Hobbing In High Gear

The length of shank-type hobs creates a large shiftable cutting zone allowing each hob to cut a number of parts.

Gleason's Genesis shank-type hob has a D-drive mechanism that contacts the entire surface of the hob key for transfer of torque. Typically, the flat-on-flat drive configuration of most machines results in a slight misalignment of the interface under torque resulting in high stress line contact.

The highly competitive, global market requires gear manufacturers to push the limits of productivity and shrink production costs. This means higher hobbing speeds and feed-rates with hobs made of premium materials that are protected by high-performance coatings. Because premium tools command a premium price, some shops resist the use of high-performance hobs without analyzing all aspects of the production process to determine what really counts — the total cost to produce each part.

The quest for higher gear-cutting productivity has led tool designers to develop high-performance, multithread, shank-type hobs. Shank-type hobs are made from a solid bar of tool steel. They have smaller diameters, and typically are longer than bore-type hobs. They are located by journals on each end when mounted in a hobbing machine, rather than mounted on an arbor as are bore-type hobs.

Solid shank-type tools are commonly used to cut worm gears that required a hob with a diameter close to that of the worm. Optimized shank-type hobs are best suited to high-volume, high-production applications. Large-diameter, coarse-pitch gears, such as used in truck transmissions or in mining applications, usually are produced with bore-type hobs.

The factors that contribute to reduced cutting time for both shank-type and bore-type hobs include increasing feed-rate and hob speed and the number of threads on the hob. As tool designers try to optimize performance, they look to smaller diameter tools because smaller diameters allow for higher cutter speeds at given surface feet per minute. Because shank-type hobs have smaller diameters and no bore, they can have greater length than bore-type hobs for similar diameters. This creates a greater shiftable cutting zone, and allows more workpieces to be cut per pass across the hob. The hob shifts along its axis bringing sharp teeth into the cutting zone.

"The high productivity of shank-type hobs is the result of more shift positions along the hob per pass, and more passes due to the high wear resistance of hob materials and tool coatings, yielding more parts per use," said Glenn Schlarb, engineering manager at Gleason Corp. (www.gleason.com).

The hobbing process can be run with or without coolant. However, many users prefer to hob dry because that eliminates environmental concerns and the disposal costs associated with coolants. Dry cutting requires hobs with premium coatings with high temperature stability and low thermal conductivity to protect the cutting edge at dry, high-speed hobbing conditions.

Because shank-type hobs are solid pieces and because there are fewer components used in mounting them in a machine, they are stiffer assemblies than bore-type hob assemblies. Hobs can be inspected as mounted between centers for sharpening and mounting and inspection. Compared with bore-type hobs, shank-type hobs eliminate a stack-up of runout tolerances between a bore and arbor, as well as wobble inaccuracy due to spacers and mounting nuts.

Premium grade shank-hob steels have high wear resistance and high levels of both room temperature and hot hardness when cutting. Although premium steels and coatings are more costly than materials with lower performance capabilities, the resulting higher cutting speeds possible with small-diameter shank-type hobs result in faster cycle times and lower machining cost per part. (See Table). For appropriate applications, this gain in machining cost per part is generally greater than the increase in tool cost per part. That results in savings on a total cost per part basis.

COATINGS ARE CRITICAL
The use of the appropriate coating for a hob application can increase productivity and lower costs of machining. Tool life can be doubled , depending on the application, by an appropriate coating, and that provides savings on tool costs while requiring fewer tool changes and less machine downtime.

Dr. Dennis Quinto, technical director at Oerlikon Balzers Coating (www.oerlikon.com) said cutting edge preparation has a profound effect on coating performance, and is often an uncontrolled aspect of tool performance.

Sharp cutting edges produce low cutting forces and low heat generation, but give rise to edge chipping of the coating, and should be used only with light feeds. Honed cutting edges, with a heavier coating thickness, are more stable against chipping, but produce higher cutting forces. Chamfered and honed cutting edges are the most stable against chipping and are suitable for brittle tool materials, such as carbide, and heavy feed-rates.

Because the cutting speeds and depth of cut in hobbing decrease with increasing tensile strength of the workpiece, proper choice of coating allows for higher speeds and, in some cases, higher feed-rates. Almost all hobs are protected by a coating that is applied by a physical vapor deposition (PVD) process. Standard titanium nitride (TiN) coatings are effective at low speeds, with lubrication. Titanium carbo nitride (TiCN) coatings have increased hardness and wear resistance and are good for abrasive applications. Titanium aluminum nitride (TiAlN) coatings were developed for dry cutting because of their toughness and high heat resistance. A more recent coating developed for dry applications is aluminum chromium nitride (AlCrN) with even higher wear, heat and oxidation resistance properties.