Optimal interplay between innovative gear cutting tools and powerful bevel gear cutting machines results in highly productive cutting processes with extremely high, reproducible gear quality. According to Klingelnberg, the gear cutting process itself can be seen as the link between the two elements.
Increased competition and the pressure that goes along with it — particularly in the automotive industry — call for new and cost-effective production processes to meet growing quality requirements. This also affects the machining of bevel gear teeth to a significant degree, which is why it is essential to exploit the full potential of all components involved in the process: The key is to utilize the machine performance and the tools that are used to their full potential, thereby reducing the cycle time.
Bevel gear cutting is characterized by continuously varying cutting conditions, meaning that “simple” feed regulation-based spindle utilization alone, and without sufficient analysis or knowledge of the process sequences, is far from optimal. Thanks to the ongoing development of the Klingelnberg machine operating software, users can now gain insights into the process sequence. The company’s Smart Process Control software module allows technicians to plot the machine utilization and allocate it to the respective process sequences in an initial step. In a second step, the machine response to process changes can be precisely analyzed and tracked. This creates a new basis on which to enable highly productive gear-cutting processes.
Criteria that make for an optimal bevel gear tooth cutting process:
- The component quality corresponds exactly to the requirements.
- Machining time is minimized.
- The gear cutting tool can be used for as long as possible.
- Primary benefits include reduced machine and tool costs.
This is a very simple checklist at first glance. However, when it comes to streamlining the bevel gear cutting process, conflicts arise between the objective and the technical implementation. Anyone wishing to improve the process has two objectives clearly in mind: higher productivity and improved process reliability aimed at meeting the strict requirements for geometric component accuracy (pitch and topography) in a reproducible and reliable manner without compromising the surface quality of the tooth flank, which acts as a measurement surface. This elevated productivity requires an increase in the machining parameters, though, and this is exactly where the conflict lies, since increased machining parameters have a tendency to worsen the wear behavior of the tools. This in turn heightens the risk of damaging the flank surface due to surface defects such as scratches or even welding marks — scenarios that must be avoided at all costs.
Process Design Limits
The options for optimizing a bevel gear cutting process are limited by various factors, the first being the component type and the different process characteristics associated with it. For non-generated components (ring gears only), it is the wear on the tool. For generated components (always pinions but often ring gears as well), it is often the formation of surface defects that sets the limit. The bevel gear cutting machine, on the other hand, usually plays a subordinate role.
The control units in modern CNC machines allow the precise recording of signals. This makes it possible to collect and store various types of information at any given point in a process. The technician’s task is to identify which key criteria should be used to define a process as optimal. Whereas the “machining time” criterion is easy to read off at the end of the process, the “flank surface damage due to surface defects” criterion is more difficult to assess and evaluate. This is because the wear on a tool changes throughout its service life, and material batch influences also play a part. The most difficult criterion to evaluate is tool load and the resulting cutting edge wear.
Contrary to many other machining processes, the contact situation of the tool cutting edges changes continually during the process sequence in the bevel gear cutting process. The load on the cutting machine also varies accordingly. The special feature of this process is that the resulting tool wear that occurs varies in appearance along the entire cutting edge because of the local loads. If a technician wishes to use information from the machine control unit as a basis for evaluating local tool wear, the only way to do this is by taking into account the cutting edge length involved in the cutting process.
Recording the utilization of the tool spindle helps with optimization as well as the monitoring of the process sequences. In bevel gear machining, the processes are defined by interpolation points from the working position and feed rate. Varying these parameters brings about a change to the tool spindle utilization. The Klingelnberg Smart Process Control software, which is directly available to the user on the machine, makes it possible to carry out precise visualization of the machine load for every machining situation. The process sequence can therefore be assessed and easily understood at a glance. Following a short training session provided by an experienced company technician, machine users will then be able to analyze and subsequently optimize their processes independently using the control. Automatic real-time logging of the spindle utilization (for a meaningful number of workpieces) enables continuous process monitoring over a freely definable time period. The data documented during this time can easily be evaluated using the usual Microsoft Office programs. This long-term monitoring makes it possible to work out the impact of changes to the process.
Long-term monitoring can also be used to identify material irregularities or changes to the wear characteristics of the tool.
Bevel gear teeth are typically manufactured in a multiple-step process. For most components, the tooth profile is generated by means of a rolling motion of the gear cutting machine. After the initial plunging process and subsequent roughing-generating, finishing-generating is the final step in the bevel gear tooth cutting process. Figure 1 shows the plotted utilization curve (in blue) of the tool spindle used in this type of finishing-generating process. Since the example deals with a gear set that obtains its final quality by means of lapping after heat treatment, the main focus during machining is on component accuracy. The process to be optimized (see Figure 1) starts with a plunging operation (I) at a lambda working position of approximately 312°. Once the tool has reached its working depth, it generates an initial area of the tooth flank (roughing-generating, II from lambda 312° to 321°). After a short infeed (III), the gear teeth are finish-generated at the tool utilization shown (WA [%]). The tool starts the finishing-generating (IV) at a lambda generating angle of 321° and stops at a lambda generating angle of approximately 301°. When doing so, it passes through the roughing-generated area. The load increase (or load surge) around the 312° generating angle is clearly visible. This is the result of the transition from the roughing-generated area to the non-generated area.
A measurement of the component under observation showed abnormalities in the flank topography that were exactly in the vicinity of the load increase, i.e. at 312°. Based on this information, the technician is able to identify the need for process modification. As a modification, the working position of the plunging operation was moved from lambda 312° to lambda 308° to enlarge the roughing-generating area. An adjustment was also made at interpolation points P1 to P4 by entering suitable chip thicknesses (SW [mm]). The result is spindle utilization that is significantly more uniform during finishing generation, without a local load increase around the plunging position lambda 308° and with a normal flank topography. On this basis, it is now possible to raise the uniform load level by shifting the interpolation points P1 to P4 at the same time, thereby achieving a shorter machining time.
The use of the Klingelnberg Smart Process Control also makes it possible to increase the feed rates for the plunging and roughing-generating area in a controlled way. In total, a 7-percent reduction in productive time was achieved in this example process, as illustrated in Figure 2.