Interactive Spur Gear Generation Using Parametric Programming
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Vol. 6(22), Oct. 2016, PP. 3172-3187 Interactive Spur Gear Generation Using Parametric Programming with CNC End Milling N K Mandal1, N K Singh2 and UC Kumar3 1, 3 National Institute of Technical Teachers’ Training and Research Block- FC, Sector-III, Salt Lake City, Kolkata-700106 2 Indian School of Mines, Dhanbad-826004, Jharkhand Phone Number: +91-033-66251973 *Corresponding Author's E-mail: [email protected] Abstract he development of computer technology brings new opportunities in all sphere of manufacturing. Traditional methods of gear manufacturing with conventional machines have many T disadvantages. The traditional methods are basically two types: Gear forming and Gear generation. Gear forming uses form cutters that are normally bought off-the-shelf and one cutter can be used repeatedly in machining many similar gears provided they are of the same module. Moreover, if the demand of the gear is very few, then buying a gear cutter for that purpose only may not be economical. In addition to that, all types of form cutter may not be available at all times. In this research work, an interactive program called MACRO is developed by which almost all types of gear can be manufactured by a simple end mill cutter in a CNC vertical milling machine only by changing some parameters. In this MACRO program, an algorithm describes the point to point movement of the cutting tool of the machine resulting generation of gear tooth profile more accurately which ensures minimum mechanical losses during power transmission. Keywords: Parametric Programming, CNC VMC machine, gear module, gear generation, Accuracy, CAD/CAM 1. Introduction It is often required to transmit power from one component to another component to increase the speed of the output component. Any power transmission system consists of one input devices, one- output devices and component, that is connected the two. Generally, the input device is a power source or prime mover. Input devices may be an engine, electric motor, PTO shaft of a tractor, etc. The output device or driven device receives the power and does productive work. Output device may be a pump, a conveyor belt etc. Connecting devices exist between the input device (power source) and the output device (point of use of the power). Couplings are used to connect sections of shafts or to connect the shaft of driving machine to the shaft of a driven machine. Clutch is used in between the input and output devices. Chains can be used for comparatively larger distance (5-8 meter) for parallel shafts and has high efficiency (98%). It has a high production cost, the operation is noisy and the design is complicated. Gear is one of the most efficient mechanical power transmission device. Advantages of power transmission by gear over other conventional power transmission system may be summarized as i) It transmits exact velocity ratio ii) It may be used to transmit larger power iii) It may be used for small centre distance of shafts iv) It has high efficiency iv) It has reliable service iv). It has compact layout. 3172 Article History: Received Date: Mar. 14, 2016 Accepted Date: Oct. 21, 2016 Available Online: Dec. 02, 2016 N K Mandal et al. / Vol. 6(22), Oct. 2016, pp. 3172-3187 There are many types of error encountered in conventional gear manufacturing. These includes: Profile Shifting: In normal practice of gear cutting by hobbling, one gear cutter is used to cut a range of number of teeth per gear like from 35 to 54 numbers of teeth. For big range of gear tooth number, error may occur in tooth profile. It is observed that in one of the tooth there is a tendency for the thickness to reduce at the tip. This is because the involute curve tends to shift inwards as the number of teeth reduces, thus reducing the base circle. Undercutting at root surface: When the number of gear teeth to be cut becomes small, the generating tool sweeps out its path, remove some of the profile and produces an undercut tooth form. To prevent under cut some correction must be introduced. Runout error of gear teeth: This error defines the runout of the pitch circle. It is the error in radial position of the teeth. Most often it is measured by indicating the position of a pin or ball inserted in each tooth space around the gear and taking the largest difference. Alternately, particularly for fine pitch gears, the gear is rolled with a master gear on a variable centre distance fixture, which records the change in the centre distance as the measure of teeth or pitch circle runout. Runout causes a number of problems, one of which is noise. The source of this error may have a detrimental effect on accuracy and ruggedness of the cutting arbour and tooling system. Lead error of gear: Lead error is the deviation of the actual advancement of the tooth profile from the ideal value or position. Lead error results in poor tooth contact, particularly concentrating contact to the tip area. Modifications, such as tooth crowning and relieving can alleviate this error to some degree. Tooth profile error: Tooth profile error is the summation of deviation between actual tooth profile and correct involute curve which passes through the pitch point measured perpendicular to the actual profile. The measured band is the actual effective working surface of the gear. However, the tooth modification area is not considered as part of profile error. Spur gear teeth profile may be either of cycloidal or involute. A cycloid is the curve traced by a point on the circumference of a circle which rolls without slipping on a fixed line. Cycloid form may be of two types, one is epicycloids and another is hypocycloid (Fig. 1). When a circle rolls without slipping on the outside of a fixed circle, the curve traced by a point on the circumference of a circle is known as epicycloids. On the other hand, if circle rolls without slipping on the inside of a fixed circle, then the curve traced by a point on the circumference of a circle is called hypocycloid. An involute of a circle is a plane curve generated by a point on a tangent, which rolls on the circle without slipping or by a point on a taut string, which is unwrapped from a reel. In Fig. 2 being the starting point of the involute. The base circle is divided into number of equal parts. The tangents are drawn and the lengths equal to the arcs are st off. Joining the points, A, A1, A2, A3 etc. we obtain the involute curve AR. Fig. 1 Construction of Cycloidal Teeth of a Gear 3173 International Journal of Mechatronics, Electrical and Computer Technology (IJMEC) Universal Scientific Organization, www.aeuso.org PISSN: 2411-6173, EISSN: 2305-0543 N K Mandal et al. / Vol. 6(22), Oct. 2016, pp. 3172-3187 Fig. 2 Construction of Involute Teeth In actual practice, the involute gears are more commonly used as compared to cycloidal gears, due to some advantages. The most important advantage of the involute gear is that the centre distance for a pair of involute gear can be varied within limits without changing the velocity ratio. This is not true for cycloidal gears which require exact centre distance to be maintained. In involute gears, the pressure angle, from the start of the engagement of teeth to the end of the engagement, remains constant. It is necessary for smooth running and less wear of gears. But in cycloidal gears, the pressure angle is maximum at the beginning of engagement, reduces to zero at pitch point, starts increasing and becomes maximum at the end of engagement. This results in less smooth running of gears. The face and flank of involute teeth are generated by a single curve whereas in cycloidal gears, double curves (i.e. epicycloids and hypocycloid) are required for the face and flank respectively. 2. Conventional Methods of Spur Gear Cutting Conventional gear manufacturing are two types: i) gear forming and ii) gear generation methods. In forming method, the tooth spaces are produced on the blank by a formed tool. The tool profile is regenerated on the machined surface. The gear teethes are milled consecutively with a formed cutter of disk. The gear milling is done with form cutters having the profile of the tooth space of the same module. The production of the gears by the generating method reproduces the meshing of a pair of the gears; here, one component of the pair is the gear blank while the other is the cutting tool. The spur gears may be generated by hobs, gear shaper cutters, and rack-type generating cutters. This is further classified as i) gear hobbing and ii) gear shaping. In gear hoboing, i) hob rotates at the cutting speed, (b) interacting rotation of the blank (generation), and (c) hob feed along the axis of the blank. The hob, in rotation, cuts metal continuously with its teeth and all the teeth of the gear are consecutively cut, without interruptions in cutting for advancing and withdrawing the hob. In gear shaping, gear shaper cutter is applied for producing both external and internal spur gears. A gear shaper cutter is shaped like a gear of the same module as the gear to be cut. The shaper cutter and the gear blank rotate together as if they were two gears in mesh. The teeth of the shaper cutter are relived to obtain the geometrical form required by a metal-cutting tool. Kibet et. al. [3] have investigated that there is a problem of accurately machining gears by conventional means.