Grinding has been employed in manufacturing for more than 100 years although the earliest grinding practice goes back to neolithic times thanks to the Homo Sapiens activities. Size reduction is infact the oldest engineering process started right in prehistoric times.
As the years go by there is an evolution from stones, mortal and pestle, saddlestone mill, larger querns driven by animals, water wheels until the 20th century that is defined as the modern grinding process birth. Indeed, the grinding process achieves his scientific basic from 1914 thanks to the seminal publications by Alden and Guest. Since then grinding is defined as a machining process thant employs an abrasive grinding wheel rotating at high speed to remove material from a softer material.
Nowadays in modern industry the grinding process is carried out thanks to machine tools which combine computer-controlled feed-drives and slide-way motions, allowing complex shapes to be manufactured free from manual intervention.
Grinding evolution has also increased grinding wheel speed and removal rates: they boosted by two to ten times over the last century.
Also the abrasive types have evolved by ensuring advances in productivity. As a matter of fact, new ceramic abrasives based on sol gel technology have been introduced together with the development of superabrasive cubic boron nitride (CBN) and diamond abrasive based on natural and synthetic diamond.
Productivity has also been increased thanks to the insertion of new grinding fluids and methods of delivering grinding fluid. These fluids have also been an essential part in achieving higher removal rates and maintaining quality.
In recent years, grinding evolution has passed through the inclusion of high-velocity jets, shoe nozzles, factory-centralized delivery systems, neat mineral oils, synthetic oils, vegetable ester oils and new additives. Minimum quantity lubrication provides an alternative to flood and jet delivery aimed at environment-friendly manufacturing.
Thanks to all of this innovation, today grinding is a key technology for production of advanced products and surfaces in a wide range of industries.
An important element of the gear hob design is the tip which represents a relief of the tooth profile and a part of the Archimedean spiral.
The Archimedean spiral is a curve traced out by a point moving in such a way that its movement towards or away from the center is uniform with the increase of its vectorially angle from the starting line. This curve is used to ensure the continuity of the tooth profile during the resharpening.
For a resharpening with a zero-rake angle, the cutting profile lies on a plane that passes through the hob axis and the Archimedean spiral must ensure that the profiles that are on this perpendicular section are constant. This curve is used in the tooth profiles of the gear hob.
The characteristics of the Archimedean spiral are:
- In Archimedes spiral, any ray passing through the origin does intersect successive turns of the spiral with a constant distance which is equal to 2 π Therefore, it is also named „arithmetic spiral“.
- An Archimedes spiral, there are two arms – one arm for θ> 0 and another for θ < 0. These two arms are connected to the origin. One arm is shown in the graph. If we take the mirror image of this arm along Y axis, the other arm will be obtained.
- Usually the term Archimedean spiral is used for more general spirals. The normal one occurs when in its equation, n = 1. Other spirals that fall into this group are the Fermat’s spiral and the hyperbolic spiral.
- All the static spirals are logarithmic spirals in nature, except for Archimedes spiral.
A gear hob is a gear cutting tool composed of different parts. The main parameter characterizing a gear hob is the module, which is the result of the division between the pitch gear diameter and the number of the teeth.
Another element of the hob is the pressure angle which coincides with the nominal pressure angle of the gear. The normal pressure angle αn – measured on a section perpendicular to the direction of the thread – and the axial pressure angle αt – measured on an axial section of the hob – are easily distinguished.
Besides, helix angle β, also called thread angle, is measured on the pitch diameter of the hob depending on this value and the number of starts. The angle is direct proportional to the diameter, pitch, module and number of threads. So, when this increase also the other parameters increase consequently. It is important to highlight that the helix direction of the hob is chosen in agreement with the helix direction of the gear.
Gashes are another fundamental geometrical element of a gear hob. These are serrations that cut the threads either in an axial or in a perpendicular direction. The number of gashes should be chosen during the design phase on the basis of many factors, including final quality and working conditions. They depend on the hob diameter and on the number of starts. The current tendency is to design hobs with a larger number of gashes due to the fact that the hobbing time is reduced and the feed speed is higher. Moreover, the gash determines the cutting face by intersecting the thread. The cutting face is radial in the majority of cases. In any case, to realize a positive or negative rake angle, the hob must be reshaped with a grinding wheel positioned out of axis by a distance that depends on the rake angle itself and on the hob diameter.
There are also other important gash’s elements which influence the cutting efficiency of the tool, just like:
- the tooth back angle;
- the bottom radius of the gash;
- the gash depth.
These are just some of the most important elements of a gear hob, to know the others read the news.