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END USER: Mercury Marine, (920) 929-5000, www.mercurymarine.com. CHALLENGE: To increase productivity when machining driveshafts and propeller shafts. SOLUTION: A lathe with a hob assembly. SOLUTION PROVIDERS: Okuma America Corp., (704) 588-7000, www.okuma.com
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With more than 70 years in the industry, Mercury Marine has maintained its leading market position with constant innovation, foresight and the latest technology. These characteristics are shown throughout 220,000-sq.-ft. Plant 4 in Fond du Lac, Wis., where Mercury Machine produces high volumes of driveshafts and propeller shafts for outboard and stern-drive marine propulsion systems. The plant has about 220 employees and contains 15 to 20 Okuma lathes of various types and vintages.
Mercury turns the driveshaft blanks on a twin-spindle LT10 Okuma lathe, without live tooling. The machined shafts are then manually transferred to a Gleason P60 hobbing machine to cut the splines.
Mercury Marine’s Okuma LB3000 EX BB MYW800 is equipped with a Wes-Tech overhead gantry for loading and unloading shafts. Image courtesy Okuma America.
Recently, Mercury added a LB3000 EX BB MYW800 from Okuma America Corp., Charlotte, N.C., which has live tools, a Y-axis with 5" of travel, a W-axis subspindle and a hob assembly from WTO Inc., Charlotte. The new machine produces a complete driveshaft in 3 to 4 minutes. It is equipped with a Wes-Tech overhead gantry for loading and unloading shafts.
A close-up view of the hobbed pinion spline. It is a ¾“ spline that rises to a taper, at which point the pinion gear is mounted. Image courtesy Okuma America.
“In the main spindle, we start with a blank that has been carburized,” said Kurt Lefeber, a Mercury process engineer. “We turn some of that carburizing off, giving greater definition to the part shape. We rough and finish one end and then transfer the shaft to the subspindle, where it’s roughed, finished, center-drilled and hobbed.”
Okuma has a unique method of transferring a shaft from the main spindle to the subspindle. There’s a pneumatic pusher system inside the main spindle, so when the gantry places a raw piece into the spindle, the piece goes to a backstop. The backstop has an air cylinder with a piston. When the first operation is finished, an M code in the control fires the air cylinder, which pushes the shaft out of the main spindle and transfers it to the subspindle.
“The spline I’m hobbing is only about ¾" long,” Lefeber continued. “But we’re hobbing into a taper. The spline follows a diameter for 0.400" and then goes into a taper, which is another 0.350".”
Mercury purchased the LB3000 EX BB MYW800 for added flexibility and as backup for the cell. Now one operator runs the LT10/P60 machine and the new lathe. “I’m getting an additional 75 percent output in the cell without an increase in manpower,” Lefeber said.
Machining the propeller shafts used to take multiple steps. The parts were turned on a Cincinnati lathe, taken to a labor-intensive WWII-vintage Barber Coleman hobbing machine to cut the spline and then ground. The turning operation alone took 3 minutes.
Now Mercury Machine is turning and hobbing the shafts on one machine in 3 minutes, 40 seconds using the Okuma LB3000 EX BB MYW1000 equipped with a hob assembly. Instead of using the two older machines, Mercury achieves similar throughput with one machine performing the operations. The new machine improves part quality, requires less part handling and frees up the operator.
A propeller shaft is hobbed by the WTO hobbing unit. Image courtesy Okuma America.
“We’re turning the shaft and then cutting the splines that drive the propeller,” said Bill Cusick, a Mercury process engineer. “The propeller shafts are machined from 4820 or 8620 steel, and 630 stainless for corrosion resistance. Basically, the shaft has an alloy steel section friction-welded to a stainless steel section. The stainless is in bar stock condition, and we turn the shaft down, starting with the stainless, crossing over the weld and onto the alloy steel section. Then we hob a spline that’s about 2⅛" long into the stainless section.”
Okuma has made significant progress in developing hobbing in a turning center. One unique function is Autoretract. If there’s a problem with the hob, the operator pushes the stop button and Autoretract retracts the hob out of the cut without damaging the hob or the splines it is cutting.
Okuma also developed a programming tool, called spindle-speed variation, to eliminate chatter. The tool is useful when machining propeller shafts. “These are smaller shafts, 1" to 1¼" in diameter, and fairly long,” Cusick said. “If you’re turning the center of this shaft across the friction weld between the chuck and the tailstock, you can get some chatter. But, for example, if you’re turning at 1,000 rpm, you can use the spindle-speed variation to increase or decrease the speed by 20 rpm for a tenth of a second. The result is a winding sine wave-type action. This takes the chatter completely out.”
As for the lathe itself, “it has the rigidity to do the hobbing work,” Lefeber said. “And that goes for the WTO unit as well—it has to be able to hold the hob cutter. Previous styles of holders we used held the hob on one end and the cutting end was not supported. WTO allows support on both ends of the hob. Now cutter life and spline quality are better.”
Related Glossary Terms
- carburizing
carburizing
Absorption and diffusion of carbon into solid ferrous alloys by heating, to a temperature above the transformation range, in contact with a suitable carbonaceous material. A form of casehardening that produces a carbon gradient extending inward from the surface, enabling the surface layer to be hardened either by quenching directly from the carburizing temperature or by cooling to room temperature, then reaustenitizing and quenching.
- chatter
chatter
Condition of vibration involving the machine, workpiece and cutting tool. Once this condition arises, it is often self-sustaining until the problem is corrected. Chatter can be identified when lines or grooves appear at regular intervals in the workpiece. These lines or grooves are caused by the teeth of the cutter as they vibrate in and out of the workpiece and their spacing depends on the frequency of vibration.
- chuck
chuck
Workholding device that affixes to a mill, lathe or drill-press spindle. It holds a tool or workpiece by one end, allowing it to be rotated. May also be fitted to the machine table to hold a workpiece. Two or more adjustable jaws actually hold the tool or part. May be actuated manually, pneumatically, hydraulically or electrically. See collet.
- corrosion resistance
corrosion resistance
Ability of an alloy or material to withstand rust and corrosion. These are properties fostered by nickel and chromium in alloys such as stainless steel.
- lathe
lathe
Turning machine capable of sawing, milling, grinding, gear-cutting, drilling, reaming, boring, threading, facing, chamfering, grooving, knurling, spinning, parting, necking, taper-cutting, and cam- and eccentric-cutting, as well as step- and straight-turning. Comes in a variety of forms, ranging from manual to semiautomatic to fully automatic, with major types being engine lathes, turning and contouring lathes, turret lathes and numerical-control lathes. The engine lathe consists of a headstock and spindle, tailstock, bed, carriage (complete with apron) and cross slides. Features include gear- (speed) and feed-selector levers, toolpost, compound rest, lead screw and reversing lead screw, threading dial and rapid-traverse lever. Special lathe types include through-the-spindle, camshaft and crankshaft, brake drum and rotor, spinning and gun-barrel machines. Toolroom and bench lathes are used for precision work; the former for tool-and-die work and similar tasks, the latter for small workpieces (instruments, watches), normally without a power feed. Models are typically designated according to their “swing,” or the largest-diameter workpiece that can be rotated; bed length, or the distance between centers; and horsepower generated. See turning machine.
- turning
turning
Workpiece is held in a chuck, mounted on a face plate or secured between centers and rotated while a cutting tool, normally a single-point tool, is fed into it along its periphery or across its end or face. Takes the form of straight turning (cutting along the periphery of the workpiece); taper turning (creating a taper); step turning (turning different-size diameters on the same work); chamfering (beveling an edge or shoulder); facing (cutting on an end); turning threads (usually external but can be internal); roughing (high-volume metal removal); and finishing (final light cuts). Performed on lathes, turning centers, chucking machines, automatic screw machines and similar machines.
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