Hard Turning Perfects Finished Parts for Small-Batch Production

Shops that process high-volume automotive jobs often use hard turning for finish work, but the cost of cubic boron nitride (CBN) inserts makes this strategy look out of reach for smaller projects. New advances in the efficiency of CBN insert technology make the tooling more efficient and cost effective for small-batch jobs, enabling shops to skip the use of grinding equipment altogether and gain the advantages of single-machine processing.

Finish work typically means moving workpieces to grinding machines for secondary processing, but multi-process machines add economies of scale, as one piece of equipment handles numerous processes in a single clamping. Not only does that approach save time, but it also eliminates the errors that can creep in when operators must unclamp and reclamp workpieces back and forth from one machine to another.

If you're focused on upfront costs alone, this strategy looks more expensive than the alternative. After all, CBN inserts for hard turning can cost 10 to 20 times the price of conventional tooling, and that disparity isn't likely to disappear, given the material costs involved. But a machine with a fast spindle and CBN inserts that feature today's advanced geometries can leverage tool life, cost per edge and overall productivity to produce up to 300 times more per-part cost effectiveness than conventional tooling.

Conventional wisdom suggests that adding more equipment also adds more capabilities. That's true – but it's only more efficient if that additional equipment performs functions that a shop's existing machines can't handle. To keep up with high-mix, low-volume production, shops need versatility and production flexibility. Lathes offer both, especially compared to grinding machines – and when hard turning enables a lathe to rough and finish a workpiece, even a less-advanced model can handle the work, provided that it offers the rigidity and vibration damping necessary for good tool life with CBN tooling. CBN tends to be more brittle than carbide or steel, so proper stability and toolholding are a must for successful hard turning.

Wiper geometries can make a big difference in machining at the higher speeds and feeds necessary for hard turning. Because of those machining parameters, heat develops at the insert edge and softens the surface in the cutting zone, essentially plasticizing the workpiece surface. Coolant extends tool life in these continuous cutting applications, so long as it doesn't make contact with the hot tool tip, where it can cause thermal shock and damaging microfractures.

Seco Tools offers numerous innovations in CBN tooling to master new and challenging materials, including workpieces with transitions between varying degrees of hardness. The Secomax™ Flowing Radii Chipbreaker, for example, uses a new smooth radius chip breaker to handle those types of hardness variations. We laser machine this chip breaker to give it a continuous radius along the cutting edge, which promotes consistent chip formation and evacuation as the tool moves from hard to soft surface areas.

Along with specialized chip breakers, we also maximize the number of cutting edges with solid-style instead of brazed-tip inserts. Solid inserts can feature as many as 20 cutting edges, compared with only between two and eight on brazed-tip inserts. Several grades also feature bimodal distribution of grain sizes to deflect cracks and extend tool life.

CBN tooling brings high-end solutions to manufacturers who previously skipped hard turning on smaller jobs because of the initial costs involved. Shops that spend the additional amount up front can realize substantial long-term savings – especially through process optimizations. Seco Tools continues to build inserts that handle new materials, approaches and challenges.

Maximize Tool Life in Micromilling

by Jay Ball, Product Manager, Seco Tools LLC

Micromilling compresses the challenges of maintaining tool life into a tiny working scale at which the cutting edges of these tools measure about the same size as the grain of the grinding wheels used to produce them. Fortunately, micromilling typically does not create the harmonic vibrations and chatter that often cause problems with long standard-size tools or during heavy conventional roughing operations. Instead, the basics of maximum micromilling tool life start long before the cut even begins, with criteria as fundamental – and as diverse – as tool design and selection, toolholding and material properties.

A well-balanced, rigid toolholder and the proper selection of machine tool feeds and speeds go a long way toward maximized micromilling tool life, as does starting with the right machine tool – and the proper cutter for the job – in the first place. Tool geometry improves surface finish and boosts tool life, if the tool geometry matches the specific workpiece features at hand. Additionally, coatings provide a thermal barrier to protect the tool from wear when it machines extremely hard materials, and the alloy chosen for the tool substrate makes a critical difference in terms of toughness, which helps promote a greater bond between substrate and coating.

First and foremost, of course, alloy selection translates to greater tool life when the properties of the tool match the hardness and abrasiveness of the part material. Substrate hardness, particularly in carbide tools, heads the list of criteria related to tool life. Hardened tool steels run between 48 and 65 HRc on end mills and slightly lower on inserts. A tool needs greater hardness than the material it cuts, and some materials present two forms of hardness, one on the surface and the other below it. A material with a hard surface and a hard, abrasive particle, for example, presents a special challenge where tool selection is concerned because it cuts like a harder material than its specifications indicate.

Along with tool-selection criteria, shops also must monitor such parameters as toolholder collet-bore cleanliness, machine stability and consistent operating temperature to ensure best results. A poorly cleaned, neglected toolholder can harbor chips and grit that cut tool life when they cause wear or interfere with secure clamping. For that matter, a subpar toolholder chosen for its price alone and not for its clamping capabilities will cause higher runout – and high TIR is the enemy of tool life. Runout of only about 0.0004" will cut tool life in half, so drastic toolholder economizing rarely yields the performance necessary for optimal tool life.

In addition to tool setups, shops need to make the right choices in programming their equipment for micromilling. Today's machine tools offer feeds and speeds that were considered impossibly high in the past, so it's understandable that shops want to take advantage of such full capabilities and run as fast as possible. But the intricate geometries of many micro machining workpieces – complex mold structures, for example – require slower speeds to keep up with the feed rates necessary for proper chip formation.

If machine speed exceeds the point of proper chip formation, the tool rubs instead of cuts the workpiece, pushing the material around and roughing up the surface. Rubbing produces a visibly inferior surface finish and makes for a difficult finish cut. With a proper chip load, the tool produces a consistent finish that offers 50% more surface integrity, which reduces the amount of additional work required to achieve completed parts. (Check out a previously run Seco blog that goes into greater depth on the importance of maintaining constant chip load.)

To maximize tool life and workpiece quality, therefore, shops need to let feed rates determine machining speeds. As a general rule, a machine tool should maintain its programmed feed rate for 80% of the expected cycle time. When speeds and feeds prove unsustainable, reduced RPM and feed rate create a sustainable working pace.

To set up micromilling projects for success, shops can rely on toolmaker data to help them optimize RPMs, feed rates and chip production. Seco Tools develops and distributes cutting data that represent optimal combinations of angle of engagement, feed rate, stepover, surface footage value and material type, providing baselines that enable shops to achieve micromilling part production success – and longer tool life – without all the guesswork.