Gear Shaping vs. Gear Hobbing
Used for shaping gear teeth in metal milling and gear manufacturing, gear shaping and gear hobbing both do a great job for cutting gears but have different approaches and benefits. Both require the use of a specific machine to effectively carry out gear tooth cutting operations. Gear shaping is handled on a gear shaping machine, while gear hobbing requires the use of a gear hobbing machine. But what are the differences between these two different machining types, and which one do you need to ensure your operation can work efficiently across the processes?
What is Gear Shaping?
As one of the most common ways to create a gear from a gear blank, gear shaping creates gears by cutting teeth into the gear blank, typically a piece of hardened metal designed to withstand significant torque. The cutter is pinion-shaped and goes between reciprocating and rotating motions to create the teeth. This method uses gear shaping to create the gear, with cutting occurring at either the machine’s downstroke or upstroke. During this process, the cutter and gear axes are parallel to the cutter, which is rotating with the gear blank. Because the gear blank and cutter both move at the same velocity in terms of the pitch cycle, it creates a better surface finish. A train of gears drive the relative motion between the gear blank shaft and the cutter, which is often positioned perpendicular to the axis of the gear blank. This cutting process ‘s finished product includes internal gears, external gears, and pinion gears.
Pros and Cons of Gear Shaping
Generally speaking, gear shaping is a simple, replicable machining process for gear production. Because it’s fairly versatile and can be accomplished easily, it helps avoid having to purchase another piece of equipment for your shop. It’s most commonly used for creating either external or internal gears, though it does have the capability for integral gear-pinion arrangements. In many situations, it’s used most often for creating gears that have a location close to a flange or similar obstruction in the final machinery. The outstanding accuracy makes it a great option when you have gears that have low tolerances in kinematic accuracy. This allows the gear to be produced without further grinding, sanding or shaving before being put into production. In addition, in situations where small and large teeth appear on the same gear, gear shaping can provide outstanding results, due to the adaptability of the cutter head. Because the cutter head is perpendicular to the gear’s axis, it’s able to quickly cut a great many simple gears, making it a great option to consider if high-speed, efficient production and manufacturing of the same part on an almost continuous basis is of importance.
However, because the gear hobbing machine goes through fewer redundant motions, gear shaping tends to be a less efficient operation. The exception to this is when large gears require a small tooth width, at which point a gear shaper is the most effective solution. Though gear shaping does provide fast results, especially when using a pinion-shaped cutter on your equipment, it does provide high accuracy in your surface finish. Should you require greater accuracy, a different approach may be needed to get the job done, whether by switching to hobbing or by using more conventional processes, including honing, lapping, shaving or grinding. The orientation of the cutter head to the gear’s axis is a drawback if you need to cut gears with complex geometry, as the cutter head’s relatively fixed position makes it difficult at best to cut the type of modules, or teeth, needed for a helical, spiral or worm gear, or to cut splines.
What is Gear Hobbing?
Gear hobbing is a similar gear cutting tool, but is more accurate in gear motion, especially when cutting complex gear geometries. Similar to gear shaping, the hobbing process is very versatile and widely used in production for gear cutting. However, gear hobbing requires a specific tool, referred to as a gear hobbing machine. As a subset of milling machines, a hobbing machine may be an index hob or a master hob. Hobbing machines are also able to effectively cut splines and sprockets, as well as advanced geometry gears such as spurs, helical, crowned, worm, and chamfer gears. Because the machine uses an automated hob to cut the teeth into the gear blank, it cuts the teeth as the gear blank is in rotation. Given the accuracy of advanced-geometry cuts and the requirement of specialized equipment, gear hobbing is most often used for spur gears and helical gears, allowing businesses that do not produce these types of gears to save on equipment costs. This is because the cutting head can reach a wide range of geometrical positions, making it more adaptable to changing positions.
Pros and Cons of Gear Hobbing
Gear hobbing machines create progressively deeper cuts into the surface of the gear blank, which allows it to have a superior level of overall accuracy in the gear as used. It’s great for delivering superior results and faster production times, making it suitable for medium to high volume gear production. Similarly, gear hobbing produces a high level of motion accuracy, providing you with superior results in high-quality mechanical work and reduction of system wear on higher-end products. Further, the progression of the cuts into the gear blank’s surface makes it an excellent option for prototyping, giving you the opportunity to check final machine performance before settling into gear production as part of your overall manufacturing process. The overall approach of the cutting surface to the gear blank can be adjusted, allowing you to make not only standard gears, but also worm and helical gears, splines, and other less usual gear types. This means that the gear hobbing machine will provide you with more versatility in your shop, making it an excellent option for your metal shop if you take on a lot of different projects.
The main drawback of using gear hobbing is that you may not receive as smooth of a finish on your gear’s surface as you would from a gear shaping machine. This can be especially problematic if the gear is visible or part of an aesthetic appearance, such as using brilliantly shined brass gears in a high-quality grandfather clock. This means that if you do end up needing to provide a high-gloss shine on the metal and use a gear hobbing machine, you’ll need to provide for extensive finishing operations to regain the high-gloss finish you’re looking for. However, the production and prototyping capabilities of the gear hob can outweigh the drawbacks, providing you with strong options to consider when investing in equipment.
Gear Manufacturing Applications
When you need a gear that requires a superior surface finish, gear shaping will often provide you with superior results. This can come from a wide range of needs, from surface finishes that may be viewed by the consumer, whether intentionally or unintentionally, to gears that have a narrower tolerance in terms of depth, but which are easily reached using a gear shaper.
For gears that have a complex geometry, such as spiral, worm, helical, and similar gear types, the careful accuracy of a gear hobbing machine will deliver strong results. It’s also a great option when a high level of accuracy and precision is needed, such as in small, fine gears or for high-precision tolerances such as aerospace and similar applications.
There is a similar breakdown by module, or tooth size. Above 5 mm, gear hobbing machinery will provide the fastest production efficiency, while below 2.5 mm, gear shaping equipment provides the best efficiency. If your gears require a module between 2.5 and 5 mm, either machinery will be about as productive, and you’ll want to fall back to your finish and geometry requirements to decide what type of equipment you’ll need to use.
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metal machining industry. Worker or service engineer operating cnc milling machine at factory
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Gear Shaping vs Gear Hobbing
Gear shaping and hobbing are perhaps the two most common methods of gear creation. Gears are essential to the running of many machines, so creating the right gear is critical for smooth operations.
An Introduction to Gear Shaping
What is Gear Shaping?
Gear shaping is one of the most common methods of gear creation. Using the gear shaping method, gears are created using a machine to cut teeth into a piece of metal. A pinion shaped cutter rotates and reciprocates, creating the teeth.
The cutting can occur either at the upstroke or downstroke of the machine. During the creation process, the gear and cutter axes are parallel with the cutter rotating in motion with the gear blank. Both the cutter and the gear blank move at the same pitch-cycle velocity, with a train of gears producing the motion between the cutter and gear blank shafts.
When is Gear Shaping Used?
Gear shaping is generally known as a relatively simple and reliable gear production method. Gear shaping is also a convenient and versatile method of gear cutting. The gear shaping method is most often used to create internal gears and external gears, as well as integral gear pinion arrangements.
Gear shaping is also commonly used to create gears that will be located close to flanges or any other obtrusive surface in its destination machine. Because of the impressive accuracy of gear shaping, gears with low requirements for kinematic accuracy are often produced through gear shaping, without the need for further shaving or grinding of the gear.
What are the Advantages of Gear Shaping?
Gear shaping with a pinion shaped cutter, as described above, can be very cost-effective. The complexity of the mechanical gear shaper means the gear shaping produces a high level of accuracy in surface finish. If you need greater accuracy for your gear’s intended application, however, you can always finish your gear through another, more conventional process, such as grinding, honing, lapping, or shaving the gear.
An Introduction to Gear Hobbing
What is Gear Hobbing?
Like Gear shaping, gear hobbing is also a versatile and widely used process for gear creation. The process of gear hobbing requires a particular tool known as a gear hobbing machine, a kind of special milling equipment. Types of hobbing machines include the index hob and master hob. The hobbing machine can also cut splines and sprockets. The process of gear hobbing involves the use of an automated hob to cut teeth into a circular blank (or flat cylinder) piece of metal, or a “blank” gear. The hobbing machine works to cut the teeth as the gear blank rotates.
When is Gear Hobbing Used?
Most spur and helical gears are produced by the gear hobbing method. The versatility and productivity of gear hobbing make it a very popular gear production process. It is also used to cut splines and sprockets.
What are the Advantages of Gear Hobbing?
Gear hobbing is a flexible process, with particular flexibility afforded to the working angle. Many different types of gears can be produced with this method. It’s comparatively inexpensive, but it is pretty accurate as well.
What’s the Difference Between Gear Shaping and Gear Hobbing?
Which is More Accurate, Gear Shaping or Gear Hobbing?
As described above, gear shaping produces very high accuracy in surface finish. Gear hobbing, however, gives great movement accuracy. So, both have pros and cons depending on what kind of accuracy your gear needs. Overall, most experts would say gear shaping is more accurate than gear hobbing.
The transmission chain used in mechanical gear shaping is more complex than that of the gear hobbing process. The tooth profile error level of gear shaping is also lower than gear hobbing. With newer gear shaping machines, the transmission chain is greatly shortened, and transmission error is reduced greatly as a result. Gear shaping as a process is incredibly precise.
Which is More Efficient, Gear Shaping or Gear Hobbing?
Gear hobbing is generally regarded as being more productive, or efficient, when compared to gear shaping. The gear hobbing machine has fewer redundant movements, and can typically be a more cost-effective process, especially during the production of larger gears with fewer teeth, for example. Efficiency can be further improved by the addition of more gear blanks being stacked and cut at one time.
A notable exception to this general rule is that gear shaping may be more productive than gear hobbing when the required gear has a large number of gear teeth and a small tooth width. This is because the gear hob cuts using a high-speed rotating motion. The gear shaper is not as quick when it comes to cutting speed.
In creating gears with a module greater than 5 millimeters, gear hobbing is more productive than gear shaping. In creating gears with a module of fewer than 2.5 millimeters, the efficiency and accuracy of gear shaping are superior to that of gear hobbing. Finally, in the creation of gears with a module between 2.5 and 5 millimeters, gear shaping and gear hobbing are equally productive, or equally efficient
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The 3 Essential Methods for Gear Machining
Time to read: 7 min
Historically, gear fabrication methods have been classified into three main categories: generation, forming, and form cutting. As new gear manufacturing technologies are developed, new methods will be added to the list, but for now, those three are still the most common ways to machine a gear:
- Gear generation uses cutting tools in the shape of the desired gear profile to create the gear (i.e. rack cutters, gear shaping, and gear hobbing)
- Gear form cutting involves tools that are used to create the gear profile (i.e. gear milling, shaping, slotting, planing, and EDM)
- Gear forming creates gears without using cutting tools (i.e. rolling, casting, powder metallurgy, 3D printing)
Gear Generation
Sunderland Method
Rack-type cutters are one of the main gear manufacturing methods. The gear rack cutting process is also known as the Sunderland Method or the Sunderland System. This method utilizes a gear machine which consists of a rack cutter with rake and clearance angles that creates the teeth profile on a gear blank. It uses the specific relative motion between the workpiece and the cutter during machining to produce teeth profiles and is similar to a rack and pinion, which is depicted below.
Sunderland method
The teeth profiles follow a geometry that consists of an involute of a circle — basically, a spiraling curve traced by the end of an imaginary string unwinding itself from that stationary circle — or if you trace the point of contact from one tooth to another as shown in the following image.
Involute curve geometry
The Sunderland Method is excellent at creating teeth of uniform shape, and all gears that are cut by the same cutter will, in theory, gear correctly with one another. So, gear designs that require high precision — even double helix gears — can be fabricated with this method. The Sunderland Method is also versatile and cost-effective, especially for medium to high-volume production runs. And because the Sunderland system has been actively maintained since its invention, manuals and documentation are widely available and relevant even for new machine designs.
Gear Shaping
In this method of gear generation, a cutter and gear blank are connected by gears so that they don’t roll together as the cutter reciprocates. The cutter starts carving its way to the desired depth, as seen in Figure 3, then the cutter and gear blank rotates slowly as the gear teeth are cut.
Gear generation – shaping
Gear shaping is commonly used for cutting spur gears, herringbone gears, and ratchet gears. This method can be used for other types of gears, but since it uses a cutter that reciprocates the gear shape, it’s often utilized for the gear types mentioned above. Since there isn’t a complex programming requirement (it’s just cutting the same as the rack cutter has into the gear blank), the setup during production is simpler. You can still manufacture these types of gears with other methods, but gear shaping provides speed, design, and setup advantages during mass production. Gear shaping is not the best option for internal gears and worm gears, due to the position of the cutter (on the outside) and cut direction.
Gear Hobbing
Gear hobbing produces gear teeth by rotating a cylindrically shaped cutter called a “hob” (hence the term gear hobbing) depicted in the gear hobbing machine interface below. The hob can be single-threaded, depending on how many teeth per revolution should be generated.
Gear generation – Hobbing
Spur gears are most often fabricated with this method, although a variety of other gears — like cycloid gears, helical gears, worm gears, ratchets and sprockets — are all made by hobbing. A well-designed hob is critical, especially when cutting a complex geometry, and this process typically does not work for internal gears. Similar to gear shaping, hobbing has setup advantages, but only for exterior cuts since the hob is cutting the outside of the gear blanks.
Gear Form Cutting
Form cutting generally isn’t preferred because of its limitations — low productivity and poor quality. However, the various form-cutting techniques are useful alternatives for repair and maintenance when necessary.
Shaping, Planing, and Slotting
When shaping, the workpiece is fixed and the tool on the ram is moved back and forth across the workpiece.
Gear form cutting – Shaping
When planing, the tool is fixed and the workpiece travels on the table back and forth under the tool.
Gear form cutting – Planing
When slotting, the workpiece is held stationary and the tool on the ram is moved up and down across the workpiece.
Gear form cutting – Slotting (stationary workpeice and tool/cutter Primary Motion is up and down)
Both shaper and planer gear machining tools cut in straight lines. However, the shaper handles small-size geometries, while the planer is built for larger applications. Shapers can be used to create slots, grooves, and keyways. Slotting is essentially a vertical shaper that cuts internal gears and grooves.
Milling
The milling form cutting method is relatively limited in its use but is well-suited to create complex gear geometries. CNC milling is utilized to machine helical and spur gear wheels for various industries, including automobile transmissions, and hob cutters.
Gear form cutting – Milling
Milling gears is slow because it creates extensive heat transfer to the workpiece, which means that successive teeth should not be milled one right after the other.
Electrical discharge Machining (EDM)
EDM is an electromechanical manufacturing process where material is removed from a workpiece by applying a series of current discharges between two electrodes separated by a dielectric bath liquid. Rather than cutting, the spark acts as a ‘cutting tool’ that actually erodes the material instead.
EDM is good at cutting complex geometries of all sizes, even as a gear cutting process, but the process has its limitations. If you don’t have good control and precise programming, it’s easy to damage part surfaces — especially curved teeth profiles that are challenging for CNC programs to execute. But high-quality and intuitive 3D modeling and CAM software — like Feature CAM, Autodesk Fusion, Master CAM, and others — can produce the smooth motion needed to cut curved teeth.
Electrical discharge machining (EDM)
EDM machines have improved in recent years, which has minimized issues with surface finishes, and improved cutting precision and resulting material properties (microstructure, mechanical properties, etc). The process can achieve tight tolerances as small as thousandths of an inch and create both small (diameter of fractions of an inch) and large gears (diameter of over 20 inches). This process is used for both delicate applications in watches and clocks and to cut more robust gears like those used in race cars.
Gear Forming
Rolling
Rolling is one of the oldest gear forming processes that hot or cold rolls a blank workpiece through two or three dies, as seen below.
Gear forming – Rolling
When material saving is a critical concern during manufacturing, rolling is a good option since there’s no chip generation. However, to get an efficient process, you must consider rolling parameters, deformations, and microstructure effects before ramping up production.
Casting
Casting is a forming process whereby molten metal is poured into a mold cavity to form shapes. Gear casting is used to create gear blanks (that are then machined) and full gears with cast teeth profiles. Tolerances and accuracy are crucial considerations when casting gears, and creating casting molds entails a lot of up-front costs. However, once the mold and process parameters are determined, large production quantities justify the investment.
Gear forming – Casting
Sand casting is used primarily to produce gear blanks to be used in other processes. Fully functioning spur, helical worm, cluster, and bevel gears are all made by gear casting and are used in washing machines, small appliances, hand tools, toys, and cameras.
Powder Metallurgy
Powder metallurgy is a high-precision forming method that’s a cost-effective alternative to conventional, machine-finished steel and cast iron gears. However, this method is not suited for larger gear sizes but is adept at creating small, high-quality spur, bevel, and spiral gears. Because of the porosity of the formed material, larger gears have less fatigue and impact resistance, though a sintering process can be used to improve their mechanical properties.
Powder metallurgy is also particularly useful when gear designs include holes, depressions, and different surface levels or projections. You’ll find these gears in appliances, farm, lawn, and garden equipment, automobiles, trucks, and military vehicles.
Additive Manufacturing
Also known as 3D printing, additive manufacturing constructs a three-dimensional object, layer by layer, from a CAD 3D model. Due to the nature of the process, additive machines can form complex designs with lattice structures modeled to achieve mass reduction not easily obtained through conventional methods. This type of geometry is often created using 3D topology optimization and generative computer design.
Gear geometry made using 3D printing
Conventional and non-circular gears can be fabricated with additive manufacturing processes, and high-quality 3D printers are relatively affordable and widely available. Because of this availability, it’s become the choice for repairs and mechanical projects like educational toys or other gadgets needing fully functional gears. You can also include additional features and even combine geometry with the gear shapes to add customized shafts, keys, or grooves to the same solid.
So, now you know more of the many ways to fabricate gears, whether you’re generating, forming, or cutting them. For help with all your CNC design projects, including gear design, check out our helpful eBook, CNC Design Guide.
