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Monday, July 28, 2014

Multi-Tasking Machines Turn Up Possibilities on Big Gear-Set Jobs

Many manufacturing shops increasingly incorporate full 5-axis multi-tasking machine tool technology. It allows them to handle basically any kind of part that comes through the door and do so with one type of machine in single setups, thereby increasing productivity. But rarely, if ever, do these shops realize that large spiral bevel gear sets are among that wide mix of part-processing capability.

With a larger-sized 5-axis multi-tasking vertical machining center, for instance, shops can cost-effectively and quickly produce those occasional, low-volume spiral bevel gear sets in-house. Thus, a shop can avoid the long turnaround times associated with farming the job out to a specialty gear shop. These specialty gear makers are few and far between because only a small amount of them have the equipment big enough to do the job, and if they do, that equipment is always at full capacity and heavily backlogged.

Credit: Mazak

Credit: Mazak

Additionally, big gear sets — those involving ring gear pitch diameters over 3.5 feet, and processed on a multi-tasking machine — require minimal matching sequences, if any at all. Special 3D software that creates the 3D model of the gear teeth makes this possible. With a gear’s geometry data, the software automatically generates the geometries required for the mating gear and the contact pattern.

The model with the proper contact pattern is then loaded into a computer-aided machining (CAM) system that allows the gears to be programmed the same way as any other 5-axis part. And from this point forward, the machine tool simply cuts to the model. So a properly designed model together with a well-maintained machine tool will generate consistently accurate gear sets.

A multi-tasking 5-axis machine tool can cut both ring and pinion gears so accurately and consistently that a shop could, for instance, separately cut five pinion gears and five ring gears. It could then pair the gears up with one another in no particular order to create five gear sets.

Plus, when not machining the occasional gear set, a shop can use its multi-tasking machine to cut all of its other parts, including the gear boxes the sets go into and any other associated transmission components, to maximize machine spindle utilization. It is this production flexibility that attracts shops to multi-tasking machines.

In process-development testing, Mazak machined a spiral bevel gear set that included a 29-tooth, 22-inch-diameter pinion gear and a 114-tooth, 6-foot-diameter ring gear on a multi-tasking 5-axis machining center. This gear set was completed in days as opposed to months. The set was also produced completely using only one machine.

The machine was a standard Mazak model with a two-pallet changer, performing all required machining processes, such as milling, turning, boring, drilling, and tapping. Its tilting spindle and turning table allowed for machining at any angle or cross-cutting position, as well as for contouring operations. The machine control provided fast, small-increment processing for high-accuracy 5-axis simultaneous machining and superior part surface finishes.

Also for development testing, Mazak used no special tools to machine the ring and pinion. They were standard, off-the-shelf, and readily available tools.

Process testing was performed on the machine with a 10,000-rpm, 50-hp spindle with a B-axis travel of -30 to +120 degrees. The machine’s 50-hp, 300-rpm turning spindle rotates 360 degrees in the C axis in 0.0001-degree minimum increments.

The machine’s round 55.1-inch-diameter face plate pallets, with four jaws, support maximum loads of 11,000 pounds. The pallets accommodate workpieces measuring up to 78.7 inches diameter and 52.9 inches high. The machine’s rapid traverses are maximum 1,653 inches per minute in the X, Y, and Z axes. A standard 40-tool magazine provides ample storage to support continuous machining operations.

Pinion gears for the sets were machined from solid cylindrical pieces of 8620 steel, while the ring gears were machined from forged 4340 carbon steel rings that had about 0.250 inches of additional stock for machining but lacked any near-net teeth shapes. Both gears were first rough-machined in their non-hardened states. Then pinion teeth were carburized to 62 Rockwell hardness (Rc), ring gear teeth induction-hardened to 55 Rc, and both finish-machined in hard-milling operations.

For this particular gear set’s machining operations, a pinion gear was fixtured on one of the multi-tasking machine’s two pallets and a ring gear on the other. While one gear was being machined, a machinist set up the other at the pallet load station. While this capability is not critical to gear set machining operations on a multi-tasking machine, it does, however, increase spindle utilization and machine output for other jobs.

After it is rough-machined, the pinion gear was moved out of the work envelope, and the ring gear on the second pallet was moved in for its rough-machining. A shop that employs this process would then send out both rough-machined gears for their hardening processes.

With its teeth hardened, the pinion gear was re-located back onto the pallet fixturing, and the machine finish-cuts the gear’s bore as well as hard-mills the flanks and edge radii of its teeth to eliminate any secondary deburring operations. All of the operations were done in the same setup to ensure the gears’ features ran true with one another. Likewise, the ring gear was re-fixtured and moved into the machine, where its bore was finish-cut and all of its gear teeth, including radii, were hard-milled to finish size.

If the gears had been produced using traditional methods, shops would have had to match-test them as sets at this stage of production. The drawback to these exclusively matched sets, however, is that pinion gears, simply because they rotate more, tend to wear quicker than do ring gears. When the pinion gear is worn, shops will make a whole new gear set, even though the ring gear may have working life left. They produce a new set because the odds of generating another pinion gear that will roll perfectly to the existing ring gear’s pattern are extremely low. However, many gear specialty shops continue to try and accomplish this almost-impossible task using reverse engineering.

Of all the challenges involved with machining gear sets, imparting the correct involute form on the pinion gear is the most daunting, and in machining, Mazak uses several proprietary techniques to minimize close-out error. In doing so, the company is able to determine the minimal amount of stock to leave on the pinion gear to correct for warping from the heat-treating process, yet also leave enough stock to fulfill the case-hardened depth requirement. Removing too much material during the finish-machining operation can compromise the gear’s case-hardened depth, thus affecting the gear’s operation and its working life.

While multi-tasking machines can generate consistently precise-matched large spiral bevel gear sets, the process is also dependent on a well-maintained machine tool. Worn and out-of-tolerance machines will be unable to drive consistently precise 5-axis toolpaths, which is key to accurate gear-set machining, as well as any other 5-axis machining application for that matter. Machines must be reliable and square, and deliver repeatable motion.

As the shortage of capacity and long delivery times for large bevel gear sets continue, those shops that lack the volumes to justify the purchase of a big gear cutting machine will look to multi-tasking machines as a viable option for producing the occasional gear set. With a standard, full 5-axis multi-tasking machine, shops can cost effectively process all of their larger, low-volume parts, including the occasional spiral bevel gear sets.

 

Mike Finn is process development engineer at Mazak Corp., a supplier of horizontal and vertical machining centers, including 5-axis and multi-tasking machines. Based in Florence, Ky., it offers the INTEGREX e-1550V/10 multi-tasking machining center with a two-pallet changer capable of large spiral bevel gear set production. The company also supplies its PALLETECH automated production cell system. For more, visit www.mazakusa.com

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