Study of Machining of Gears with Regular and Modified Outline Using CNC Machine Tools

Classic methods of machining cylindrical gears, such as hobbing or circumferential chiseling, require the use of expensive special machine tools and dedicated tools, which makes production unprofitable, especially in small and medium series. Today, special attention is paid to the technology of making gears using universal CNC (computer numerical control) machine tools with standard cheap tools. On the basis of the presented mathematical model, a software was developed to generate a code that controls a machine tool for machining cylindrical gears with straight and modified tooth line using the multipass method. Made of steel 16MnCr5, gear wheels with a straight tooth line and with a longitudinally modified convex-convex tooth line were machined on a five-axis CNC milling machine DMG MORI CMX50U, using solid carbide milling cutters (cylindrical and ball end) for processing. The manufactured gears were inspected on a ZEISS coordinate measuring machine, using the software Gear Pro Involute. The conformity of the outline, the tooth line, and the gear pitch were assessed. The side surfaces of the teeth after machining according to the planned strategy were also assessed; the tests were carried out using the optical microscope Alicona Infinite Focus G5 and the contact profilographometer Taylor Hobson, Talysurf 120. The presented method is able to provide a very good quality of machined gears in relation to competing methods. The great advantage of this method is the use of a tool that is not geometrically related to the shape of the machined gear profile, which allows the production of cylindrical gears with a tooth and profile line other than the standard.

1. Introduction

It can be said with certainty that cylindrical gears are still the most frequently used gears. They are mainly processed by the hobbing method with modular hob cutters, as well as with Fellows chisels, or a finishing process by grinding according to the Maag, Niles, or Reishauer methods. Involute cylindrical gears with straight teeth are most commonly used in drive units in many machines and devices. They are most often machined using shaping, as presented by Litvin and Fuentes [1]. In addition, in unit production, they can be shaped with modular disc or finger cutters. The study of gear machining with modular disc cutters was presented in the work by Pasternak and Danylchenko [2], who simultaneously analyzed the distribution of cutting forces in the process.

The accuracy of the machined gear to the greatest extent depends on the technology and tool used, which can be guaranteed by the use of common tools such as modular hobs, while an even greater increase in the accuracy of these tools will allow for wider use of multipurpose universal CNC machine tools, as investigated by Piotrowski et al. [3]. These are special and usually expensive tools, and the machining is carried out on special machines.

The indicated technologies for the construction and design of gears were described in the works by Radzevich [4], Skoć and Switoński [5], and Nieszporek [6]. At present, there is a noticeable tendency to inspect manufactured gears using contactless methods; for example, the method of optical inspection of gears made of polymer using an injection method was presented by Urbas et al. [7].

Polymer materials are more and more often used as construction materials of gear elements. The polymer material itself can be modified by annealing in order to improve machining conditions with small allowances, as assessed by Gnatowski et al. [8]. Quite often, gears are machined with special tools, as presented in the work of Boral et al. [9] while analyzing the accuracy of the described method.

The use of a mathematical model in the process of designing the geometry of a special tool was presented in an article by Xin-Chun et al. [10], which can be the basis for optimizing the parameters of the milling cutter used to machine the gear. Gear machining technology is becoming increasingly demanding in terms of both quality and efficiency. It has also become very important that, in the manufacturing process, the machined gear does not have to undergo finishing processes on another machine tool. Thus, the done-in-one machining philosophy, i.e., the integration of all machining processes from clamping the raw material to finishing in a single machine, is increasingly becoming a priority [11]. The development of the done-in-one technology involves the integration of selective laser hardening with a multi-axis machining platform. This allows gears to be machined and hardened in one production center, as concluded by Peeters et al. [12].

One of the ways to improve the quality of gears after heat treatment is grinding, which is necessary in the finishing process; problems related to errors in the finishing process and their further consequences in the operation of the gears were presented in work by Gołębski and Szarek [13]. It is also possible to apply a new approach to the finishing of gears using non-Newtonian liquid polishing technologies, as described by Nguyen et al. [14]. The quality of the entire gearbox is determined by the location of the mating area of interacting elements, whereby mating analysis is mainly used to determine kinematic errors that are the result of machining and errors in gear assembly, optimization, and the synthesis of the gears (especially for the analysis of spatial gearing), as described in the work Nieszporek et al. [15].

The predictability of machining parameters, the characteristics of chip formation during machining, tool wear, and the generation of forces in the cutting process are currently the subjects of intensive industrial research, constantly contributing to the development of modern areas of gear machining, as presented by Bouzakis et al. [16]. With the increasing demand within the automotive industry around the world, a highly efficient method of gear manufacturing is required. Gear transmissions are used not only in cars but also in all kinds of mechanical products, including airplanes, ships, and power-generating equipment, and the production volume of gear boxes is huge. Álvarez et al. [17] investigated development in the field of design and construction of CNC machine tools allowing for the development of dedicated gear machining technologies, allowing for a free approach in terms of the shape and geometry of gears.

The advent of multitasking machines in the machine tool sector presents new possibilities for processing large-size gears in a single production on these machines. However, the possibility of using standard tools in conventional gear machining machines presents a technological challenge in terms of the quality of the product. However, the use of appropriate solutions and developing methods in CNC machining technology gear seems fully justified, as investigated by Skoczylas et al. [18]. One example of this is the power skiving technology as an efficient method for producing internal high-precision gears, as investigated by Inui et al. [19]. Klocke et al. [20] presented an analysis of the influence of geometrical parameters for the power skiving method on the quality of the manufactured gear. In the process of this machining method, a very important role is played by the visualization of the resulting machining shape, whereby we can precisely analyze the changes in the shape of the gear during the machining process, as well as the volume of the workpiece material removed, as noted by McCloskey et al. [21]. The method of machining according to the power skiving technology in the field of machining of internal gears is unmatched in terms of both quality and efficiency; however, machines dedicated to this method are very expensive, as presented by Chung-Yu et al. [22]. Software in which we can prepare the technology allows for the geometrical assessment of the machined gear by simulating the manufacturing process of a gear wheel, as pointed out by Spath et al. [23]. In contrast, Hyatta et al. [24] presented a very bold thesis that CNC machine tools with the use of appropriate technology of gear machining can offer a much higher productivity and quality level.

The possibility of generating the tool path and verifying the model of the machined part at the stage of technological preparation of production gives great opportunities for using the technology offered by computer control systems of machines, as presented by Yi et al. [25]. Karpuschewski et al. [26] presented an analysis of the surface integrity states for various variants of gear machining, concluding that the skiving milling method can compete with the finishing machining processes for small modular gears. Scherbarth’s [27] patented method of gear machining InvoMilling developed by DMG MORI and Sandvik using universal and simple tools represents a highly effective use of universal machining centers for productive and accurate gear manufacturing. Continuous development in the field of CNC control systems has created opportunities for technologists and programmers, leading to increasingly wider applications. Golebski [28] presented the advantages of parametric programming of CNC machine tools and the possibility of using this technology in the strategy of machining gears by the multipass method, while Jiang et al. [29] used the method of parametric programming to generate the NC code of gear machining for the case of machining with a modular hob. Another important idea in the whole process of using numerically controlled machines is that of using machine tools with the simplest kinematics, with particular emphasis on three-axis machine tools, which, in the case of machining straight gears, seems to be fully justified, as noted by Suh et al. [30].

A spur gear allows transmitting rotational movement and load adequate to its work. Therefore, the modification of the tooth line should be determined from the strength conditions of the gear. The first study on the optimization of the gear structure, with particular emphasis on the tooth profile, was conducted by Novikow, who proposed an innovative type of gear with a concave–convex profile, described in a study by Markowski and Batsch [31]. As a result of affixing, the trace of the mating teeth, modified under load, transforms into a point contact and should be located only in the area of the useful tooth surface. Spur gears with straight teeth with a longitudinally modified profile are characterized by insensitivity to assembly errors and the ability to transfer large loads. The assessment of the influence of the torsion angle of the axes on the mating area of gears and the position of the trace path of overlapping tooth was presented in work of Gołębski et al. [32], showing that, in the case of involute gears without longitudinal modification, the impact of errors in the gear assembly is significant for the mating area of gears.

According to the literature review, it can be stated with certainty that the topic taken up in the work is up to date and worthy of interest. It has been shown that gears can be machined using many methods, with particular emphasis on machining using CNC machine tools. The proposed method of machining cylindrical gears with straight teeth with longitudinal modification of the tooth line on universal CNC machines, using universal tools as cylindrical or spherical end mills, can represent an alternative to the industrial methods described in the work. It should be noted that the undertaken task is universal and may be further developed thanks to the application of a tool that is not geometrically related to the shape of the machined gear profile, which allows the production of cylindrical gears with a tooth and profile line other than the standard. In the context of the indicated features of the tested method, the analysis carried out in the work can answer the question of whether the accuracy of the developed method is adequate and can be used to produce the advisable types of gears.