Machining: A turn for the better

The drive for machining process improvements at automotive shops has led to several developments in tooling for steel part turning 

While much has been documented about hard turning techniques in recent years, turning non-hardened steel has passed largely under the radar. However, as a number of interesting tooling developments highlight, there is no lack of appetite for technologies that can help boost the productivity of this important process.
 
There are countless CNC turned parts in a typical vehicle, from propshafts, camshafts and sideshafts through to axle components, CV joints, bearing housings, compression fittings, flow nozzles, gear blanks, couplings, pinions, bushes, king pins and so the list goes on. Some are machined from raw bar stock while others, typically larger more complex parts, are machined from forgings or castings.
 
Of course, numerous automotive parts are machined to a finish from bar or billet, typically from materials such as low-to-medium carbon steels, alloy steels and annealed steels.
 
Increasing tool life
Turning insert grades such as KT1120 from Kennametal score well in this arena. The micro-grain structure of KT1120, a TiCN-based cermet (ceramic material in a metal matrix) grade, is said to provide great toughness and thermal shock resistance, resulting in enhanced tool life, surface finish and application versatility.
 
In a featured application involving the turning of an automotive connector component made from AISI 1010 carbon steel, KT1120 outperformed a comparable competitor insert by offering a 300% gain in tool life (delivering some 900 parts per edge) while achieving the required 12.5Rz surface finish. The cutting data was the same for both inserts: 200m/min cutting speed; 0.3mm/rev feed rate; and 1.4mm depth of cut.
 
Also from Kennametal, the Beyond range is an entire platform of turning products comprising various grades and geometries to suit a host of automotive applications. Beyond features a post-coat surface treatment that is said to improve edge toughness, reliability and depth-of-cut notch resistance. It also micro-polishes the surface to reduce friction and built-up edge (BUE). Kennametal says that its fine-grained alumina layer is responsible for the potential increases in cutting speed.
 
For example, KCP40 for roughing steel shows speed and depth-of-cut increases of 10-20% in tests against competitor grades, while KCP05, a Beyond grade for finishing operations, shows feed increases of 30-40%. All Beyond inserts are CVD coated, but whereas conventional CVD coatings are under tensile stress, the aforementioned proprietary post-coat treatment ensures that this is reduced.
 
Balancing wear
As an operation, steel turning needs to address and balance many factors. Of particular importance is the condition of the edge line. If this is broken, rapid breakdown occurs that can result in unacceptable parts and loss of machining security, particularly at the more challenging end of the steel material spectrum. Limiting continuous, controllable wear and eliminating discontinuous, often uncontrollable wear, is the secret of steel turning success. This is why insert grade manufacturers work hard to produce solutions that tackle the mechanisms which lead to premature breakdown, as can be seen with Sandvik Coromant’s evolution of GC4325 for steel turning.

“As a representative example, one type of steel that we have succeeded far and above expectations with GC4325 is the steel used for bearings,” says Mia Pålsson, senior manager turning tools at Sandvik Coromant. “These specifically challenge the cutting edge, often generating rapid crater wear. With this in mind, we have developed the insert substrate and coating to better withstand diffusion wear at high temperatures, thus reducing the effect that causes crater development on the rake face. In this way we have also made sure that GC4325 is capable of maintaining an ideal fluid flow zone when generating the chip. This means being able to use a higher cutting speed but with the edge security required in unmanned machining.”
 
GC4325 is equipped with Inveio, a technical breakthrough of unidirectional crystal orientation. Normally, crystal orientation in CVD alumina coatings is random, but with Inveio, Sandvik Coromant has found a way to control the crystals, making them all line up in the same direction – towards the top surface. These tightly-packed crystals create a strong barrier towards the cutting zone and chip.
 
Among the early adopters of GC4325 is Bifrangi, a 430-employee producer of machined steel forgings for the automotive industry based at Mussolente near Vicenza in northern Italy. Here, customers include BMW, Getrag and Deutz to name but a few.
 
Using a Famar CNC vertical turning lathe featuring a Coromant Capto C4 coupling, the first task for GC4325 was roughing a 200mm diameter automotive hub. External axial turning and facing operations were required on a workpiece made from forged steel (CMC code 02.1). With a time-in-cut of 26 seconds per part, Bifrangi could achieve 116 components using the previous generation grade GC4225 before the insert required replacement. However, using the new grade in the same style (CNMG square inserts with PR chip breaker geometry), the company can now achieve 160 components, thus demonstrating a 38% increase in tool life. The cutting parameters are identical: 200m/min cutting speed; 318rpm spindle speed; 0.36mm/rev feed; and 2mm depth of cut.
 
Challenges with soft steels
Many would consider the turning of so-called softer steels to be fairly straightforward, but in reality the ductility of materials such as low carbon steel can often cause larger, inconsistent chips which limit success. As a result, good chip forming and breaking is the key to maintaining high productivity levels in these materials.

The relationship between cutting depth and the insert nose radius has a big influence on the level of chip breaking that can be achieved. For best results in this area, machine shops should try to achieve a cutting depth greater than, or at least close to the nose radius value. If there are no opportunities to do this, it is beneficial to change to a wiper insert with smaller nose radius, if possible, to maintain the feed capability.
 
Feed rates, when machining low carbon and pressed materials also have a big influence on chip breaking. A low feed rate will result in thin chips which are very hard to break. Low feeds, in combination with smaller cutting depths also mean that the chip is not able to reach the chip breaker. To solve these problems, always aim for the highest feed possible, taking into consideration the stability of the workpiece, tool, clamping and surface finish requirements.
 
For best chip forming performance, always choose a cutting direction which provides an effective entering angle as close to 90° as possible – back turning should be avoided as this gives a very small effective entering angle. Better chip formation can be achieved with a downwards cutting direction on the workpiece which also minimises the risk of vibration.

Of course, many automotive supply chain shops will be tasked with turning diverse materials, not just steel. With the benefits of reduced inventory in mind, Walter has developed an addition to its Tiger.tec Silver insert series. Here, the use of a new, thinner CVD coating has resulted in WMP20S, a universal wear-resistant insert with sharp cutting geometries that is designed as a universal grade for machining on both stainless steel ISO M and carbon steel ISO P materials.

Until now, says Walter, dedicated sharp-edge inserts would traditionally be applied to stainless steels in the ISO M group, while heavier edged tooling would be used for ISO P steels. WMP20, however, combines the best of both worlds, thus reducing inventory costs.
 
Featuring a CVD coating that is only half as thick as conventional CVD – courtesy of a subsequent treatment to the Tiger.tec Silver technology (cooling the insert after the CVD coating) – the material’s tensile strength is transformed into compressive stresses, which in turn produces a special residual stress. The result is increased cutting speed and longer tool life.

Turn for the better
Among recent developments in the steel turning arena, Kyocera has unveiled four new grades in its CA5 series which are said to enhance abrasion resistance and fracture resistance. Featuring a new CVD coating, Kyocera says its know-how has allowed the realisation of 1.4-times higher adhesion to the carbide substrate compared with previous grades.

AP2420 is another new steel turning grade, this time from Arno. The grade is said to feature a substrate and coating structure that is very different from original CVD multi-layer coatings. The individual layers are better connected to one another, with a cutting edge that is solid and resistant to chipping, says the company.

Widia’s latest Victory TN7100 series is a new range of coated-carbide inserts specifically for finishing, medium-duty and rough machining of all types of alloyed and unalloyed steels. The inserts offer a combination of wear resistance and toughness with a coating that is micro-engineered in both composition and post-coat treatment to emphasise coating adhesion and resistance to BUE.

Last but not least, WNT’s range of HCR1135 inserts are for steel turning under unstable and difficult machining conditions. Operating in the ISO P35 range, the HCR1135 inserts are designed for applications where the component has intermittent or difficult-to-machine surfaces, such as forgings and cast steel components with hard skins, or where the cut is heavily interrupted.