Flexible Approaches to Gearcutting

Flexible Approaches to Gearcutting

Multiprocess or multitasking machine tools are typically thought of as machines combining operations such as milling, turning and perhaps grinding within a single machine tool. A more focused metalcutting operation that multitasking machines might also perform is gearcutting.

But when does gearcutting on a more versatile CNC machine make more sense than a conventional dedicated hobbing machine? I spoke with Mazak Senior Applications Engineer Mike Finn and Cybertec Hybrid Multi-Tasking Manager Joe Wilker to find out. We discussed the Mazak Integrex i-630V AG Hybrid multitasking machine, which is designed to cut gears through CNC milling and skiving.

Multitasking machines benefit from being able to perform multiple machining operations in a single setup. Here, we see an Integrex i-630AG using a skiving operation to cut teeth into the ID of a part. Photo Credit: Mazak

To Hob or Not To Hob

Although dedicated gear hobbing machines are the dominant production method for making metal gears, this process has faced competition in recent years from milling and skiving as customer demand has changed. Just-in-time and lean manufacturing philosophies have led many traditional gear customers to avoid keeping large backstocks, which has led to smaller order sizes across many industries.

While hobbing is the fastest method of gear manufacturing, its economic viability is questionable for shops with high-mix, low-volume work. According to Finn, the flexibility of a multitasking machine makes up for the difference in speed. “While specialty gear machines are best for high-volume gear applications producing tens of thousands of units of a single part, the trade-off is what happens to this specialty gear machine once the job is over,” he says. “Multitasking auto-gear machines like the i-630AG can be easily changed over to a new job or even a different process.”

The reason it is so easy to change to a new job with a CNC milling or multitasking machine is simple: While a hob must be designed with the final profile of the gear teeth in mind, a single end mill can cut numerous gear tooth geometries without needing changed. The economics of scale mean that high-mix, low-volume shops will get far more use out of their tooling using CNC end mills that can cut numerous features, while low-mix, high-volume shops will get more use out of a few hobs that can produce identical gears more quickly.

MIlling gears on a multitasking AG machine provides flexibility to the user. An end mill can be used to machine numerous features, which means they can be useful for complex parts with gear features, as well as low-volume work. Photo Credit: Mazak

Another draw to multitasking machines is their ability to fully machine parts with splines or gear teeth in a single chucking. Rather than using different machines for the gear features and the milling or turning processes, the user can produce a complete product in a single setup. This reduces the amount of time a shopfloor worker must spend loading and unloading machines, making them much more competitive in smaller batch sizes.

According to Finn, we also cannot discount the changeover between different processes. “A multitasking auto-gear machine has a quick changeover from gear skiving to gear hobbing to gear milling,” he says. “This enables users to more quickly produce complex gears with multiple features.”

Flexible Gearcutting in Five Axes

According to the company, the i-630AG is capable of five-axis machining and is designed to produce large, complex parts. Additionally, it can machine difficult materials such as hardened steel using cutting tools with a maximum diameter of 10.24 inches and max length of 19.69 inches. With large, complex gears being ideal for low-mix, high-volume work, it is well positioned for outcompeting dedicated hobbing machines in its niche.

There are other benefits specific to multitasking machines, according to Wilker. “Using a multitasking machine simplifies the programming, in comparison to using both a hobbing machine and a mill,” he says. Additionally, it enables in-process gear measurement, making it easier to avoid scrap. And according to Wilker, “Datum points can be held in relation to gear teeth with one chucking, improving part accuracy.”

Every machine shop must make its own economic calculations on how to make purchases. For some, the batch sizes of customers’ orders can justify investing in a dedicated hobbing machine. However, the flexibility of being able to skive the OD while milling complex features without changing machines will appeal to others. Photo Credit: Mazak

Both the C and B axes are monitored in the i-630AG using rotary-axis scale feedback, and both are synchronized to prevent fluctuations in the spindle speeds from producing out-of-spec parts. The machine uses Mazak’s Mazatrol SmoothAI control and includes Smooth Gear Cutting software, which automatically adjusts cutting parameters if either the milling or turning spindle drifts away from the target speed. “Thanks to the machine’s synchronization, we’ve increased productivity,” says Wilker. “Additionally, heat-treated materials can be cut with carbide cutters and cut small-to-large / heavy-ID or -OD gears based on machine models.”

According to the company, the machine’s control is designed to enable the user to completely program a job at the machine, rather than offline or at a dedicated CAM system. “This lets users create a part program in its entirety, including turning, drilling, milling and gear-tooth machining on the machine control without additional software and without a part model.” For users nervous about programming entirely on the machine control, each gear-cutting module can be verified through toolpath simulation accessible on the control.

For a machine shop, it is dangerous to rely on old orthodoxy when it comes to growing the business, as we can see with this machine. While experience can guide shops well, it is important to dispassionately interrogate the way parts are processed and decide if that is still the way forward. Where once productivity and precision were the only measures of a machine’s value, it seems that more shops are performing this calculus and concluding that flexibility is vital to their success.

A Guide to Gear Hobbing - Avon Machining

What Is Gear Hobbing?

Gear hobbing is a quick and versatile process that is fundamental for gear manufacturing. Gear hobbing machines utilize a rotating cutting tool, or hob, to generate a tooth profile on gears as they are fed through the machine. The precision and accuracy of gear hobbing makes it the ideal method for gear manufacturing.

How Gear Hobbing Works

A gear hobbing machine contains two skew spindles. One of these spindles houses the hob, while the other houses the gear blank. The angle at which these spindles are placed relative to each other depends largely on the type of gear being manufactured.

Once the spindles are placed at the proper angle, the machine is set up to begin rotating the shafts at a speed ratio that suits the gear type. As the shafts rotate, the hob gradually cuts the teeth into the gear with the proper depth. To facilitate faster production, manufacturers can stack multiple blanks on the spindle together, allowing the hob to cut teeth into multiple gears at once.

Just like there are multiple different angles, speeds, and techniques manufacturers can use to customize the process, there is also a wide variety of hobbing machines available for use. Most hobbing machines specialize in distinct applications. Hobbing machines are built to handle gears of a particular size and come in two different varieties: single-threaded and multi-threaded hobs. Multi-threaded machines allow for increased production, but they aren’t as precise as single-threaded machines.

Differences Between Gear Hobbing and Gear Milling

Gear hobbing and gear milling are both processes that can be used for gear manufacturing; however, they utilize different cutting techniques.

Gear Hobbing

Gear hobbing is the process of generating gear teeth with a helical cutting tool, or hob. The gear blank and hob rotate continuously until all gear teeth are cut into the blank. The speed of this process makes it ideal for large production runs.

Gear Milling

The gear milling process uses a cutter, known as a form cutter, that moves axially to produce a gear tooth at the proper length and depth. After one tooth is cut, the cutter is withdrawn to allow the gear blank to rotate to the next position. Once the blank is rotated, the form cutter cuts the next tooth, and this process continues until all teeth are cut into the blank. This process is slower, making it a low production process.

Gear Hobbing

High precision gears are components that can determine the quality, performance, service life, safety and reliability of high-end power transmissions. While there various methodsof manufacturing these gears such milling or grinding, hobbing is the most widely used method of gear manufacturing. Generally, when compared with the grinding process, the hobbing process has the advantage of high precise, efficient at lower cost. (1) Though the application of this process can be extremely limited by part geometry, hobbing is still by far the most productive form of gear tooth generation for external spur and helical gears. (2) 

Three important parameters are to be controlled in the process of gear hobbing indexing movement, feed rate and angle between the axis of gear blank and gear hobbing tool (gear hob). The schematic diagram of the set up of a gear hobbing machine can be seen in the figure.  The aims of the hob are set at an inclination equal to the helix angle of the hob with the vertical axis of the blank. If a helical gear is to be cut, the hob axis is set at an inclination equal to the sum of the helix angle of the hob and the helix angle of the helical gear. The operation of gear hobbing involves feeding the revolving hob until it reaches the required depth of the gear tooth. The process of the gear hobbing is types according to the directions of feeding the hob for gear cutting, but not to be confused with the two different strategies that can be used in gear cutting that will discussed in the next paragraph. The different types of gear hobbing are: hobbing with axial feed, hobbing with radial feed, or hobbing with tangential feed. Axial feed is when the hob is fed along the face of the blank and parallel to axis. This method is used in spur and helical gear manufacture. Radial Feed is when the gear blank and hob axises are set normal to each other and the hob is fed against the gear blank in a radial direction. This type of feed is used in the generation of worm wheels. Tangential feed, which is used for worms, is a case where the hob is held with its axis horizontal but at a right angle to the axis of the blank. The hob is set at the full depth of the tooth and then fed forward axially. The hob is fed tangential to the face of the gear blank. This method is also used to manufacture of worms. (11)

There are two different cutting strategies when gear hobbing, climb cutting and conventional cutting. The term “climb” and “conventional” cutting refer to the direction of hob feed into the workpiece with reference to the table or spindle nose. For maximum stability during cutting, it is recommended that conventional cutting be used when possible. In conventional hobbing, the hob is fed into the work, moving toward the table or spindle nose, parallel to the blank axis. In climb hobbing, the hob is fed moving away from the table or spindle nose, parallel to the blank axis. (1) To determine which hobbing strategy should be used in a particular application the direction and axial feed must be defined by the three distinct motions of the gear hob: the tool rotation about its axis, tool axial displacement, and the workpiece revolution about its axis. (5) A general rule of thumb is that climb hobbing yields better tool life and accuracy while conventional hobbing yields a better finish. (6)

In hobbing multiple teeth of the tool, the hob’s teeth, are in contact with the workpiece at the same time. Together with the continuous coupling of the tool and the workpiece rotational motion, these teeth create the involute tooth gaps in a generating way. (9) In hobbing one of the complexities is arranging the cutting configuration in a way that will produce the desired tooth profile whether standard or modified in some way as the cutting configuration corresponds to the selected design structure specifying the geometric and kinematic parameters of the tool’s tooth profile such that the tooth trough in the gear is produced in an orderly manner and the cut layer is of the specified shape.(8) The kinematics of this complex process is based off of three relative motion between the gear and the workgear due to the synchronization needed to produce gear teeth. In gear hobbing, the hob tool and the workgear move in a linked revolution ratio. These revolutions are synchronized with hob axial-feed and are dependent on the number of starts in the cutter and the number of teeth of the workpiece. (2)

The generation process is one of the most fundamental concepts in the gear hobbing. The cutting tool geometry can determine cycle times and the tool wear of the process among other factors of gear manufacturing. The hob itself is essentially a worm with gashes cut across it to produce the cutting edges. The hob could also be considered a series of racks positioned around the circumference of a cylindrical tool. Each successive rack is shifted axially to create a worm, typically a single thread. It is possible to design the shape of a cutting tool to produce modified tooth forms. The advantages of the different modification is shown in Table 1: (6)

Modification

Advantages

Topping Hob

• Cuts the outside diameter of the gear tooth

• May eliminate the need for finish turning

Semi-Topping

• Eliminates the sharp corners between the tooth flank and outside diameter

Protuberance

• Provides a uniform stock for the finishing tool

• Provides a blend between the hobbed root area and the finished flank

Tool wear is also concept of gear manufacturing that is highly researched as it is very difficult to model and predict due to the generating-rolling principle that governs the hobbing kinematics. Referring to the wear influence of the tool geometry, there are critical gearing parameters that can influence the tool life time in a very positive or negative way. Modifications can be made to the tooth profile of the hob in order to optimize the production of a particular workpiece. These special profiles are widely used in industrial applications whereas only company internal knowledge currently exists. (9) The variant chip formation on each cutting tooth during hobbing provokes different wear laws and usually leads to an unequal wear distribution on the hob teeth. (7) The hob will also have certain defects that contribute to tool wear: for example, the simultaneous and intermittent participation of two or more cutting edges of the tool teeth in hobbing. consequently , pronounced local wear can appear at the tooth tip, with associated loss of tool life. These defects need to be taken into consideration on certain jobs in which a custom hob must be made with longer runs. (8)

There are many factors to be taken under consideration during the optimization of the gear hobbing process; the cutting forces involved in this process are one of the most important factors as the cutting forces will contribute to the chip formation and the tool wear of the hobbing process. The tool and machine wear caused by the forces can thus affect the pricing of the part in production. There are various models in which authors have created models to predict the cutting forces, but presently there is not a way to accurately measure the cutting forces, tool wear or chip formation process due to the complex nature of the kinematics of gear hobbing. Cutting forces are generally separated into using small cutting edge elements as shown in Figure 1. (4)

Besides the geometries of the gear workpiece and the hobbing tool, the material properties also affect the gear hobbing process. Common materials of the hob tools are high speed steel and carbide. The material used for the hob is dependent on the material of the gear blank and the speeds at which you would like to run the hobbing process. Steel gear workpieces, for example, are typically formed by the hobbing tools made from solid tooling material, such as tungsten carbide. (5) Studies have also shown that a major increase in the applied cutting speed is possible when matching the substrate with a convenient coating. (9)

New suggestions and methods to improve the precision and efficiency of hobbing have been introduced by researched by researchers such as modeling the kinematics of the hob.