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The most efficient hard parts turning methods

January 03, 2015

Traditionally, when the hardness of the used material is HRC45 or above, specific grinding equipment and machining processes should be used.

Because of the stable development in cutting and machining technology, manufacturers of automotive parts are provided with more and more possibilities to explore new machining solutions. In comparison to using a grinding machine, using cutting tools has several advantages, namely, (1) the cost for cutting is much lower, (2) a multitude of different automotive parts can be processed, which greatly improves production flexibility, and (3) the pieces that are chipped off during cutting can be recycled, while ground pieces cannot. Although grinding can produce parts with a precision of up to ± 0.005mm, cutting can be used in most other production applications.

When cutting automotive parts, one of the most important parts is choosing the right insert. Carbide inserts are a good choice for producing products with a hardness between 40 and 50 HRC. Ceramic inserts are a good choice for processing materials between 50 and 55 HRC, such as carbon steel, which has a low carbon content and usually requires high wear resistance due its hardened surface.

PCBN inserts are a good choice for materials exceeding 55 HRC, such as hardened steel and most integral quenched steel, since they require a special kind of resistance to prevent wear of the cutting surface. As a material, CBN (cubic boron nitride) stands out above the rest for heavy machining processes. Thanks to its high wear resistance and it being able to cope with high temperatures, it is the second hardest material out there.

For materials exceeding 55 HRC, it is of the utmost importance to decide the CBN’s carbon content. To make sure corrosion and wear of the cutting surface is avoided, a lower cutting speed of about 400 inches/min should be applied. As an exception to the rule, there is no need take wear into consideration when working with materials with a low carbon content. CBN inserts with a ceramic binder still have sufficient wear resistance at a cutting speed of 600 inches/min.

Beside carbon content, the design of the automotive parts will also affect the choice of the CBN insert. They depend on three cutting types: (1) for continuous cutting, inserts with 40% CBN are a good choice, (2) for intermittent cutting, inserts with 50 to 60% CBN are needed for its wear resistance, and (3) for heavy intermittent cutting, only inserts with 70 to 75% CBN will be able to meet the machining requirements.

The right shape of the blade on the insert also has an effect on the cutting process. Under different machining conditions, different angles are required and thus different inserts types are needed. For example, T-shaped inserts are the most common for cutting. They disperse the cutting force evenly due to their round angle. S-shaped inserts, on the other hand, though similar to the T-shaped inserts, cut the surface in a different angle as to improve their wear resistance.

Lastly, using Wiper grooving inserts will greatly improve quality and efficiency. Comparing the Wiper with a standard insert’s radius, it can generally reach the same surface, but with less than two times the feed rate. The reason for this is that the cutting force of the Wiper insert is stronger, thus with the same feed rate as a standard insert, a Wiper insert can do more.
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