Many shops increasingly incorporate full 5-axis multi-tasking machine tool technology because it allows them to handle any kind of part that comes through the door, with one type of machine, in single setups to increase productivity. But rarely, if ever, do these shops realize that large spiral bevel gear sets are also among that wide mix of part-processing capabilities.
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 rare, 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.
Additionally, big gear sets, those involving ring gear pitch diameters over 3.5’, 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. (Figure 1)
The model with the proper contact pattern is then loaded into a CAM system that allows the gears to be programmed the same as any other 5-axis part. And from this point forward, the machine tool simply cuts to the model. Therefore, 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 sets, a shop can use its multi-tasking machine to cut all of its other parts—including the gearboxes 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”-diameter pinion gear and a 114-tooth, 6’-diameter ring gear on a Mazak INTEGREX e-1550V/10 Multi-Tasking 5-axis Machining Center. This gear set was completed in days, as opposed to months, using only one machine.
The INTEGREX e-1550V/10 is a standard model with a two-pallet changer. The machine performs all required machining processes such as milling, turning, boring drilling, and tapping. Its tilting spindle and turning table allows for machining at any angle or cross cutting position as well as for contouring operations. The machine’s Mazatrol Matrix 2 CNC Control provides 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. All tools were standard, off-the-shelf, and readily available.
As a standard model, the INTEGREX e-1550V/10 provides the power and torque needed to machine the gear sets. The machine has a 10,000-rpm 50-hp spindle with B-axis travel of -30 to +120 degrees. The machine’s 50-hp, 300-rpm turning spindle rotates 360 degrees in the C-axis at 0.0001-degree minimum increments.
The machine’s round 55.1”-diameter face plate pallets with four jaws support maximum loads of 11,000 lbs. The pallets accommodate workpieces measuring up to 78.7” in diameter and 52.9” in height.
For minimizing non-cut time, the machine rapid traverses at a maximum speed of 1,653 ipm in the X, Y, and Z axes. A standard 40-tool magazine provides ample storage to support continuous machine operations. (Figure 2)
Pinion gears for the sets are machined from solid cylindrical pieces of 8620 steel, while the ring gears are machined from forged 4340 carbon steel rings that have about 0.250” of additional stock for machining, but lack any near-net teeth shapes. Both gears are first rough machined in their non-hardened states, then pinion teeth are carburized to 62 Rc, ring gear teeth induction hardened to 55 Rc and both are finish machined in hard milling operations on the Mazak machine.
For this particular gear set’s machining operations, a pinion gear is fixtured on one of the multi-tasking machine’s two pallets and a ring gear on the other. While one gear is being machined, a machinist sets 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 moves out of the work envelope, and the ring gear on the second pallet moves in for its rough machining. A shop would then send out both rough-machined gears for their hardening processes.
With its teeth hardened, the pinion gear is re-located back onto the pallet fixturing, and the machine finish cuts the gear’s bore as well as hardmills the flanks and edge radii of its teeth to eliminate any secondary deburring operations. All the operations are done in the same setup to ensure the gear’s features run true with one another. Likewise, the ring gear is re-fixtured and moves into the machine where its bore is finish cut and all of its gear teeth, including radii, are hardmilled to finish size. (Figure 3)
If the gears had been produced using traditional methods, shops would 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 this happens, shops will make a whole new set, even though the ring gear may have additional working life left. They produce a new set because the odds of generating another pinion gear that will roll perfectly matched 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, 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.
