Thread cutting in high-tensile tool steel

For tool makers and manufacturers, machining material 1.2714 is a regular challenge when it comes to the tool life achieved during thread cutting. A company based in Saarland introduced the Walter Prototyp Paradur HT some time ago and is now able to report that its tool lives have more than doubled.

“The demands of our customers in terms of the materials used have been increasing continuously for years. High-quality and high-tensile tool steels are becoming even more important to us for the manufacturing of machine parts,” explains Markus Weber, factory manager at Schneider Werk St. Wendel GmbH & Co. KG. The medium-sized business based in Saarland particularly found that thread cutting in material 1.2714 (high-alloy heat-treated tool steel) was increasingly becoming a source of problems.

“The diverse selection of tools that we were using was unable to meet our tool life needs during tapping operations”, Markus Weber points out in particular. “The thread cutters would soon become blunt and occasionally erupt at the cutting edges.” At that time, a cutting quantity of 15 threads was the best that could be achieved. “With this material, the chips pulverise the cutting edges and so our average was just two to fifteen threads per drilling unit.” As an alternative to cutting, milling was given a try but, again, the cutting edges were pulverised and soon became unusable. “Even milling very quickly turned out to be totally uneconomical,” explains company boss Christoph Schneider.

To improve the thread cutting process, the managers of Schneider Werk got in touch with Walter AG in Tübingen, their tooling partner of many years. Having already had good experience with milling the material 1.2714 using indexable inserts, it was an easy decision. Thanks to their good relationship with field service employee Michael Diesel, Technical Support and Sales Walter Deutschland GmbH, they were happy to consider the benefits of tapping. The expert Walter adviser, who possesses a highly technical background, has worked regularly for a number of years as a consultant for demanding and unusual machining challenges. He was quick to identify the nature of the problem and was able to provide an effective solution.

Thread rescheduled to the end of the process

Before a thread is manufactured, it is necessary to drill the core hole. The quality of this hole has a considerable influence on the cost-effectiveness and process reliability of the subsequent threading operation. The production of internal threads is one of the most demanding machining tasks in production engineering. A particular challenge stems from the fact that the workpiece does not receive its threads until the end of the manufacturing process.

To produce internal threads, there are essentially three production methods available: tapping, thread forming (the non-chipping alternative), and thread milling (which offers a particularly high degree of process reliability). Crucial to selecting the appropriate production method is having as broad a knowledge as possible of the advantages and disadvantages of each method as well as their practical limitations. Ultimately, technical and economical parameters will prove critical.

The better matched the drilling and threading tools are, the better the result will be. In respect of true-to-gauge threading, tolerance, chip formation, avoiding bird nesting and oversizing, machinists demand the best possible performance. Only optimally matched, seamless solutions produce the perfect thread.

For production at Schneider Werk, Michael Diesel proposed the use of the Paradur HT tap from the Walter Prototyp product range and suggested adapting the cutting strategy to suit the material. This tap is made from universal high-performance HSS-E-PM and is suitable for use in a broad material spectrum. This range of materials includes long-chipping materials of medium to high strength, short-chipping materials, abrasive materials with tendency to weld, structural steel and high-tensile steel (350 – 1300 N/mm2), stainless steel, spheroidal graphite and malleable cast iron as well as long-chipping copper and aluminium alloys. The tool has proven its worth above all when used for long and short-chipping materials with a strength of up to 1,300 N/mm2 on conventional machine tools equipped with synchronous spindles. It has a THL hard material coating and additional surface treatment for longer tool life with no cold welding. Standard tool versions are available with axial or radial coolant outlet specifically for use with minimum quantity lubrication (MQL).

To manufacture the blind hole thread, the Paradur HT features an R45 helix angle with chamfer form C or E, long flutes and a special flute geometry for optimum chip formation and good chip removal even in deep threads. A variant with short chamfer form E makes it possible to produce the thread almost to the very bottom of the hole. Thanks to the tapered thread runout, the risk of eruptions is avoided.

A series of tests provided the information

In a series of tests, emulsion, grease lubrication and various cutting parameters were set and tested until the desired tool life was eventually achieved. “Today, we achieve tool life quantities of up to 50 tapped holes”, Markus Weber is pleased to say. “The results have since turned out to be very economical,” adds Christoph Schneider. “Walter has succeeded in giving us a performance capability five to ten times that of other providers.”

The Paradur HT guarantees greater process reliability even with relatively deep blind and through hole threads because it safely masters chip formation. Universal usability across a broad material spectrum means that fewer tool changes are needed and high cutting speeds and long tool lives help to ensure optimum machine capacity utilisation. At the same time, cooling lubricant costs can be reduced thanks to support for dry or MQL machining in steel, cast iron and aluminium alloys. “Since the introduction of the Paradur HT, our thread cutting has become significantly better and more cost-effective,” says Christoph Schneider in summary. “Of course, the cutting speed defined back then and the use of grease lubrication must be given due consideration.”