PRODUCT SHOWCASE: Index Develops Bevel Gear Process for Turn-Mill Centers

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Index has developed a bevel gear hobbing package, which consists of a specially designed control cycle and four Index cutter heads with module-dependent inserts. Equipped with these features, the Index R200 and Index R300 turn-mill centers become gear-cutting machines on which spiral bevel gears can be produced from bar stock with front- and rear-end machining, complete in one setup or as a pure two-spindle gear cutting machine. 

By hobbing using a continuous indexing method, which corresponds to the Klingelnberg Cyclo-Palloid® method, spiral bevel gears can be produced with constant tooth height in a module range of 0.6 to 4 mm.

Compared to the conventional process chain with classic gear-cutting machines, users can achieve shorter cycle times and better geometry and position tolerances. And it is much more flexible.

“The starting point of the development by Index lies in its own manufacturing governed by the principle: quality-determining components are made in-house,” said Dr. Volker Sellmeier, head of technology development at Index-Werke. “When the toolholder production was reorganized several years ago, the decision was made to produce the required bevel gears ourselves.”

Bevel gear cutting requires a machine with high rigidity and a B-axis as the basis. Due to their excellent static, dynamic, and thermal properties, the innovative turn-mill centers of the Index R-series are well-suited to gear cutting, provided they are equipped with Index’s bevel gear hobbing technology package. The R machines’ axis configuration with two milling spindles on Y-B-axes running in hydrostatic bearings makes it possible to machine on the main and counter spindle simultaneously in five axes.

The ability of the turn mills to accomplish complete gear-part machining on the front and rear ends simultaneously results in shorter total cycle times and lower cost per piece. 

“When we machine typical bevel gears with module 1.15 mm and approximately 25 teeth for our tool holders completely from bar stock, we achieve a cycle time of less than 3 minutes,” Dr. Sellmeier said. “The share of gear cutting amounts to about 30 seconds.”

In a classical gear process chain, the workpiece has to be set up on several individual machines for turning, drilling, milling, gear cutting, and deburring. Index’s approach is to run all operations on the turn-mill center. Bevel gears are turned, drilled, milled, and finally cut on a single machine. Even brushes for deburring can be set up. Thus, the soft machining process is completely autonomous with a process-reliable gear quality of IT5, according to Index. This is then followed by hardening. A final finishing process is usually required only for the mounting distance and the polygonal shaft/hub connection.

The Index solution works for contract manufacturers, which need to produce small lot sizes with high flexibility, as well as for mass producers, which want to produce bevel gears in large quantities at minimal cost. The investment is relatively low compared with specialized machines. Also, the consumption costs are kept within manageable limits, since the cutter heads are equipped with interchangeable inserts. 

In addition to bar stock machining that’s best primarily for small quantities, the R machine can be used as a pure gear-cutting machine for series production, working on the main and counter spindles simultaneously.

 “This requires the use of an automated workpiece loading and unloading system that loads the blanks and removes the finished parts gently,” said Dr. Sellmeier. “We offer a quadruple gripper with two stations on the main and counter spindle that picks up the finished parts, rotates, and then loads new blanks. This way, we use the machine as a kind of double-spindle machine, cutting the time per piece in half.”

The tools developed by Index for the process are especially important. Two cutter heads are required per bevel gear. They differ slightly in their cutting circle radius in order to produce the longitudinal crowning. Index offers the cutter heads in four different sizes that can be fitted with up to six carbide inserts and feature internal cooling. 

In contrast to the typical Cyclo-Palloid® method with an interlocking cutter head, the Index method uses two separate cutter heads per bevel gear. 

“The two cutter heads provide a larger number of cutting edges,” Dr. Sellmeier said. “This allows us to achieve a higher cutting performance. We also have more freedom for flank modifications and correction of the contact pattern.”

A control cycle developed by Index is another essential part of the technology package. The user enters the same parameters as on a conventional gear-cutting machine. These include, for example, machine distance, eccentricity, and auxiliary angle. The cycle translates these values into the movements of each axis so that, at the end, the same relative movements are effected as on a conventional gear-cutting machine.

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