2008 schindler zipshape

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ZipShape A Computer-Aided Fabrication Method for Bending Panels without Molds Christoph Schindler designtoproduction GmbH, Zurich, Switzerland, schindler@designtoproduction.com www.designtoproduction.com

Abstract. ZipShape is a universal method to fabricate single curved panels from any plain material without molds. The system uses two individually slotted panels that interlock when bent to the predefined curvature. As non-radial curves require individual teeth geometry, the method makes use of automated detailing with corresponding software routines. ZipShape is a fusion of information processing and material processing based on comparatively simple software technology and standard workshop machinery. With help of case studies, this paper evaluates the method’s claim to be a variable and feasible solution for the realization of non-planar shape. Keywords: ZipShape; Digital fabrication; Cold wood bending. Introduction The manufacturing of curved parts from plain materials is laborious. Special molds are needed to define the curvature during the forming process – a factor that is time consuming and costly, especially in individual production and small series. The company designtoproduction, a consultancy for the digital production of complex designs, is frequently asked for advice how to realize architectural designs involving curved surfaces. Within the scope of a privately funded research project, we sought for a universal method that makes it possible to fabricate single curved panels from any plain material without molds. Several methods for bending plain panels are established in the market, all of them using wood or composite wood: for example Plywood (Ngo and Pfeiffer, 2003), wood products with regular slots

(Spannagel, 1954; e.g. Glunz Topan Form) and methods relying on cold bending of laminated wood (e.g. by Finnforest Merk, see Übelhack, 2007). All these methods do require molds to define the panel’s desired curvature. Instead of optimizing the production of molds, we were looking for a way to get around them: We moved the laborious steps from material processing to information processing and managed to automate them with comparatively simple software routines.

ZipShape information processing The whole method is built upon a geometrical idea. A curved element is assembled from two slotted panels that interlock only when bent to the desired shape (Figure 1). The curvature is defined by the difference between the angles a and a’ of the teeth’s flanks.

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B’ E ’

A’ a

B

D

C

D

C A

Figure 1 Detail section of the two panels before and after assembly. Note the different geometry of the two panels (defined only by the difference between a and a’) that define the curvature of the assembled element.

D’

C’ a’

A

E

B

Established methods use molds e.g. from wood composites to define a curve. The ZipShape method replaces the mold by adding details to the drawing: The curvature is not formed by material, but described by information. A ZipShape curve consists of a large number of teeth.

N2 5 0 N2 5 2 N2 5 4 N2 5 6 N2 5 8 N2 6 0 N2 6 2 N2 6 4 N2 6 6

1 Geometry definition

2 Detailing

In case the curve’s radius is variable like a Bézier curve’s geometry, all angles might be individual and differ from each other only in some tenth of a degree. Not to lose the advantage of skipping the mold with time-intensive CAD-detailing, we automated the steps between curve definition and developing the panel with simple software routines written in the scripting language of the CAD software Vectorworks. To keep the planning as flexible as possible, the detail geometry such as tooth dimensions and incline as well as a layer for the glue between the panels were described as variable parameters. In the case of a chair, for example, a furniture designer would establish a curve using CAD software. The ZipShape script then determines the geometry for the teeth and creates the necessary machine code to mill that pattern (Figure 2).

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X8 8 . 5 4 7 Y4 2 8 . 7 2 2 X8 8 . 5 4 7 Y4 8 1 . 9 8 3 X2 7 2 . 2 5 0 Y4 8 1 . 9 8 3 X2 7 2 . 2 5 0 Y4 2 8 . 7 2 2 X2 7 2 . 2 5 0 Y1 0 7 9 . 4 2 8 X2 7 2 . 2 5 0 Y1 1 3 2 . 6 8 9 X8 8 . 5 4 7 Y1 1 3 2 . 6 8 9 X8 8 . 5 4 7 Y1 0 7 9 . 4 2 8 X2 7 2 . 2 5 0 Y1 5 5 6 . 7 9 8

3 Development

4 Machine code

ZipShape material processing Although the backbone of the ZipShape construction is an immaterial geometrical principle, it is only valuable if compatible with appropriate production methods. We had to find answers to two questions of fabrication: • How to cut individually shaped slots? • How to fix and press the interlocked panels during the hardening process of the glue? Materializing the geometrical principle turned out to be the most challenging and time-consuming part of the project. Soon we realized that we needed a specialized industrial partner for each question. The first question we solved with a long-term partner, the carpentry Bach Heiden and their 5-axis router. Because each tooth has a different geometry, it was not possible to cut a slot with a single tool movement. The fabrication of a single slot had to be subdivided

Figure 2 Concept of automated curve definition with software routines


Figure 3 Grooving the teeth’s first flank with a 5-axis-controlled saw blade. Figure 4 Grooving the second flank.

Figure 5 Manual assembly of the panels Figure 6 Hardening of the glue with uniform pressure in a vacuum bag.

into three operations of the CNC-router: Two cuts with a saw blade (Figure 3, 4) and one cut with a milling bit to remove the remaining edge between the saw blade cuts. Therefore, the fabrication process was slower than we expected. It takes approximately one hour to cut both panels for one square meter of assembled ZipShape. The second question was more difficult to solve. In the first try-outs, after manually assembling the panels (Figure 5) we fixed them with ordinary screw clamps, but it turned out that the insufficient pressure between the clamps caused irregularities in the joint (Figure 7). Finally we found a solution in cooperation with Schilliger Holz AG, a manufacturer for engineered lumber products. The company uses large vacuum bags to glue wall panels. With a vacuum bag

produced for our purposes, we could apply uniform pressure to our interlocked element while the bag’s shape could be adjusted to any desired curve (Figure 6).

Material constraints ZipShape’s geometrical principle is universal, but the characteristics of the chosen material determine its maximal deformation. We applied cold bending to MDF, plywood and massive wood. According to several specialized publications on wood bending (Stevens and Turner, 1948; Stevens and Turner, 1970; Taylor, 2001), a simple empirical formula is applicable both to cold bending of wood and composite wood to determine its minimal radius:

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Figure 9 Full view of the ZipChaise.

rmin = t ∙ 50 (rmin = minimal radius, t = material thickness)

As our slotted single panel consists of two thicknesses and a variable triangle, we could adapt the formula to our purposes (disregarding the triangles): rmin = (t1 ∙ 50 + t2 ∙ 50)/2 (t1 = total thickness, t2 = remaining slot thickness)

According to this empirical formula, a wooden board with a thickness of 20mm could be bent without hot steam to a radius of 1m. With ZipShape, assuming a remaining slot thickness of 2mm, the panel theoretically could be bent to a radius of 550 mm; almost half of the radius of cold bending.

However, the results of our experiments got far beyond empirical forecast: Our MDF-samples reached a factor of r / t = 10 (t = 20 mm, r = 100 mm, see Figure 7). Even the ZipChaise’s massive oak panels in Figure 8 and 9 have a ratio of r / t = 12.5 (t = 20 mm, r = 250 mm); half of the result of our altered formula and a quarter of the result of the original formula for cold bending of massive material. We assume that the vacuum bag’s equal support during the bending process helped significantly to improve the results.

Figure 7 Three samples, from top to bottom: Plywood, (t =12 mm, r = 400 mm), MDF (t = 15 mm, r =200 mm) and MDF color (t = 20 mm, r = 100 mm). Note the irregular joint caused by the screw clamps.

Project analysis

Public recognition The outcome of this research was received warmly. ZipShape was recognized by several independent institutions and won four respected awards: • holz21 Award 2007 by the Swiss Federal Office for the Environment • Design Plus Material Vision 2007 by German Design Council • iF Material Award 2008 by iF International Forum Design • MTechnology Award 2008 Silver by ZOW and iF International Forum Design MTechnology Award’s jury stated enthusiastically: “This system is based on a super intelligent method of making new, completely individual forms out of

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Figure 8 Detail of ‘ZipChaise’ prototype, fabricated in massive oak (t =20 mm, r = 250 mm).


wood. It is tantamount to developing a new material. It opens up countless opportunities to create something completely new. A real innovation that creates an infinite number of new applications.”

Cost analysis Although the four awards helped to get a lot of international publicity, the ZipShape method has not been applied yet to any other project than the case studies represented in this paper. To get some insights into the method’s potential market value, we will briefly analyze it under economic aspects. Table 1 Cost analysis of case study ‘ZipChaise’ with lot size = 1 (the gray rows only show hypothetical sums saved with automated software routines, i.e. they are not part of the total production cost)

a look at the cost structure of the process (Table 1): If the ZipShape method had been carried out without automation, the steps of ‘Manual CAD detailing of the drawing’ and ‘Manual CAM machine code programming’ would sum up to 40% of the total cost. Second, the ZipShape-method saves the cost for the mold. A mold is a considerable investment; e.g. a mold for plywood fabrication is about 3000 EUR. Figure 10 explains the relation between investment and lot size: For lot sizes smaller than fifteen units, the cost per unit of a ZipShape product is lower than a plywood product.

5

Work description

Hours

Cost

Percentage

Manual CAD detailing of the drawing

3h

230 EUR

20 %

Manual CAM programming

3h

230 EUR

20 %

Material cost (1.2m2 massive oak)

370 EUR

32 %

CNC 5-axis milling machine cost

1.3h

140 EUR

Vacuum gluing machine cost

0.5h

110 EUR

12 % 9%

Surface treatment

1.5h

80 EUR

7%

machine

code

Table 1 First of all, to understand the potential of autoCost analysis of case study "ZipChaise" with lot size = 1 (the gray rows only show hypothetical sums mating process steps with software routines, we had saved with automated software routines, i.e. they are not part of the total production cost) Figure 10 Comparing cost per unit of plywood and ZipShape for a theoretical chair: Because plywood production requires a cost-intensive mold, ZipShape is the more economical method for lot sizes smaller than 15 units

3'000 EUR 2'500 EUR 2'000 EUR

Figure 10 Comparing cost per unit of plywood and ZipShape for a theoretical chair: Because plywood production 1'500 EUR requires a cost-intensive mold, ZipShape is the more economical method for lot sizes smaller than 15 units 1'000 EUR

500 EUR

Market area 0 EUR ZipShape addresses a very specific market area: Its40 Units panel size50can be bigger than 1 Unit 10 Units 20 Units 30 Units Units plywood, but at the same time its radii are not able to compete (as long as only cold Plywood bending is applied). Its panel size and radii come closest to bending methods using ZipShape regular slotting such as Glunz Topan Form. ZipShape does not have a backside; both sides of the element have the same high Session 19: Generative Design - aesthetic eCAADe 26 value 799 surface quality. Another point that should be mentioned is the unique of its joint, which reveals the technical background of its formation (Figure 8). It


seems recommendable to use the method for freestanding applications that bring out this quality. But most prominently, ZipShape’s saves the cost for the mold, which qualifies it clearly for individual production. To make the set-up of an individual production cost• Testing other materials than itwood and wood Market areaa certain number of individual copies effective, is required; namely reflects the composites ZipShape addresses a very specific market area: Its principle of “mass customization”. The profile of an individual serial production of panel size can be bigger than plywood, but at the stand-alone objects moderate radii not seem to meet the demands of large same time its radii are notwith able to compete (as longdoesConclusions shares in the market. Its profile is suitable as only cold bending is applied). Its panel size and for rather specialized applications such as radii come closest to bending methods using regular This is method illustrates emblematically how a new x Corporate interior design, e.g. chain stores slotting as Glunz Topanspaces, Form. such as concertconstruction and even a new aesthetic emerges from x such Large interior halls ZipShape does not have a backside; both sides of shifting economically crucial process steps from max Individual furniture like office dividers the element have the same high surface quality. An-

terial processing to information processing.

Method

Max. panel size

Min. radius rmin / t

Mold cost

Plywood (hot steam bending)

ca. 1.00 · 1.50 m

1

3000 EUR

Topan Form (cold bending)

1.03 m · 2.80 m

20

300 EUR

Finnforest Merk (cold bending)

4.50 m · 14.50 m

500

3000 EUR

Zipshape (cold bending)

1.20 · 4.60 m

20

Table 2

other point that should beZipShape mentioned is the unique wood bending However,technologies the analysis of the project’s feasibilMarket area: Comparing with established aesthetic value of its joint, which reveals the techniity and competitiveness demonstrates that many of cal background of its formation (Figure 8). It seems the method’s decisive factors remain on the material recommendable to use the method for freestanding side, such as material-related radii and the challenge applications that bring out this quality. of an appropriate gluing method. But most prominently, ZipShape’s saves the cost To realize this approach, neither sophisticated for the mold, which qualifies it clearly for individual software solutions nor high-end production tools production. To make the set-up of an individual prohad to be invented. A key to innovation in conduction cost-effective, a certain number of individutemporary construction seems to be skillful interal copies is required; namely it reflects the principle weaving existing tools of material and information of ‘mass customization’. The profile of an individual processing. serial production of stand-alone objects with moderate radii does not seem to meet the demands of Acknowledgements large shares in the market. Its profile is suitable for rather specialized applications such as This research project was carried out within the • Corporate interior design, e.g. chain stores tight budget of a small private start-up company. • Large interior spaces, such as concert halls Therefore, it could not have been realized without • Individual furniture like office dividers the grant of IKEA Foundation (Switzerland) and the generous support of Franz Bach and Hansueli Dumelin from Bach Heiden AG in Heiden, Switzerland and Further investigations Ernest Schilliger from Schilliger Holz AG in Küssnacht There are still several open questions we would like am Rigi, Switzerland. to investigate further: • Universal fabrication of ruled surfaces • Reducing the radius by using hot steam instead of cold bending

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Table 2 Market area: Comparing ZipShape with established wood bending technologies


References Ngo, D. and Pfeiffer, E.: 2003, Bent Ply: the art of plywood furniture, Princeton Architectural Press, New York. Schindler, C.: 2007, ZipShape – Gekrümmte Formstücke aus zwei ebenen Platten durch geometrisch variables Verzinken, in Bulletin Holzforschung, 15(1), Schweizerische Arbeitsgemeinschaft für Holzforschung SAH, Dübendorf, pp. 9-11. Spannagel, F.: 1954, Der Möbelbau : Ein Fachbuch für Tischler, Architekten und Lehrer: auch ein Beitrag zur Wohnkultur, 10th ed., Maier, Ravensburg. Stevens, W. C. and Turner, N.: 1948, Solid and Laminated Wood Bending, Department of Scientific and Industrial Research, Her Majesty’s Stationary Office, London. Stevens, W. C. and Turner, N.: 1970, Wood Bending Handbook, Ministry of Technology, Her Majesty’s Stationary Office, London. Taylor, Z.: 2001, Wood Bender’s Handbook, Guild of Master Craftsmen, Sterling, New York. Übelhack, A.: 2007, Produktion und Einsatz von gekrümmten Brettsperrholz- und Hybridkonstruktionen, in: Praktische Anwendung von Massivholzplatten, 39, Fortbildungskurs 7-8 November 2007 in Weinfelden (Switzerland) Conference Proceedings, Schweizerische Arbeitsgemeinschaft für Holzforschung SAH, Dübendorf, pp. 59-61

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