Digital wood bending paper

Digital Wood Bending Melissa Lim

Jia En Chiew

Jessie Tang

Minh Chau Nguyen

Singapore University of Technology and Design melissa_lim@mymail.sutd.edu.sg

Singapore University of Technology and Design jiaen_chiew@mymail.sutd.edu.sg

Singapore University of Technology and Design jessie_tang@mymail.sutd.edu.sg

Singapore University of Technology and Design chau_nguyen@mymail.sutd.edu.sg

ABSTRACT

Wood bending is a process employed in architectural, engineering and product design applications such as timber lattice structures, building interiors, boat hulls, musical instruments and furniture design. The key tenant of wood bending is in preserving the natural fiber continuity and grain of the material while changing the curvature characteristics of a typically linear or planar stock. This reflects the desire to retain both the aesthetics and the mechanical behavior of the organic composite within freeform end product geometry. Bending timber is a complex process due to the irregular grain of the natural material; the requirement of heat treatment in order to relax the fiber bonds that allow the material to acquire a new form; and the complications of retaining the final shape due to mechanical behaviors such as the spring back.

We present an integrated digital design and fabrication process for shaping thin profile linear timber elements into curved geometries using a novel thermoforming approach that does not require complex molds and clamping. Our system is comprised of: (a) a tooling apparatus comprised of a stationary steaming, bending jig and a six-axis industrial robotics pneumatic grip end-effector; (b) a digital fabrication protocol motion planning and control of the robotic wood bender; (c) a parametric modeling component that approximates the bent curved timber elements to a mathematically defined curve.

Keywords

Design Computation; Digital Fabrication; Architectural Robotics; Wood Bending.

INTRODUCTION

Industrial robotic has been greatly influential to the design and research of Architecture (Braumann et al, 2014) since the rapid development of both the Computer Aided Design and Manufacturing field as well as the digital fabrication area (Anderson, 1992). The use of industrial robotics has progressed from onsite construction (Cousineau, 1998) to designers leveraging on robotics to explore new approaches to design and construction (Braumann et al, 2014). They have been used for creating new fabrication techniques that were previously unachievable by traditional craft techniques such as precision based incremental forming of sheet metal and acrylic (Kalo et al, 2014) creative assembly of brick walls (Gambao, 2000) as well as extrusion 3D printing (Brell-Cokcan et al, 2013).

Among those fabrication processes, robots have been used as a form finding method. Robotic wood crafting has been widely explored in the areas of subtractive methods of fabrication. One such example is the translation of typical CNC milling to flank milling with the robot arm (BrellCokcan et al, 2013). In traditional steam wood bending, timber strips are pre-treated, bent and cooled to shape while being clamped onto a mold. Our project seeks to tackle the technical difficulty of working with wood, experiment and evaluate the possibility of mass customization through the integration of robotic fabrication.

DIGITAL AND PHYSICAL PROCESS OVERVIEW

The goal of this project is to design a mass customization method of producing wood bends of varying curvature that can be input into a parametric model for curve characterization. The project is thus organized in the following research and design tasks: (a) Material Modeling, (b) Wood Bending apparatus, (c) Motion Planning for Robotic Fabrication, (d) Geometrical Computation of Wood Bends Material Characterization

Wood is a natural, versatile material used in small-scale artifacts to large-scale buildings. Wood has excellent properties - It is readily and economically available; displays exceptionally good strength to weight performance and a remarkable record for durability and performance as a structural material; a good heat and electrical insulator; its grain patterns and tonality make it aesthetically pleasing (Bender, 1999); it is composed of cellulose- the most abundant natural fiber in the world – a renewable and biodegradable resource. Because it is a natural composite of cellulose fibers in a matrix of lignin, it exhibits irregular anisotropic behaviors that is a challenge that requires skill to tackle.

Wood Bending Methods

In the wood processing industry, there are three commonly used methods of bending wood applied for industrial manufacturing: a) Steam Box Wood Bending; where wood is placed in a box and steamed under high moisture and heat for a period of time. This method is often employed in the area of creative furniture design, art, boat hulls, and musical instruments. b) Laminate Wood Bending – finely sliced layers of wood are glued together and pressured into shape as the glue sets. Kerfing Wood Bending – Wood is kerfed at intervals and bent inwards to achieve the curve. Kerf bending is a new technique made possible through CNC

milling. The steam method is an effective way of wood bending while retaining most of its inherent properties unlike the laminated wood bending where multiple layers of bent wood are glued with an excessive amount of adhesives; and Kerfing which disrupts the connective fibers in wood, weakening the original wood.

Steam bending was chosen due to its non-invasive nature and retains most of the wood during the process without the use of adhesives.

From our preliminary studies, Hardwood has been recommended to be better for steaming than their softwood counterparts. Straight-grained timber was found to be less susceptible to cracks and catastrophic failures during the bending procedure. For our initial test we used Paulownia wood, a material often used for boat and eventually we used pine as our material for the final material due to pine having straighter grain.

Manual steam bending experimentation

To better understand the original steam bending process, we created a simple set up experiment for steaming wood (Figure 2). The experiment was carried out using Paulownia wood pieces of 6mm x 600mm and 4mm x 900mm. These wood pieces are steamed for an incremental amount of time before bending as recorded in Table 1.

Figure 2. Steaming Experiment Set up Upon steaming the wood via the set up, we devised a modular jig to hold the wood in shape to dry. The modular jig enabled us to bend wood at different angles through the rearrangement of pins in jig. A collation of our wood bends results on the jig is shown on Figure 3.

The experiments gave us insights to how we should modify the production process. The bendability of the steamed wood decreases rapidly after it is extracted out of the steam bag. Our experiments showed a repeatability issue as the strips showed discrepancies even when the same bending conditions were applied due to the grain of the wood (Figure 4).

Figure 1. Methods of Bending Wood

Figure 3. Manual Steam bending Experiment We believed part of the issue was the inconsistency in the time taken from taking wood out of the steamer to before the bending process. As a result of human factor and inaccuracy, the pressure that was exerted to bend the wood was inconsistent in magnitude and applied at different positions. To achieve bending without fracture, we had to bend the wood across several points along the wood and this differed based on the person bending it and the quality of the wood. The high temperature of the wood made it scalding for the bender, causing quality of each bend to be dependent on the heat tolerance of the bender, resulting in differences in each bend. The wood takes about 24 hours to set in the Jig, which is time consuming and inefficient for production.

There is also a need to reevaluate on how the wood upon bending sets in the production process.

Tool Building for Wood Bending

We developed a new method for steaming by adapting the traditional steam box method. From our initial experiments with the steaming method, we narrowed down to 2 factors that affected the bendability of wood through steam. The first is the moisture content of the wood, and the second is the amount of heat applied to the wood at the time of bending. Instead of viewing this as a singular process of steaming, we decided to separate the process into 2 parts – first is increasing moisture of the wood and second is applying heat on the wood with high moisture content.

First Iteration: End Effector Heat Application Method

Figure 4. Repeated Curves Showing Discrepancies This experiment’s observations guided our next iteration by highlighting that human factor was significant in compromising the repeatability of wood bending; and that the wood bending process has to be sped up in the steaming process or integrated with the bending process in order for the steam bending to work for the robotic mass production process. In addition, we found out that the effective bending of wood requires not a singular force exertion process but a progressively bending process along the length of the wood.

Figure 5. Initial Concept on End Effector Heat Application Wood Bending Apparatus Our initial tool concept was for the robot to carry a heat supplying gripper as the end effector to bend soaked wood shown in Figure 4. To simulate how this would work, we obtained a handheld heat supplier (Hair Straightener) to test out the viability (Figure 5).

Figure 6. Initial concept for tool We pre-soaked pinewood of 2mm thickness for a period of 2-6 hours before applying it to the heat supplier. Although we obtained successful results as documented in Figure 5, we foresee several difficulties of the implementing this method on the robot. The first is the heat supplier could not double up as a gripper. This is because the most heated soaked portion is located at the heating region that is rigid and does not allow bending. The heated portion has to be pulled out before it can be bent.

Figure 9. Results of Hand Held Heat Supplier Wood Bending Test Second Iteration: Stationary Detached Heat Application with Simple Gripper Method

For our second iteration, we inverted the process in iteration one for wood bending. Instead of using the heat supplier as the end effector and gripper, we devised a stationary heat supplier that dispenses heated wet wood for the simple end effector of the robot to effect on (Figure 9).

Figure 7. Observations on heat and bend locations The direct contact of the heat supplier on the wood also created a situation where the surface of the wood loses moisture much more rapidly than the gain in heat, causing some surface of the wood to be burnt. The success in bending the wood is only achieved upon incrementally heating the wood while bending it at intervals to create an even curvature across the wood. The desired curve is recomposed as a polyline with multiple kinks created by bends.

As wood does not bend sharply at an angle but exerts resistance to it; we get a strip of wood that is between the ideal curve and a polyline.

Figure 8. Wood curve discretized to Polyline

As an improvisation on the heating component of the system, we used a steam iron instead of the hair straightener. The steam iron provided a larger heating surface and steaming functions to retain the moisture of the wood during the heating process. The set up involves a jig attached to a stable table surface. The jig has 3 layers, the bottom protective layer, the middle slit layer and the top securement layer. The steam iron is placed on the top layer while the wood is placed in the middle slit layer (Figure 10). The Iron is slightly elevated to prevent direct contact with the wood preventing surface toasting of the wood.

Figure 10. Section of Wood Bending Apparatus with Detached heat source

Figure 11. Robotic Fabrication Plan with Detached Wood Bending Apparatus Our first jig was made using MDF Board due to its strength.

However, it was replaced after several uses due to intense warping from the high moisture from the steam iron. The jig was improved incrementally to include other features such as extruded bending outlet and angled bending abilities as documented in Figure 12.

Figure 11. Evolution of Wood Bending Apparatus Motion Planning for Robotic Fabrication

This project uses the Universal Robot (UR10) together with the Jeneratiff digital fabrication library that allows the translation of geometry from CAD to the robot by generating robot joint axis angle instructions through inverse kinematics from a list of planes and transforms them into linear tool paths in physical space by the robot.

We were able to perform rapid changes to the robot motion using the direct network connection offered by the Jeneratiff digital fabrication library while simulating the motion digitally on the computer. The key benefits of this approach is the ability to create a sequential production of

custom curve angles for bending based on the parametrically modeled design without the need to go through time consuming tedious process of understanding the formation process.

First Motion Planning Iteration: Simple Passive Gripper Motion Path

Motion planning for wood bending fabrication process includes translating digital design curves to robotic operations. This takes into account properties such as machine acceleration, travel speed and breaking which are correlated to the heat retention of the wood during bending as well as the bending motion of the curves which affects the eventual shape of the bend. Our main challenge was to define the paths such that the end effector can accurately locate the contact position with the wood each time after bending.

Figure 12. Simple Passive End Effector

Our first iteration involves the motion path for a specially designed simple passive gripper made of laser cut plywood (Figure 12).

The passive gripper grips wood by twisting its prongs against it, and reverse its twist to un-grip. Based on this process of gripping, we designed the motion path as follows: The robot wrist twists to grip the wood and pulls it out, then pushes the wood down in a circular arc to bend against the jig-edge, pause to let the bend set for 5s, pull out a length for the next bend, untwist its wrist to un-grip, move away from the wood into the 'safety zone' and repositions itself at the gripping plane and repeat. We planned for the safety zone in consideration that the wood will have a spring back action upon bending at any angle. To prevent the wood from possibly interfering with the gripper, the gripper will retract to this safety zone. From our first attempt, an unplanned contortion occurred in the wood strip. The wood was bent between the gripper fingers instead of bending at the mouth of the jig (Figure 13). We realized that this was caused by the strong twisting effect the passive effector created while trying to bend the wood.

Second Motion Planning Iteration: Pneumatic Gripper Motion Path

Upon realizing that the passive effector not only did not produce the desired effect expected from the motion path, it is also obstructing the bending, we changed to a commercial pneumatic gripper. This action changed our motion path planning, as we no longer need to twist the gripper in order to move the wood. In this robot motion with the pneumatic gripper, the gripper receives signals that causes it to open and close onto the wood before pulling it out, pushes the strip down in a circular arc to bend against the jig-edge, pause to let the bend set for 5s, pull out a length for the next bend, receives signals to open its grip, moves away from the wood into the 'safety zone', repositions itself at the gripping plane and repeat.

The active pneumatic was able to grip the wood precisely and bend the wood strip at the edge of the jig as intended unlike the passive gripper in the first motion planning.

However, it is at this point when we noted that when the robot gripper moved out of the wood bending plane into a 'safe-zone', the wood sprang back against the bend really quickly as there was not enough time for the wood to set.

By retracting back to the safe-zone, we are also unable to accurately direct the gripper back to the wood as the wood becomes a little curved. It is here we realized that the safezone, which we have planned for, is in fact obstructing the wood bending process.

Figure 15. Spring back Effect on Wood Final Motion Planning Iteration: Pneumatic Gripper Motion Path

Figure 13. Passive Gripper Effect on Wood

Our final iteration is a simplified motion path that removed the safe-zone motion. This new motion path is summarized as follows: 1.

2.

3.

4.

Figure 14. Pneumatic Gripper with Teeth

5.

6.

Positioning of robot gripper to end of the wood Robot gripper closing to clamp wood and pull it out by a sufficient distance (3cm) for bending Robot gripper making a circular motion with respect to the edge of the jig to bend the wood downwards at a specified angle (30 degrees) to the edge of the jig and holds it there to set ( 5seconds) Robot gripper pull the wood out by 6cm ( bending interval length) Robot gripper releases wood and returns to its initial bending position, 3cm away from jig while sliding up the bend of the wood.

Robot gripper repeats step 2 to step 5 while having variable inputs for bending angle and bending interval length.

Figure 16. Pneumatic End Effector Motion Path When bending the wood, we realized if the edge we are bending against is slanted, the bend could be given a direction away from the initial bending action, result in twirls and spiraling effects. The jig design was improved to allow the wood freedom to slant with respect of the bending edge, causing twirls at varying angles to become possible.

This discovery motivated us to explore how we can improve our motion path to integrate the spiral.

We planned the robot motion path for wood twirling in two ways. First, the robot pulls out the wood with the gripper and rotate the strip about an XY plane at the bending point, bend it, pull out a length for the next bend, reposition the wood 90degrees in the XY plane to the bending plane and repeat. Alternatively the wood can be fixed at a predetermined slant before the robot performs the whole sequence of slant bends (Figure 17).

The first method enables the robot to bend varying angles across the wood as each angle is separately positioned and repositioned by the robot itself. Though the second requires the human helper to set the initial angles to which the robot would work on, it would be more efficient at bending the wood as it does not have to position and reposition the wood. Ultimately, we chose to proceed with the 2nd option for the twirling effect. Despite the range of angles afforded by the robot setting out each different angle for slant bending, this motion planning would result in several fast twists by the robot wrist that would cause the robot to jam.

The range of motion performed by the robot in the first motion is also much greater and varied, making its actual

path harder to predict and results in the motion clashing into our jig or the wood. The second method is more constrained, as it is restricted to performing same slant repeatedly allowing for more predictable results that is suitable for production.

Figure 17. Tool Bending Effector with Slant Bends for twisting Compared to conventional steam box bending method, our robot bend is a quick approach to performing consistent wood bending under constant conditions without the use of molds and clamps. As the wood is not clamped to molds, each curve is achieved by spring back from the over bending of the wood at an exaggerated angle (Figure 18). Only by multiple over-bends at short intervals can an overall arch emerge. At larger angles of 80-60 degrees these over-bends at the bending edge would tear apart wood fibers on the upper surface that are in tension, weakening the wood.

Hence, this process could be considered to be destructive to the overall tensile nature of the wood and could be placed between kerfing and steam box bending in its degree of maintaining wood properties.

Figure 18. Robotic Fabrication Process As the wood spring back is an anticipated and integrated part of our wood bending process, the bending angle of the wood cannot be expected to give an accurate one-one curve.

However, this could be used as a means to make flexible wood strips of varying curvature that would spring back into a curve.

After many bends, cracking was observed to occur over the eyes of the wood that is a break in the continuity of the grain. As such, adding adhesives to bend the crack may be considered. Otherwise using a thicker strip of wood may improve its durability from fracture.

Geometrical Computation of Wood Bends

From the previous sections we have heavily experimented on creating curve bends and twirl bends. In order to utilize our findings, we attempted to digitalize the wood curves we obtained into digital curves so as to assist us in our final Design.

Since our approach to bending was creating incremental bends of various degrees with a constant bending distance, we expected an arc-based approximation. We attempted to approximate the curves based on 3 different graphs: the arc; the parabolic curve; the cubic curve by overlaying the digital curves onto a photo of the wood bend.

For arcs, 3 points are located on the wood bend for the approximation.

For parabolic curves, many sets of 3 control points are first introduced to create parabolic curves. These curves are then evaluated to locate the highest points. The distance between the highest point on the wood bend and the highest point on the parabolic curves are calculated and the parabolic curves with the shortest height difference is used. For cubic curves, instead of 3 control points, 4 control points are used to create the approximate curve.

The results were different from our expectations of an arcbased curvature. Shown in Figure 11 is the overlay of the 3 curves with the original wood bend. It is found that the parabolic curve best describes the wood bend curvature.

This could be due to the approach we took to bend the wood. As mentioned above that we used sectioning to

bend our wood incrementally, however, our starting and ending sections of the wood bend are not bent. This could have affected the overall bend and hence resulting in it performing more like a parabolic curve.

Based on the parabolic curve approximation, we can thus create a relationship diagram between the digital curve and the physical angle we used for the incremental bending. The parabolic curve is described the equation ax2 + bx + c. The Parameters a,b,c are evaluated based on the curve, the start point, end point and the highest point.

With these information, we can find the relationship between the curvature of the parabolic curve by mapping the value of a to the bending degrees we used for the incremental bending. The results are as shown below.

This relationship can be described by a linear equation: bending angle =-69.832a+2.48. We envisioned that should we have a design we can get the exact bending angle we should program the robot to perform. However as these approximations are performed on 80cm length wood strips, it is hard to extrapolate the results to wood of other lengths.

Figure 19. Graph of Bending Angle Vs Coefficient a

Different arc-based approximation with different properties

Parameters to describe an arc: r

Parameters to describe an parabolic curve: a,b,c where ax2 + bx + c = 0

Parameters to describe a cubic curve a,b,c,d where ax3 + bx2 + cx +d=0

Mapping physical curve to digital curve

Height Width Building arcs

Building parabola curves

Building cubic curves

Choosing the best model to describe the curve

Results of best fit parabola curve 90

60

30 Degrees

y = (-0.000394)x2+ (-0.0466)x + 11.3

40 Degrees

102

y = (-0.000623)x2+ (-0.291)x - 1.17

755 50 Degrees

710

720

y = (-0.000745)x2+ (2.097)x - 502

70 Degrees

y = (-0.000106)x2+ (2.825)x - 600

Figure 20: Physical curve approximation

y = (-0.000631)x2+ (1.337)x - 294.7

140

138

740 60 Degrees

90

760

780

80 Degrees

y = (-0.000111)x2+ (4.210)x - 1044

Figure 21. Artist Expression with Robot Fabricated Curves

CONCLUSION

Overall, this project challenges typical steam bending of wood by creating a new method of fabricated wood curvatures with the aid of robotics fabrication technologies.

Bridging the method of kerfing and steam bending, this method achieves incremental bending without the use of molds or setting fixtures. The reduced heat supply in our process also reduced production time and energy savings for the potentially similar applications. Moreover, these curvatures can be further designed to include more complex geometries. Given the research and experimentation done to fabricate such wood bends, it holds many possibilities for future wood bending production, especially in the area of artistic explorations.

ACKNOWLEDGMENTS

We thank Singapore University of Technology and Design, our tutor and mentor Stylianos Dritsas and all publications support and staff, who wrote and provided helpful comments on previous versions of this document.

Jobscope Assignment: Design: Melissa, Jia En, Jessie, Minh Chau Computation: Jia En, Melissa Motion Planning: Jia En Tool Building: Melissa, Jia En, Jessie Prototyping: Melissa, Jia En, Jessie Paper: Jessie

REFERENCES

1. UNESCE: 2005, Press Release ECE/STAT/05/P03, Geneva, 11 October 2005, World Robotics Survey, last accessed July 2013, http://www.unece.org/fileadmin/DAM/press/ pr2005/05stat_p03e.pdf.

2. Anderson, R.E. Social impacts of computing: Codes of professional ethics. Social Science Computing Review 10, 2 (1992), 453-469.

3. Cousineau, Leslie, and Nobuyasu Miura. Construction Robots: The Search for New Building Technology in Japan. Reston, VA: ACSE, 1998. Print.

4. Kalo, Ammar, and Michael Jake Newsum. "An Investigation of Robotic Incremental Sheet Metal Forming as a Method for Prototyping Parametric Architectural Skins." Robotic Fabrication in Architecture, Art and Design 2014 (2014): 33-49. Web. 5. Gambao, E., C. Balaguer, and F. Gebhart. "Robot Assembly System for Computer-integrated Construction." Automation in Construction 9.5-6 (2000): 479-87. Web. 6. Brell-C̦okcan, Sigrid, and Johannes Braumann. "Digital

by Material : Envisioning an Extended Performative Materiality in the Digital Age of Architecture."

Rob/Arch 2012: Robotic Fabrication in Architecture, Art and Design. Wien: Springer, 2013. 12-27. Print.

7. Braumann, Johannes, Sigrid Brell-Co̧kcan, Aaron Willette, Wes McGee, and Mónica Ponce De León.

Robotic Fabrication in Architecture, Art and Design 2014. Print 8. Sigrid Brell-Çokcan, Martin Reis, Heinz Schmiedhofer, Johannes Braumann, Digital Design to Digital Production, 2013 9. Bender, D. Wood Handbook: Wood as an Engineering Material. Madison, WI: USDA Forest Products Laboratory, 1999. Print.