapted for the Vidy theater [125]. One of the necessary adaptations was a special postprocessor for the CMS machine of the industry partner company. ...
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... In this case, three-axis laser cutting machines allow only perpendicular cuts to the working surface, making oblique cuts along the thickness of the panels impossible. Furthermore, joints with multiple degree of freedom (DOF) would have prevented the simultaneous sliding of several components during the assembly, because 1DOF-joints constrain relative movements between the parts to a single remaining motion path, as seen in Robeller and Weinand [3]. For these reasons, the orthogonality condition between the normals of each pair of panels was considered essential for the correct functioning of the system, identifying, through experimentation with 1:2 scale elements, an acceptable range within which the condition can be satisfied: 88° < α < 92°. ...
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This paper proposes a new construction system consisting of double frame crossed panels, to be used for the design and construction of free-form architectural structures. The aim of this study is to combine geometric, productive and constructive aspects with form-finding processes, keeping their self-compatibility with designers' intentions, Pottmann [1]. For this purpose, a parametric tool has been developed to adapt the cross-panel system to a wide number of target surfaces, by changing the joints angles and, at the same time, maintaining the orthogonality between panels. The experimentation went on with a case study involving a production and assembly phase using digital fabrication tools, for the construction of a temporary wooden pavilion. The study comprises three main phases: 1. Design of the construction system, through the identification of the elements, their interlocking methodology and the construction constraints. 2. Creation of the parametric tool, to fit the construction system to a given freeform, assessing its adaptability and compatibility. 3. Implementation with digital fabrication, transposition of the digital parameterized model into a buildable prototype.
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... He developed an innovative wood-wood connection inspired by traditional joinery [3]. Such connections particularity is that they are an integral part of the panel, therefore they require a customized automated prefabrication: connectors are cut with the panels in a single operation thanks to CNC (Computer Numerical Control) machining [4]. Similar work was carried out by T. Schwinn et al. [5], where a pavilion made of timber plates was connected with finger joints. ...
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Automation is increasingly present in the construction industry, whether at the design, manufacturing or assembly stage. Thanks to new technologies, such as robotics, new ways of designing structural elements can be imagined and implemented. Complex methods and sequences of assembly can be set up quickly as well. Numerous studies have been carried out in this direction at the laboratory for timber constructions IBOIS (EPFL) especially on wood-wood connections called Integral Mechanical Attachments (IMA). This paper is focused on the mechanical characterization of a prefabricated structural element entirely made of Oriented Strand Board (OSB) panels produced by a fully robotized line. In order to avoid bonding process due to cost, ecological and time reasons, IMA using OSB have been chosen to connect each prefabricated element. Experimental tests and numerical models have been developed to determine the mechanical response of such structural elements.
... When considering more complex geometries with possibly varying MTSJ geometrical parameters, for which the computation time would increase exponentially, using the spring elements could prove to be more efficient. Additionally, using spring elements would be more appropriate for geometry cases with nonparallel adjoining plate edges, as using strips would result in their inconsistent widths along a single edge (for example, timber plate-shell geometries as shown in [28]). ...
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Timber folded surface structures assembled using semi-rigid multiple tab and slot joints (MTSJ) have been shown to form feasible structural systems with high load bearing potential. However, for their further development and use on large building scales, a pertinent model for prediction of their structural behaviour has yet to be developed. This paper focuses on simplified numerical methods for accurately modelling the semi-rigid structural behaviour of bidirectional timber folded surface structures with multiple tab and slot connections. Within this scope, the structure behaviour is considered to be in the elastic stage. Three practical methods of analysis for such structural systems are presented. The first two approaches use the Finite Element Method (FEM), where the theory of plates and shells are applied. In the first method, the MTSJs are modeled using strip element models, while, in the second strategy, spring models are used. The third modeling strategy elaborates on the new macroscopic mechanical models, referred to as macro models. Sets of one-dimensional (1D) elements are used to represent the mechanical behaviour of the entire system. Both linear and geometric nonlinear analysis are performed for all three modeling strategies. The numerical results are then validated against the large scale experiments. Comparison of the strip and spring element model results have shown that the strips represent more accurately the experimentally obtained values. Concerning the macro modelling approach, very good agreement with both detailed FE modelling approaches, as well as experimental results, were obtained. The results indicate that both linear and nonlinear analysis can be used for modelling the displacements within the elastic range. However, it is essential to include geometric nonlinearities in the analysis for accurate modelling of occurring strains as well as for displacements when considering higher load levels. Finally, it is demonstrated that including semi-rigidity in the numerical models is of high importance for analysing the behaviour of timber folded surface structures with MTSJ.
... It consists of a male tab and female slot and enables precise alignment and assembly of components, but contains an inherent instability in the direction of component insertion. This can be partially abated through a fine control of part tolerance to achieve a friction-only fit (Robeller, 2015), or through interlocking geometry which prevents the movement of two parts in all but one direction (Robeller & Weinand, 2016), but such measures can also offset the ease of assembly. In terms of structural capacity, press-fit structures can possess compressive capacity approaching that of the glued sections, but can also be subject to a catastrophic 'pop-off' failure mechanism where sudden loss of friction cohesion causes an explosive bifurcation and complete disassembly (Al-Qaryouti et al, 2016). ...
This paper presents a detailed geometric design-to-fabrication procedure for a new type of timber sandwich structure that combines a folded assembly method and integral mechanical joints. The paper also investigates the hypothesis that such a combination creates a fast and highly-accurate assembly method for modular timber construction. A digital design procedure is first presented and includes a computational method to segment and unfold a target building profile and a computational method to digitally-fabricate segments as timber sandwich panels with integral press-fit and rotational press-fit (RPF) joints. Two structures were built to validate the procedure and hypothesised construction speed: a 30m² house comprising 6 identical folded timber arches, built in one week; and a 42m² canopy structure comprising 4 pairs of varying folded timber `wings', built in two weeks. The as-built structures were 3D scanned and a defect analysis was conducted to assess the reliability and precision of assembled geometries. Both structures were highly accurate, with average absolute surface error generally less than the thickness of the timber material and an average angular defect for RPF joints generally less than 1 degree. In the few regions where larger surface error was observed, a strong correlation was seen with angular defect error and joint interlock failure in RPF joints at that location. Preliminary structural testing of the system also showed it to have a good load carrying capacity for its weight.
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Integral Mechanical Attachment, highlights on one of the world's oldest technologies and makes it new again. Think of buttons and toggles updated to innovative snaps, hooks, and interlocking industrial parts. Mechanical fasteners have been around as long as mankind, but manufacturers of late have been re-discovering their quick, efficient and fail proof advantages when using them as interlocking individual components as compared with such traditional means of joining materials like welding, soldering, gluing and using nuts bolts, rivets and other similar devices. For many years, it has been virtually impossible to find a single-source reference that provides an overview of the various categories of fastening systems and their various applications. Design engineers should find this book to be an invaluable source of detailed, illustrated information on how such fasteners work, and how they can save time and money. Students, too, will find this book to be extremely useful for courses in mechanical design, machine design, product development and other related areas where fastening and joining subjects are taught. This will be the first reference book to come along in many years that will fully illustrate the major classes of integral mechanical fasteners, replete with examples of typical assembly and ideas and suggestions for further research. * Covers all major techniques for integral mechanical attachment within the context of other types of joining including chemical (adhesive) bonding, melting and solidification (welding, soldering, brazing), and mechanical joining (fasteners and part features) * Includes specific chapters for particular attachment considerations by materials type, including metals, plastics, ceramics, glass, wood, and masonry * Provides unique coverage of mechanical/electrical connections for reliable contact and use.
Folded-plate structures provide an efficient design using thin laminated veneer lumber panels. Inspired by Japanese furniture joinery, the multiple tab-and-slot joint was developed for the multi-assembly of timber panels with non-parallel edges without adhesive or metal joints. Because the global analysis of our origami structures reveals that the rotational stiffness at ridges affects the global behaviour, we propose an experimental and numerical study of this linear interlocking connection. Its geometry is governed by three angles that orient the contact faces. Nine combinations of these angles were tested and the rotational slip was measured with two different bending set-ups: closing or opening the fold formed by two panels. The non-linear behaviour was conjointly reproduced numerically using the finite element method and continuum damage mechanics.
Machining plays a basic role in most procedures of the conversion of timber. Machining usually alters the shape, size and surface quality of wood. Machining occurs by cutting in most cases, and chips are the by-product. Some chips are used for the production of chipboard and some are used to generate energy. The chip may be the product in certain cases, and then we talk about technological or target chips. Chips are deliberately produced for the production of plywood, chipboard and wood-wool for packing.
This paper focuses on multiple tab-and-slot joints (MTSJ) for load-carrying assembly of structural wood elements, which are inspired by traditional cabinetmaking joints and adapted for automatic fabrication and engineered wood panels such as laminated veneer lumber. First a numerical method is presented for the estimation of the connections’ semi-rigid properties based on box beam samples. In a simplified model, the mechanical behavior of the joint is represented by potential elastic slips at interfaces. They are handled by the model with the help of rigidity modulus, which is determined by matching simulated and experimental deflections. Finally, the influence of tab length and contact face angle on the semi-rigidity of the joint is discussed considering a dovetail geometry of the MTSJ. Glued and screwed samples serve as references in this study.
A double-curved Vault Structure built from Timber Plates-Multi-constraint optimization for Assembly, Prefabrication and Structural Design, accepted in Advances in Architectural Geometry
  • C Robeller
  • M Konakovic
  • M Dedjier
  • M Pauly
  • Y Weinand
C. Robeller, M. Konakovic, M. Dedjier, M. Pauly and Y. Weinand. A double-curved Vault Structure built from Timber Plates-Multi-constraint optimization for Assembly, Prefabrication and Structural Design, accepted in Advances in Architectural Geometry 2016, Springer Verlag.