A turn of the screw: medical fastener production with thread whirling.

The small parts business aspect of human body repair is booming. The surgical repair of bone fractures of all types, plus a panoply of replaceable hip, arm and leg joints, spinal repairs, dental bridges, implants and posts--all have increased the need for a multitude of specialized fasteners.

Every bone screw manufacturer develops a custom thread for its screws. The reason is that the human bone is not homogenous, it consists of a hard outer shell and a softer, sponge-like core. The self-tapping bone screw must be hard enough and sharp enough to penetrate the outer shell, and its thread form must be able to resist pulling out of the bone's softer core. The FDA provides guidelines for bone screw thread design; however, each manufacturer designs its threads for what it feels will provide the best all-around performance.

Take another look at the bolts sticking out from Boris Karloff's neck in "Frankenstein"--obviously a poor fit because of crude machine shop equipment. Medical screws have traditionally been problematical to produce en masse due to unusual length-to-diameter ratios--size variations can range from [less than or equal to] 1 mm dia x 3 mm in length, to over 8 mm dia x 80 mm in length. To ensure complete biological compatibility and prevent any risk of rejection, these parts are made from materials such as stainless steel recast under vacuum, or titanium. However, such materials are difficult to machine. Many types of the mid to large screws also have very long through holes, leading to thin wall sections and unique thread geometry.

In past, bone screws were machined with a single threading cutter requiring numerous thread passes, but suffered from poor finish quality and tool life problems. These problems often require corrective secondary operations such as tumbling, polishing, and grinding. Because of increased demand, the job facing medical parts manufacturers is how to select the most efficient metalworking techniques from numerous methods.

The thread whirling alternative

Swiss turning machines, which have proven themselves by producing high-precision parts for the automotive, electronics connector, and the general mechanical engineering field, first earned their reputation for automatic production of small precision parts for the Swiss watch industry (hence the name). These machines have evolved over many years, and can now produce small diameter (but often very long) parts, including all normal secondary operations, in a single pass. Milling, drilling, tapping, hobbing, polygon turning, and thread whirling are all possible on today's Swiss-type machines.

Thread whirling has become popular for efficiently cutting quality threads on bone screws. One source estimates that about 60% of all threads for dental and medical devices are currently whirled.

According to Steve Rose, president of RTSI, Solon, OH and a teacher of professional shop practices, "Thread whirling is essentially a threading process in which a hollow cutter, part of a machine-mounted attachment, rotates around the outside diameter of a part. As the process takes place, the part rotates slowly while the cutter rotates (or 'orbits') around the part at a high spindle speed--both part and cutter rotating in the same spindle direction.

As opposed to thread cutting and tapping, thread whirling produces clean contours. There is no part deformation, as cutting forces are low and no burrs are produced. Chips are no problem as only fine particles are produced. Continues Rose, "The process of thread whirling is comparable to thread milling or gear hobbing--either of which tend to produce a faceted surface finish due to the interrupted cut of the tool contacting the workpiece. The orbital cutter path of thread whirling produces an are contact between the cutter and the part, resulting in a superior surface finish and fast cycle time. The cutter (or the part) is advanced in the Z-axis at the thread pitch--the direction of Z-axis movement determines either left or right hand of the thread.

"CNC Swiss-type machines are well applied to this technique because the moving headstock feed can be programmed to synchronize with the thread-whirling attachment. On most bone screws, the process involves first whirling the thread on the main spindle, then handing the part off to the subspindle to perform backworking operations to cut the screw head and driver connection."

An external thread-whirling tool is basically a hollow, ring-type cutter with multiple inserts mounted on the ID. Typically, the part is rotated at 15 to 20 rpm, while the cutter speed may be 5000 to 20,000 rpm. With the addition of driven attachments and aftermarket electric spindles with up to 50,000 rpm, internal threads can be produced down to very small internal diameters. Internal thread whirling has the advantage of a net increase in productivity and precision, when compared to the use of a conventional tapping tool. Internal and external thread whirling can carry on simultaneously on many machines machine, producing complete thread inlets and outlets with no deteriorating threads in only one pass.

The whirling head

Almost every machine that can perform thread whirling uses an add-on head. Such interchangeability simplifies the changing of cutters for a different task. Says one shop manager, "Most of our operators keep a spare head at the machine and simply swap heads to minimize downtime. That's the most efficient method, since it avoids idling the machine for the amount of time it takes the operator to index or replace the [three] inserts. The machine is back into production quickly, and the operator can index or replace the inserts on the head that was removed while the machine is making parts."

An operator might also switch out units for different motor capacity. Many thread whirling heads have an adjoining motor, mounted parallel or perpendicular to the cutting axis, which drives the cutter via a gear train. There are also machines in which the thread whirling attachment is installed directly on the machine's "live" tool turret head, an assemblage of different tools selected by the machine on cue from the machine controller. A motor in the turret powers the cutter is powered, and any other tools that entail independent motion.

An improved tool head

Rem Sales Inc. of East Granby, CT, is the North American importer of Tsugami Swiss-machining tools, in studying the efficiencies of thread whirling, the staff engineers found that there were some negative factors in the tooling and methods currently in common use. The list of problems included: an expensive cutter body with a limited number of sources; expensive insert technology with limited sources; longer than acceptable set-up times; the need for precise setting and inspection of inserts off the machine; the need for extended nose guide bushing adapters; extreme expense of owning a variety of cutter bodies with differing swept diameters for small to large work; and cutter rpm limitations on most competitive products.

The solution was to design a cutter to use standard, off-the-shelf components in a wide variety of cutting material grades. The cutter body can be purchased or manufactured locally at a small fraction of the cost of the traditional body. The cutter inserts are available from any branded source locally, at a fraction of the cost of traditional specialty inserts. The insert pockets (where the inserts are installed in the cutter body) are designed so that no pre-setting is required further, the inserts are presented to the work very close to the guide bushing, eliminating the need for expensive and unique guide bushings.

Since the whirling cutter speed is the determining factor in calculating the part rotational speed and the resulting part finish and cycle time, the cutter was modified so that a part with a threaded length of 10 threads that would take 1 minute to produce on another machine, could be completed on the Tsugami machine in 20 seconds.

In the Tsugami line of Swiss-type sliding headstock machines, there are 11 models suited to producing medical products--23 if the tally includes aerospace, hydraulics-pneumatics, and information technology products. Parts can be produced from [less than] 1 up through 38 mm dia. Parts are commonly made exceeding 20 in. in overall length.

It is enough to keep even Frankenstein's head firmly in place.

Circle 130--Rem Sales Inc., or connect directly at www.rsleads.com/509df-130