Gear Cutting: Definition, Processes, and Types
Gear cutting is a group term used to describe several processes that are used to create gears out of gear blanks. The gear-cutting process involves the cutting or abrasion of material from a gear blank to create a gear. Different types of gear cutting include: grinding, shaping, finishing, broaching, hobbing, and milling. Each of these processes has its characteristics.
This article will discuss what gear cutting is, how it works, and the types.
Gear cutting is the process in which a gear is cut using a milling machine, as opposed to the gear being cast or stamped. Gear cutting can be achieved using a few different processes including: grinding, shaping, broaching, hobbing, and milling.
Gear cutting has a few different methods, but they all work on a similar principle. A gear blank, which is a block shape resembling the final shape of the gear, is given gear teeth using either cutting or abrasion. Cutting uses a special cutting tool mounted on a milling machine which cuts one tooth at a time, whereas the abrasion method uses a shaped grinding disc to abrade away material.
Gear cutting is used to create a series of different gears, which mesh with one another to transmit a rotational force. Most gears are used in the transmission of mechanical power. Several gear-cutting techniques offer the ability to create different gear types. The different gear types have different characteristics including: increased power transmission, reduction in friction, reduction in noise, or low cost.
The use of any form of gear cutting is to create a gear. The different types of gear cutting have different uses. For example, gear grinding is used for high-precision gear cutting, while gear shaping is used for medium production runs for high consistency. Meanwhile, gear finishing is used to increase the precision of other gear cutting methods, gear broaching is used for the quickest production, gear hobbing is used for the best speed and accuracy, and gear milling is used for low-volume custom gears.
Some gear-cutting methods are forms of milling, so they do share some common ground. However, there are also differences which are listed below:
- Milling usually produces one cut at a time, whereas gear cutting can make multiple cuts at once.
- Milling tolling is often more expensive than gear cutting.
- Milling has a much wider array of uses as it can cut holes, slots, undercuts, fillets, chamfers, and surface cuts. Whereas gear cutting specifically relates to the formation of gear teeth.
- Milling often requires deburring, whereas gear cutting often does not.
Milling is by far the most common machining application due to its versatility and necessity in the creation of so many different parts for different applications, whereas gear cutting is limited only to the production of gears.
There are a few ways in which gears can be cut. The most common are discussed below:
Gear grinding uses an abrasive wheel to remove material from a gear blank. The abrasive wheel will rotate at high speed and make contact with the gear blank, imparting its profile into the gear. Gear grinding does come at a high tool and energy cost, and is slower than other methods. However, this process is very precise. Figure 1 is a photo of gear grinding, the pink wheel is the grinding disc which images with the gear blank on the right of the picture:
Image Credit: Shutterstock.com/Dovzhykov Andriy
Gear shaping is one of the oldest methods of cutting gear teeth. In the process, the profile of the cutter is the same geometry as the desired tooth. This cutting tool uses a linear motion to cut away material from the gear blank. The linear motion consists of a cutting and return stroke. The gear shaping tool has the same pitch as the pitch required for the gear tooth. Although the pitch of the tool has to meet the pitch of the teeth cut, the cutter can be used on gears that require a varying number of teeth. The advantage of gear shaping is it has a shorter production time, produces consistent results, and can create most gear types apart from the worm gear. The disadvantage of gear shaping is the cutting arm has low rigidity, so the accuracy is low. Gear shaping is a good solution for medium-sized production runs.
Gear finishing is a process used to increase surface finish and accuracy. In gear finishing, an abrasive wheel which has the same profile as the gear teeth is used to smooth the meshing surfaces of the gear. Gear finishing is very similar to gear grinding but the abrasive wheel is much finer, removing less material.
Gear broaching is a machining process that uses a special type of cutting tool called a broach. A broach is a multi-toothed cutting tool specifically used for cutting gear teeth as seen in Figure 2:
Image Credit: Shutterstock.com/Dmitry Kalinovsky
The broaching tool design leads to one of the fastest gear-cutting processes. Each broach is designed to cut a specific style/size of gear tooth, and so the price of tooling for his method is high. For this reason, gear broaching is only used for high-production parts. Gear broaching can be performed on any lathe, milling machine, machining center, or turning machine.
Gear hobbing is performed using a helical cutting tool. Hobbing is a fast process that is best suited for medium to high production volumes. Gear hobbing is performed using a CNC hobbing machine, in which the gear blank and hobbing tool are rotated on a perpendicular axis and brought together until the teeth are all cut. This is the most accurate way to manufacture gears. Gear hobbing cannot be used to produce internal gears or spline gears.
Gear milling uses a form-cutting tool on a milling machine to cut gear teeth from a gear blank. The form cutter spins to cut each gear tooth individually. Between each cut, the form cutter moves away to allow the gear blank to rotate for the next cut. Gear milling is a slow process that limits its production to only low-volume manufacturing. Gear milling is also not as precise as other machining methods. Gear milling is used because it can machine one-off custom cuts without special tooling. Figure 3 is a picture of gear milling, the black form tool can be seen in the upper middle:
Industrial gear milling.
Image Credit: Shutterstock.com/Dmitry Kalinovsky
There are several common variations of gears produced as a result of gear cutting as listed and described below:
A worm gear has a singular tooth that wraps around a cylindrical body. Worm gears intermesh with a worm wheel, which looks similar to a spur gear. This gearing type suffers from high friction due to the rotation of the worm gear against the worm wheel. In a worm gear/wheel pair hard material is used for the worm gear and soft material is used for the worm wheel. Worm gears are compact and able to provide good speed reduction. Therefore worm gears are used in small engines, conveyors, and ship rudders. Figure 4 is a picture of a worm gear and worm wheel from top to bottom respectively:
Image Credit: Shutterstock.com/AndreyProekt
Bevel gears are used to transmit power between two shafts, whose axes intersect. Bevel gears have a conical face with gear teeth (Figure 6). There are different types of bevel gear including: straight, spiral, zerol, hypoid, and mitre. Bevel gears have a large intermeshing surface area allowing for high power transmission, but are expensive to manufacture. Bevel gears are used in power plants, transmissions, vehicle differential gears, helicopter power transmissions, and accessory gearboxes. Figure 5 is a picture of two engaged bevel gears:
Image Credit: Shutterstock.com/AndreyProekt
Spur gears are cylindrical bodies with pitched surfaces around their circumference. They have parallel shafts and teeth lines which are parallel to the axis of rotation. Spur gears are the most common gear type and are easy to produce. They have no axial load and are used in transmissions, speed reductions, pumps, motors, and machine tools. Figure 6 is a picture of three spur gears:
Image Credit: Shutterstock.com/GLYPHstock
Helical gears are similar to spur gears, as they are cylindrical bodies with pitched teeth around their circumference. The difference is the teeth on helical gears are not parallel to the axis of rotation but wrap around the circumference. This difference in tooth design allows helical gears to transmit higher loads with less noise. This makes them more suitable for high-speed applications. Due to the tooth design, there is an axial load which will necessitate the use of thrust bearings. Helical gears are used in transmissions, conveyor belts, elevators, steel rolling, and earth-moving equipment. Figure 7 is a photo of a helical gear:
Image Credit: Shutterstock.com/AndreyProekt
Herringbone gears exploit the advantages of helical gears but reduce the thrust load. This is achieved by essentially pairing two helical gears that have opposite tooth-winding directions, see Figure 8. Below is a picture of a herringbone gear, which resembles two mirrored helical gears:
Image Credit: https://www.globalspec.com/learnmore/motion_controls/power_transmission/gears/herringbone_gears
Herringbone gears are manufactured as one piece. However, due to the expense of manufacturing, it is common for two helical gears to be joined using a machined center. They are used in lathes and milling machines, gas turbines, gearboxes, and marine propulsion systems.
To select a type of gear cutting the gear type, production volume, budget, and design tolerance must all be known. Then these criteria must be compared against the different types of gear cutting to choose the most appropriate type. For low-production, custom parts chose gear milling. For high precision choose gear grinding or gear hobbing. For high production, high-budget applications chose gear broaching, and for high production, lower budget chose gear stamping.
Choosing the right gear-cutting process is necessary as each process has its unique advantages and disadvantages. For example, gear milling has a low tool cost as it uses more generic tooling, but is only cost-effective for low production volumes. Gear broaching, on the other hand, requires special tooling but is much faster at cutting gears, making it only cost-effective for high-production runs. By choosing the right type of gear cutting, the right balance between cost, quality, and speed can be achieved.
To gear cut using gear hobbing, first, choose the right hob for the gear to be cut. Each spindle cuts a different gear tooth size and pitch. Fit the chosen hob to the tool spindle of the machine and the chosen gear blank to the work spindle. Then set the angle that the spindles must intersect to cut the teeth either parallel or at an angle to the gear blank’s axis. Next, set the speed ratio between the spinning speed of the tool spindle and the work spindle. Once the parameters are set the machine drive can be engaged and the spindle brought into contact with the gear blank. The process should be monitored to ensure the cutting speed is correct and the work receives enough coolant. Once cut, the gear should be removed for any post-processing.
Gear cutting should be used when manufacturing any of the following parts: worm gear, bevel gear, spur gear, helical gear, and herringbone gear. This is because gear-cutting methods provide the fastest and most accurate methods for the production of gears.
There are several reasons why gear cutting is used in the production of gears. The advantages of gear cutting are listed below:
- Ability to cut different gear types.
There are some disadvantages to gear cutting which are listed below:
- Limited to the creation of certain features.
- Sometimes requires specialized tooling.
- Sometimes requires specialized machinery.
- Limited size.
- Produces significant waste.
Below are a few tips that will help ensure successful gear cutting:
- Choose the right type of gear cutting for your application.
- Take time to ensure the right cutting speed and feed speeds are calculated.
- Depending on the gear-cutting type, ensure the work receives plenty of lubrication during cutting.
- Cut the smaller gears first to save material and time in the event of an error.
No, with the correct tools and some experience gear cutting is not hard. As with any machining process, there are hard cuts and easy cuts, and experience always makes the process easier. Generally, however, gear cutting is not considered to be one of the harder machining processes.
Yes, gear cutting is better than grinding in the production of gears as it is generally faster and cheaper. Grinding excels at high precision and creates fine surface finishes. Although sometimes this is required in the production of gears, it is not the most important factor, and so gear cutting is better suited in the production of gears.
This article presented gear cutting, explained it, and discussed its various processes and types. To learn more about gear cutting, contact a Xometry representative.
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