Burr-Free Micro Thread Whirling

Micromachining is a significant presence in the Swiss industry. In order to qualify as Swiss-made, the majority of the parts of a product such as a watch must be assembled in Switzerland, and at least half the tiny parts of the movement have to be made in the country. To provide these parts, the Swiss need an industry that can make things small. Precision manufacturing is a selling point, and it serves other industries in Switzerland, including the manufacture of medical devices.

To meet the demands of these industries, DC Swiss (available in the U.S. directly through DC Swiss in Switzerland or through the Mikron Tool Group, official representative for DC Swiss in the U.S.) specializes in the development and manufacturing of high-precision threading tools, forming and tapping solutions as well as various SCS-certified control devices for a variety of materials and applications in the 0.3-mm to 160-mm (0.0118" to 6.294") thread diameter range.

DC Swiss’ latest development is the thread whirling tool GWi5000, which was developed for customers in the medical, watch-making and aerospace industries facing larger series coupled with shorter delivery times. “We conceived a tool to produce the ‘perfect internal thread’ for micromachining applications in demanding materials such as titanium, but also alloyed steels and aluminum,” explains Sergio Galantino, development and technology, marketing manager at DC Swiss SA in Switzerland. “A perfect thread is a burr-free thread that does not require any post-treatment and can be produced lights out in one clamping without operator intervention in large quantities.”

The Tool

The GWi5000 is a multi-tooth, full-profile carbide thread whirler available in sizes from 0.8 mm to 6.35 mm (series UNC, M, MF, UNF, S) diameter that combines three different operations in one. Featuring either three or four teeth (depending on the diameter), two teeth rough and finish-machine the flanks while one tooth (or two when machining larger diameter threads) machines the core diameter, ensuring burr-free profiles and edges of the thread. “The tool is unique in its ability to complete-machine the flanks and the core diameter in one cut,” Mr. Galantino explains. “There is no other tool in the market that can guarantee a burr-free thread without extra deburring operations.”

The ultra-fine grade carbide tool is coated with a special VS coating for low wear and good heat resistance in high performance machining. The internal cooling is realized through special half-moon shaped channels (see photo) to ensure the coolant is delivered to the cutting edges where it is needed. According to Mr. Galantino, this design ensures perfect chip evacuation and high surface qualities (Ra 0.2).

The Process

To generate “the perfect thread,” a “perfect hole” is required, Mr. Galantino says. Perfect here means drilling the hole with the right diameter for this particular thread whirling process. “Since we are not only machining the flanks, but also the core diameter, the general formula for calculating core diameters using the thread minus pitch does not work. There needs to be enough material for the thread whirler to machine the core diameter during the threading process.”

After the centering and drilling operations, the GWi5000 enters the hole with a counter-clockwise rotation, left-hand cut, cutting the thread from top to bottom with a clockwise interpolation (see image). Generally, conventional thread whirling works from bottom to top with a right-hand cut.

After reaching the bottom, the tool moves to the center of the hole and then vertically back to its starting position. According to Mr. Galantino, in 60 to 70 percent of all cases, one cut finishes the thread; hard and demanding materials might sometimes require two cutting passes.

Moreover, DC Swiss ensures a reliable, stable process, requiring few if any NC corrections during machining. The following example illustrates this point.

The Application

While smaller series threads can be more efficiently machined using a combination tool such as the DC Swiss SA product ZBGF circular thread milling cutter, which combines drilling and threading in one tool, separate drilling and threading operations with the GWi5000 will pay off when machining a larger series.

A customer specializing in medical and dental products was looking for a highly precise thread that could be produced in high batches on a Swiss-type Tornos Evo Deco without post treatment and 100 percent according to the required standard. Made from Titan Grade 4, the part needed a blind hole with a thread of M1.8 6H by 0.35 and a depth of two times diameter. Using cutting oil as lubricant, the GWi5000 produced 2,500 threads at a cutting speed of 30 m/min and a feed rate of 0.03 mm/tooth. Only one NC correction was required (the operator adjusted the NC program during the process because of tool wear), which makes the GWi5000 suitable for series production, Mr. Galantino explains. “Other products required as many as five NC corrections for the same job. Our special tool geometry and coating ensures minimum wear and, hence, a more reliable, accurate process. We can also calculate the point where the NC correction needs to happen to organize automated production.”

While the process requires machines with a certain spindle power and rpm, most new generation turning and milling machines are suitable to ensure a reliable process, Mr. Galantino adds. Most companies in the aerospace, medical, automotive and watch-making industries have already upgraded most of their equipment, he says, but older machines with lower rpm can also be used for thread whirling by retrofitting them with a HSM Jet spindle, for instance, which fits on a range of CNC machines without modifications or any additional power source.

For quality control, DC Swiss also offers ring and plug gages to control the profile shape, thread flanks and surface qualities.

Mikron Corp. Monroe | 203-261-3100 | us.mikrontool.com/en

DC Swiss | dcswiss.com/en