[Editor's note] Parts are machined under surface hardening conditions (whether hardened or hardened) to help achieve higher accuracy in a more efficient manner. As an increasingly widely used processing area, hard part turning has always been the focus of tool development. There are many parts in the automotive industry that require hard machining, including transmission gears, fuel injection pump nozzles, bearing races, steering gears, brake discs, axles, camshafts, valve seats, pistons and cylinder liners, while hard machining is cutting The tool puts forward higher requirements.
Recently, the two manufacturers have significantly improved the production efficiency of hardened parts processing by 116% and 322% respectively. The reason is that they have neither replaced the equipment nor introduced new processing methods. However, when the turning process is performed on a CNC lathe, changing the type of indexable inserts has greatly improved the production efficiency. Compared to before, this new type of insert increases the feed rate of the tool by a factor of three, saving machining time and reducing production costs.
The blade that has achieved such a significant increase in productivity is the Sandvik Coromant Xcel insert, which guarantees a higher feed rate while maintaining a low surface roughness. This cutting edge technology is innovative for hard part turning. It has had a profound impact.
Hard part turning requirements
Hard part turning has unique requirements in terms of cutting force, and under normal conditions, the tool is subjected to more pressure than other materials. In addition, hard workpiece materials are abrasive, so the tool is more susceptible to wear and generates more heat. The hardness of cubic boron nitride (CBN) tools is second only to diamond, which is very suitable for this processing field. Today, a range of high-performance inserts with the main component of CBN pellets can be used for hard part turning.
The Xcel blade concept improves the performance of hard part turning by a level. For the finishing process, the superiority of the Xcel insert is fully demonstrated as long as the surface roughness is met. Moreover, for the roughing process, the feeding ability is also quite good. When normal stability is guaranteed, the Xcel blade can be used to achieve a feed rate of up to 0.5 mm/r with sufficient clearance along the shoulder. If you match the Wiper blade with a feed speed twice that of a normal tool, you can ensure a stable process and predictable machining results.
For hard part turning, if the lathe is replaced with a lathe, the best solution is to use a Xcel insert for roughing, followed by a Wiper insert with a WG groove for the finishing process.
Xcel blade features
Ideally, the Xcel insert has a feed rate of 0.3 to 0.5 mm/r. When using Xcel inserts, the recommended maximum depth of cut is 0.25mm. When machining with a CNGX insert to the shoulder, it must be specified due to the special shape of the insert: at least 2.7 mm is avoided unless there is an undercut at the shoulder. The cutting speed can reach up to 300m/min, but the recommended value is usually between 150 and 200m/min. When cutting, first determine the feed rate to obtain a higher processing rate, and then determine the cutting speed to optimize production efficiency.
The ideal parts for hard turning are large diameter shafts, sleeves and gears, which involve longitudinal turning and end face machining with good stability. The workpiece material is typically hardened steel or case hardened steel, including 16MnCr5 (hardness > 60HRC) or 17CrNiMo6. In particular, hard-soft cutting occurs when machining gears, where the material hardness at the outer diameter is 62 HRC, and the material hardness at the center is reduced to 30 HRC. CBN is not recommended for workpiece materials with hardness below 45HRC, but CBN can be combined with carbide tools for applications and materials.
Cutting can be continuous or intermittent, as with gear teeth or shaft characteristics, which can be optimally optimized by selecting the right insert material. The surface roughness value can usually reach Rz = 1 μm or Ra = 0.25 μm, and the dimensional tolerance is kept within 0.01 mm. Good surface quality can be obtained by high feed: surface roughness Ra = 0.3 μm at a feed rate of 0.3 mm/r; surface roughness Ra = 0.5 μm at a feed rate of 0.4 mm/r; feed rate When it is 0.5 mm/r, the surface roughness Ra is 0.8 μm.
In terms of semi-finishing capabilities, Xcel inserts are extremely efficient.
Hard part turning is an alternative to hardened part grinding, and the two methods complement each other. Modern CBN materials mean more rugged knives, and current blade technology has resulted in cutting edges with increased productivity and finishing capabilities.
When the maximum chip thickness is used as the basis for the feed rate, Xcel provides a higher feed capacity than a standard rounded tip blade. When the lead angle is small, the thin chips generated by the straight edge provide greater productivity and space.
Mechanically locked CBN insert fillets provide the strength and safety required for multi-edge inserts, making them economical and optimized for reliability. The Safe–Lok multi-fillet technology features a securely welded CBN fillet that allows it to be kept away from the hot cutting zone formed during the turning of hard parts (see photo).
Safe–Lok multi-rounded blade
Xcel blade stability
Stability is a success factor for all CBN blade applications. To ensure optimum Xcel blade processing, the workpiece aspect ratio should be kept within 4:1.
Standard round head inserts produce the lowest cutting forces, but feed rates are strictly limited by the relationship between tool nose radius and surface roughness. Wiper inserts with WH groove shape still achieve good surface quality at lower feed rates and lower stability requirements; while at very high feed rates, better surface quality is achieved, but Need more stability. When using Xcel inserts, high surface quality is achieved at high feed rates, but the stability requirements are higher; even at lower feed rates, Xcel inserts produce good surface quality. . The focus of attention is on reducing the cutting time as much as possible, and the relationship between feed rate and surface roughness should be emphasized.
When the blade uses a very small lead angle (10°), the depth of cut is always limited during hard part turning. With conventional round head inserts, the chip thickness varies along the cutting edge, but with Xcel inserts, the chip thickness is thin and uniform, which provides high feed processing capability.
Tool life
For standard round head inserts, the grooves on the cutting edge are very abrasive, which greatly limits their life. With Xcel inserts, the cutting edge wear is evenly distributed, resulting in improved tool life. Improvements to the tool with Xcel inserts are due in part to the lower cutting contact times resulting in lower temperatures and diffusion along longer straight edges. Therefore, even with a large increase in feed rate, the Xcel insert can process more parts.
Xcel inserts are available in two types based on cubic boron nitride (CBN). This tool material has a very high thermal hardness and is resistant to plastic deformation, allowing for a higher combination of feed and cutting speeds when machining hardened materials. The materials currently used also have good toughness, high strength, thermal shock resistance and abrasive wear resistance. These materials have a CBN content of 40% to 65% and contain ceramic binders for higher chemical resistance at elevated temperatures. In addition, there are also materials with a higher CBN content, in which a metal binder is added for higher toughness.
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