Closed-loop gear manufacturing system speeds design to manufacturing

The complex geometry of spiral bevel and hypoid gears makes it a tedious, timeconsuming process to turn a gear design into a finished product that works as intended. Gear manufacturers must go through a trial-and-error process of design, prototype manufacture, inspection, and adjustment of machine settings to refine the part geometry. And they often must repeat the entire process more than once to obtain a dimensionally acceptable gear.

Over the years, advances in computers and software have eliminated much of the tedium involved. But still, this trial-and-error process takes several days.

Now, an integrated, closed-loop gear development and manufacturing system brings the three key elements of this process together so gear manufacturers can perform the trial-and-error steps more efficiently, thereby reducing the time from days to hours, and minimizing the potential for human error. These key elements include:

• Gear design, using software that analyzes the tooth surfaces and their mating contact characteristics before parts are generated.

• Machine settings (CNC program) for the gear-generating machine.

• Gear evaluation, using software to verify that the generated gear matches the design.

In the early 1980s, The Gleason Works, Rochester, N.Y., developed the software for this closed loop system: a gear design and analysis package that was available to gear manufacturers online, via modem. Users could move sequentially through design and analysis programs to calculate machine settings for an optimum gear design. The late ‘80s brought enhancements such as graphic capabilities, and multi-tasking, letting users perform these tasks on desktop workstations.

Evaluating a gear before it exists

The first step in the closed-loop process is to establish an optimum theoretical gear design. Using Gleason’s design and analysis software package, called CAGE (Computer-Aided Gear Engineering), the design engineer enters basic gear parameters, such as number of teeth, diametral pitch, and face width. The software helps the engineer develop and optimize gear tooth geometry of bevel and hypoid gears based on customer application requirements.

As part of the design process, the analysis portion of the software predicts how the theoretical gear will mesh with its mating gear, so the engineer can “test” gearset performance before any metal is cut. At this point, the gear only exists in the computer as a set of mathematical parameters, along with data on the desired contact conditions between the two gears and their desired uniformity-of-motion.

Tooth contact analysis is the primary method for determining the optimum tooth geometry. Software programs such as CAGE simulate tooth contact between two mating gears, commonly called the gear and pinion, in a gearset. The software displays the contact pattern that would occur if the gears were rolled together in a test machine, as well as the smoothness of the rolling motion. It also calculates tooth bending and contact stresses under the expected load. This allows the engineer to optimize the contact pattern — uniformly distributing the load to minimize tooth contact stresses.

Once the theoretical gear design is established, the software produces machine instructions, which the operator transfers to a computer disk and loads into the CNC controller of a gear cutting machine. With Gleason’s Phoenix cutting machine, for example, these instructions enable a Fanuc controller to simultaneously control six axes of machine motion, three linear and three rotational, that generate the tooth shape. An operator then uses the machine instructions to cut a trial gear.

Each axis of motion uses a separate motor — five ac servo motors and one spindle motor in all — to drive the cutter and work head at the required velocities and accelerations. Incremental rotary and linear encoders provide position feedback information for the rotary and linear axes, respectively.

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Evaluating a “real” gear

Next, operators compare the gear tooth shape “as manufactured” in the trial gear to the theoretically correct tooth shape. Gleason’s G-AGE gear inspection and evaluation software, combined with a high-resolution coordinate measuring machine (CMM), provides the tools to accomplish this step. Typically a CNC-controlled Zeiss CMM or a Zeiss/Hofler gear measurement CMM is used, with a resolution of 0.4 to 0.5 microns.

Using a probe, the CMM measures both sides of four teeth (located 90° apart around the gear circumference) and averages the results. Because of the gear complexity, a 3D probe is required. Using CMM measurement software, a built-in CNC positions the probe to take measurements. Servomotors drive the probe in at least three axes to optimize its contact with the gear tooth surface.

The G-AGE software interfaces with the CMM software and evaluates the differences between measured dimensions on the trial gear and the theoretical gear dimensions. Then it produces corrective settings for the gear cutting machine. Typically, the machine operator enters these changes into the CNC. As an option, the G-AGE software can automatically enter the changes on a floppy disk for updating the machine controller.

In most cases, the second trial gear matches the theoretical design. If not, operators repeat the process to develop a third set of instructions and cut another trial gear. When the trial gear dimensions are sufficiently accurate, production can begin.

Cut gears are hardened and sent to a Phoenix grinder. Settings on the grinder, like those for the gear cutter, are generated from the original design file. The operator installs a cut gear on the grinding machine and runs the grinding program. As before, this trial gear is inspected on the CMM, corrections are made in the machine settings, and a second trial part is ground. Again, grinding typically requires only one iteration before accurate machine settings are established.

Reverse engineering a gear master

A new use of this closed loop system is the process of reverse engineering a gear master. Several major U.S. automotive axle manufacturers are doing this, as are some Chinese gear makers.

Certain gears that were manually developed by trial-and-error methods years ago are proven designs for given power transmission tasks. Their gear masters are used to set up production machines and verify the accuracy of production gears in roll testing machines. But today, manufacturers need to produce these gears using the new, more productive CNC machines. Reverse engineering is the answer.

Gleason’s George Grana, application engineer, Gear Metrology, explains: “First, an engineer uses CAGE to create CMM measurements and G-AGE instructions based on a master gear’s original design. Then an operator imports this data and measures the gear on a CMM. The G-AGE software interprets these measurements and transmits them back to the CAGE program. The engineer then uses CAGE to create new cutting or grinding machine settings to produce a gear that duplicates the master.”

Closing the loop

When gear cutting or grinding machines are integrated into a closed-loop system with the gear design and evaluation software, much of the time required, and potential for human error, can be taken out of this iterative design-to-manufacturing process. John Swanger, senior project research engineer at Gleason, explains: “The G-AGE software produces corrective settings based on evaluation of the trial gear. The operator enters these new settings into the Phoenix machine’s Fanuc CNC. Another gear is cut, then inspected by the CMM and simultaneously evaluated by the software to verify the results.”

For example, the Arrow Gear Co., Downers Grove, Ill., worked closely with Gleason Works to integrate computerized design, inspection, and correction into their manufacturing operation for spiral bevel and hypoid gears. The resulting closed-loop gear manufacturing system shortens gear development time and reduces the need for operator interpretation.

Using machine settings generated by CAGE, operators cut a trial gear on a Phoenix 250HC gear cutter, then inspect it using G-AGE software on a Zeiss/Hofler CMM. The G-AGE software makes corrections to the machine settings and the operator cuts a second gear with the corrected settings. In most cases, the second trial gear accurately corresponds to the master file, so gear production can begin.

After the gears have been cut and hardened, Arrow Gear uses the same closed-loop process to develop grinding machine instructions. Again, this usually requires only one iteration to establish accurate machine settings.

Since installing the closed-loop system, Arrow Gear reduced its throughput time (from customer order to shipment) by an average of 20% for jobs that require new gear designs. In addition, the computer-controlled system satisfies customer demands for just-in-time inventory and small production runs.

In addition to developing new gears, some gear makers use this closed-loop system to audit production and ensure on-going quality. John Swanger adds: “Acceptable tolerances are determined for flank form errors, based on each point on the teeth measured by the CMM. Changes are made to bring the cutting or grinding machine performance back to spec if the tolerances are exceeded.”