A conventional center lathe machine has been selected as a good candidate for the CNC retrofit. The main spindle motor power is 7.5 kW and main spindle speed range is (12–1200) revolution per minute (RPM) which are quite satisfactory for a spinning process. Static acceptance tests are performed on the machine to ensure its mechanical accuracy. The test includes straightness and flatness of slideways, alignment of slideway and axes, alignment and true running of spindle, pitch error of lead-screws. The results are found to be within the acceptable limits given by Altozano [14]. A chart representing the sequence of retrofitting operations to convert the lathe into a CNC spinning system is shown in Fig. 2. This requires the addition of actuators, sensors, CNC controller, CNC software, and some mechanical elements.
Fig. 2Stages of CNC retrofitting procedure
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2.1
Actuators and sensorsThe actuator for the mandrel is the built-in AC motor with a gear box driving the lathe spindle with 7.5 kW power and (12–1200) RPM rotational speed. The feed gear box of the lathe had been dismantled and replaced by AC servo motors and power-screws to control and synchronize the movements of the forming roller in the axial and lateral directions (Fig. 3). AC servo motors have been utilized due to their high intermittent torque, high torque to inertia ratio, high speeds, good velocity control, less maintenance and quiet performance. Selected AC servo motors specifications are listed in Table 1. Power calculation has been checked in the longitudinal direction based on the axial spinning force (5 kN) and axial feed speed (800 mm/min.) which are found to be greater than both the radial force (2 kN) and feed speed in the transverse direction [15].
Fig. 3Transmitting motion from servo-motors to lead screws
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Table 1 AC servo-motors’ specificationsFull size table
To provide longitudinal motion, the carriage lead screw has been extended. Longitudinal and transverse motions could be achieved using AC servomotors connected to the lead screws via pulleys and timer belts (Fig. 3). Motion in each direction is controlled by drives and a CNC controller.
Two types of sensors have been used; limit switches (two for each axis) and proximity sensors (one for each axis). Limit switches are used for safety against exceeding work space limits, while proximity sensors are used to adjust the Home Position in the CNC control (Fig. 4). The Home Position is defined by the clearance between the roller and mandrel which equals the blank thickness. Wiring system of the actuators (motors) and sensors is illustrated in Figs. 5 and 6. The connections for both servomotors are the same, so only the wiring for Z-axis servomotor is presented.
Fig. 5Z-axis servomotor wiring diagram
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Fig. 6Spindle induction motor and safety buttons wiring diagram
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2.2
CNC controller and softwareA CPU5A Economy Series—USB CNC Controller connected to a personal computer (PC) using USB cable has been used. Using software integrated with the controller, full synchronization with the two servo motors moving the forming roller can be achieved. The system is equipped with safety buttons for emergency, start, stop, and a selector (ON/OFF) to enable the motors drives. A schematic diagram of the new CNC system is illustrated in Fig. 7 along with the control panel.
Fig. 7The new CNC system with control panel
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The software used is Eding CNC Release 4.00.46 provided with the controller. The tool path is first drawn using CAD software (Fig. 8) and saved in (.dxf) format, then imported to the CNC software where a G-code is generated based on selected process parameters such as feed rate.
Fig. 8Simultaneous X–Z tool paths in forward and backward motion
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2.3
Mechanical elementsThree cylindrical mandrels of tool steel D2 [15] with different diameters are designed and used to resemble the profile of the final component and support the sheet during deformation. The mandrel, in use, is clamped in the lathe chuck (Fig. 11). The three diameters give three spinning ratios 1.7, 1.48, and 1.26 respectively; spinning ratio (SR) is the ratio between sheet blank diameter (240 mm) to the mandrel diameter.
The roller is a rigid tool that forms the sheet over the mandrel. Music et al. [16] and Satpute et al. [17] introduced two equations for selection of roller diameter Dr, and nose radius ρr in terms of the blank diameter Do.
$$\begin{aligned} D_{r} & = \, 0.1D_{o} + \, 80 \pm \, 40 \, \left( {\text{mm}} \right). \\ \rho_{r} & = \, (0.012 \sim 0.05)D_{o} \left( {\text{mm}} \right). \\ \end{aligned}$$
Blank diameter Do is 240 mm. For the roller design to be used in this work the selected roller diameter is 140 mm and roller nose radius is 9.5 mm. The roller tool is made of tool steel O1 to the desired shape, hardened to 60 HRC; Rockwell C Hardness Scale, and then polished. The complete assembly is shown in Fig. 9. The fork is used for mounting the roller on the tool post. The roller rotates around a tapered roller bearing [18] and a thrust ball bearing.
Fig. 9Roller geometry and complete assembly of the tool
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The sheet holder is a circular disk clamping the sheet against the mandrel face. It is flat to fit over the mandrel and further support the sheet while it is being formed. Figure 10 shows a sectional view through the components of sheet holder. The rod end is tapered to be mounted into the lathe tailstock. The holder is free to rotate on a thrust ball bearing to overcome friction. The connector makes it simple to change the holder according to mandrel shape and size. Figure 11 shows the mounting of mechanical components on the machine. Figure 12 shows the complete CNC spinning machine.
Fig. 10The holder design
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Fig. 11Mechanical components mounted on the machine
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Fig. 12Developed retrofitted CNC spinning machine
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