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Mechanical engineering |Машиностроение

Lesson 10

Read the text: Hobbing and honing

A hobbing machine is a special form of milling machine that cuts gears. It is the major industrial process for cutting, as opposed to grinding, spur gears of involute form. The machine forms the gear via a generating process by rotating the gear blank and the cutter (called a hob) at the same time with a fixed gearing ratio between hob and blank. The hob has a profile given in cross-section by the fundamental rack for the gear tooth profile and is in the form of a helix so that the sides of the teeth on the hob generate the curve on the gear. The helix has a number of cuts parallel to the axis to form the cutting teeth and the profile is suitably relieved to provide cutting clearance.

For a tooth profile which is a theoretical 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 other than the theoretical pitch circle diameter. 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 is invariably used to produce throated worm wheels, but it is not possible to cut all useful tooth profiles in this way; if any portion of the hob profile is perpendicular to the axis then it will have no cutting clearance generated by the usual backing off process, and it will not cut well. The NHS Swiss tooth standards give rise to such problems. Such small gears normally must be milled instead.

For cycloid gearing each module, ratio and number of teeth in the pinion requires a different hobbing cutter so the technique is only suitable for large volume production. Hobbing machines are characterized by their capacity of module or pitch of gear hobs that can be loaded on the machine.

Honing

A hone is a machine tool used in the manufacture of precision bores to improve the geometry, surface finish and dimensional control of the finished part. This process is called honing.

Typical applications are the finishing of cylinders for internal combustion engines, air bearing spindles and in gear manufacturing. Types of hone are many and various but all consist of one or more abrasive stones that are held under pressure against the surface they are working on. The hone is usually turned in the bore while being moved in and out. Special cutting fluids are used to give a smooth cutting action and to remove the material that has been abraded. Machines can be portable, simple manual machines, or fully automatic with gauging depending on the application.

Modern advances in abrasives have made it possible to remove much larger amount of material than was previously possible. This has displaced grinding in many applications where "through machining" is possible. External hones perform the same function on shafts.

1. Match the left part with the right:

2. Complete the sentences with the suggested words: hobbing; pinions; space width ; straight-sided ; tooth profile; diameter; flat top; gear ratio; the correct pitch

For a_____which is a theoretical involute, the fundamental rack is_____, with sides inclined at the pressure angle of the tooth form, with_____and bottom. The necessary addendum correction to allow the use of small-numbered_____can either be obtained by suitable modification of this rack to a cycloid form at the tips, or by_____at other than the theoretical pitch circle______. Since the _____between hob and blank is fixed, the resulting gear will have ______on the pitch circle, but the tooth thickness will not be equal to the_______.