Hobbing

Last updated
A hob -- the cutter used for hobbing. MillingCutterHobbingInvolute.jpg
A hob — the cutter used for hobbing.

Hobbing is a machining process for gear cutting, cutting splines, and cutting sprockets using a hobbing machine, a specialized milling machine. The teeth or splines of the gear are progressively cut into the material (such as a flat, cylindrical piece of metal or thermoset plastic) by a series of cuts made by a cutting tool called a hob.

Contents

Hobbing is relatively fast and inexpensive compared to most other gear-forming processes and is used for a broad range of parts and quantities. [1] Hobbing is especially common for machining spur and helical gears. [2]

A type of skiving that is analogous to the hobbing of external gears can be applied to the cutting of internal gears, which are skived with a rotary cutter (rather than shaped or broached). [3]

Process

Spur gears on horizontal CNC gear hobbing machine Affolter-cnc-gear-hobbing-machine-gear-cutting-spur-gear.jpg
Spur gears on horizontal CNC gear hobbing machine
Spur gears on horizontal CNC gear hobbing machine Affolter-cnc-gear-hobbing-machine-gear-cutting-spur-gear-brass-metal.jpg
Spur gears on horizontal CNC gear hobbing machine

Hobbing can create gears that are straight, helical, straight bevel, faced, crowned, wormed, cylkro and chamfered. [4] A hobbing machine uses two skew spindles. One is mounted with a blank workpiece and the other holds the cutter (or “hob”). The angle between the hob's spindle (axis) and the workpiece's spindle varies depending on the type of part being manufactured. For example, if a spur gear is being produced, the spindle is held at the lead angle of the hob, whereas if a helical gear is being produced, the held at the lead angle of the hob plus the helix angle of the helical gear. The speeds of the two spindles are held at a constant proportion determined by the number of teeth being cut into the blank; for example, for a single-threaded hob with a gear ratio of 40:1 the hob rotates 40 times to each turn of the blank, producing 40 teeth in the blank. If the hob has multiple threads, the speed ratio is multiplied by the number of threads on the hob. [5] The hob is then fed up into the workpiece until the correct tooth depth is obtained. To finish the operation, the hob is fed through the workpiece parallel to the blank's axis of rotation. [4]

Often during mass production, multiple blanks are stacked using a suitable fixture and cut in one operation. [5]

For very large gears, the blank may be preliminarily gashed to a rough shape to make hobbing more efficient.

Equipment

A CNC gear hobbing machine Affolter AF160 CNC Gear Hobbing Machine Gear Cutting.png
A CNC gear hobbing machine
A horizontal hobbing machine Walzfrasmaschine CNC 2001.jpg
A horizontal hobbing machine
A horizontal hobbing machine Hobbing machine.jpg
A horizontal hobbing machine

Hobbing machines, also known as hobbers, come in many sizes to produce different sizes of gears. Tiny instrument gears are produced on small table-top machines, while large-diameter marine gears are produced on large industrial machines. A hobbing machine typically consists of a chuck and tailstock to hold the workpiece, a spindle to mount the hob, and a drive motor. [6]

For a tooth profile which is theoretically involute, the fundamental rack is straight-sided, with sides inclined at the pressure angle of the tooth form, with flat top and bottom. The necessary addendum correction to allow the use of small-numbered pinions can either be obtained by suitable modification of this rack to a cycloidal form at the tips, or by hobbing at a diameter other than the theoretical pitch. Since the gear ratio between hob and blank is fixed, the resulting gear will have the correct pitch on the pitch circle but the tooth thickness will not be equal to the space width.

Hobbing machines are characterized by the largest module or pitch diameter it can generate. For example, a 10 in (250 mm) capacity machine can generate gears with a 10 in pitch diameter and usually a maximum of a 10 in face width. Most hobbing machines are vertical hobbers, meaning the blank is mounted vertically. Horizontal hobbing machines are usually used for cutting longer workpieces; i.e. cutting splines on the end of a shaft. [7]

The hob

A gear hob in a hobbing machine with a finished gear. Walzfraser und gefraste Verzahnung.JPG
A gear hob in a hobbing machine with a finished gear.
Gear-hob.jpg

The hob is a cutting tool used to cut the teeth into the workpiece. It is cylindrical in shape with helical cutting teeth. These teeth have grooves that run the length of the hob, which aid in cutting and chip removal. There are also special hobs designed for special gears such as the spline and sprocket gears. [6]

The cross-sectional shape of the hob teeth are almost the same shape as teeth of a rack gear that would be used with the finished product. There are slight changes to the shape for generating purposes, such as extending the hob's tooth length to create a clearance in the gear's roots. [8] Each hob tooth is relieved on its back side to reduce friction. [9]

Most hobs are single-thread hobs, but double-, and triple-thread hobs are used for high production volume shops. Multiple-thread hobs are more efficient but less accurate than single-thread hobs. [10] Depending on type of gear teeth to be cut, there are custom made hobs and general purpose hobs. Custom made hobs are different from other hobs as they are suited to make gears with modified tooth profiles. Modified tooth profiles are usually used to add strength and reduce size and gear noise.

Common types of hobs include:

Hob dimensions.png

Uses

Hobbing is used to make the following types of finished gears:

Hobbing is used to produce most throated worm wheels, but certain tooth profiles cannot be hobbed. If any portion of the hob profile is perpendicular to the axis, the hob will not have the cutting clearance generated by the usual backing off process and will not cut well.

Cycloidal forms

For cycloidal gears (as used in BS978-2 Specification for fine pitch gears) and cycloidal-type gears, each module, ratio, and number of teeth in the pinion requires a different hobbing cutter, so the hobbing is ineffective for small-volume production.

To circumvent this problem, a special war-time emergency circular arc gear standard was produced giving a series of close-to-cycloidal forms which could be cut with a single hob for each module for eight teeth and upwards to economize on cutter manufacturing resources. A variant on this is still included in BS978-2a (Gears for instruments and clockwork mechanisms. Cycloidal type gears. Double circular arc type gears).

Tolerances of concentricity of the hob limit the lower modules which can be cut practically by hobbing to about 0.5 module.

History

Christian Schiele of Lancaster England patented the hobbing machine in 1856. [11] [ self-published source ] It was a simple design, but the rudimentary components are all present in the customary patent drawings. The hob cutting tool and the gear train to provide the appropriate spindle speed ratio are clearly visible. Knowledge of hobbing within the watchmaking trade likely precedes his patent. The next major step forward was in 1897, when Herman Pfauter invented a machine that could cut both traditional “spur” gears and helical gears, driving production further forward. [12]

See also

Related Research Articles

<span class="mw-page-title-main">Gear</span> Rotating circular machine part with teeth that mesh with another toothed part

A gear is a rotating circular machine part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth, which mesh with another (compatible) toothed part to transmit rotational power. While doing so, they can change the torque and rotational speed being transmitted and also change the rotational axis of the power being transmitted. The teeth on the two meshing gears all have the same shape.

<span class="mw-page-title-main">Lathe</span> Machine tool which rotates the work piece on its axis

A lathe is a machine tool that rotates a workpiece about an axis of rotation to perform various operations such as cutting, sanding, knurling, drilling, deformation, facing, threading and turning, with tools that are applied to the workpiece to create an object with symmetry about that axis.

<span class="mw-page-title-main">Rack and pinion</span> Type of linear actuator

A rack and pinion is a type of linear actuator that comprises a circular gear engaging a linear gear. Together, they convert between rotational motion and linear motion. Rotating the pinion causes the rack to be driven in a line. Conversely, moving the rack linearly will cause the pinion to rotate. A rack-and-pinion drive can use both straight and helical gears. Though some suggest helical gears are quieter in operation, no hard evidence supports this theory. Helical racks, while being more affordable, have proven to increase side torque on the datums, increasing operating temperature leading to premature wear. Straight racks require a lower driving force and offer increased torque and speed per fraction of gear ratio which allows lower operating temperature and lessens viscal friction and energy use. The maximum force that can be transmitted in a rack-and-pinion mechanism is determined by the torque on the pinion and its size, or, conversely, by the force on the rack and the size of the pinion.

<span class="mw-page-title-main">Shaper</span> Machine tool which linearly cuts or grinds the workpiece

In machining, a shaper is a type of machine tool that uses linear relative motion between the workpiece and a single-point cutting tool to machine a linear toolpath. Its cut is analogous to that of a lathe, except that it is (archetypally) linear instead of helical.

Broaching is a machining process that uses a toothed tool, called a broach, to remove material. There are two main types of broaching: linear and rotary. In linear broaching, which is the more common process, the broach is run linearly against a surface of the workpiece to produce the cut. Linear broaches are used in a broaching machine, which is also sometimes shortened to broach. In rotary broaching, the broach is rotated and pressed into the workpiece to cut an axisymmetric shape. A rotary broach is used in a lathe or screw machine. In both processes the cut is performed in one pass of the broach, which makes it very efficient.

<span class="mw-page-title-main">Edwin R. Fellows</span> American inventor & engineer (1865-1945)

Edwin R. Fellows was an American inventor and entrepreneur from Torrington, Connecticut who designed and built a new type of gear shaper in 1896 and, with the mentoring of James Hartness, left the Jones & Lamson Machine Company to co-found the Fellows Gear Shaper Company in Springfield, Vermont, which became one of the leading firms in the gear-cutting segment of the machine tool industry. Fellows' machines made a vital contribution to the mass production of effective and reliable gear transmissions for the nascent automotive industry. By the conclusion of World War II, Fellows Gear Shaper Company machines were in defense contractor plants, manufacturing geared components for aircraft engines, tanks, instruments, cameras, fuses and other war-time materiel.

<span class="mw-page-title-main">Turning</span> Machining process

Turning is a machining process in which a cutting tool, typically a non-rotary tool bit, describes a helix toolpath by moving more or less linearly while the workpiece rotates.

Milling cutters are cutting tools typically used in milling machines or machining centres to perform milling operations. They remove material by their movement within the machine or directly from the cutter's shape.

<span class="mw-page-title-main">Metal lathe</span> Machine tool used to remove material from a rotating workpiece

In machining, a metal lathe or metalworking lathe is a large class of lathes designed for precisely machining relatively hard materials. They were originally designed to machine metals; however, with the advent of plastics and other materials, and with their inherent versatility, they are used in a wide range of applications, and a broad range of materials. In machining jargon, where the larger context is already understood, they are usually simply called lathes, or else referred to by more-specific subtype names. These rigid machine tools remove material from a rotating workpiece via the movements of various cutting tools, such as tool bits and drill bits.

Gear cutting is any machining process for creating a gear. The most common gear-cutting processes include hobbing, broaching, milling, grinding, and skiving. Such cutting operations may occur either after or instead of forming processes such as forging, extruding, investment casting, or sand casting.

<span class="mw-page-title-main">Spur gear</span> Simplest type of gear

Spur gears or straight-cut gears are the simplest type of gear. They consist of a cylinder or disk with teeth projecting radially. Viewing the gear at 90 degrees from the shaft length the tooth faces are straight and aligned parallel to the axis of rotation. Looking down the length of the shaft, a tooth's cross section is usually not triangular. Instead of being straight the sides of the cross section have a curved form to achieve a constant drive ratio. Spur gears mesh together correctly only if fitted to parallel shafts. No axial thrust is created by the tooth loads. Spur gears are excellent at moderate speeds but tend to be noisy at high speeds.

Gear manufacturing refers to the making of gears. Gears can be manufactured by a variety of processes, including casting, forging, extrusion, powder metallurgy, and blanking. As a general rule, however, machining is applied to achieve the final dimensions, shape and surface finish in the gear. The initial operations that produce a semifinishing part ready for gear machining as referred to as blanking operations; the starting product in gear machining is called a gear blank.

<span class="mw-page-title-main">Profile angle</span>

The profile angle of a gear is the angle at a specified pitch point between a line tangent to a tooth surface and the line normal to the pitch surface. This definition is applicable to every type of gear for which a pitch surface can be defined. The profile angle gives the direction of the tangent to a tooth profile.

A spline is a ridge or tooth on a drive shaft that matches with a groove in a mating piece and transfers torque to it, maintaining the angular correspondence between them.

A herringbone gear, a specific type of double helical gear, is a side-to-side, rather than face-to-face, combination of two helical gears of opposite hands. From the top, each helical groove of this gear looks like the letter V, and many together form a herringbone pattern. Unlike helical gears, herringbone gears do not produce an additional axial load.

<span class="mw-page-title-main">Spotface</span>

A spotface or spot face is a machined feature in which a certain region of the workpiece is faced, providing a smooth, flat, accurately located surface. This is especially relevant on workpieces cast or forged, where the spotface's smooth, flat, accurately located surface stands in distinction to the surrounding surface whose roughness, flatness, and location are subject to wider tolerances and thus not assured with a machining level of precision. The most common application of spotfacing is facing the area around a bolt hole where the bolt's head will sit, which is often done by cutting a shallow counterbore, just deep enough "to clean up"—that is, only enough material is removed to get down past any irregularity and thus make the surface flat. Other common applications of spotfacing involve facing a pad onto a boss, creating planar surfaces in known locations that can orient a casting or forging into position in the assembly; allow part marking such as stamping or nameplate riveting; or offer machine-finish visual appeal in spots, without the need for finishing all over (FAO).

In manufacturing, threading is the process of creating a screw thread. More screw threads are produced each year than any other machine element. There are many methods of generating threads, including subtractive methods ; deformative or transformative methods ; additive methods ; or combinations thereof.

<span class="mw-page-title-main">Gear shaping</span>

Gear shaping is a machining process for creating teeth on a gear using a cutter. Gear shaping is a convenient and versatile method of gear cutting. It involves continuous, same-plane rotational cutting of gear.

Gashing is a machining process used to rough out coarse pitched gears and sprockets. It is commonly used on worm wheels before hobbing, but also used on internal and external spur gears, bevel gears, helical gears, and gear racks. The process is performed on gashers or universal milling machines, especially in the case of worm wheels. After gashing the gear or sprocket is finished via hobbing, shaping, or shaving.

References

  1. American Society for Metals, Cubberly & Bardes 1978 , p. 334.
  2. Drozda et al. 1983 , p. 13‐34.
  3. Weppelmann, E; Brogni, J (March 2014), "A breakthrough in power skiving", Gear Production: A Supplement to Modern Machine Shop: 7–12, retrieved 2014-03-11.
  4. 1 2 Degarmo, Black & Kohser 2003 , p. 769.
  5. 1 2 Jones 1964 , p. 289.
  6. 1 2 Todd, Allen & Alting 1994 , pp. 59–60.
  7. Endoy 1990 , p. 6.
  8. Jones 1964 , p. 288.
  9. Degarmo, Black & Kohser 2003 , p. 768.
  10. Degarmo, Black & Kohser 2003 , p. 770.
  11. "The Original Hobbing Machine". Evolvent Design. Retrieved 2021-01-17.
  12. Machine, Federal Gear and (2016-12-06). "A Short History of Gears and Where Gear Manufacturing Is Today". Federal Gear. Retrieved 2024-02-08.

Bibliography

Further reading