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Bone Screws
Replace Your Screw Machine Whirling with Leistritz Whirling
For those looking for a faster, more-economical method of producing Bone Screws, whirling as a separate operation -is just what you're looking for.

By Ralph Wehmann

Over the past several years we have seen the whirling process come of age, so to speak. This very efficient and fast method of producing threads has recently been integrated into swiss type screw machines. This combining of operations goes against the "lean" ideals of high productivity by performing as many operations in a single machine. It is the philosophy of Leistritz, arguably the most renown and experienced machine builder in the field of whirling, that separating operations is more productive! The very features that make swiss-type turning centers unbeatable for turning small diameter and long shafts, screws or spindles are exactly what make them less efficient whirling machines. Feeding of the bar material through a guide collet has the disadvantage of not permitting continuous rough and finish turning over a long length, so whirling a long screw requires incremental turning and whirling steps until the whole screw length is achieved or whirling the screw without turning the blank to the thread major diameter first! Both of these alternatives are not only time consuming but inaccurate as well.

The Leistritz concept is to turn bone screws blanks in existing swiss-type turning centers without the thread. This would allow an even shorter cycle time and greater throughput per machine. Then, with a whirling only machine, whirl the thread. Leistritz whirling machines have the necessary following supports to permit whirling of screws with any profile in virtually unlimited lengths in the shortest possible time. Whirling by Leistritz can outperform whirling in a turning center by a factor of 2 or possible 3:1! Most Bone Screws even in their longest designs and in either 316L stainless steel or Titanium are whirled in under 60 seconds!

The Leistritz LWN-65MED machine has a very unique automatic parts loader and hopper system. The pre-turned blanks with either finished or unfinished heads are dumped into the vibratory hopper which aligns the screw blanks to be oriented tip first and funneled to the staging area. Once "on deck" for the loading process, each screw is measured for length with a light beam and the proper CNC program is selected to run the appropriate screw length. This is a valuable tool since nearly all bone screws are produced in families of the same diameter. Typical Bone Screws of the same family vary only in length from 10 to a maximum of 100mm in 5mm increments. With the Leistritz LWN-65MED machine all screws within the same family can be produced simultaneously without any set-up.

This processing also allows cell integration of a number of turning centers to each whirling machine. Typically 4 or possibly 5 turning centers produce the blanks (of various lengths) for a single LWN-65MED whirling machine.

Leistritz-a machine tool builder based in N├╝rnberg, Germany, with a branch office in Allendale, New Jersey-has developed a complete line of multi-axis CNC whirling machines. These machines have been designed specifically for whirling and have the inherent rigidity for cutting deep, wide profiles in tough materials and the necessary accuracy for cutting precision, close-tolerance screws. These machines are ideally suited for production environments where there is minimal time to do routine maintenance.

Medical screw manufacturers throughout the country and in Europe are fast discovering this better way to process their production. When production volumes increase to the point where more than one turning center is required, this separation of operations saves even more by maximizing the utilization of all machines in a cell. The low tooling costs combined with short cycle times and mostly no set-up time really proves beneficial to their bottom line.