Gear hobbing has been an integral part of gear making. Whereas milling would be used to produce the basic gear shape, gear hobbing performs the much more intricate task of shaping the gear. Various facets of gear teeth emerge once hobbing is complete, including its thickness, profile and addendum.
Gear hobbing is a specialised process of gear cutting, spline cutting and sprocket cutting. The central equipment in the gear hobbing process is the milling machine. This gear hobbing machine does the task of cutting the spline or teeth using a series of cuts using a hob. Gear hobbing is often used for spur gear and helical gear making, because of its ease of use and cost effectiveness.
Construction Of Gear Hobbing Machine
Hob– In this process, the gear blank is rolled with a rotating cutter called a hob. Gear hobbing is done by using a multipoint cutting tool called gear hob. It looks like a worm gear having a number of straight flutes all around its periphery parallel to its axis. These flutes are so shaped by giving proper angles to them so that these work as cutting edges.
In gear hobbing operation, the hob is rotated at a suitable rpm and simultaneously fed to the gear blank.
The gear blank is also kept as revolving. Rpm of both, gear blank and gear hob are so synchronized that for each revolution of gear bob the gear blank rotates by a distance equal to one pitch distance of the gear to be cut.
The motion of both gear blank and hob is maintained continuously and steady. A gear hob is shown in Figure and the process of gear hobbing is illustrated in Figure
The hob teeth behave like screw threads, having a definite helix angle. During operation the hob is tilted to helix angle so that its cutting edges remain square with the gear blank.
Gear hobbing is used for making a wide variety of gears like spur gear, helical, hearing-bone, splines and gear sprockets, etc.
Types Of Hobbing Process :
The process of gear hobbing is classified into different types according to the directions of feeding the hob for gear cutting. The classification is described as given below.
• Hobbing with Axial Feed
In this process the gear hob is fed against the gear blank along the face of the blank and parallel to its axis. This is used to make spur and helical gears.
• Hobbing with Radial Feed
In this method the hob and gear blanks are set with their axis normal to each other. The rotating hob is fed against the gear blank in radial direction or perpendicular to the axis of gear blank. This method is used to make the worm wheels.
• Hobbing with Tangential Feed
This is also used for cutting teeth on the worm wheel. In this case, the hob is held with its axis horizontal but at the right angle to the axis of the blank. The hob is set at full depth of the tooth and then fed forward axially. The hob is fed tangentially to the face of the gear blank.
The process of gear hobbing is as follows:
• The gear hobbing machine is checked for any mechanical issues. This includes the synchronisation of the rotating gears for the workpiece and the hob.
• The gear hobbing machine has two skew spindles, one of which has the gear blank while the other one has the hob.
• The spindles are set at an angle from each other, depending on the type of teeth to be set on the gear blank.
• The two spindle shafts are then rotated proportionally with the hob cuts on the gear blank. Having them properly rotating is the key to making the right gear teeth on the gear blank.
• The hob would then be applied to the gear blank until the correct tooth depth has been reached.
• The hob is then run through the gear blank’s axis of rotation as a final step.
• The process is repeated for each gear blank, which is often stacked one atop another. The rotation ensures identical gear spindle depth and a consistent number of teeth on each gear blank.
• The completed gears are removed from the milling machine.
• The central tool to gear hobbing is the hob, a worm-shaped cutter. The hob makes successive cuts on the gear blank to create the gear teeth. In order for the cutting to be precise, the gear blank and the hob must be synchronised in the rotation.
The 5 Advantages of Gear Hobbing
Gear manufacturing gives us lots of options. There are a handful of different processes we can use to make dozens of different gear types from dozens of different materials. Accordingly, when we choose a gear manufacturing method, it’s because it’s the best one for the job.
Gear hobbing is just one way we can manufacture gears. It relies on a special form milling machine with a tool known as a hob, which generates the teeth in both gears and splines. So what are the advantages of this process?
• Speed. Some gear manufacturing processes take a long time, but gear hobbing is relatively fast. The machine is simple, so it doesn’t require as much operational attention, and for some gears, we can stack multiple units to hob them all at the same time.
• Precision. That said, not just anyone can operate a hobbing machine. With the right expertise, hobbing can be highly precise, resulting in high quality gears.
• Flexibility. There’s more than one type of hob—and more than one type of hobbing machine. There are countless variations that cater to specialized applications, so you always have options available.
• Applications. While hobbing is often used for spur gears, the process can be used for a variety of other gears, such as cycloid gears, helical gears, worm gears, ratchets, splines, and sprockets (as long as you have the right tools for the job).
• Related processes. One of hobbing’s only weaknesses is that it does not work for internal gears (with inward-facing teeth). However, there’s a related process called shaping that can be used instead—with all the same advantages of hobbing.
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