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When Is Hard Turning the Right Solution?

Aaron Eller
By Aaron Eller Product Manager, Seco Tools

Hard turning has long been used for finishing when it comes to high-volume applications. Now, tooling suppliers have pushed cubic boron nitride (CBN) insert technology further, with geometric innovations that further increase the efficiency—and cost-effectiveness—of hard turning. As a result of these innovations, any shop machining with tolerances above the sub-micron level may want to consider hard turning in place of grinding, even for smaller run batches.

This may seem counterintuitive to those focused on sticker price, as CBN inserts designed for hard turning tend to be 10 to 20 times more expensive than conventional tooling. These costs are hard to reduce, as cemented carbide will always be cheaper than what amounts to man-made diamonds. Instead, the cost reductions have come from improved machine tools and spindles.

With a rigid machine and fast spindle, shops have achieved up to 300 times more effectiveness in terms of cost per part per edge, tool life and overall productivity. And that’s before one factors in the process optimization made possible by combining processes with a single machine.

The state of the art in lean manufacturing now relies on multi-process machines that prioritize accomplishing as much as possible in a single setup. This reduces the risk of scrapping a part due to human error while transferring it from one machine to the next or mispositioning it while re-clamping. It also frees up operators for other tasks to add value up or downstream.

Advanced hard turn tooling makes it possible to eliminate grinding machines altogether for all but the most precise operations with standard CNC lathes and ISO standard-geometry inserts.

Naturally, some applications cannot be hard turned, particularly those that require custom grinding solutions to create hard-to-turn part features. But grinding machines can be expensive, single-purpose investments, which means hard turning can significantly lower the barrier to entry for precision machining with hardened materials.

In addition to the cost savings realized when the same machine that roughed out a part can finish it, most lathes can be more versatile than most grinders. This means job shops can maintain the processing flexibility they need to keep up with the pace of high-mix/low-volume production. These CNC turning machines do not need to be particularly advanced models, either. The main requirement for a hard turning machine is high rigidity and vibration damping, necessary qualities for optimal tool life. As CBN inserts are typically more brittle than traditional carbide or steel cutting tools, any process instability will chip inserts and reduce tool life.

Innovative new CBN tooling solutions now enable hard turning for equally innovative materials, including parts with transitions in hardness. The Secomax CH2540, for example, makes these transitions easier to handle, thanks to a new flowing radius chipbreaker. Made with laser machining instead of traditional tool grinding, these chipbreakers have a continuous radius along the cutting edge rather than a hard angle. Now, when the tip of the tool moves from a hard surface to a soft area chip formation and evacuation remains consistent.

Hard turning involves running machines at higher speeds and feeds to achieve a softened cutting zone at the insert edge via elevated temperatures—finishing via controlled plasticization, more or less. And tooling options like the CH2540, for example, feature wiper geometries that enable 300 percent faster feed rates than standard inserts, enabling even greater speeds. Coolant can be an effective way to extend tool life but only in continuous cutting operations, as coolant hitting the extremely hot tool tip causes thermal shock and microfractures.

To improve the cost effectiveness of these tools, Seco utilizes solid-style inserts rather than brazing the tips. Brazed-tip inserts typically come with two to eight cutting edges whereas solid inserts can have up to 20, dependent on depth of cut, which reduces cost per part per edge by a factor of four. And with bimodal CBN grain size distribution, coarser grains help to deflect cracks, extending tool life even further.

These tools have been used in larger factories. Now, CBN tooling has made this solution accessible to job shops that may have previously been scared off by the sticker shock. The upfront cost remains higher, but by any measure the long-term cost savings just in terms of tooling are significant. When additional process optimization is factored in, many shops will find hard turning a good choice for their finishing operations.

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