What is gear nomenclature

Planing - Hogback Hill

A hob - the cutter used for planing.
Method of cutting teeth into gears

Planing is a machining process for gear cutting, spline cutting and sprocket cutting on one Hobbing machine , which is a special type of milling machine. The teeth or splines of the gear are gradually made through a series of cuts with a cutting tool called the Hob is cut into the material (a flat, cylindrical piece of metal). Compared to other gear forming processes, it is relatively inexpensive, but still quite accurate. Hence, it is used for a wide range of parts and quantities.

It is the most common gear cutting process used to make spur and helical gears, and hobbing cuts more gears than any other process because it is relatively quick and inexpensive.

A type of peeling similar to hobbing external gears can be applied to cutting gears internal gears that are ground (rather than shaped or nosed) with a rotary cutter.

process

Spur gears on a horizontal CNC gear milling machine
Spur gears on a horizontal CNC hobbing machine

Hobbing uses a hobbing machine with two helical spindles, one with an empty workpiece and the other mounted with the hob. The angle between the spindle (axis) of the hob and the spindle of the workpiece varies depending on the type of product being made. For example, if a spur gear is made, the hob is angled equal to the pitch angle of the hob. When making a spur gear, the angle must be increased by the same amount as the helix angle of the helical gear. The hob features for gears are straight, spiral, straight bevel, face, crowned, worm, cylinder, and chamfer. The two shafts are rotated in a proportional ratio that determines the number of teeth on the blank. Example: For a hob with a thread and a gear ratio of 40: 1, the hob rotates 40 times for each revolution of the blank, creating 40 teeth in the blank. If the hob has multiple threads, the speed ratio must be multiplied by the number of threads on the hob. The hob is then inserted into the workpiece until the correct tooth depth is achieved. Finally, the hob is guided through the workpiece parallel to the axis of rotation of the blank.

Often several blanks are stacked and then cut in one operation.

In the case of very large gears, the blank

equipment

A horizontal CNC hobbing machine
A horizontal hobbing machine
A horizontal hobbing machine

Hobbing machines, also called planers, are fully automatic machines that come in many sizes because they must be able to make everything from tiny instrument gears up to 3.0 m (10 feet) in diameter. Each gear milling machine typically consists of a chuck and a tailstock for holding the workpiece or a spindle, a spindle on which the hob is mounted, and a drive motor.

In the case of a tooth profile, which is a theoretical involute, the base frame is straight, with sides inclined at the pressure angle of the tooth shape with a flat top and bottom. The necessary to enable the use of small numbered pinions can be obtained either by suitable modification of this rack in a cycloid shape at the tips or by hobbing at a pitch diameter other than the theoretical. Since the transmission ratio between hob and blank is fixed, the resulting gear has the correct pitch on the pitch circle, but the tooth thickness does not correspond to the width of the room.

Hobbing machines are characterized by the largest module or the largest pitch diameter that they can produce. For example, a machine with a capacity of 250 mm (10 inches) can produce gears with a pitch diameter of 10 inches and typically a maximum face width of 10 inches. Most hobbing machines are vertical planers, meaning the blank is mounted vertically. Horizontal hobbing machines are typically used to cut longer workpieces. i.e. cutting wedges at the end of a shaft.

Hob

A gear in a hobbing machine with a finished gear.

The hob is a cutting tool that is used to cut the teeth into the workpiece. It has a cylindrical shape with helical cutting teeth. These teeth have grooves that run the length of the hob and make it easy to cut and remove chips. There are also special hobs that are designed for special gears such as splines and sprockets.

The cross-sectional shape of the hob teeth closely matches the shape of the teeth on a rack and pinion gear would be used with the finished product. There are minor changes to the shape for generation purposes, e.g. B. extending the tooth length of the hob to create clearance in the roots of the gear. The back of each hob tooth is relieved to reduce friction.

Most hobs are single threaded hobs, but two and three threaded hobs increase production rates. The downside is that they're not as accurate as single-thread hobs. Depending on the type of toothing to be cut, there are tailor-made hobs and all-purpose hobs. Custom-made hobs differ from other hobs in that they are suitable for producing gears with modified tooth profiles. The tooth profile has been modified to increase strength and reduce size and gear noise.

This list shows the types of hobs:

  • Roller chains sprocket hobs
  • Worm gear hobs
  • Wedge hobs
  • Bevel hobs
  • Spur gear and helical gear hobs
  • Straight wedge hobs
  • Involute wedge hobs
  • Toothed hobs
  • Half-coupling hobs

use

The following types of finished gears are produced by hobbing:

Hobbing is used to make most of the fillet worm gears, but certain tooth profiles cannot be milled. If part of the hob profile is perpendicular to the axis, it will not have the cutting distance created by the usual retraction process and it will not cut well.

Cycloid forms

For cycloid gears (as used in BS978-2 specification for fine pitch gears) and cycloid gears, the module, ratio and number of teeth in the pinion each require a different hob, so the technique is only suitable for mass production.

To circumvent this problem, a special circular arc gear standard for war emergencies was made that includes a series of near-cycloidal shapes that can be cut with a single hob for each module for eight teeth and more to conserve resources in the manufacture of tailors save up. A variant of this is still contained in BS978-2a (gears for instruments and clockwork mechanisms. Cycloid gears. Double circular arc gears).

Concentricity tolerances of the hob limit the lower modules, which can be practically cut by planing, to around 0.5 modules.

history

Many manufacturing companies that maintain museums about the manufacture of products in the past have examples of manual transmissions that helped make gears before the gears of the 19th century and earlier. Along with these fully manual gear hobs, examples of some of the first semi-automatic gear hobs are shown, and finally examples of newer technologies that demonstrate the fully automatic process that modern gear hobs use to make gears today. Some gear hobs manufacturers also have interesting literature on the history of gear hobs, including details of how modern gear hobs can produce thousands of gears in an hour.

See also

credentials

bibliography

  • American Society for Metals; Cubberly, William H .; Bardes, Bruce P. (1978), Metals Handbook: Machining, 16(9th, illustrated edition), ASM International, ISBN 978-0-87170-007-0 .
  • Degarmo, E. Paul; Black, J T .; Kohser, Ronald A. (2003), Materials and Processes in Manufacturing (9th ed.), Wiley, ISBN 0-471-65653-4 .
  • Drozda, Tom; Wick, Charles; Benedict, John T .; Veilleux, Raymond F .; Society of Manufacturing Engineers; Bakerjian, Ramon (1983), Tool and Manufacturing Engineer's Handbook: Machining, 1(4th, illustrated edition), Society of Manufacturing Engineers, ISBN 978-0-87263-085-7 .
  • Endoy, Robert (1990), Planing, Shaping, and Shaving Gears (Illustrated Edition), Society of Manufacturing Engineers, ISBN 978-0-87263-383-4 .
  • Jones, Franklin D. (1964) Machine Shop Training Course (5th Illustrated Edition), Industrial Press Inc., ISBN 978-0-8311-1040-6 .
  • Todd, Robert H .; Allen, Dell K .; Alting, Leo (1994), Reference Manual for Manufacturing Processes, Industrial Press Inc., ISBN 0-8311-3049-0 .

further reading

  • Burstall, Aubrey F. (1965), A History of Mechanical Engineering, MIT Press, ISBN 0-262-52001-X, LCCN 65-10278 . On p. 303, "The hobbing process conceived by Christian Schiele in 1856 became a practical process for production as soon as involute gears replaced the cycloid type in the 1880s, since the involute hob, like the involute frame, has straight sides (z The worm is a shape of a Continuous frame. To turn a worm into a hob, all you have to do is drive a few teeth into the worm so that the blank is cut off when it is turned. "
  • GB 185702896, Schiele, Christian, "Machines for Cutting Nuts, Bolts and Gears", published December 6, 1856, published June 5, 1857; Patent not found on eSpaceNet prior to 1890 (see British Library Notes); See reprint from Google Books with sheets 1 and 2 missing.
  • Woodbury, Robert S. (1958), History of the Gear Cutting Machine: A Historical Study of Geometry and Machines, MIT Press, OCLC 1689960 . On p. 105, "But it had been recognized that the worm was a shape of a continuous rack and all that was necessary to cut gears with it was to put cutting edges on it - to make a hob (Fig. 45). Teeth were from probably first cut this method by Ramsden in 1768. "
  • Woodbury, Robert S. (1972), "History of the Gear Cutting Machine." Studies in the History of Machine Tools, Cambridge, Massachusetts: MIT Press, ISBN 978-0-262-73033-4, LCCN 72006354, OCLC 609185

External links

  • Gimpert, Dennis (January 1994), "The Gear Hobbing Process" (PDF), Gear Technology, 11 (1): 38–44 . Contains circuit diagrams of the hobbing machines in Figures 8-10.