Thread Whirling--One Of The Oldest New Products In Town

There's not much new about whirling. It's been used in Europe to machine helical parts for more than 50 years. It's time-tested, and it's proven to machine screws and thread forms at speeds often as much as four times faster than traditional thread milling or hobbing, and typically without the need for secondary finishing operations. But that's in Europe.

What's new is whirling's recent and growing acceptance in the United States. It has become widely used in the manufacturing of "large" parts, such as steering gears, worm gears, acme threads, ballscrews and other threaded parts. To make these, however, has required installation of large, costly and dedicated whirling machines.

Now there's an alternative. It's a new whirling attachment that's designed specifically for use on Citizen-Cincom Swiss turning centers. This new tool brings the advantages of whirling to the production of "small" parts, such as bone screws and threaded surgical implants. It also introduces this new application to a non-dedicated machine tool, while preserving the machine's multi-axis turning, milling and drilling versatility.

According to John Antignani, vice president of sales and marketing for Marubeni Citizen-Cincom (Allendale, New Jersey), whirling threaded parts solves problems that have long plagued bone screw manufacturers. "Bone screws are machined from hard metals, typically 316L stainless steel or titanium. They're characterized by high length to diameter ratios, by deep helix buttress thread forms, and by wide differences between root and head diameters. This combination of part length, material hardness and depth of cut mandates either numerous machining passes or high cutting forces. The first of these lengthens cycle times; the second introduces the risk of bowing or deflection.

"Traditional threading operations have limitations," Mr. Antignani continues. "Single point threading works well on shorter screws. But for longer screws, pre-turning and special supports are needed. Die heads prove suitable for 316L, but not for titanium. Thread milling involves pre-turning, special cutters and special supports. Thread grinding serves almost exclusively as a secondary finishing operation following traditional threading operations. In addition to the special tooling, supports and frequent need for secondary operations, a number of passes are needed to keep the chip load low to prevent deflection."

By combining the speed of whirling with the rigidity afforded by a sliding-headstock, Swiss turning center, helical products can be machined precisely, and in a single pass, without the need for special supports.

Citizen's whirling attachment consists of two components: a cutting head that contains the cutting inserts, and a whirling head that mounts the same way as a cross drill in a live tool station. Three circular form tools are mounted along the inner diameter of the toroid-shaped cutting head. The tools are mounted at the height and rake required for the threaded part. Once the tools are installed, the cutting head is secured in the whirling head and rotated to the appropriate helix angle. The entire assembly is then mounted on the Swiss turning center, using any of the live positions on the machine's tool turret or tool block.

In operation, the ring rotates at speeds up to 3,000 rpm around a slowly rotating (approximately 10 to 20 rpm) workpiece. The three tools rotating at 3,000 rpm are equivalent to a single tool operating at 9,000 rpm—eight or nine times faster than typical for single point threading of deep buttress threads. This allows a smaller chip load, minimizing the risk of deflection, while significantly reducing cycle times. High precision parts, such as bone screws and other threaded surgical implants can be produced from rough stock at high speeds in a single pass.

Mounting the whirling attachment on a sliding headstock turning center eliminates the risk of deflection. On a sliding headstock machine, all operations are performed within an eighth of an inch from the machine's guide bushing. With the distance between the bushing and tool minimized, and with whirling's minimal chip load, pressure on the workpiece is minimized, eliminating the risk of bowing and deflection.

At present, nine whirling attachment packages are available for use with Cincom model C, E, L or M series Swiss turning centers. They are suited for helical parts up to ½ inch diameter, with the length limited only by the stroke of the machine. Maximum depth of cut is 0.18 inch on diameter, and typical feed rates range from 1 inch to 2 inches per minute.