Original Date: 09/20/2004
Revision Date: 01/18/2007
Best Practice : Hobbing – Fine, Medium, and Coarse Pitch
By purchasing and maintaining modern, high-tech equipment and cutting tools, Forest City Gear has the unique capability to hob high quality, specialty gears. This has enabled the quick turn out of novel gears in quantities as low as one piece, and expanded the company’s customer base to more than 900 companies.
The average age of gear hobbing equipment used by the U.S. Gear Industry is 25 to 40 years. Many of these competitors’ machines have been upgraded with a Computer Numerical Control (CNC) interface; however, they remain old machines with many, possibly worn, mechanical components, each contributing to variations in hobbed gears. In addition, the machines lack the rigidity and high-speed spindles to take advantage of state-of-the-art cutting tools.
Forest City Gear (FCG) reinvests 25% to 40% of its gross revenues into the company every year, much on state-of- the-art hobbing equipment. The average age of FCG’s gear cutting equipment is six years. The company often purchases unique equipment that features such new technology that it acts as a beta site for the machine manufacturer. The company’s CNC modern technology hobbing machines offer a number of unique gear hobbing capabilities. The machines are rigid, which enhances reliability and maintains consistent size control. Often the machines can cut gears for an entire shift without needing to make size adjustments, unlike the older generation machines that required constant size adjustments. Many of the machines feature robotic automation for loading and unloading parts, which enables high production with one operator running three or four machines. The CNC controls permit enough programmed variation to allow the cutting of unusually shaped gears.
The modern controls allow the machines to make multiple cuts with varied speeds and feeds (e.g., making a roughing cut with lower spindle speed and high feed, and a finishing cut with higher spindle speed and low feed, all in the same set-up). The high rigidity machines with high spindle speeds allow the use of carbide hobs to re-cut hardened gears. This process yields nearly the same quality as finish grinding at a higher production rate without the risk of burning the gears. The machines have non-contact probes that examine all of the teeth before re-cutting, and allow the machines to reposition the work piece for accurate, consistent stock removal on gear flanks.
In addition, many of the machines can be loaded with two hobs, which allows the machines to hob multiple gears with different numbers of teeth and/or pitches, maintaining precision alignments between the teeth or other features on both spur and helical gears (Figure 2-3). Some of FCG’s hobbing machines have been purchased with precision mechanical components. This allows FCG to hob high precision gears (American Gear Manufacturers Association [AGMA] Quality 14 or 15 pitch and spacing) for use in telescopes, missiles, gun sighting, and other applications that require precise alignment. Typically, such gears can only be produced by finish grinding at a much higher cost.
FCG pays as much attention to its gear cutting hobs as it does in obtaining the best hobbing machines. The hobs are purchased from leading manufacturers world-wide and are often solid carbide for re-cutting hardened gears. Other hobs are made from advanced tool steels such as Rex 121, that can outperform carbide in some applications. Hobs are coated with ceramic (TIN, TICN, TIALN, or multiple layer) coatings to improve cutting and reduce heat generation to allow higher cutting speeds. FCG strives to obtain precision hobs, often paying a premium to obtain class AA hobs.
Figure 2-3. Gear Hobbing Operation
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