Gears are essential components in many machines and devices, such as cars, airplanes, robots, and wind turbines. Gears transmit power and motion between rotating shafts, and they need to be durable, efficient, and quiet.
To achieve these qualities, gears are usually case hardened, which means they have a hard outer layer and a tough inner core. However, case hardening introduces distortions and surface defects that affect gear performance and quality. Therefore, gears need to undergo a hard finishing process after case hardening to remove unwanted material and improve surface finish and geometry. Continuous generating gear grinding provides a stable, repeatable process that helps manufacturers ensure gear quality. Manufacturers of machines that perform this operation include Reishauer, KAPP, Gleason, and Liebherr.
Getting the most out of the gear-grinding process requires attention to several factors, including grinding wheel selection, incoming part quality, and machine fixturing and setup.
What is continuous generating gear grinding?
Continuous generating gear grinding, sometimes referred to as CGG grinding, is one of several types of gear grinding, and it’s commonly used in high-production applications. It is a threaded-wheel process that uses a large-diameter grinding wheel with a thread pre-formed on the outside diameter, similar to a hobbing tool. This threaded wheel is dressed, allowing the side of the thread tooth to have the same pressure angle as the workpiece, and the workpiece and wheel rotate in a fixed ratio. The workpiece is fed axially past the wheel while the generating motion occurs.
So, why grind gears? The grinding portion of the process finishes the profile of the gear flanks for optimized performance. High-production applications that must produce a lot of gears quickly can gain many benefits from this type of gear grinding because it helps operations meet tight tolerances and optimize gear performance. Benefits of CGG grinding include:
• Corrects dimensional errors created by the roughing and heat treat process.
• Enhances the surface finish of the tooth flank, allowing increased torque transfer and helping reduce noise in the gear assembly.
• Provides the capability to remove up to 1 mm of material per flank in one pass, reducing cycle time and the number of passes required.
• Offers very short idle times, as the grinding worm can be moved quickly between different gears or workpieces.
• Has a high degree of automation and integration with other machines, such as measuring devices or washing units.
CGG grinding challenges
One key challenge in CGG grinding is choosing the right grinding wheel for the job. The grinding wheel is the cutting tool that generates the gear profile by rolling with the gear. It must have a high cutting ability, durability, profile accuracy, and free-cutting properties. It’s also important to ensure the wheel has a suitable shape, size, module, pressure angle, helix angle, number of starts, pitch diameter, width, abrasive material, grain size, bond type, hardness grade, and porosity level to match application needs.
The grinding area can be a bottleneck in a gear-manufacturing facility, especially if the wrong grinding wheel is used. The choice of wheel can significantly affect cycle time and productivity and have a negative or positive impact on costs. Wheels come in a variety of abrasive combinations and bond types, as well as different widths, diameters, and thread types. It is important to match the wheel to the gear material. Different materials have different grinding characteristics, which affect the choice of the grinding wheel. For instance, a softer material will need a different type of wheel than a harder material. Likewise, the type of wheel used for grinding steel will not be the same as the one used for grinding Inconel.
Labor can be another challenge in CGG grinding. Many manufacturers use automated CNC machines for gear grinding that require specific training and experience to properly operate. It is challenging to find skilled operators for these types of machines. This inherently makes it harder for gear manufacturers to apply new grinding wheel technology correctly and to improve overall processes by ensuring the correct wheel is being used.
Following some best practices for gear grinding can help operations improve results while reducing cycle times and costs. Consider these six tips:
Tip 1: Choose a wheel with the correct grain and bond
Grinding wheels are composed of grains, which are held together by bonds that vary in strength. Examples of wheel grains include aluminum oxide, zirconia, mono crystalline, and CBN. Each type of grain and bond offers different properties. For instance, some wheels are designed to be stronger and have better form holding, offering longer product life. Other wheels are designed to fracture readily so they maintain sharpness and withstand severe abuse — which can increase the versatility of the application and offer more flexibility when selecting geometries to grind. To choose the right wheel, consider these factors:
Gear material: The wheel should be made of abrasive grains that are harder than the gear material. For example, for case-hardened gears, choose fused or sintered aluminum oxide grains with high hardness and toughness.
Gear quality: The wheel should have a fine grain size relative to the gear geometry to achieve the required surface finish and precision geometry of the gear.
Material removal rate: The wheel should have a grain size related to the geometry of the gear tooth that allows high material removal rates while also achieving gear quality.
Dressing frequency: The wheel should have a suitable bond type and porosity level to maintain shape and cutting ability during the grinding process. For example, for gears with a high dressing frequency, use vitrified bonded wheels with medium to fine porosity (dependent on gear geometry) to facilitate chip removal and coolant penetration.
Operations that choose grinding wheels based solely on purchase cost may not realize that paying more for a high-performance wheel can significantly prolong the life of the tool, enhance the productivity of the process, and improve the quality of the output. These advantages can lower the total costs of grinding in the long run. Therefore, grinding wheels should not be selected based on purchase cost alone. Sometimes a different wheel can solve problems or issues in the grinding cell.
Finding and applying the correct grinding wheel specification requires knowledge and experience. It’s best to work with the wheel’s manufacturer (like Weiler Abrasives, for example) to help determine the correct specification. There are many wheel options and many factors and priorities to consider, such as performance vs. cost and the associated tradeoffs. In addition, new grain technologies available today have accelerated wheel performance, so it’s important to work with an application engineer and abrasives specialist to properly apply the technology.
Tip 2: Train operators properly
After the correct wheel is chosen, proper operator training is key to getting the best results. Different wheels or materials may require an adjustment in machine parameters. Some wheel manufacturers provide hands-on product training, either on-site at the gear manufacturer or in their own facilities. Weiler offers abrasives and machine training for all precision products.
Tip 3: Ensure incoming part quality
A good first step in optimizing the gear grinding process is to consider prework and the parts coming into the grinding cell. The stock coming into the cell must be of high quality to achieve the best results. When incoming parts aren’t high-quality — perhaps they are too big or have heavy burrs — it adds time and cost to the process. Often, operations will make parts bigger than necessary upstream with the intention of cleaning them up during grinding. They do this to avoid creating excessive scrap. However, this method increases grinding cycle time and reduces throughput. Ideally, the incoming part should be as close to the necessary finished dimensions as possible to streamline the process.
Tip 4: Check the fixturing
The fixturing or work holding inside the grinding machine should be correctly dialed in, clean, and in good working order. As issues happen inside the machine, the work holding may slip or move, which can break the grinding wheel. Be sure to periodically check the fixturing to ensure that it’s located properly. Also, use a good coolant inside the machine for the grinding process. Mobil and Castrol make quality coolants for grinding applications.
Tip 5: Properly mount and maintain the wheel
Proper wheel mounting helps optimize grinding results and maintain safety. Be sure to follow all safety recommendations for wheel mounting and conduct regular inspection of the wheel, spindle, and blotters. Operators should consult ANSI B7.1: Safety Requirements for the Use, Care and Protection of Abrasive Wheels. Improper mounting can damage the wheel and cause more downtime and added costs. Take advantage of on-site training offered by some wheel manufacturers to ensure safe machine and wheel setup.
Tip 6: Document the entire process
If the grinding process is a bottleneck, look at it from start to finish to see where efficiencies and improvements can be found. Chances are, solutions are available to increase capacity of the process and reduce cycle time or increase throughput with longer wheel life and fewer wheel changes. A review of the entire process can help pinpoint challenges.
The process parameters are the variables that control the grinding process. Parameters that can be reviewed include grinding and workpiece speed, infeed, feed rate, shifting distance, coolant type, and coolant flow rate. These factors affect material removal rate, surface finish, profile accuracy, temperature, wear, vibrations, and noise of the grinding process. Therefore, they must be optimized to achieve the desired results and avoid problems.
While such an analysis may seem daunting, it can deliver significant benefits that save time and money. This analysis can be made easier by working with an expert who will review processes, document challenges and problems, and then create and develop solutions and specifications for each application. If the solution involves a grinding wheel change, the application engineer can help mount the tool, do a test run, collect data, and document results.
Some wheel manufacturers offer programs to help document processes, see where the challenges are and provide solutions based on that data. The Weiler Process Solution (WPS) program helps users better manage grinding wheel costs and increase productivity by testing and evaluating different options to compare product life, reduce cycle times, and eliminate bottlenecks.
The program also helps eliminate process variability, rejections, and rework. Consider a real-world example from one grinding wheel customer who was at capacity in its gear production operation and was outsourcing some of that work.
A full evaluation of the entire process resulted in grinding wheel change and decreased cycle times in the gear-grinding process, so they were able to take on more work.
Final thoughts
In high-production manufacturing applications that must produce many gears quickly — while meeting tight tolerances and optimizing gear performance — CGG grinding can deliver numerous benefits if best practices are followed for choosing the right wheel, operator training and system setup. Selecting the correct grinding wheel is critical and will help operations optimize throughput and reduce cycle times in gear manufacturing.
Because CGG grinding requires specific knowledge and experience, an application engineer is critical to ensure process efficiency. Abrasives experts can provide necessary advice and guidance to help manufacturers further optimize the process.
