Gears are used widely throughout modern industry to transfer power and motion or control speed and torque in a wide range of mechanical devices. Most gears are produced on a hobbing machine, which, when compared to other methods, has proven efficient and inexpensive. This article will take a closer look at the process of gear hobbing, how it has evolved into the modern method widely used today, and what customers should look for in a vendor’s hobbing processes to ensure quality.
A brief history of the hobbing machine
The first hobbing machine patent, British patent number 2896, was issued to Christian Schiele in Decem- ber 1856. Unfortunately, he was unable to capitalize on his patent, and no significant progress was made on Schiele’s gear-hobbing machine until 1900, when Herman Pfauter of Germany built upon Schiele’s original design. Pfauter was issued several patents for his advancements that significantly progressed Schiele’s design into a more universal machine capable of making spur, helical, and worm gears of any number of teeth.
Gears produced by these machines are made using a rotating tool mounted on an arbor, which was itself mounted on a swiveling axis used to produce a helix angle. The gear blank is mounted on the table of the hobbing machine, and the hob (cutting tool) is fed into the blank, which removes material from the blank to create the gear teeth.
The first hobbing machines were manually operated and used a set of differential and indexing gears as well as feed gears, in some cases, to set the feed rate. Many of these hobbing machines are still in use today. With some care and maintenance, they can still produce good quality gears.
Today’s hobbing machines
Like almost all other machine tools in the modern era, gear-hobbing machines have evolved into CNC machines. The operation of these machines has been made much more operator friendly than their earlier counterparts with the introduction of conversational controls. Today’s operators need very few basic inputs of gear geometry, hob cutter information, process cycles, and speeds and feeds to output a precision gear.
Modern hobbing machines offer a wide variety of cutting options to enhance their performance. These options include standard hobbing programs using a standard hob-cutting tool, single index programs for using heavy-duty gashing cutters, and other programs for jobs such as diagonal hobbing and worm hobbing.
Along with the conversion to CNC, today’s gear-hobbing machines have also become much more powerful, robust and rigid, and require a larger footprint. This is done so that, contingent upon the quality of the cutting tool, the machines can take a heavier depth of cut at higher speeds and feed rates. For example, in replacing two manual 1,500 mm hobbing machines placed side by side, with a new CNC hobbing machine of the same size, the new machine required the same footprint previously taken up by the two older manual machines.
Like the hobbing machines themselves, gear-cutting tools have also evolved into more productive and efficient pieces of tooling. Many early hobs were made from M2 high-speed tool steel. This has evolved into M4, T15, M48, or REX 76™, and many other higher quality options. Each alloy added into the steel has helped optimize a specific part of the cutting process.
Manufacturing of high-speed tool steel also has evolved. The use of PM tool steels has improved the overall metallurgical structure. These are high-performance, highly-alloyed super-hard powder metal tool steels. This translates into improved wear resistance, higher hardness levels, and higher toughness needed in today’s demanding tooling requirements for higher productivity and longer production runs.
Another improvement in optimizing tool life of high-speed tools is the use of coatings. The fore runner of tool coatings is TiN (titanium nitride), which was engineered to allow for better cutting action of the tool, reduced friction, and overall increased tool life. Today, one of the most popular coatings is AlCrN (aluminum chromium nitride), which has allowed for more improvements in the performance of cutting tools. In the early years when coatings were first introduced, the manufacturers using them learned many lessons. The biggest take away being that a coating is only as good as the base material. This lesson remains highly applicable to today’s gear-hobbing processes. Manufacturers should not take a 20th century tool steel and use a 21st century coating over the material, as they will not obtain the results they expect.
One of the recent big breakthroughs in hob-cutting tools has been the use of ICI (indexable carbide insert) hobs. These hobs were developed after noticing the benefits found in using carbide inserts in other machining processes such as turning, mill drilling, and others. The use of ICI hobs, with their ability to run higher speeds and feeds to cut gear teeth and optimize productivity, is what compelled machine-tool manufacturers to build the rigid robust gear-hobbing machines that can handle the much higher cutting forces produced when using this type of cutting tool.
What should customers look for?
Like any venture, customers should decide what they’re looking for in a gear cutting vendor and materials supplier. This should include required quality levels, any heat-treatment requirements, and what dimensional inspections are required.
The most important lesson to remember is good gears start with good blanks. The quality of the gear blank to be hobbed directly affects the quality of the hobbing process. One of the most important keys in the process is to keep the diameter where the gear teeth will be cut concentric with the axis of rotation. For both external and internal gears, that generally means the outside diameter is kept concentric with the bore or other specified datum. Finally, the other important feature is the mounting face. This surface is where the blank is mounted to the workholding tooling, which should be kept flat and perpendicular to the axis of rotation or bore.
Conclusion
Hobbing is a fast and efficient way to produce gears of all sorts and sizes, especially with the leaps and bounds it has made from its beginnings as a manual process to the highly technical operation it is today. But customers should spend the time to certify their gear provider is employing the right materials and processes to ensure the appropriate standards for quality are always met.
