A New Edge On Gear Milling

April 01, 2010

New insert manufacturing techniques from Sandvik Coromant play a critical role in elevating production results for an increasingly demanding marketplace.

What does a new generation of indexable inserts mean to the gear manufacturing industry? Minimizing lead times in gear production is becoming an increasingly competitive factor, and for many machine shops cost-competitiveness while maintaining or improving quality consistency will be decisive in achieving success in new and existing markets. And, as is always the case in machining, it all starts at the cutting edge.
The Cutting Edges
When applying a milling operation to manufacturing, a number of basic factors are always routinely assessed: workpiece, material, operation type, machine tool, and quality demands. This then leads to a number of optimization possibilities, with the main ones being method, tool, tool paths, tool positioning, and cutting data, all with the aim of achieving machining to the right quality level, the lowest manufacturing costs, and the best throughput time. Today the role of insert grades as part of the tool needs to be highlighted in particular, as recent developments have changed the possibilities to directly improve gear milling.

Coated indexable inserts have proved themselves to be the new benchmark in the milling of most materials, especially as they have such good resistance to heat- and chemical-related wear. They have the capacity for high cutting speeds, long and reliable tool lives even when feed rates are high, and can resist the formation of the edge cracks that make a lot of inserts unsuitable for the thermal variations of milling, even with coolants, as is still the case with much of gear machining.

Insert-grade dedication is required when productivity and security are more prominent issues than versatility in machining. Gear milling is an area where a dedicated insert grade will optimize the application as it regards performance, providing high metal removal rates, short cycle times, and predictable tool life. Crater wear, thermal cracking, and even plastic deformation are among the risks to cutting edges when high cutting speeds, large cutter diameters, varying edge engagement times, and harder materials are involved, as is the case in gear milling. These wear types act to shorten tool life and impair predictability consistency of the quality achieved.

Thus, to achieve higher machining security at high cutting data in steel milling, wear resistance often needs to be a priority over toughness. But in some operations—as is the case with the milling of bigger gear modules—toughness needs to be the dominant cutting edge property.

Types of Toughness
Gear milling is characterized by interrupted cutting action, and in addition to thermal variations on the cutting edge, mechanical interruptions make demands on high insert toughness and strength. Generally the insert shape and geometry, as well as the entering and exit conditions, determine the degree of toughness needed from the tool material. In gear milling the demands for edge toughness are also influenced by the relatively small radial depths of cut in combination with the large tool diameter. The use of coolant will emphasize the demand for toughness, sacrificing wear resistance, which will have a limiting effect on cutting data.

Dry machining, without any coolant application, is to be preferred when putting the cutting process in the center because in today's high machining rates it is hard for even high amounts of coolant to have much effect at the cutting edge. Coolants in gear machining are very much inherited from the use of high speed steel tooling. Most of the coolant in modern milling will be vaporized and what remains will only intermittently cool the insert as it goes in and out of the cut. Thermal variations are thus amplified, which has more of a negative effect on the insert than what high temperatures do.

A modern coated insert grade has the capability of providing high security at the temperatures generated by high cutting speeds and feeds. Having said that, however, a lot of grades will function well in both dry and wet conditions, when the coolant is necessary for chip evacuation and maintaining component temperature for keeping within dimensional tolerances, and so on.
Critical Coating
When it comes to coated indexable inserts, there are two coating methods which produce two characteristically different grades. CVD (chemical vapor depostion) coated inserts make it possible to make coatings that provide a high degree of wear resistance by resisting chemical, abrasive and thermal breakdown of the edge. These grades are coated at relatively high temperatures, giving them something of a tendency for being brittle, but this is offset through edge preparation today. This gives the CVD-coated grades an ability to machine at high cutting data while still retaining a high level of bulk toughness. Fig. 1

PVD (physical vapor depostion) coated inserts have coatings suitable for relatively sharper and more positive cutting edges. They do not have quite the same level of wear resistance as CVD coatings but provide a high degree of toughness localized more at the cutting edge line. These grades are coated at lower temperatures, which does not provide the same level and nature of wear resistance as CVD-grades but does give them added edge toughness, thus making them advantageous for sharper edges. Modern manufacturing techniques, however, have meant that the basic differences between CVD and PVD grades have become smaller, with grades overlapping in application areas and providing much more capable optimization instruments. This is certainly the case for gear milling, where new insert generation grades have elevated performance.

A new CVD-type ISO P40 grade for steel milling has established itself as a solution for many demanding operations where cutter entry and exit in the workpiece needs a strong edge. Typical for CVD applications are that chips are usually thicker, as in medium and heavy milling with high feeds, and cutting speed is generally high. CVD edges will typically provide  a high degree of edge durability thanks to their resistance to heat and chemical loads, as well as abrasive wear.

A new PVD-type ISO P30 grade provides the type of edge strength often needed when there is some instability, such as when longer tool overhang, difficult chip evacuation, and more difficult cutter entry and exits, as can be the case with short cutting edge engagements. In these operations, chips are typically thinner and cutting speed somewhat lower. Surface finish and tolerances are more likely to be to closer limits and the PVD edge may be more suitable as it is sharper.

Assessing Operations
Assessing the gear milling operations at hand may well lead to one grade being shown to have an optimizing effect against the other. This is partly because toughness in gear milling is made up of more than one aspect. A stable operation which needs to be run at high cutting data can be improved by using a CVD-grade insert, while some operations can benefit from the easier-cutting, PVD-grade, more-positive insert.

For example, the CVD-coated grade GC4240 is a tough, steel-milling grade for applications where cutting edge strength is needed for more severe demands both as regards material and machining conditions. Generally this grade will solve most operations where a combination of high insert toughness in combination with resistance to heat and chemical breakdown is necessary. It will provide a winning, reliable solution, especially for medium to large diameter cutters, when other insert grades will not stand up to the demands of insert strength required by the operation. Fig. 2

Complimentary to this, the PVD-coated grade GC1030 is a steel milling grade for applications where a more-positive, sharper, and tougher cutting edge line is needed. This grade is generally ideal when radial depths of cut are smaller, vibration risks apparent, chip evacuation tricky, cutter entry/exit demanding, and functions better when coolants are applied.

When compared to traditonal high speed steel tools for gear milling, the new generation of coated cemented carbide insert grades offer a completely new scope for reducing machining times and improving the manufacturing cost for components with gear teeth. When compared to the insert grades that have been used so far in gear milling, the new generation of CVD and PVD steel milling grades will very likely provide a substantial upgrade of machining, with better throughput times and machining security as a result.

When it comes to milling normalized case-hardening steel, the advantage of carbide inserts is particularily highlighted in the form of higher productivity, while for tempered steel the advantage is especially apparent in longer tool life.

Another important aspect to the success of indexable insert in gear milling is that of insert geometry and insert manufacturing quality. Edge geometry and edge preparation are important combination factors with insert grades for achieving the best solution. Establishing the right microgeometry in the form of lands, chamfers, and edge rounding decisively affects the strength characteristics of the cutting edge, as well as durability. This has a direct influence on cutting data capability, security and, very importantly, the predictabilility of quality consistency.

Gear milling cutters have a large number of large inserts, and the position of one cutting edge will affect the others as regards load during cut. If these loads vary too much they can negatively affect machining performance and the ability to achieve the right quality level. In this context new insert manufacturing techniques, producing inserts to much closer tolerances, play a critical role in achieving solutions to elevate results for a marketplace where quality demands are constantly escalating.   

About The Author

Christer Richt

is a technical editor for Sandvik Coromant, based in Sandviken, Sweden. He has been writing about metal cutting and the industry for the past 40 years, including gear hobbing since 1980. For more information about the company’s InvoMilling process, call 1-800-SANDVIK, or log on to www.sandvik.com.