TheThe WHIRLINGWHIRLING PROCESSPROCESSfor for InternalInternal ThreadsThreads By Ralph Wehmann

With the advent of “internal whirling,” Leistritz has introduced an innovative new way to machine internal threads, thereby increasing efficiency, productivity, and quality.

20 GEAR SOLUTIONS • MARCH 2005 • gearsolutionsonline.com an industry leader in the field of whirling tech- nology, Leistritz has developed an innovative solution to difficult internal threads. The process, which is called “internal As whirling,” can be performed in both soft mate- rial using carbide tooling and in steel as hard as 65 hrc with CBN. In each case the process is performed dry, without coolants. The process was developed in response to pressure from the ball screw industry, where the process of choice has been grinding with internal thread grinding machines, which has always been the slow- est and most troublesome operation in producing a ball nut. Internal whirling eliminates thread grinding and eases the manufacture of all types of internal threads. Grinding has several inherent disadvantages. It requires very high cutting speeds, and with the small-diameter that’s needed for internal grinding, this translates into spindle speeds in excess of 30,000 rpm. Another problem when grinding involves coolant and the difficul- ties associated with distributing it to the exact interface between the grinding wheel and the workpiece. Very often, sophisticated coolant filtration and auxiliary pumping systems are required. The third drawback to grinding is the thread profile or form. Because the grinding wheel cannot be angled to the helix angle, it must be dressed with a compensated or narrowed contour to gener- ate an acceptable thread profile; in many cases with less than ideal results. The wheel is grinding askew to the thread profile, and a larger than necessary contact with the workpiece can cause high heat and burn the work, creating micro cracks. Also, the grinding wheel quill must be as long as the internal thread that needs to be ground. In the case of long threads—and especially those with small diameters—the wheel can be a long dis- tance from the spindle bearings. Subsequent deflection from the grinding forces requires using very conservative grinding parameters and, as a result, very long cycle times.

gearsolutionsonline.com • MARCH 2005 • GEAR SOLUTIONS 21 Internal whirling has none of these disadvan- tages. The spindle speed is normally under 2,000 rpm. Fast enough for CBN tooling, but still man- ageable and safe. Internal whirling is normally done dry. The ideal cutting conditions, com- bined with the optimal cutting edge preparation on the insert, create an environment in which nearly all of the heat is carried away by the chips. An air blast through the whirling spindle main- tains a temperature of both the and the work below 150°F. The tool oscillates axially in and out so that the insert profile is very nearly the same as the desired thread form. The Leistritz Internal Tool System uses a supporting with a bearing right up close to the cut, enabling very large length-to-diameter ratios. This system also pro- duces very accurate threads and excellent surface finishes, comparable to grinding. In most cases this internal whirling process eliminates a rough threading operation. Leistritz has developed its CBN technology to a state that the system can reliably and economically remove the full-hardened steel to reveal a precision, fin- ished thread in one pass. This is unheard of with grinding, which normally requires minimal stock removal to reduce the cycle time. Internal whirling

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22 GEAR SOLUTIONS • MARCH 2005 • gearsolutionsonline.com is always performed in a single pass, and the cycle time is the accuracies, and having the on top also prevents any same if removing full material or just simply finishing, so chip buildup inside the workpiece bore. most manufacturers choose to take full benefit of this tech- The machine has been designed for a maximum thread length nology by eliminating the roughing operation. This, however, of 200mm (8"), but longer lengths are possible. The total inter- requires that the nut blank be hardened to a depth at least as nal whirling system is capable of producing pitch great as the profile so that a sufficient hardness of the material threads. This means that a screw with a bore diameter of at the finished profile remains. Ø25mm and a pitch of 25mm is possible. The production time ACME, rope, ball, gothic, multiple start, or standard V savings can be huge. In many cases internal whirling will pro- threads are all ideal for this process. Interrupted cuts— duce a thread 10 times faster than grinding, and with compara- where a thread needs to be machined through a groove, ble quality. In some extreme cases the savings is even greater. For channel, or holes—are no problem and have no negative example, a Ø32mm thread with a 5mm pitch 80mm long needs effect on the tooling. Although internal whirling produces only three minutes to whirl in full, hardened steel. Another a thread in a single pass, it never removes more than the example of a Ø1.0" minor diameter with a .3125" pitch, 2" long calculated chip thickness, which is normally between can be whirled in about 60 seconds. 0.001-0.004". Yet another advantage to this system is the ease in integrating Leistritz’s Innovation 200 machine is an inverted vertical automatic loading. Since the chuck remains above the work, machine with the chuck on top. The “Z” axis longitudinal and the combined “X” and “Z” axes have expanded travels, the travel moves the chuck and its rotary “C” axis up and down. chuck can be used as its own gripper system to go to a known The internal whirling head remains stationary below. This position and pick up a blank workpiece. By adding a parts con- concept isolates any vibration caused by the oscillating mass- veyor that travels through the machine base, a simple “pick and es directly into the massive “minerlit” machine bed. Minerlit place” loading system using the machine axes is provided is an epoxy based polymer granite offering maximum ther- (please see accompanying photo). A sliding automatic door mal stability and superior vibration damping characteristics separates the loading and cutting areas. compared to cast iron. It is mixed as a viscous solution and To increase the machine utilization and guarantee the best then poured into a mold and cured. Locating surfaces and possible concentricity to critical diameters, Leistritz has added mounting holes are then machined for assembly. Hydrostatic , , and probing capabilities to the Innovation guideways for all axes guarantees the highest precision and 200. The probe can be used to find the true position of existing

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gearsolutionsonline.com • MARCH 2005 • GEAR SOLUTIONS 23 timing holes or surfaces, so the thread can be precisely timed for the start point, etc. Hard turning of the bore prior to whirling is REQUIREMENTS? suggested to improve the overall thread quality. Even the slightest amount of run-out can negatively effect the finished thread. Of • BUILDER OF NEW AND course, and OD turning is available and provided for. The REMANUFACTURED TURNKEY Innovation 200 is a fully capable, 75HP, CNC . Using a block BROACHING SYSTEMS tool holder system, fixed are mounted directly to the • EIGHT STANDARD LINES machine bed; thus a turret is eliminated, so chip-to-chip times are OF NEW MACHINES reduced while increasing rigidity for hard turning applications. • SPECIAL MACHINES The standard hydraulic, 10" chuck is mounted to a DIN 55026 FOR UNIQUE BROACHING spindle capable of 4,000 rpm. Additionally, a high speed, 12,000 APPLICATIONS rpm, 20HP milling spindle with HSK 63 spindle adapter is avail- • WORLD LARGEST INVENTORY able for auxiliary operations such as finishing of the ball return OF USED BROACHING MACHINES holes for the crossover mechanisms of ball nuts, etc. AND BROACH SHARPENERS • PARTS AND SERVICE FOR ALL TYPES OF C AXIS BROACHING MACHINES • BROACH CUTTING TOOLS AND PRODUCTION BMS Electro-Mate, electromechanical drive, table up internal broaching machine BROACHING SERVICES INTERNAL WHIRLING SPINDLE 25180 SEELY RD. NOVI, MI 48375 Z AXIS PH: (248) 471-4500 FAX: (248) 471-0745

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“LEISTRITZ HAS DEVELOPED ITS CBN TECHNOLOGY TO A STATE THAT THE SYSTEM CAN RELIABLY AND ECONOMICALLY REMOVE NEW HOFLER HELIX 400 CNC GEAR GRINDER WITH ONBOARD GEAR ANALYZER THE FULL-HARDENED STEEL • Precision Ground Spur or Helical Gears up to16 inches in diameter. Achieving and TO REVEAL A PRECISION, Exceeding AGMA Class 12 Quality • Prototype to Medium Production Quantities FINISHED THREAD IN • Holfer ZP350 Analytical Gear Analyzer ONE PASS.” to Insure Quality As the tool rotates at cutting speed (approx. 540 sfm), it also oscillates axially. The amount of oscillation is calculated from the thread parameters and tooling circle diameter. The tool circle diameter is approximately 75 percent of the bore diameter of the workpiece. This means that, if the tool was exactly on center, it wouldn’t cut at all. So the “X” axis is used to control the cutting depth and to compensate for taper control. Higher lead threads 4884 STENSTROM ROAD • ROCKFORD, IL 61109 PHONE: 815-874-3948 • FAX: 815-874-3817 require larger oscillation amounts. The oscillation continually www.raycargear.com maintains the cutting insert “normal” to the helix angle. This

24 GEAR SOLUTIONS • MARCH 2005 • gearsolutionsonline.com cutting action maintains the most true profile and ideal cutting characteristics. When the tool rotates far enough so that it’s no Call us NOW for your needs! longer in the cut, it oscillates back to the beginning while contin- WE SPECIALIZE IN uing to rotate. While this continues for each spindle rotation, the WE SPECIALIZE IN Barber-Colman Gear Cutting workpiece (“C” axis) is slowly rotated in the same direction as & Hob Sharpening Equipment the tool and the “Z” axis of the machine is advanced at lead until Used, Reconditioned or Rebuilt the desired thread length is achieved. The rotational rate at which New and Used Parts for Sundstrand the work turns determines the chip thickness removed by each • Mills • Omnimills pass of the insert. At this point the spindle is stopped, the “X” Fellows #6 & #7 Parts • Barber-Colman axis is retracted, and the entire tool is withdrawn. Normally a OEM completed thread is achieved in a single whirling pass. Atlantic Van Norman Mills • Atlantic Borer

COLOR KEY: SUPPORT MANDREL WHIRLING SHAFT CLAMPING PLATE WHIRLING INSERT Hob & Shaper Cutter Resharpening Service We Service Barber-Colman Hobbing & Hob Sharpening Machinery, Sunstrand Mills, Lathes and Omnimills Atlantic Van Norman & Atlantic Jig Borer Repair Parts, Inc. 2415 Kishwaukee St. • Rockford, IL 61104 Changeovers are very fast, simply selecting a program in the 815-968-4499 CNC, if the profile remains the same and the diameter is similar. Fax 815-968-4694 The oscillation amount is programmed directly into the CNC [email protected] • www.repair-parts-inc.com part program as a “W” axis value. In the accompanying diagram, the support mandrel does not rotate. It has a supporting bearing very near the cutting forces. The whirling shaft is driven from inside the main internal whirling head (not shown) and rotates at a maximum speed of 2,285 rpm. This whirling shaft also oscil- lates axially with each revolution. There are air blast lines inte- grated into the support mandrel to blow chips away and cool the tool, and the insert is rigidly held in place with a clamping plate. These components can be quickly and easily changed from one workpiece to another. The workpiece and application determine which, if any, components need to be changed. The entire concept has recently premiered, but it has already proven itself in Europe with several ball screw manufacturers. In another installation an Innovation 200 is producing an auto- motive component with a six-start thread used in the suspen- sion of an SUV to control automatic vehicle leveling. This tier 1 supplier is using the machine to do both hard turning and hard internal whirling to produce a finished nut that is assembled with the mating screw—though hard whirled in a different machine—to deliver more than 200,000 units annually.

ABOUT THE AUTHOR: Ralph Wehmann is national sales manager for Leistritz—a builder offering internal and external whirling solutions, rolling and burnishing equipment, and hydraulic keyseating machines. The company is based in Nuernberg, Germany, and its products are sold, supported with spare parts, and serviced by Leistritz Corporation of Allendale, New Jersey. Wehmann can be reached at (201) 934-8262 ext. 14 or via e-mail at [email protected]. The company’s Web site is [www.leistritzcorp.com].

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