gear shaping, Skiving and Hob sharpening
| tip relief |
Tip relief can be divided to addendum and dedendum relieves. Addendum relief is mostly used for axles. As standards and gear levels are different from one country to another, tip relief varies. The strengths are interference mitigation, noise reduction and increased life of gears; however, bad engagement might happen if there is too much tip.
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| Crowning & relieving |
Crowning and relieving are two ways of correcting the direction of lead. For the former method, lead is trimmed to a crown shape so that the tooth surface can concentrate on the central part of the tooth width/range. If crowning is conducted overdone, bad impact will be imposed on the strength of gears. For the approach of relieving, both ends of lead are semi-topped slightly so that the crowning effect might be achieved.
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| topping and semi-topping |
When gears are machined with cutting tools, scraping of the outer diameter pf gears is called topping, which can reduce external diameter variation. Appearance may be improved by removing deburs generated by the tooth tip. When the topping of the addendum is cut but not the external diameter, it is called semi-topping. JIS and DIN have different standards.
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Gear-shaping
Gear shaping is one of the most common ways for gear machining. Its application is very extensive such as the processing of general spur gears, helical gears, internal gears and herringbone gears, etc. After adding certain special measuring tools, the procedure can be used for machining surface gears or double-lead worms. The precision of shaped gears is pretty high and could reach Class 3 above the JIS standard.
Gear shaping is based on the theory of conjugation of involute gears. It is presumed that the cutting tool is a completely rigid object (not deformed against any external force) and the gear workpiece is a completely elastic object (plasticity resulted from external force). Both objects have to engage to each other and rotate with a fixed rotation rate so that the gear workpiece will be deformed in response to the tooth shape of the cutting tool.
For practical application, in addition to joint rotation with the workpiece, the cutting tool also moves back and forth along the axis of the workpiece so as to remove the material in the empty part and form gears. Therefore, the required shaping angle of the tooth shape of the cutting tool is slightly different from the actual tooth shape.
Skiving of gears after heat treatment
Heat treatment is often conducted after gears have been shaped and machined in order to increase strength. Machining after general heat treatment can be divided into grinding and skiving. A brief introduction of skiving is made as follows.
| Hob sharpening tool for skiving | General material is carbonized wolfram (WC); i.e. the so-called carbide. The body itself is Hss with carbonized wolfram blades. |
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| Generally speaking, the rack angle at the cutting edge is not zero and negative dip/inclined angles are used mostly. | |
| Greatest hardness up to HRC55° | |
| Hob sharpening tool for pre-machining | Material is Hss mostly. |
| Generally speaking, the cutting edge is a zero angle. | |
| It is used for machining of soft material prior to heat treatment with a depth of tooth 2.35*M. | |
| Notice for Hob Sharpening | The more blades of the hop sharpening tool, the more form blades are required for fine processing. Besides, error of the tooth surface becomes smaller and re-grinding times become less relatively. The optimum value can be selected. |
| The greater the negative dip/inclined angle of the hop sharpening tool, wear resistance becomes greater as well. | |
| The bore diameter of the hop sharpening tool should be increased as much as possible to enhance the strength while machining. | |
| The lead angle of the hop sharpening tool should rotate to the same direction as the machined workpiece does so as to reduce vibration of the bench. |
