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ScienceDirect Procedia Engineering 184 (2017) 163 – 170

Advances in Material & Processing Technologies Conference

Design and Development of a Hybrid Machine combining Rapid Prototyping and CNC Milling Operation A.N.M. Amanullaha, Murshiduzzamana, Tanveer Saleha*, Raisuddin Khana a

Smart Structures, System and Control Research Laboratoty, Department of Mechatronics Engineering, International Islamic University Malaysia, Kuala Lumpur 50728, Malaysia

Abstract Nowadays, two most important processes, namely rapid prototyping (RP) and CNC machining are being used to produce prototypes. CNC machining (subtractive method) is relatively more precise and accurate, but it is tough to create stuffs with complex features. RP (additive method), by contrast, is able to form parts with sophisticated features, that consents materials to be utilized more efficiently. But its entire automation emanates with conciliations in the qualities of material and geometry. Combining both subtractive and additive process on a single platform has significant advantages. However, this has several challenges such as control system integration and maintaining accuracy of alignment during the change over process. This research attempts to assimilate both of these processes and propose a new design of hybrid machine with the purpose of overcoming the drawbacks related with different control panel and misalignment issues. Fused deposition modeling (FDM) is considered as the RP process in this study. A CNC cutting spindle and an FDM heat extruder has been designed to be placed respectively in front of a rotary stage which will be used to overcome the misalignment with the help of IR sensors. The proposed design allows CNC milling and FDM process in a single setup thus attaining the benefit of decreasing expenses for additional actuators. ©©2017 Authors. Published by Elsevier Ltd. This 2017The The Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the Advances in Material & Processing Technologies Peer-review under responsibility of the organizing committee of the Urban Transitions Conference

Conference.

Keywords: Hybrid machine, CNC, Rapid Prototyping, FDM, Rotary Stage, IR sensors

* Corresponding author. Tel: +603-6196-5709; fax: +603-6196-4433. E-mail address: [email protected]

1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the Urban Transitions Conference

doi:10.1016/j.proeng.2017.04.081

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A.N.M. Amanullah et al. / Procedia Engineering 184 (2017) 163 – 170

1. Introduction Rapid Prototyping (RP),as well known as ‘Layered Manufacturing’ (LM) and ‘Additive Manufacturing’ (AM) is the “basic term for several technologies that allow components to be made without the need to involve the services of expert model-manufacturers”[1]. This technology has pinched significant attention due to its capability to overcome many drawbacks of old-fashioned manufacturing techniques. Its ability to form almost any geometric feature or shape is a great advantage of AM[2]. However, some new limitations have been introduced, which come out from its manufacturing methodology. CNC (Computer Numerical Control) machining process is the perfect way out to the downsides which occur in the RP process. Although high-precision, high-flexibility, high-accuracy and high-speed and so on desired properties are offered by modern CNC technology, the flexibility of the final products obtained from CNC machining is quiet pretty imperfect as paralleled to the RP process. Among the vast area of RP, a number of methods are used: Stereolithography (SLA), Fused Deposition Modelling (FDM), Laminated Object Manufacturing (LOM), Selective Laser Sintering (SLS), Multi-jet Modelling or Solid Imaging (SI), Selective Laser Cladding (SLC), 3D Printing (3DP) or Selective Binding, Laser Engineering Net Shaping (LENS) [3,4].The basic technology of any RP system is LM technology that permits the production of three-dimensional parts layer-by-layer [5]. The principle of RP is that the original three-dimensional geometrical part is firstly disintegrated into two-dimensional profile layers. After that material is adjusted layer-by-layer until the completion of the final part of most RP systems whereas material is removed in machining processes. Without the assistance of any tooling, RP process allows parts to be manufactured directly from CAD descriptions [6]. Based on using various forms of raw material the Rapid Prototyping process is classified as solid base, powder base and liquid base. A light source is used layer-by-layer in the liquid-based RP Process for solidifying the liquid polymer, till the product is manufactured. The main difference between the powder-based RP process and liquid-based RP process is that powder is used as a raw material instead of liquid, and a glue ejector takes the place of the light sources. There is no need of support material in the powder based RP process for manufacturing the overhang feature since the powder is able to act as the support material itself. Among various solid-based RP processes FDM is extensively used [7]. FDM has the outstanding geometric capability as like as other RP processes and comparing to other RP processes it is quite simple and has relatively higher accuracy, but it is not capable of handling any metallic product as like as machining processes. In the FDM process, a plastic material (ABS or PLA) is extruded through an extrusion nozzle layer-by-layer to buid a product. Usually, this material is supplied in the form of a filament. Resistive heaters are contained by the nozzle that help to keep the temperature of the plastic just above its melting point. So, the plastic can easily flow through the nozzle and create the layer. After that, it hardens instantly and bonds to the previous layer. This procedure is continued till the shape of the prototype is being created is shown in fig.1 .

Molten material Solidified model

Fig. 1. Illustration of FDM process


The FDM can yield parts with complex features: nevertheless, for the reason of having residual stresses and shrinkage which occurred during running the RP process, the precision and accuracy of the produced product is usually low. Staircase effect causes poor surface quality that is another limitation of FDM [8]. The layer thickness was found out as the most important aspect for the finished surface in FDM process [9]. If we want to reduce the layer thickness, then we have to increase build time that will create another issue. So, for the solution of the above problems we are influenced to assimilate the CNC milling and FDM operation to attain a hybrid machining system. A hybrid process was developed with the combination of three-axis milling and SLC where the milling spindle and the laser head were mounted on two vertically isolated axes [10]. In that research, SLC process was applied to form a mold, followed by milling to the desired accuracy. Similar to this another process was developed where CO2 laser welding was used as the RP process [11]. Any type of machinable material could be used for the fabrication of the product with that method but may not be simply valid to products with a complex shape. However, The milling spindle and the laser head were mounted on the similar side of the vertical axis in both of the methods, and therefore could obstruct with each other throughout the process, hence restraining the traveling spaces of the X and Y axes. Some other researches have been done where the only change in the implementing of RP process for example gas metal arc welding (GMAW)[12] and arc welding[13]. Another type of hybrid process was developed by using laser cladding (LC) technology and five-axis machining as RP process and machining process respectively. In this system, the laser cladding nozzle was mounted just afterward to the milling spindle. But the limitation of that machine was its complicated mechanism [14]. A different approach was adopted for the combination of conventional machining process and RP process [15]. In that research the product was divided into numerous portions, machining was used to form each of those portions over the sheet material. Finally, the portions were glued together. For the construction of the undercut feature, a mechanism was applied to reverse the material. Any type of machinable material could be used for the fabrication of the product with that method but may not be simply valid to products with a complex shape. This drawback was solved by using multi-axis (5-axis) milling machine to construct the undercut feature, lastly screws and pins were used to make the assembly[16]. A more robust assembly was reported in which ultrasonic welding was used to connect sheet material [17]. Recently a hybrid RP system has been developed with the combination of five-axis machining and FDM. The FDM extruder was installed in a relation to the cutter spindle so that two modes could be interchangeable by 180º rotation of the axis which prevented the interference between the cutter spindle and FDM extruder [7]. One more advantage of integrating both of the machining and FDM capabilities in one single platform is that it lessens the errors and also time affected by refixing parts. But at the same time, this design was bulky. In this study, the new design of the hybrid machine is suggested, which has achieved the benefits of the singlesetup process of significant accuracy of CNC milling and FDM. We believe that this new design of the hybrid machine, which will incorporate a combined RP concepts, will offer an optimal manufacturing resolution by taking up the benefits of the CNC and RP systems.

2. Proposed Design of the Hybrid Machine: Firstly, the proposed mechanical design of the hybrid machine is described in this section. Finally, the outline of the control system of the hybrid machine is discussed. 2.1. Mechanical Design part of the Hybrid Machine: One of the major issues with respect to designing this hybrid machine was determining how to incorporate the cutting tool spindle and the FDM extruder together without unnecessary upsurges in the complexity of mechanism. The main issue for this one was the alignment. The advanced design idea was using a rotary stage on which to incorporate the CNC cutting spindle and heat extruder of FDM separately. For maintaining the alignment, a rotary stage and two IR sensors have been used. In front of the rotary stage, there is an attachment where the CNC Milling Spindle and FDM multi-nozzle extruder is attached respectively just parallel to the rotary stage. The rotary stage can be operated both manually and automatically. The IR sensors are attached to another attachment that is perpendicular to the rotary stage. The sensors will be used to omit the problems of misalignment during exchanging the equipment of CNC milling and FDM process. The misalignment problem is overcome by the differences of the

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analog signal coming from the IR sensors. The rotary stage is moved according to the signal from the IR sensors. When the two reflected IR signals come from at the same distance then there is no difference between the signals that means voltage difference is zero which indicates that the object (CNC spindle or FDM heat extruder) is at the correct position. Similarly, if the signals come from different distances then there will be voltage difference which indicates that the object is not at the correct position. This principle is shown in fig 1(a) and 1(b). a

b

Fig. 2. When the object (CNC spindle or FDM extruder) is (a) in aligned position; (b) in misaligned position

After that, we will implement the CNC spindle for doing the milling operation. After doing the milling operation, we will go for the Rapid Prototyping (FDM) process The FDM system will be developed using two materials known as modeling material and support material where modeling material constitutes finished object and supporting material act to support the object. We will use multi-nozzle heat extruder form FDM process. Fig. 3(a) and 3(b) shows the proposed design for the Hybrid machine with CNC spindle and FDM extruder respectively. a

b

Fig. 3. Proposed design of the Hybrid machine with (a) CNC milling spindle; (b) FDM extruder

One more benefit of having the CNC machining and FDM capabilities in single platform is that it lessens the time and errors affected by re-fixing the stuffs. Deprived of this proposed hybrid machine, still people can use an FDM system to form FDM item and then take it to a machine tool for subsequent machining. However, it is tough or time-consuming to line up the milling spindle with the reference of the FDM object because of inexact dimensions or lack of references of the FDM parts[18]. The proposed design of the hybrid machine will eliminate this problem.


2.2. Control system for the proposed design The total control system for the proposed design was PC-based. The whole control unit for the machine was developed using separate controller, circuitry to handle the CNC milling and FDM process. For our research purpose, we have used LabVIEW 2014 for programming. The total control structure of the proposed hybrid system is shown in Fig.4.

Fig. 4. Control Structure of the proposed system

For controlling the whole process we are using NI PCI-7344, UMI-7764 and USB 6211. It was suggested to use three servo motors, one stepper motor one rotary stage and two IR sensors which are desired to be controlled: three motors for the X, Y and Z axis, and the other one is for the FDM heat extruder. NI PCI 7344-four-axis motion was suggested to use for controlling the four motors. This controller is able to achieve simultaneous three-axis motion trajectories through linear, spherical, circular or helical interpolation, which will placate all our requirements for concurrent three-axis machining and also for the FDM operation. The full control panel was in the same Graphical User Interface (GUI) which had given a unique position for our proposed Hybrid Machine is shown in the following fig.5.

Fig. 5. The single GUI for both CNC milling and FDM operation

3. Investigation of the Characteristics: Some cases were studied to demonstrate the capability and the features of the proposed design of the hybrid machine. Firstly, the JOG operation and three axes movement operation is done with the control panel.

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Then, experiment was done to check the alignment using IR sensors on that same control panel. The next case was to control the temperature at a fixed point which is necessary for the FDM extruder and also for the heat bed. All these features were done in the same control panel which was our one of the most important objectives of this research. 3.1. Control of the three axis: To perform the CNC milling operation and FDM extrusion operation we need to move the Cartesean robot into 3 axis. To do this operation we made a program in LabVIEW 2014. In this case we have successfully done the JOG operation and also have done the linear and circular interpolation which are useful for performing CNC milling and FDM extrusion operation. A sample operation is shown as writing “IIUM” shown in fig.6 with the program where both linear and circular interpolation operation were included.

Fig.6. “IIUM” movement pattern

3.2. Fixing the alignment problem One important issue of our design was to overcome the misalignment of the position of the object. We have used two IR sensors for this purpose. From the sensors the Analog signal goes to the NI USB 6211. According to the signal we could aligned the position of the CNC spindle and FDM extruder. Fig 7(a) and 7(b) shows the reflected IR signals when the object is in aligned and mis-aligned position respectively b

a Unwanted noise from sensor

Voltage difference = 0 Voltage difference = 0

Fig. 7. The difference of reflected IR signals as analog voltage when the object is in (a) aligned and (b) mis-aligned position.

The white and red signal is coming from the upper and lower IR sensor respectively as shown in fig 3(a) and 3(b). In fig 7(a) the two signals are overlapped with each other which means there is no voltage difference. It


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indicates that the object (CNC spindle or FDM heat extruder) is in aligned position. In fig 7(b) there is a voltage difference between the two signals which implies that the object is not in the aligned position. Getting information from this voltage difference we rotate the rotary stage accordingly to overcome the misalignment issue. In both of the figures 7(a) and 7(b), there are some spikes in the signal which is only for some unwanted noises. It will not affect significantly on the alignment issue. 3.3. Temperature control: For FDM extrusion process the main important part is to control the temperature of the heater and heat bed. To do this observation, we have used the 12V-40W heater and one 100k NTC thermistor. Thermistor is one kind of resistor which resistance varies with the changing of temperature. The data acquisition device measures voltage instead of resistance. So, we had to construct a voltage divider circuit to measure the resistance. Fig.8 shows the schematic for the total temperature control circuit.

Fig.8. Circuit diagram for temperature control

Here Vi is a fixed voltage from the data acquisition device Then we measured the temperature of the thermistor as a function of voltage using data acquisition device. After reading the temperature it was necessary to keep the temperature at a fixed point. To do so we needed to make a circuit with MOSFET which is regulated by the PWM signal from the data acquisition device. We have created an on-off control using PWM signal to control the temperature of the heater. When the current temperature is upper than the set temperature, the PWM duty cycle goes to 0% and the heater is off. Similarly, when the current temperature is below the set-value, the PWM duty cycle rises to 100% and the heater is on. As the temperature crosses the set-value to interchange the output condition, the process temperature will be cycling repetitively, going from above set-point to below, and back above. In this way, we were able to keep the temperature at a constant value for the FDM operation. 4. Conclusion: A new innovated design for hybrid system, consisting of CNC milling and FDM, was proposed in this study. One of the novel aspects of the design consist of installing the CNC cutting spindle and the heat extruder of FDM on a rotary stage and using the IR sensors which makes the mechanism simpler and overcomes the problem with misalignment issue. The rotary stage has also given the advantage of using the 3 axis machine as a 4 axis machine. The other innovative feature of this research was the single control panel for both CNC milling and FDM operation.

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