Use of Mechatronic System to Manufacture Spiral Bevel Gear using 3-Axis CNC Milling Machine

15th. Annual (International) Conference on Mechanical Engineering-ISME2007 May 15-17, 2007, Amirkabir University of Technology, Tehran, Iran ISME2007-...
Author: Florence Black
3 downloads 1 Views 387KB Size
15th. Annual (International) Conference on Mechanical Engineering-ISME2007 May 15-17, 2007, Amirkabir University of Technology, Tehran, Iran ISME2007-1268

Use of Mechatronic System to Manufacture Spiral Bevel Gear using 3-Axis CNC Milling Machine S.Mohsen Safavi S.S. Mirian M. Karimian Sichani Assistance professor Lecturer MSc. student Isfahan University of Technology Islamic Azad University of Najafabad Isfahan University of Technology [email protected] [email protected] [email protected] R. Abedinzade MSc. student Isfahan University of Technology [email protected] Abstract Gears are crucial components for modern precision machinery as a means for the power transmission mechanism. Due to their complexity and unique characteristics, gears have been designed and manufactured by a special type of machine tools, such as gear hobbing and shaping machines. The spiral bevel gears (SBG) are the most complex type and are used to transmit the rotational motion between angularly crossed shafts. In this paper, we attempt to manufacture the spiral bevel gear by a three-axis CNC milling machine interfaced with an additional PLC module. This consists of (a) Geometric modeling of the spiral bevel gear. (b) Simulating the traditional and our new non-traditional method using a CAD/CAE system. (c) Process planning for CNC machining and PLC programming. Experimental cuts were made to discover the validity of the presented method with a three-axis CNC milling machine.

precise gears [2]. Among the various types of gears (Figure 1), the spiral bevel gears (SBG) are the most complex type and are used to transmit the rotational motion between angularly crossed shafts. Spiral bevel gear is a term generally used in case of bevel gears that have teeth curved longitudinally along the length of the teeth. The main advantage of these gears over the straight-toothed varieties lies in the fact that as more teeth are in contact at the same time because of the curved-shaped contour of the teeth, a smoother meshing action between the mating pair is ensured [3]. The design and manufacturing of spiral bevel gears is still a hot topic of research that is vital for application of such gears in helicopter transmissions, motorcycle gears, reducers, and in other branches of industry.

Keywords: Gear manufacturing, Spiral bevel gear, CNC, PLC, AC motor, Sensor, AC drive, CAD/CAM, Rotary Encoder Introduction Gears are important and precision mechanisms for industrial machinery as a means for mechanical power or motion transmission between parallel, intersecting and non-intersecting cross-axis shafts. Although hidden from sight, gears are one of the most important mechanical elements in our civilization. They operate at almost unlimited speeds under a wide variety of conditions. The machines and processes that have been developed for producing gears are among the most ingenious we have. Whether produced in large or small quantities, in cell, or job shop batches, the sequence of processes for gear manufacturing requires four sets of operations [1]: 1. Blanking 2. Gear cutting 3. Heat treatment 4. Grinding Dependent on their type and application or required strength and resistance, gears are manufactured by casting, extruding, forging, powder metallurgy, plastic molding, gear rolling and machining. Among these processes, machining is more frequently used for high

www.sakhtolid.ir

Figure 1: Various industrial gears

As far as manufacturing is concerned, the gears are machined by a special type of machine tools, such as gear hobbing and shaping machines. Recently, special CNC based gear manufacturing machine tools are used in industrial practice. Figure 2 shows one of those special machines used to manufacture Spiral Bevel Gears. This may be why literature on gear manufacturing is sparse in the open research domain. Recently, CNC based gear manufacturing machine tools have been developed and increasingly used in industrial practice. However, their kinematic structure is still inherently different from the industrial CNC milling machine, as the former is designed for a special type of cutter. [4].

tool path planning for sculptured surface machining (SSM) of SBGs) for a ball endmill which is used for cutting the surface model of the spiral bevel gears based on basic gearing kinematics and involute geometry along with tangent planes. Geometric specification of the Spiral Bevel Gears Most of the time, the geometric parameters of a gear are provided with an engineering drawing, Figure 3 shows Spiral-Bevel-Gear nomenclature. Some parameters (principal parameters) are required for defining the geometry. Table 1 summarizes these parameters including relationships among parameters for the gear which has been manufactured in our test. To calculate these parameters we have used “drive component development software” called GearTrax. The design of spiral bevel gear require high-accuracy mathematical calculations and the generation of such gears drives require not only high-quality equipment and tools for manufacturing of such gear drives, but the development of the proper machine-tool settings. Such settings are not standardized, but have to be determined for each case of design (depending on geometric parameters of the gear drive and generating tools) to guarantee the required quality of the gear drives.

Figure 3: Spiral-Bevel-Gear nomenclature Figure 2: Special machine tool and cutter for manufacturing SBGs In this paper, we attempt to present a new manufacturing procedure of SBGs by using a three-axis milling machine interfaced with a PLC module which operates as an indexing table. In terms of production rate, it is obvious that this method will be lower than that of the special machine tool. Other than production rate, this method is advantageous in the following respects: (1) the conventional method requires a large investment for obtaining various kinds of special machinery and cutters dedicated to a very limited class of gears in terms of gear type, size, and geometry; (2) by this method, various types of gears can be manufactured with the industrial three-axis CNC milling machine; (3) this method is more economical than using the special machine tool. In [4], Suh and Jih presented a complicated mathematical tool path planning (called

www.sakhtolid.ir

Table 1: Gear Data Spiral Bevel Gear Specification Term Pinion Gear Module M=3 D n 20 Pressure Angle Spiral Angle

E

35 $

Shaft Angle No. of Teeth Spiral direction Face Width Pitch Angle

6

90 $

Root Angle Face Angle of Blank Tooth Thickness

Z 1 20 Left Hand

Z 2 20 Right Hand

b=15

G 1 45 G f 1 40.802

G 2 45 G f 2 40.802

G k1

G k2

49.198

K=4.4584

49.198

Some of the formulas which are used to calculate the above data sheet are as follows; G 1 tan 1 Z 1 Z 2 (1) G 2 90  G 1 (2)

T f1

tan 1 h f 1 R

h

R

(3)

Tf2

tan

G k1

G1 T f 2

(5)

G k2

G 2 T f1

(6)

G f1

G1 T f 1

(7)

G f2

G 21  T f 2

(8)

1

f2

(4)

Manufacturing the SBG As it is discussed in the introduction, by machining all types of gears can be made in all sizes and machining is still unsurpassed for gears that most have very high accuracy. Form milling is one of the most common machining processes used to manufacture any types of gears. The cutter has the same form as the space between adjacent teeth. Standard cutters usually are employed in form-cutting gears. In the United States, these come in eight sizes for each diametral pitch and will cut gears having the number of teeth indicated in standard tables. To manufacture the SBGs with the 3-axis CNC milling machine, we first test the process by developing a CAD/CAM system composed of geometric modeling and graphic simulation modules. The commercial software SolidWorks is used for creating CAD model and Visual Nastran is used for simulating the process of gear manufacturing. As far as machine tool configuration is concerned, it is obvious that a rotational motion of the workpiece is required for NC machining of the SBGs. Based on the machinability analysis [5], at least four-axis controls are required for NC machining of SBGs by one set-up. Thus, a rotary table to be interfaced with the three-axis milling machine is required. Form cutting or form milling is used in our tests. The tool is fed radially to ward the center of the gear blank to the desired tooth depth, then across the tooth face, while the rotary table rotates the workpiece around its center, to obtain the required tooth width. When one tooth space has been completed, the tool is withdrawn, the gear blank is indexed using a dividing head, and the next tooth space is cut. Basically, this method is a simple and flexible method of machining SBGs. The equipment and cutters required are relatively simple, and standard 3-axis CNC milling machine is used. However, considerable care is required on the part of tool feed which should be a small value in each step to prevent any spoil. Although the form cutting of this kind of gears is currently done on universal milling machine, using an indexing head, the process is slow and requires skilled labor and operator. Figure 4 shows the arrangement that is employed when the work is done on a universal machine in Isfahan-Ferez workshop. As it is shown, the cutter is mounted on an arbor, and a dividing head is used to revolve (required to cut the gear tooth) and index the gear blank. The table is set at an angle equals

www.sakhtolid.ir

to the spiral angle (35 degrees), and the dividing head is geared to the longitudinal feed screw of the table so that the gear blank will rotate as it moves longitudinally. In the presented method we have used an AC motor interfaced with a worm gearbox. Worm gearbox is used to reduce the output speed of AC motor and also to set the angle between the tooth trace and the element of the pitch cone, known as spiral angle [6]. Because of the needed synchronization between tool path planning and rotary motion of outward shaft of worm gearbox is required, we used a mechatronic system to control all the four axes (one-axis motion for the rotary table and three-axis motion for the cutting tool) simultaneously.

Figure 4: A photograph of manual cuttig on universal milling machine This mechatronic system contains four major parts: (1) PLC (Programmable Logic Controller), one of electronic equipments, which can be called “Sequence Controller”. It uses internal microprocessor to execute the calculation of sequence of the work according to the status and the value of the input signal of Encoder and Sensors; (2) Inverter, which is used to operate a standard 3-phase motor and act as an intermediate device between PLC and AC motor. It has different input terminals which can be used to rotate AC motor in Forward/reverse direction; (3) Rotary Encoder, a feedback system which determine the position of outward shaft or measure the revolution of workpiece mounted on outward shaft; (4) Proximity sensors, which produce an output signal if a metal object (form cutter in our experiment) enters the electromagnetic field of its oscillator. Two proximity sensors are placed on the system where define the start and end of the tool path. We used ladder diagram, common program language, to operate the PLC in the mechatronic system. The operation of the PLC based on the ladder diagram is as follows: Step 1: Read the external input signal, such as the status of sensors or rotary encoder. Step 2: Calculate output signal, according to the value of the input signal in the step 1 and send it to AC drive (Inverter) to run the AC motor in Forward/reverse direction or turn the motor for a special angle (Circular pitch) using a rotary encoder. The procedure of the whole system is accomplished in five stages;

Stage 1: Form cutter reaches the first proximity sensor and sends a signal to the PLC. As it’s mentioned before, PLC sends out an output signal to the AC drive to run the motor in forward direction. Stage 2: Form cutter machines the rotating workpiece. Stage 3: Cutting tool reaches the second proximity sensor and sends the second signal to the PLC. PLC sends a stop command to the inverter. Stage 4: Milling tool withdraws the stopped workpiece and returns to its first place. Simultaneously, the PLC uses the measured value of rotary encoder to rotate the workpiece in reverse direction until it reaches the first position. Stage 5: Stage 1 through stage 4 continues till the first tooth space is cut. PLC count a number for each of the above four stages till it reaches the predefined number of machining sequences. Then it sends out a signal to the AC drive to index the gear blank a diametral pitch and all the above stages will be repeated again. The traditional procedure (Figure 4) is quite slow, and considerable care is required by the operator. The procedure utilized is essentially the same as on a universal milling machine, except that, after setup, the various operations are completed automatically. The machining configuration and a photocopy of the experimental cutting are shown in Figures 5 and 6 and 7.

Figure 1 A photograph of electrical panel including PLC and Inverter

Machining Strategy The workpiece is wood and the blank material is premachined as a conic (Face Angle of the gear) form by turning operation. Standard cutter No.5 used in our experiment is mounted on the machine spindle and the gear blank is mounted on outward shaft of worm gearbox. The tool is fed subsequently (around 30 machining sequence to prevent spoiling work) toward the center of the gear blank to the desired tooth depth. When one tooth space has been completed, the tool is withdrawn; the gear blank is indexed using the AC motor based on explained procedure, and then is followed by cutting the next tooth space.

Figure 5: A photograph of the experimental cutting

Figure 6: A photograph of tooth cutting and AC motor utilized as an indexing head

www.sakhtolid.ir

Conclusions In this paper we attempted to manufacture the spiral bevel gear using a three-axis CNC milling machine based on form milling method. Basically, form cutting uses a simple and flexible method of machining of gears. The equipment and cutters required are relatively simple and inexpensive, and standard CNC milling machine is used. Compared to the conventional gear cutting method in which dedicated machine tools is required, the presented method can easily be modified to produce any type and size of SBGs or any other types of gears. Also it is a multi-deception system which is a dynamic and constant evolving technology.

References

[1]

[2]

[3] [4]

[5]

[6]

E. Paul DeGarmo, J.T Black, 1957, Materials and Processes in Manufacturing, Prentice-Hall International, Inc. J. Edward Shigley, 1986, Mechanical Engineering Design, McGraw-Hill Publishing Co. G.M. Maitra, 1994, Handbook of Gear Design, Tata McGraw-Hill Publishing Co. S. Suh, W.S. Jih, H.D. Hong, D.H. Chung, 2001, Sculptured surface machining of spiral bevel gears with CNC milling, International Journal of Machine Tools & Manufacture 41 833–850 S. Suh, J. Kang, 1995, Process planning for multi-axis NC machining of free surfaces, International Journal of Production Research 33 2723–2738. D.W. Dudley, 1962, Dudley’s Gear Handbook. The design, manufacture and application of gears, McGraw-Hill book Compan

www.sakhtolid.ir

Suggest Documents