Threading, Three Ways
- May 24, 2019
Time-saving alternatives to traditional threading techniques
No machining process is perfect. Single-pointing is slow. Tapping’s inflexible. Thread mills are expensive, and to newcomers at least, can be challenging to program. Despite their perceived shortcomings, however, these established thread-making processes continue to trump alternative, oftentimes far more efficient ways to produce screw threads.
This could be due to a lack of understanding. It might be because there’s no time to explore what’s available. Or it could be a simple case of that’s the way we’ve always done it, so why change? Whatever the reason, you owe it to your shop’s profit margins to explore different manufacturing technologies, especially if it means improving one of the most common of all machining processes: threading.
One of these is thread whirling. Sandvik Coromant milling products and application specialist Brian MacNeil describes it as a “very fast threading process,” albeit one that’s largely limited to medical parts. It is almost exclusively performed on Swiss-style CNC lathes, although there’s no real reason (barring the unavailability of machine-specific attachments) why it couldn’t be done on a multitasking lathe or even a mill-turn machine.
The process is similar to thread milling, except it cuts external threads rather than internal ones, and is used to produce components with long length-to-diameter ratios such as bone screws and cannulas. Whirling uses a ring-shaped cutter body containing six or more full-profile inserts, mounted circumferentially around the ring’s inner diameter. The body itself is mounted in a special rotating adapter, which is tipped to the helix angle of the thread and positioned in the lathe’s X axis to achieve the correct root diameter.
While the workpiece rotates at a relatively slow speed—say 10 to 20 rpm—the cutter rotates around it in the same direction, spinning at whatever rpm is needed to achieve a suitable surface speed for the material. The workpiece is then fed into the ring along the Z axis (longitudinally), cutting the complete thread in a single pass.
“Our CoroMill 325 has proven very effective in titanium, cobalt chrome, and other difficult materials,” MacNeil says. “It can be used for single or multi-start threads, and is available with a range of standard bone screw inserts as well as specials. Chip control is excellent, as is tool life and part quality, making thread whirling the first choice for medical manufacturers that specialize in threaded orthopedic components.”
Edwin Tonne, training and technical specialist at Horn USA Inc., agrees. He says the company’s new line of Jet-Whirling heads solves the problem facing many would-be whirlers: insufficient coolant flow. “Jet-Whirling was developed through collaboration with our partner company W&F Werkzeugtechnik,” he says. “It is the first modular whirling system to apply coolant directly to the insert edge, improving tool life and part surface finish while doing a much better job of clearing the chips.”
Horn has also recently introduced its High-Speed Whirling system, which uses a front-mounted insert to simultaneously rough and finish turn the workpiece while whirling. “Both of these systems are modular, so where a medical company might have a family of parts, they can simply change out the module on the front of the whirling unit, tweak the program, and go,” says Tonne. “It reduces setup time and cuts down on the hassle.”
Another way to reduce hassle is with thread rolling. Like whirling, rolling has been around for many decades. It is a chipless cold-forming process that basically “squeezes” external threads to the desired shape within seconds, not unlike a roll-form tapping operation. And as with form tapping, a wide variety of materials can be rolled to produce very high quality and mechanically superior threads.
Tilo Knobelsdorff, manager of LMT Tools USA’s Center of Competence Thread Rolling in Chicago, says the first axial rolling system was developed in Germany shortly after World War II, and was used to eliminate the secondary threading operations that were once done on dedicated rolling machines. So-called tangential rolling heads came along some years later, followed by the first radial thread rolling system in 1973. Today, LMT Fette uses 3D printing to produce the arms for its rolling heads, allowing the company to create integrated cooling and flushing nozzles while increasing the product’s strength through topology optimization.
Like whirling, rolling can be used to produce deep and complex thread forms in virtually unlimited part lengths. Unlike whirling, though, thread rolling is suitable for a wide array of machine tools, including CNC lathes and machining centres, multispindle screw machines, rotary dial and transfer lines, and even manual lathes and drill presses. It can also be used to knurl, spline, burnish and swage parts. All that’s required is the correct style of rolling head, a set of dies and a little know-how.
About the only limitation to rolling, says Knobelsdorff, is the material—it must have an elongation factor of at least 5 per cent. “Hard metals up to 50 Rc are fine, but stainless steels, ductile steel alloys, and many superalloys are ideal. Because it displaces material, slight elongation of the workpiece can occur, but we can adjust the roll pitch to compensate if this becomes a problem. And contrary to what some might think, rolling does not require large amounts of torque or spindle power to drive the tool. With production speeds up to ten times faster than cutting or milling and the ability to produce threads from 1.4 mm (#0-80) up to 230 mm (9 in.), it’s simply the fastest, most dependable threading process available.”
Thrilled to meet you
For an internal thread, one of the best ways to cut cycle time is by reducing the number of tools needed to machine it. That’s according to Patrick Maigatter, cutting tools product manager for Ceratizit Chicago Inc., who points to the company’s Komet brand of combined thread milling and drilling tools—or thrillers—as the solution.
“A thriller is a solid carbide tool that drills and chamfers the workpiece, backs out of the hole slightly, then interpolates the thread just like any thread mill,” he says. “We offer several styles, including ones that can counterbore the hole, or helical interpolate it as you would using an end mill.”
This all sounds great, but there is a catch: multi-purpose tools like these are designed for softer materials like aluminum, plastic and some cast irons; attempt to thrill holes in Inconel or 316 stainless steel and you’re likely to destroy a relatively expensive cutter. For the right application, though, thrilling can generate significant savings—Maigatter noted that one customer reduced the time spent drilling and tapping M6 threaded holes in high-silicon aluminum from 9.1 seconds to 3.2, a 65 per cent reduction. Another shaved 12 seconds per part off an aluminum gear box containing six M4 threaded holes, while enjoying tool life of 30,000 holes per thriller.
Still, Maigatter admits drilling and thread milling with the same tool isn’t for everyone. “The sweet spot comes when you’re machining aluminum or gray cast iron parts, and parts that have a small number of threaded holes,” he says. “That’s because you’re eliminating at least one and possibly three tool changes. But if you’re doing manifold work, for example, and have 10 or 15 holes to make, it might be faster to take a conventional approach. Either way, I encourage people to take a look at thrilling, especially for longer running and repeat jobs. It can make a big difference.” SMT