77 Asian International Journal of Science and Technology in Production and Manufacturing (2008) Vol. 1, No.2, pp. 77-87 Development of CNC Pipe Bending Process in Small Batch Manufacturing Environment John P.T. Mo, Fareed Al-ayid and Tony G.Z. Chen School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Victoria, Australia Abstract In truck manufacturing, the exhaust and air inlet pipes are specialized equipment that requires highly skilled, heavy machinery and small batch production methods. This paper describes a project to develop the computer numerically controlled (CNC) pipe bending process for a truck component manufacturer. The company supplies a huge range of heavy duty truck parts to the domestic market and is a significant supplier in Australia. The company has been using traditional methods of machine assisted manual pipe bending techniques. In a drive of continuous improvement, the company has acquired a pre-owned CNC bending machine capable of bending pipes automatically up to 25 bends. However, due to process mismatch, this machine is only used for single bending operation. The researchers studied the bending system and changed the manufacturing process. Using an example exhaust pipe as the benchmark, a significant drop of manufacturing lead time from 70 minutes to 40 minutes for each pipe was demonstrated. There was also a decrease of material cost due to the multiple bends part in one piece without cutting excessive materials for each single bend like it used to be. Keywords: Tube bending; CNC bending; small batch manufacture; automotive components manufacture. 1 INTRODUCTION In truck manufacturing, the exhaust and air inlet pipes are specialized equipment that requires highly skilled, heavy machinery and small batch production methods. A truck component manufacturer supplies a huge range of heavy duty truck parts to the domestic market and is a significant supplier to truck manufacturers and spare parts network throughout Australia. The company’s manufacturing division has the ability to adapt to small and large volumes. They have specialist equipment enables them to manufacture exhaust pipes between 2.5 inches to 6 inches. In the last few years, vast improvements in the automotive industry have been made and part quality becomes inherent with the use of numerical controlled work centres. Recently, in an attempt for continuous improvement of quality and reducing cost, and simultaneously increasing product offerings, the company bought a computer numerically controlled (CNC) bending machine capable of bending pipes automatically up to 25 bends. However, when this machine came to the factory, due to lack of training and support, it was used to make single bends only. It is the desire of management to improve the quality and accuracy of their products. The company needs to change the manual operations to this CNC bender to reduce the time of production and to improve the process of manufacture. The following problems related to the CNC bending machine were identified: 1) The operation manual was written in Chinese. There were some notes in English but it was very brief, and was far from sufficient to support the operators to manipulate the machine in multiple bends mode. J. P.T. Mo et al. 78 2) Due to lack of proper on-site training and knowledge of the principles of multiple bends, the operators and the supervisor did not know how to program and operate the machine. 3) Although there were a big range of products available, they were not well categorized according to the machines and processes, especially for multiple bends operations. The lack of systematic method of capturing design and manufacturing prevented further improvement to manufacturing technology. This paper describes the process improvement project that led to over 40% reduction in manufacturing time and significant materials savings within six months of the investigation. 2 THE BENDING PROCESS The tube bending process is widely used to manufacture parts in aerospace, automotive, oil and other industries [1]. During the last four decades bending technologies development could be divided into three periods: Traditional Bending (late 1970s to early 1980s), New Bending (mid 1980s to mid 1990s), and Advanced Bending (mid 1990s to present). Initially, the operator bent the tube pieceby-piece. This method was very slow and did not work with large demands. With continuous improvement in this field, new bending technologies with computer controls made bending less operatordependent. Computer aided manufacture software and other advances allowed for even faster setup times and the ability to handle greater part complexity. CNC tube benders were developed to circumvent the problems associated with the labour intensive methods. Modern computer technology linked with servo-mechanical control offers an excellent method for controlling the three bending axes. The mechanics of CNC benders operate very similar to the manual methods, except that servo drives control the distance between bend and plane of bend. A carriage/collet system is used to assist feeding. The functions of tooling movement and sequencing, part data storage are controlled by the computer automatically. Rotary draw bending is the most popular bending method in bending tube, pipe and solids for applications like: handrail, frames, roll cages, handles, lines and others [2]. This type of bending uses “die sets” which have constant Centre Line Radius (CLR). A die set consists of two parts: the forming die has the shape to which the material will be bent; the pressure die pushes the material into the forming die while traveling the length of the bend. Figure 1 shows how the rotary draw bender works. Forming die Pressure die Clamp block Tube Pressure die rotating tube on forming die Wiper die Figure 1: The Rotary Draw Bending Machine Bends the Pipe In many rotary-draw benders, a mandrel is inserted into the material before bending and removed afterwards to further reduce any visible deformation through the bend area (Figure 2). The mandrel helps to achieve tighter bend radiuses when using a mandrel (compared to a regular rotary-draw bender) [3]. Figure 2: Tube Bended with Mandrel The working principle is relatively simple. The tubing is held in position by the clamping block and is pulled around the forming die by the pressure die. The material is compressed and stretched through the bend. Some deformation is inevitable, but it is difficult to identify without measuring the tube or cutting a cross-section of the bend [4]. The followings are common defects in the manual process that the Development of CNC Pipe Bending Process in Small Batch Manufacturing Environment 79 company tries to minimize using a consistent CNC bending process. During the bending process the bending moment induces axial forces in the inner and outer fibers. The inner and outer fibers are subjected to compressive and tensile stresses respectively. This results in thinning of the tube wall at the outer section (extrados) and thickening of the tube wall at the inner section (intrados) [5]. The wall thickness variation is shown in Figure 3. to ti Outer thinning Inner thickeningFigure 3: Variation in Wall Thickness The fibers at the extrados are subjected to tensile stress. When the tensile stress induced in the tube due to the bending moment at the extrados exceeds the ultimate yield strength of the material, the tube fractures at the extrados [6]. Figure 4 shows the fracture of a tube. Figure 4: Tube Fracture As the tube is bent, the inner surface of the tube, the intrados, is subject to compressive stress. When the tube is bent into a tight radius, it is subject to high compressive stress in the intrados which leads to bifurcation instability or buckling (wrinkling) of the tube. Wrinkles are wavy types of surface distortions. As tubes are used as parts in many applications where tight dimensional tolerances are desired, wrinkles are unacceptable and should be eliminated. Furthermore, wrinkles spoil the aesthetic appearance of the tube [7]. Figure 5 shows tube wrinkling. They are hard to remove even by surface treatment methods such as spray painting and the product is in an unacceptable quality for the customers. Figure 5: Tube Wrinkling Another undesirable effect is that the cross section of the tube becomes oval instead being circular. During bending, there is a tendency of material fibers at both the ends to move towards the neutral axis. The outer fiber of the tube tends to move towards the neutral plane to reduce the tensile elongation. The common practice in industry is to provide support to the tube from inside to prevent flattening or distortion of cross section; usually a filler material or mandrel is used. Figure 6 shows the cross section distortion of tube [8]. Distorted Original cross section cross sectionFigure 6: Cross Section Distortion J. P.T. Mo et al. 80 A serious quality issue is the amount of spring backs due to the elastic nature of the tube material after the tooling force is withdrawn. During the bending process internal stresses are developed in the tube and upon unloading the internal stresses do not vanish. After bending, the extrados is subjected to residual tensile stress and the intrados is subjected to residual compressive stress. These residual stresses produce a net internal bending moment which causes spring back. The tube continues to spring back until the internal bending moment drops to zero. The spring back angle depends on the bend angle, tube material, tube size, mandrel, machine, tooling and the working condition at the time of bending. In practice, the amount of spring back is calculated and the tube is over bent by that amount [9]. Due to the enormous stresses that are induced in this process to a relatively small volume of materials, control of the process is particularly important. The use of CNC bending machine is the key to better quality output. 3 THE BENDING MACHINE The machine is a CNC rotary draw bending machine with a mandrel (Figure 7). It is a computerized and automatic 3D bending machine under the industrial control unit. The machine is designed to bend tubes from 2.5 inch to 6 inch and it is capable of bending pipes automatically up to 25 bends without interruption. Actually, the machine is designed to bend thin-walled tubes with a tight radius to ensure bend with insignificant ovalization and wrinkling. The clamp die presses the tube against the bend die while the pressure die keeps the tube where the tube is fed into the bender. The wiper die is used to eliminate wrinkling of the tube on the inner bend. The mandrel is usually inserted into the pipe to reduce cross-sectional flattening and prevent collapse during the bending. The original process to produce a pipe on the CNC multiple bends machine took 70 minutes. It was selfevident that there were many problems in this production process. The biggest issue was that only single bend mode was used. The repetitive work of cutting, bending small pieces of pipes and then welding them together into final product led to a significant waste of material and labour. The larger the job quantity was, the more waste it had. At the same time, the efficiency of work was not satisfactory because of the long manufacturing lead time. These two major disadvantages of current process prevented the company to become more competitive in the market and further development. Figure 7: Bending Machine In the beginning, the research team worked with the production manager to familiarize with the machine. A process map was developed to record every detail of the operation and the procedures to set up the machine and to change the tools. By analyzing the process maps, a better way to reduce the setup time and established some standards and templates for them to changing the tools in the future was developed. The project has two main tasks: 1. Defining Parameters: We listed all the equations which were required to find the feeding length, the angle of bending and rotation. We listed these equations with explanations and some of their real-life products to give the staff in the company a clear idea of how they should work. 2. Machine Operation: We operated the machine on different ways to see and understand how the machine works. We also developed the correct process to operate it. Furthermore, we investigated the standard method and generated a manual to setup the tools on the machine. This method is often used for these kinds of machines and it is related to the tools which they have now. 4 PARAMETERS FOR CNC TUBE BENDING The most important part of the project is the calculation of the parameters for bending, such as: feeding length, bending and rotation angle. These parameters are derived from the drawings of the Development of CNC Pipe Bending Process in Small Batch Manufacturing Environment 81 product, in our case, the sample exhausted pipe. There are two ways to get the right parameters for the multiple bends purpose. As we can see from Figure 8, the first way is to build a 3D model in a CAD software system following the geometrical relationships on the drawings. However, this method requires a CAD system and the expertise to drive it. This is not normally available to small manufacturers making small batches. Figure 8: CAD Model Instead of the method of building 3D models, the other way is to apply a systematic vector analysis method to calculate these key parameters. Some basic theories of vector analysis are given here as background. Distance is a numerical description of how far apart objects are at any given moment in time. The distance between two given points (x1, y1, z1) and (x2, y2, z2) in three-dimensional space is: d = (x1 ?x2)2 +(y1 ? y2)2 + ?(z1 z2)2 (1) Also for any given triangle ABC (Figure 9), the angles of the triangle are given by: c C BA a b α β γ Figure 9: Finding angles in a triangle cos α = b 2 + c 2 ? a 2cos β =cos γ =Equation 2 is useful for computing the third side of a triangle when two sides and their enclosed angle are known, and in computing the angles of a triangle if all three sides are known. In our case, to calculate the bending angle of one single bend, we need to build up a triangle between three points and compute the bending angles as long as we get all the sides of this triangle. The other set of parameters is on orientation, which is an integral aspect of vector analysis. Threedimensional vectors can be treated as ordered triplets of three numbers and obey rules very similar to those obeyed by two-dimensional vectors. We represent three-dimensional vectors by arrows and the geometric interpretation of the addition and subtraction of these vectors follows the parallelogram rule just as it does in two dimensions. We define unit ??????vectors i , j and k along the x, y and z axes of a Cartesian coordinate system and express threedimensional vectors as u?= u ix?+u jy ?+u kz ? (3) The dot product of two vectors is used to compute the angle between them. ???aba b caca c bbc2222 2 22 2 2+ ?+ ? J. P.T. Mo et al. 82 u v? (4) cos θ = ? ??u ? vWhere u??is the norm of u The cross product of two vectors is required to determine the working plane. It is expanded by the law of matrix. The significance of cross product is that it forms a third vector vertical to the plane that contains the previous two vectors. The third vector obeys the right hand rule. ?????? i j k ? ????? ? u× =v ?ux uyuz ????vx vy vz ???= i????uvyyuvzz ????? ?j????uvxxuvzz ????+ k?????uvxx uvyy ???? (5) ?? ? ?= (u vy z ?u v iz y) +(u vz x ?u vx z) j +(u vx y ?u v ky x)Based on the two definitions in vector analysis, we can get one of the most important parameter in bending which is the rotation angle (Figure 10). And the approach is listed as below: 1. Find two vectors in the first plane. And get the cross product n?1of these two vectors. 2. Find two vectors in the second plane. And get the cross product n?2of these two vectors. 3. Get the angle between n?1 & n?2 using the dot product of these two vectors, and this angle is the rotation angle for bending. PLANE 2 PLANE 1 2n 1n θ 1 212..nnnn=θFigure 10: Rotation angle approach 5 THE BENDING EXAMPLE The project team used a sample exhaust pipe for some models of truck as a study example. The company provided the team some initial drawings for this job with detailed dimensions and some specific requirements. Figure 11 is the model established in a CAD system according to the drawing. Figure 11: Drawing of sample exhausted pipe After investigating the available tools, which include the clamp die, bend die (radius: 114.3mm), pressure die, wiper die, and mandrel with three balls, we conclude that the job for sample exhausted pipe is impossible to conduct multiple bends from Point A to Point D, because the feeding length is less than the radius of the bend die. This means the bends can not be completed due to the interference of the former bends. So we decide to start multiple bends from Point D to Point G, which consists of three bends. Table 1 shows the geometry and parameters. Table 1. Coordinates of bending points on the products Point Coordinates C (0, 0, 0) D (313, 0, 75) E (313, 0, 875) F (313, -82, 1399) G (339, -70, 1641) 5.1 Bending Angle The Point to Point distance serves as a fundamental element for further calculation of the key parameters, such as bending angle, feeding length and theoretical length etc. Using Equation 1, we can get the distance between two points based on the given coordinates as shown in Table 2. Development of CNC Pipe Bending Process in Small Batch Manufacturing Environment 83 Table 2. Point to point distances Lin e Formula Length in mm CD (313 0)? 2 + ?(0 0)2 +(75 0)? 2 321.86 DE (313 313)?2 + ? +(0 0)2(815 75)? 2 740.00 EF (313 313)? 2 + ? ? +( 82 0)2(1399 815)?2 589.73 FG (339 313)?2 + ? +( 72 82)2 +(1641 1399)? 2243.69 CE (313 0)? + ? +2 (0 0)2 (815 0)? 2 873.04 DF (313 313)? 2 + ? ?( 82 0)2 +(1399 ?75)21326.5 4 EG (339 313)?2 + ? ? +( 72 0)2(1641 815)?2 829.37 Using Equation 2 and the three sides of one triangle, we computed the bending angles for the three bends. For example, in Figure 13, the angle between lengths CD and DE is given by: ?1 CD DE CE2 + 2 ? 2 103.47mm θD = cos =2×CD×DEC E D 321.86 740.0 873.04 φD θDFigure 12: Illustration of Bending Angle Hence, the bending angles (in degrees) required for each bend at each point are: φD = 180 – 103.47 = 76.53 φE = 180 – 172.0 = 8.0 φF = 180 – 167.57 = 12.43 5.2 Feed Length The next step is to calculate the feeding lengths we needed for bending. In Figure 13, the sustained angles at the bend determine the length at the bends. The length X1 (in mm) sustained at bends D and E are given by: X R1 = tanφD = 114.3× tan 76.3° = 90.16mm 2 2Where R = 114.3 due to the radius of the die set. Hence, the feed length from C to D is given by: fC→D = CD – X1 = 321.86 – 90.16 = 231.7 mm CFeed D to EFeed C to DDEBend radius RBend radius RX1X2φDφEFigure 13: Calculation of feed length Similarly, the feed length for the other sections can be calculated as: fD→E = DE – X1 – X2 = 740.0 – 90.16 – 8.0 = 641.84 mm fE→F = EF – X2 – X3 = 589.73 – 8.0 – 12.45 = 569.28 mm fF→G = FG – X3 = 243.69 – 12.45 = 231.24 mm 5.3 Theoretical Pipe Length The theoretical pipe length is the minimum length needed for the finished product, which is derived by adding each section together including the arcs at each point and feeding lengths. Since raw tubes come from the process which produces straight tubes, this length determines the length of raw straight tube that has to be cut before bending. The cut size of the tube is always longer than the theoretical length because