Hard turning for medical production

PHOTO COURTESY OF SECO

Geometric innovations from tooling suppliers are pushing cubic boron nitride (CBN) cutting insert technology, increasing efficiency and cost-effectiveness of hard turning operations. Medical manufacturers who machine parts with tolerances beyond the sub-micron level may want to consider hard turning instead of grinding, even for smaller-run batches. This may seem counterintuitive in terms of sticker price, as CBN inserts designed for hard turning tend to cost 10x to 20x more than conventional tooling, cemented carbide will always cost less than what essentially are man-made diamonds – but today’s machine tools and spindles provide the savings.

Rigid machine and fast spindle can increase cost-per-edge effectiveness up to 300x for medical shops, improving tool life and overall productivity. Those increases come even before the optimizations made possible by combining processes on a single machine.

State-of-the-art lean manufacturing relies on multi-process machines that accomplish as much as possible in a single setup. This reduces workpiece scrapping rise because of human error during re-clamping or a transfer from one machine to the next – key considerations with highly precise medical parts – and frees up operators for other tasks.

Advanced hard-turn tooling with standard CNC lathes and ISO standard-geometry inserts can eliminate grinding processes for all but the most precise operations. Some applications cannot be hard turned, particularly those that require custom grinding solutions for difficult-to-turn part features, but grinding machines can be expensive, single-purpose investments, and hard turning can lower the barrier to entry for precision machining with hardened materials.

In addition to the cost savings of using one machine to rough-out and finish a part, most lathes are more versatile than most grinders. Smaller shops with medical contracts can maintain processing flexibility to keep up with the pace of high-mix/low-volume (HMLV) production.

Additionally, CNC turning machines need not be particularly advanced models so long as they offer the main requirements for hard turning and optimal tool life: high rigidity and vibration damping. Typically, CBN inserts are more brittle than traditional carbide or steel cutting tools, so any process instability will chip inserts and reduce tool life. Coolant can effectively extend tool life, but only in continuous cutting operations, as coolant hitting an extremely hot tool tip causes thermal shock and microfractures.

Tooling solutions

Innovations in CBN tooling solutions enable hard turning for equally innovative materials, including parts with hardness transitions. The Secomax CH2540, for example, uses a new flowing radius chipbreaker to make these transitions easier to handle. Produced through laser machining instead of traditional tool grinding (eliminating grinding even on tool creation), this chipbreaker uses a continuous radius along the cutting edge rather than a hard angle. Chip formation and evacuation remain consistent when the tip of the tool moves from a hard surface to a soft area, such as on parts with hardened bearing or welded surfaces.

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. Tooling options such as the CH2540 wiper geometries enable 300% faster feed rates than standard inserts, allowing for even greater speeds.

To improve the cost effectiveness of these tools, Seco uses solid-style inserts rather than brazing the tips. Brazed-tip inserts typically come with two to eight cutting edges, depending on geometry, whereas solid inserts can include up to 20, depending on depth of cut, which reduces cost per part per edge by 4x. The CBN bimodal grain size distribution’s coarser grains help deflect cracks, extending tool life even further.

These tools have long been common in the automotive industry, where indexable inserts promote cost-effective mass production of ring and pinion gears, as well as other components. Now, CBN tooling has made this solution widely accessible. The upfront cost remains higher, but by any measure, the long-term cost savings in tooling are significant. Factor in additional process optimization, and most shops will find hard turning to be the right choice for their finishing operations in medical-industry production.

Seco Tools

About the author: Aaron Eller is a product manager at Seco Tools. He can be reached at 248.528.5200.