Higher gear: General Industry Coverage

All images courtesy Ingersoll Cutting Tools

An operator at Allied Specialty Precision cutting a gear from 17-4 PH bar stock using an indexable tool.

By tradition, gear makers tend to be skeptical of new ideas. Most still generate gear teeth with single-point, HSS shapers, rack cutters or hobs, believing that indexable-insert tools cost too much, can’t meet American Gear Manufacturers Association standards or are “not proven” enough.

More progressive gear makers tell a different story. By applying indexable-insert tools, many have gained substantial competitive and cost advantages. They are routinely tripling material-removal rates and tool life with no tradeoff in accuracy or surface finish. They have also gotten off the tool regrinding carousel, a bottleneck that often delays deliveries. Regrinding tools typically requires trafficking five expensive cutters through 5-week regrinding cycles just to keep a single process operating.

In many cases, these progressive thinkers have also dispensed with special-purpose hobbing machines and machine-to-machine part transfers. Some are even completing the entire gear—teeth, keyways and hubs—with “one-and-done” machining processes on general-purpose CNC machining centers.

Opening New Doors

The potential to improve gear machining stems largely from advances in indexable-insert carbide tools that enable them to repeatably hold much closer tolerances than previously possible. Indexable mills, including form tools, significantly perform better than and outlast HSS ones. They also routinely hold 0.0005 ” (0.0127mm) repeatability to datum, axially and radially. That’s more than good enough for the vast majority of gear machining applications. Moreover, many indexable tools or inserts can be replaced in the spindle, so chipmaking can resume in minutes with no touching off.

One case demonstrates many of these advantages. On an 8 AGMA 66.5 ” shaft gear, a mining equipment manufacturer reduced tooth-generation cycle time from 34 hours to 8.5 hours by switching from HSS shaping to indexable-insert hobbing. The new process uses a 1.25DP Ingersoll single-start indexable hob on a Liebherr LC 2500 hobbing machine. The change also eliminated a separate rough sawing operation and mid-cut stoppages to replace tools. Cutting edges last for two complete parts and deliver an 80-rms surface finish to the roots of the shaft gear.

A modified standard Chip-Surfer slotting tool from Ingersoll forms teeth in two passes at Allied Specialty Precision, reducing cycle time by 20 minutes.

The process is essentially climb milling. Parameters for roughing are 110 mm/min. (4.33 ipm) surface speed, 0.90mm (0.035 “) axial feed per table revolution and a 42.45mm (1.67 “) DOC. For finishing, the respective parameters are 140 mm/min. (5.51 ipm), 0.300mm (0.012 “) and 5mm (0.197 “). The application reflects the trend toward using advanced indexable hobs for higher AGMA-class work.

The dramatic reduction in cycle time stems from two key differences between the hobbing and shaping tools. First, hobbing, a rotating-tool process with multiple cutting edges, keeps a cutting edge engaged in the cut much longer than a reciprocal-motion shaping process. Second, the indexable single-start hob allows higher speeds and feeds, leading to increased mrr and shorter cycle times.

One and Done