Gear-driven machining

Sept. 14, 2009
Cutting spiral bevel gears on 5-axis machining centers gains momentum.
More shops doing low volumes of spiral bevel gears are looking into cutting them on 5-axis machining centers, such as this Mitsui Seiki.
DMG developed special software specifically for cutting spiral bevel gears on its 5-axis machining centers.

Growing markets such as wind energy, hydroelectric power and mining are driving the popularity of cutting low to moderate volumes of spiral bevel gears on 5-axis machining centers, as opposed to dedicated gear generating equipment. In addition to those markets, shops manufacturing gear cases and doing prototype work also are showing interest in 5-axis machines for spiral bevel gear cutting.

Probably the most significant benefit of gear cutting on a 5-axis machine is that shops can, on one machine and one setup, turn gear blanks, finish cut bores, finish cut face profiles, add bolt holes and generate gear teeth profiles. In some instances, they may hard mill them after heat treating, too. Then, once the job is done, they can use the machine to produce completely different parts.

Traditional gear-cutting machines may run faster. However, the tooling economics, increased accuracy through one-setup machining and improved process control, make the overall costs of 5-axis machines more advantageous, especially for low-volume runs, explained Mark Speier, a sales engineer at Mitsui Seiki USA Inc.

“Depending on the size of the gears, the ideal job volumes for 5-axis machine gear cutting can be as little as one or two pieces to as many as seven or eight. If the gears are fairly large, the cycle times, even for a dedicated gearcutting machine, can run long. A gear that takes about an hour to machine on a gear generating machine, may take seven or eight hours on a 5-axis machine using a single end mill. But, setup on a dedicated gear-cutting machine is three or four times longer as compared with that of a 5-axis machine,” said Speier.

In addition, he mentioned that a 5-axis machine eliminates manual benchwork, such as deburring gear teeth ends, because such operations can be programmed into the cutting program and done right on the machine tool.

For shops already machining gearbox cases, cutting low lot sizes of gears on a 5-axis machine is a natural transition and a chance to gain new business opportunities. In fact, one of Mitsui Seiki’s customers did just that and now machines gears measuring up to 35 in. on a Mitsui Seiki HU80A- 5X 5-axis machining center.

The benefit of producing gears on that machine is that the relative gearboxes or gear cases, and even reduction gearboxes, can be processed on it as well. The gear housing is a critically precise piece, and the accuracy of both parts directly correlates to the transmission performance over the life of a motor, and product reliability as well.

According to Speier, to machine spiral bevel gear teeth profiles, a machine obviously will need full, five-axis contouring capabilities, first and foremost. But, it must have properly synchronized drives for synchronized contouring, too. Mitsui Seiki ensures this provision by building a mechanically square trunion-style 5-axis machine with smooth A-axis trunion movement, which is square to the machine’s B axis, and square linear axes.

“The higher the quality of gear tooth profile required, the tighter a machine’s movement must be. Even in a gear’s green state, a consistent amount of stock removal must be maintained on all sides of teeth. If consistency isn’t maintained, problems may arise during the finish cutting operation,” Speier said.

Besides a competent machine tool, he added that shops should be “gear knowledgeable” and have the infrastructure for quality control and inspection. And, like the machine tool itself, the higher the grade of gear, the higher will be the grade of inspection equipment needed to check them.

Software is another crticial aspect to successful spiral bevel gear cutting on 5-axis machining centers. In particular, shops will need a program that generates optimum toolpaths and correct tooth profiles.

In the past, a lot of design software for generating gears was not quite amendable to CAM software, even for straight spur gears. Speier said that the tooth profiles were a little off, and with spiral bevel gears, there would be inconsistencies from tooth to tooth. This is why Mitsui Seiki partnered with CNC Software Inc., the developer of Mastercam, for gear design software that generates correct tooth profiles and is a more suited to CAM.

CAD software has always allowed for designing gears that were difficult to machine. So, shops would program machines manually from the CAD data, writing G-code by hand. The development of CAM lets toolpath motion be based on gear design models.

However, the CAD model data translation process is where tolerance errors can be introduced. If the model goes through multiple translators, the result could be corrupt data or gaps between surfaces.

“The best CAD/CAM system is one that can design and machine because some CAD systems can’t machine, and some CAM systems can’t design. When it comes to cutting gears on 5-axis machines, shops need to stay as close as possible to the design file without going through a bunch of translators,” said Karlo Apro, product manager for multiaxis at CNC Software.

CAM software, according to Apro, must control cut patterns, tool axes, and tool tips for spiral bevel gear cutting. An example of controlling the cut pattern would be having the tool follow the spline of the gear or maintaining a climb cutting orientation.

For tool axis control, the software would keep the cutting tool normal to gear surfaces to avoid collisions; in tool tip control, it would keep tool tips – in the case of ballnose cutters – in an optimum cutting position to the gear surface, for cleanly cut surface finishes.

Apro added that spiral bevel gears also can be machined on different configurations of 5-axis machines, and shops have to take that into consideration when generating code for the machine. No matter what the configuration – table/table, table/head or head/head – machine limitations of each come into play when generating toolpaths.

Mastercam software also performs simulations, which are extremely important in gear cutting on a 5-axis machine. Simulations not only determine potential collisions, but also present the most efficient machine motion.

“On 5-axis machines, it is very easy to hit machine travel limits. With full machine simulation, shops can identify where this will occur and test out different cutting strategies, all without touching an actual part or leaving their desks,” said Apro.

In addition, his company is working now to make gear-cutting toolpath generation even easier, via visual-type prompts and an increased amount of automatic functions specific to gear cutting, so that shops don’t have to learn all the intricacies involved.

DMG is another company that has developed software specific to cutting spiral bevel gears on 5-axis machining centers. The machine tool company has also experienced strong interest in 5-axis gear cutting from customers, especially those in wind power, hydropower and mining markets.

At a recent company open house, Bob Medler, product manager in the milling group at DMG, discussed how shops that are not using 5-axis machines to cut spiral bevel gears have to input what is known as Klingenberg data, and purchase extremely expensive machine tools.

He said that shops could spend up to $1 million or more for these dedicated gear machines, just to rough out gears shapes. Then after gears are heat treated, another just-as-expensive machine is required for grinding them.

“The hurdle in cutting spiral bevel gears on 5-axis machining centers is that no one had known how to transform Klingenberg data into a spiral bevel gear. DMG’s new patented software can create gears from that data,” said Medler.

The software allows shops to turn, mill and even hard mill gears on one DMG 5-axis machining center. Complete gear machining at a fraction of the usual cost, said Medler.

During its open house, DMG cut spiral bevel gears on one of its DMC 60U duoBlock machines. The 5-axis vertical machining center delivers short chip-to-chip times and extreme precision. A combination of a fast chain magazine, a rotary pallet changer, NC rotary table and NC-controlled swivel milling head as a B axis contribute to the machine’s high level of productivity for gear cutting.