Seco Tools' PCBN (polycrystalline cubic boron nitride) technology solutions for hard turning case-hardened steels are delivering benefits to automotive parts manufacturers. Seco Tools explains
An example of this is case-hardened steel, the material of choice for high volume manufacture of automotive gearboxes (specifically gears and drive shafts) and a significant number of components found in drive-train assemblies.
Case-hardened steel is a tough uncompromising material that features a hard and wear-resistant outer skin and an impact-resistant core.
Up until relatively recently, grinding was not only the preferred but, in many circles, considered to be the only process for machining such hard materials. However, due to the high energy costs, lack of flexibility and high industrial waste generation, PCBN tools have been accepted as 'fit-for-purpose' and a ready-made alternative.
PCBN tools are hard, thermally conductive and chemically inert, as far as ferrous materials are concerned. As a result, they provide manufacturers with improved tool life, increased part accuracies and faster processing times, and are increasingly the tool-of-choice for automotive component manufacturers.
Typical tolerance specifications for an automotive synchromesh gear wheel in case hardened steel, 58-63HRC are:
Bore: diameter ± 0.015 mm; roundness ± 0.004 mm; straightness ± 0.003 mm; parallellism ± 0.006 mm; surface roughness Ra 0.2 – 0.4 µm
Taper: cone angle ±0.04°; roundness ± 0.004 mm; straightness ± 0.002 mm; surface roughness Ra 0.2 – 0.4 µm
Bearing ratio 90% (depth 0.002 mm)
To achieve and maintain these types of tolerances relies on a number of inter-related factors.
Taking the rigidity and efficiency of the machine tool and workholding systems as givens, the next most critical factor is the cutting tool and specifically:
a) the selection of the correct grade for each machining operation,
b) the choice of insert geometry,
c) the selection of the most effective cutting strategies
Developing different PCBN grades that offer a wide range of performance attributes is an area where Seco has invested considerable resources and has resulted in the development of a number of grades for machining case-hardened steels – the latest being:
• Secomax CBN060K for continuous and light interrupted machining;
• Secomax CBN160C for interrupted machining.
The vast majority of operations using case-hardened steel involve continuous or light interrupted machining. (For example the machining of oil feed holes or O ring grooves will be where Secomax CBN060K comes into its own). Where significant interruptions occur, for example in the machining of gear faces or splines, then a tougher grade PCBN is required (Secomax CBN160C).
Developing different PCBN tools requires a thorough understanding of CBN, the different binders that can be used, and what happens when they are bonded together. With Secomax CBN060K, a super-alloy binder has been incorporated within a TiCN hard binder phase, creating a grade that has greater cutting edge reliability and a long tool life - especially in finish machining of case hardened steels.
Adding coatings to PCBN cutting tools was a process developed over 10 years ago. Initially, coatings were applied to PCBN inserts principally to help register (visually) tool wear, because the dark colour of PCBN made it difficult to identify and assess used cutting tool edges. As a result, a light gold TiN PVD coating was added to make worn edges easier to see.
However, as coating technology developed, it was obvious that coating PCBN inserts could improve tool life significantly, and the latest generation of coatings from Seco employ a PVD applied Ti,Al,Si)N coating, which, in trials with case hardened steels, have shown a two-fold increase in tool life compared with the same uncoated grade.
Possible reasons for improved cutter performance (reduced wear) can be explained by the following:
a) Sacrificial wear of the coating occurs first before PCBN wear takes place.
b) Coatings act as a chemical barrier resisting thermo-chemical wear.
c) Coatings increase the coefficient of friction allowing better chip flow resulting in edge weakening crater wear further away from the cutting edge.
d) Coating adds surface compressive stress to the cutting faces increasing cutting edge strength.
PCBN insert format can have a significant effect on a machining technique. This is especially true when machining case hardened steel gear wheels. While single-tipped or multi-tipped inserts are the conventional way to turn the gear surfaces, the option of a solid PCBN insert provides an alternative.
Seco Tools was the first and is still the only supplier to offer solid PCBN for finish machining. The most recent grade introduced being Secomax CBN060K.
With Secomax CBN060K and a solid PCBN format, it is possible to plunge turn (as opposed to conventional hard turn) a range of case-hardened steel components in a fraction of the time.
Plunging essentially uses a significant part of the cutting edge length to generate the machined surface. The concept is not new, since it has been used very successfully in the plunging of valve seats in engine cylinder heads for many years.
The process relies on a number of key factors:
a) Insert edge quality is important in achieving good surface finish and maximising tool life;
b) It is also necessary to increase cutting speeds and reduce the feed rates thereby reducing cutting forces and ensuring excellent dimensional accuracy.
To maintain dimensional accuracies, the insert is allowed to dwell at the base of the cut for 3 to 4 revolutions. To avoid the cutting edge profile affecting the surface finish, a small axial movement is made.
With such techniques and a good machine tool set-up, it is possible to achieve consistently high surface finishes and component accuracy.
In gear turning, plunging has been the method used for machining gear faces and the synchromesh taper. The longest edge machined by plunging is 16 mm.
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Author: Andrew Allcock