Shear cutting and counter shear cutting of sandwich materials

Journal of Manufacturing Processes 15 (2013) 364–373

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Technical paper

Shear cutting and counter shear cutting of sandwich materials M. Liewald, C. Bolay ∗ , S. Thullner Institute for Metal Forming Technology (IFU) – Holzgartenstr. 17, 70174 Stuttgart, Germany

a r t i c l e

i n f o

Article history: Received 20 September 2012 Received in revised form 9 February 2013 Accepted 5 March 2013 Available online 2 April 2013 Keywords: Counter shear Cutting Burr Sandwich materials

a b s t r a c t New lightweight sandwich materials challenge existing forming processes as well as following process steps. As such the manufacturing potential of shear cutting has to be evaluated. Two cutting methods are compared. Method commonly used is shear-cutting within one stroke engaged, the other one is known as counter-shear cutting, which uses two strokes. The challenges of cutting sandwich materials are variation of hole diameter within the different layers, fraying of the textiles, deformation of the hole contour and burr formation. These effects occur in conventional shear cutting as the intermediate layer and the lower sheet metal are cut by the scrap of the upper sheet instead of the cutting punch. The following methodology included shear cutting with closed cutting edge i.e. cutting of holes into five different sandwich materials. The sandwiches exemplarily represent multiple kinds of possible material designs. For instance, aluminum and steel face sheets, different thicknesses of intermediate layers and different intermediate layers materials such as integrated textile fibers have been used. Adequate cutting parameters such as die clearance and the use of a blank holder have been determined. To achieve good results a stiff machine design with good guidance and precise control of punch position was crucial. Observations of conventional shear cutting revealed the need of small cutting clearance of 4%. High burnish area is possible for the upper face sheet due to the superimposed force by the lower face sheet. The major conclusion depicted that high cutting quality of sandwich materials requires counter shear cutting. Hence, the roll-over of the lower sheet facing the intermediate layer, the burnish area at the lower sheet, good cutting quality of the fibers improve significantly and burr formation is avoided completely. Summarized this paper provides cutting parameters for sandwich materials based on experimental work. © 2013 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.

1. Introduction Due to low weight properties, sandwich materials today have gained high acceptance not only in industrial applications but also, for consumer products too. Current research work focuses on further requirements such as stiffness, strength and vibration damping. The category of layered composite materials consists of different materials such as metals, plastics, textiles and ceramics. The following classification according to Hufenbach defines three mayor fields of applications of multi layered composites [1]. By using the combination of thin face sheets with a low-density core or formed elements the moment of inertia increases in terms of a structural lightweight design. Thus, weight of the sheet metal decreases but low formability by cracking and wrinkling due to high sandwich thickness and missing support of the intermediate layer can be observed. Examples for such structural lightweight

∗ Corresponding author. Tel.: +49 711 68583821; fax: +49 711 68583821. E-mail address: [email protected] (C. Bolay). URL: http://www.ifu.uni-stuttgart.de/ (C. Bolay).

sandwiches are sandwich materials with thin face sheets combined with a light polypropylene core [2], an aluminum foam core [3] and a integrated hump technology in the intermediate layer [4]. Acoustic damping sandwiches normally consist of face sheets bonded by very thin viscoelastic intermediate layers. This allows good formability, but compared to monolithic sheet metal it exhibits reduced bending stiffness of component [5–7]. Reduced sound radiation enables secondary weight savings within subsequently applied damping material. The third category of layered composites includes sheet metals with a surface finishing [8] (Fig. 1). The state of the art discusses mainly the formability and the component properties in terms of a high stiffness and strength. No comparison of shear cutting of the shown classification exists. Technical datasheets of commercialized products recommend small cutting clearances for cutting lightweight [9] and damping sandwiches [10]. Only few scientific investigations have been conducted on shear cutting of sandwich materials until now. Cutting of double layered sheet without an intermediate layer e.g. after welding and one commercial lightweight sandwich have been investigated [11]. Within a new work different face sheet thicknesses with a viscoelastic intermediate layer for shear cutting have been analyzed

1526-6125/$ – see front matter © 2013 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jmapro.2013.03.001

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Fig. 1. Classification of layered composites.

[12]. So far no research on counter shear cutting of sandwich sheet is known. 2. Fundamentals of shear cutting and counter shear cutting

FS max = A∗S kS

(4)

lS∗ s

(5)

AS =

Fig. 2 depicts the principle of a shear cutting tool which consists of a cutting punch and a die. Either a blank holder force is applied or a blank holder is located slightly on the sheet top surface. The latter type of blank holder works as a stripper to prevent blank deformation during the back stroke of the punch after cutting. Concerning the part geometry there is a difference in piercing and trimming [13]. In this work, piercing process will be investigated in detail and the cutout in this work is defined as scrap. The clearance between punch and die is an important process parameter which depends on the guidance of cutting tool or the machine. The clearance of a cutting tool can be defined by the following two relations: absolute clearance u = (ddie − dpunch )/2

(1)

relative clearance urel = (u/s) ∗ 100%

(2)

Depending on the sheet metal different clearance values are chosen [14]. u = c ∗ s∗ 0.1∗ Tb

tance up to 60% [15]. The required cutting force can be calculated with the following formula.

(3)

 b shear fracture strength; c ≈ 0.8 (0.6 brittle and hard materials; 0.95 for ductile materials with good formability)Wear of cutting tools i.e. higher clearance can lead to an increase of the cutting resis-

AS cutting area; lS cutting length; s sheet thickness; kS cutting resistance 0.8*Rm [16], (0.8. . .0.86)*Rm [17], [2–((1 + c)/1.01)]*(1–0.005)*Rm when c = 0.005 for highest cutting quality [14] Fig. 3 shows the principle of counter shear cutting. Two strokes are performed. The first stroke acting from lower blank side the sheet metal has the cutting depth tE that is less than the material’s thickness. The second fully penetrating final stroke follows from the upper side. This process improves the cutting quality by avoiding burr formation and increasing the burnished area [18–20]. Usually the last stroke is toward the bottom side to eject the scrap. For the cutting operation appropriate punch and die diameters have to be chosen. In case of piercing the punch diameter of the final stroke defines the hole and will not be changed. To achieve a constant hole diameter equal punch diameters for both strokes from both sides are necessary. But, only small deviations that may be caused by machining tolerances of the cutting tool, the variation of sheet metal thickness or vibrations during cutting can lead to a bulge (Fig. 4). As the cutting is performed in two strokes from the lower and upper side of the sheet metal the cutting clearance of the first stroke has a major influence on the final stroke. A slight negative clearance during first stroke avoids bulging and leads to a robust process (see Fig. 3) [19]. Negative clearance means that the first punch overlaps the upper die’s diameter. Using a negative clearance cutting completely through the sandwich in the first stroke must be avoided. Otherwise the punch and the die would crash and the tool would be damaged. For the tool design the following two conditions (a) and (b) should be complied. (a) dpunch1 > dpunch2 to keep the diameter of the first stroke in terms of dimensions, roll-over and burnish area (b) dpunch2 > ddie1 to avoid bulging (see Fig. 4)

Fig. 2. Principle of shear cutting of monolithic sheet metal.

The following scheme in Fig. 5 explains measured outcome of both cutting processes. In the beginning the sheet metal is formed until reaching the tensile strength which leads to fracture by cracking. The cutting edge consists of a roll over on the upper side (red), followed by the burnish area (blue), a fracture area (green) and a burr on the lower side. The shear counter cutting edge shown right has a roll over and a burnish area on both sides instead. The scheme of the cutting edges in Fig. 5 depicts advantages and disadvantages of both processes very well. The conventional shear cutting process represents a commonly used and very efficient process. The need of cutting dies according to the part geometry

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Fig. 3. Principle of shear counter cutting of monolithic sheet metal.

Fig. 4. Principle of shear counter cutting with a bulge of monolithic sheet metal.

Fig. 5. Conventional shear cutting edge vs. shear counter cutting edge of monolithic material.

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Fig. 6. Shear cutting of sandwich materials.

Fig. 7. Effect of blank holder on delamination during back stroke.

(except nibbling machines) leads to application for high quantities. However the achievable fracture area and burr formation do not always meet the final part requirements. On the other hand the counter shear cutting also affords geometry based cutting dies and an additional second stroke. This leads to increased burnish areas and no cutting burr. The cutting bulge in Fig. 4 can be controlled through the die design. The counter shear cutting of sandwich materials promises even more advantages, as each face sheet is in contact with a cutting punch. The characteristics and objectives of cutting sandwich materials will be explained in Section 3.

3. Shear cutting of sandwich materials Sandwich materials challenge the cutting process in terms of a reduced quality of the cutting edge e.g. low burnish area and failure of workpiece due to delamination [11]. As Fig. 6 illustrates the punch cuts the upper sheet and pushes the scrap towards the intermediate layer and the lower sheet. Thus the lower sheet is cut by the scrap of the upper one which leads to different upper hole and lower hole diameters.

Fig. 8. Edges of conventional shear cutting vs. counter shear cutting of layered sandwich materials.

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Fig. 9. Tested sandwich materials.

A risk of failure occurs during the return stroke of the punch by a normal force due to elastic spring back. As Fig. 7 depicts the upper sheet can be lifted up due to friction forces between the sheet and the punch. The blank holder force acts against the force caused by friction and helps to avoid delamination. The cutting edge which is defined in this work over the complete thickness s significantly differs from a monolithic sheet as Fig. 8 illustrates. At shear cutting only the upper sheet shows a burnish area and a second roll over of the lower sheet occurs. Using counter cutting for sandwich materials promises a higher quality of the lower hole diameter, no roll over facing the intermediate layer and a second roll over instead of the burr. Also a burnish area of the lower sheet can emerge in some cases.

4. Testing material and experimental setup To represent variety of the existing sandwich materials the selected sheets for this investigation vary by the following material parameters: • • • •

Steel and aluminum face sheets Thickness of the face sheets Thick and thin intermediate layer Presence of fibers in the intermediate layer

Fig. 9 depicts different testing materials which include commercial products and materials still in development. The Lightweight Sandwich is used for structural applications but its very high

Fig. 10. Adopted cutting tools and cutting machine [21].

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Fig. 11. Methodology and cutting clearance of the experiments.

stiffness causes a very low formability. The damping sandwiches with steel and aluminum face sheets provide high acoustic damping characteristics, high formability but feature a reduced stiffness. The Textile Sandwich provides high stiffness, increased damping and formability properties. For the experimental investigations reported about in this paper a cutting machine with two controlled tool axes, which are defined as ram and active die clamping fixture as shown in Fig. 10 has been used. The cutting tool for the first stroke of shear counter cutting has a blank holder on the die and on punch side [16]. The setup for the second stroke is equal to the one used in conventional shear cutting. For the whole work a punch diameter of 10 mm has been chosen. The die diameter is determined by the needed relative cutting clearance (refer to Section 2). The blank holder diameter for the second stroke is kept at 11 mm. The testing parameters in the following experiments are the cutting clearance u, the blank holder force and the depth tE of the first shear counter cutting stroke into the lower sheet. 5. Experimental results and discussion The investigation of conventional shear cutting behavior of sandwich material has been divided in two steps. The first approach in terms of pilot testing of the five materials served to determine adequate cutting clearance (definition see formula 1). The constant punch diameter of 10 mm and different dies corresponding

to Fig. 11 were chosen. The die material was a hardened tooling steel (1.3343S 6-5-2; HRC 62). The cutting speed of 300 mm/s and forming speed (first stroke in counter shearing) of 100 mm/s was given by the range of the cutting machine. Summarizing the major variation of the relative cutting clearance in pilot testing proved that lowest roll over, lowest diameter variations and highest burnish area of the upper sheet could be achieved with a 4% relative cutting clearance. Compared to monolithic materials the cutting clearance of sandwich materials reduces about 6% of the common value of 10%. Hence the different face sheet and total sandwich thickness play a minor role and can be neglected. The recommended decrease to 4% compared to monolithic materials can be explained by the reduced thickness of the single face sheets even though the total thickness of sandwich materials usually is higher. However, poor cutting quality of the fibers is still observed. In the second step the adequate clearance of 4% has been chosen. A detailed analysis was performed and the use of a blank holder has been investigated. The process parameters were applied to the second stroke of shear counter cutting. To evaluate the holes’ diameters a microscope (enlargement values 0.5–2) was used. The burnish area has been examined in top view of the cutting edge through another microscope (enlargement values 25–50). Analyses of metallographic specimens with the second microscope provide the dimensions of roll over, burnish area and burr. The shear cutting edges are shown in Section 6.

Fig. 12. Results for shear cutting with and without blank holder.

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Fig. 13. Depth of the first stroke with two examples.

Testing different blank holder forces showed that a force below 5000 N avoids marks on the face sheets. Fig. 12 illustrates the measured average values of 5 experimental repetitions using a 4% cutting clearance. This data give an idea of the range within measurement, e.g. 100 ␮m for the diameter or 10 ␮m for the burr height.

The gray highlighted cells indicate the improved results either with active or passive blank holder. An active blank holder only allows high cutting quality for the Damping Steel Sandwich 2 respective the hole diameter and the cutting burr. The upper sheets experience high burnish area. Independent from the use of a blank holder

Fig. 14. Ground microsections of sheared cutting edges.

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Fig. 15. Burnish and fracture area of sheared cutting edges.

the lower sheet of this sandwich does not show any burnish area. The burnish area of the Textile Sandwich cannot be evaluated due to the bad cutting quality of the fibers (compare Fig. 15) and the resulting fraying. The high potential concerning the cutting quality of counter shear cutting compared to conventional shear cutting the cutting edge is shown later. The first stroke in counter shear cutting uses a negative clearance. This additional operation differ counter shear cutting from shear cutting. Counter shear cutting is defined by this first step. The machine controls the feed length of the first punch. The process starts with rolling over the hole’s edge (forming process) before actual cutting (severing process) sets in. Therefore, the resulting cut depth tE ,shear is less than the stroke feed tE ,machine (see Fig. 13a). The two examples in Fig. 13b and c show the influence of lower cutting operation on the upper sheet. The cutting depth must be sufficient to completely shear the lower sheet without shearing the upper one. This setup for the feed has been confirmed by the analyses of the final counter shearing results. Criteria were the biggest burnish area and lowest diameter deviations.

The second column in Fig. 14 depicts the insignificance of a blank holder on the cutting edge. The sheared cutting edges by conventional shear cutting of sandwich materials exhibit a roll over of the lower sheet facing the intermediate layer. It can be eliminated by counter shearing. Counter shear cutting does not form burr (Fig. 14, third column). However, counter shearing of thick layers (Fig. 14c) causes a gap between the two operations. Shearing of the intermediate layer depends on the used material and layer thicknesses. In conventional shear cutting the scrap goes ahead the punching tool. Thus elastic and textile fiber intermediate layers do not get sheared by the scrap (Fig. 14d, e, g, j, m, n). In contrast, counter shearing provides major improvements (Fig. 14f, i, l, o). When using face sheet metal with reduced stiffness such as aluminum (Fig. 14a–f), the roll over increases compared to steel face sheets (Fig. 14g–l). The highest roll over occurs when thin steel face sheets and textile intermediate layer are cut (Fig. 14m–p). The last example (Fig. 14p) shows a bulge at counter shearing due to positive clearance. It is crucial to avoid such tool diameters. An overlapping negative clearance is necessary for robust cutting.

Fig. 16. Burnish area of upper and lower face sheets in conventional shear cutting and counter shear cutting.

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Fig. 17. Accuracy of hole diameters of upper and lower face sheets at conventional shear cutting and counter shear cutting.

In Fig. 15 the visualization of the magnified cutting edges’ top view evaluates the amount of burnish and fracture area. The lower sheet at conventional shear cutting has no burnished area (Fig. 15a and c). Its cutting edge consists only of a fracture area. For Textile Sandwiches it is not possible to measure it. Fraying is caused by bad cutting quality of the fibers. The major enhancements using counter shear cutting are clearly visible (Fig. 15b and d). Now the Damping Steel Sandwich 2 and Textile Sandwich show 100% burnish area at the lower and upper sheet. Corresponding to side view in Fig. 14 cutting of the fibers is significantly improved compared to conventional shear cutting. The results in Fig. 16 focus on the burnish area. It is characterized by its percentage of the corresponding face sheet thickness (both face sheets always have the same thickness). The remaining percentage corresponds to the fracture area. The second focus is the achieved holes’ diameters especially the variation between the upper and lower one which depicts Fig. 17. Further geometric factors such as the angle of the cutting areas and height or width of the roll over and burr have been determined and will be discussed in a future article. Compared to cutting of monolithic sheet metal the upper sheet of a sandwich shows in Fig. 16a very high amount of burnish area up to 100%. The reason for this might be a superimposed force by the lower sheet while cutting the upper sheet. The lower sheet is sheared by the scrap of the upper sheet, which leads to very low or 0% of burnish area. When counter shear cutting is performed the burnish area is largely improved. Now, the aluminum face sheets used in Lightweight Sandwich and Damping Aluminum Sandwich both have a burnish area of 90% at the upper and at least 30% at the lower sheet. The sandwich materials with steel face sheets such as Damping Steel Sandwich 1 and 2 and Textile Sandwich show very big burnish area up to 100% for both face sheets. Fig. 17 illustrates the resulting hole diameters of the upper and lower sheet in shear cutting and counter shear cutting. Both diameters mainly depend on the contact to the cutting punch. By using counter shear cutting the quality of the lower sheet in terms of the reduced deviation from the set punch diameter of 10 mm is improved (see dashed line in Fig. 17). For Lightweight Sandwich, Damping Steel Sandwich 1 and 2 the deviation of the upper sheet’s diameter to the set value of 10 mm is reduced.

6. Conclusions It’s absolutely crucial to use a small cutting clearance for conventional shear cutting of layered sandwich materials. Except of textile layers good cutting edges can be achieved. Small diameter deviations between face sheets and roll over of the lower sheet facing the intermediate layer are acceptable. The main reason for reduced quality is that the lower sheet is sheared by the scrap of the upper sheet. The use of a blank holder does not improve the cutting quality. This might be different after a forming operation. A deep drawing operation, for example, exposes the intermediate layer to a previous stress. To avoid cutting burr formation and fracture of the lower sheet counter shear cutting process has to be used. As well, major improvements of the holes’ diameters are reached. When sheets with thin intermediate layers are cut, a very straight cutting edge with very high burnish area of both face sheets is possible. Thus shearing of elastic intermediate layers is improved. An adequate quality of the cutting edge of textile fiber layers only is possible with counter shear cutting. However, to reach this higher quality the process knowledge and the additional cutting punch needs to be respected. Future work will be done in cutting more complex geometries and different types of fibers. Acknowledgments This work was carried out at the IFU using financial means within industrial research alliance (IGF) brought by the European Research Association for Sheet Metal Working (EFB). The authors would like to thank Dr. T. Stegmaier, A. Vohrer and T. Hager at the Institute of Textile Technology and Process Engineering Denkendorf (ITV) for the good cooperation and the manufacturing of the textile sandwiches. As well special thank go to R. Hank and J. Kappes of the TRUMPF Werkzeugmaschinen GmbH+Co. KG for the support in experiments on the TruPunch 5000 punching machine. References [1] Hufenbach W, Adam F. Structuring and classification of steel sheet multi layered composites, research project P307. Düsseldorf: Society of Steel Appliances; 1996.

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