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.

Cut the Chatter with Stabilized Machining Strategies

Unstable milling processes produce chatter, vibration and runout that can destroy tools and ruin workpieces. In the quest for higher metal removal rates and increased part production, shops look for ways to elevate machining performance and eliminate any parameters that can introduce negative effects. The right choice of end mills can propel that quest for productivity into profitability – and tool design holds the key to taking full advantage of today's powerful milling machines without performance problems that curtail output.

On the list of troublesome milling issues, chatter ranks high, but identifying its source can require careful detective work. Overall, chatter can cause many woes, including the inability to achieve good surface finishes and work at high cutting speeds. The vibrations that cause chatter can stem from setup problems, including improper toolholding and poor-quality toolholding systems.

In some cases, however, the harmonics produced by the tools themselves can lead to chatter and pose a dramatic challenge to material removal. Most end-mill designs use symmetrical flutes with helix, rake and relief that match from flute to flute. This more-nearly conventional approach presents drawbacks because the highly regular impact of its cutting edges can lead to chatter.

Geometry that varies from flute to flute can go a long way toward promoting cutting stability, even at heavy depths, by squelching the resonant vibrations that stem from harmonic cycles. For example, our first-generation Niagara Cutter™ Stabilizer™ end mill varies tool geometry so each flute varies from the others.

To take the advantages of variable geometry to its fullest potential, however, would require production capabilities that tool manufacturers lacked – until recently. Today's production equipment and special programming approaches enable the creation of end mills with patented fully variable designs, including continuously variable rake, relief and helix for each flute. At the same time, new carbide compound formulas make the tool material itself more stable and less prone to deflection and runout, even when the tool is fully engaged.

The combination of continuously varying asymmetrical flute geometry and newly re-engineered flute shapes produces the edge strength and long tool life of Stabilizer™ 2.0, with twice the MRR, chip load capacity, feed rates and productivity of the product's first generation, even in heavy roughing cuts on 50-taper milling machines. The special new flute design, with individualized flute cavities for each tool and each size, squelches chatter because it eliminates the vibrations that produce it while simultaneously improving chip evacuation.

Stabilizer™ 2.0 targets aerospace, power generation, automotive and general engineering applications including slot milling, side-mill roughing and finishing, face milling, high-performance optimized roughing, pocketing and ramping, with two tool series, each created to work with specific materials. The ST430.2 series focuses on steels, alloy steels, copper alloys and cast iron, while the ST440.2 HT series takes on stainless steels, steels with hardness values up to 42Rc, titanium and Inconel®. This universal end mill eliminates the need for shops to invest in an ever-increasing tool inventory to complete projects in wide-ranging materials and with traditional and advanced machining methods.

GTDA Takes the Guesswork Out of Tool Life

In today’s era of high-mix/low-volume (HMLV) manufacturing production environments, traditional strategies for analyzing and correcting cutting tool failures can be nearly impossible to utilize. Comprised of short runs where cutting tools can be used across many distinct parts or part families, HMLV manufacturing strategies often involve various materials and characteristics that can make finding effective cutting parameters especially challenging. As a result, many manufacturers will prematurely change their cutting tools with every new setup to avoid any possible issues – a costly practice that prevents these shops from obtaining the full value of their cutting tools.

Instead of playing the tool life guessing game, we recommend taking a more holistic view of the manufacturing cycle to arrive at the best process and practices. To this end, Seco has developed a wear analysis methodology called Global Tool Deterioration Analysis (GTDA), which utilizes five core elements of manufacturing excellence to gain a better understanding of tool life across all areas of HMLV machining.

1.     Overall Machining Processes – By focusing on tool life alone, one misses the upstream and downstream effects of tool usage. For example, burrs typically have little to do with tool life, but their occurrence interrupts the manufacturing process due to secondary operations required to remove the burrs. As tool geometry directly impacts burr formation, a truly global tool deterioration analysis will examine its effects. To avoid time spent developing and implementing secondary or tertiary operations, it’s far superior to identify the root causes and correct them directly.

2.     Production Economics – Tools cost money, both in terms of total cost of goods sold and in terms of time spent on tool changeover and setup. A comprehensive analysis will go further and factor in related activities, like time spent acquiring and organizing tooling or loading programs into machines’ controls. To boost operational profitability, one must understand the various elements of the manufacturing process that contribute to production costs.

3.     Waste Reduction – Once you understand these production costs, you can begin to strategically address them. The following formula offers a useful way to track effective machining times in expressed as a percentage of the total time available:

            TTA = Total Time Available for Machining

            PD = Planned Downtime

            UB = Unscheduled Breakdowns

            CO = Changeovers

            PI = Minor Production Interruptions

            SP = Time Wasted on Scrapped Parts

            EMT = Effective Machining Time                

While 100 percent effective machining time is not practically attainable, a thorough analysis will get shops far closer to that level.

4.     Percentage Yield Goals – In HMLV production environments, production yield can often be all or nothing. A successful single-part run represents 100 percent yield, but if the part fails quality assurance or a workpiece is ruined, the yield is zero. Because this first-time yield is critical, avoiding tool breakage is perhaps the most important consideration. Because tool wear can be a minimal concern in these short run situations, a shop can apply, within reason, more aggressive and productive cutting parameters.

5.     Workforce Management – Of course, HMLV production depends on human workers, who must be skilled technicians to efficiently adapt to the continually changing parts and cutting conditions characteristic of these new manufacturing paradigms. That means hiring different kinds of machine operators and investing more in their ability to think on their feet and solve problems in real time.

Seco also recommends that shops pair this on-the-fly problem solving and comprehensive analysis with an equally rigorous process for determining how tools are wearing. When combined with the other data, this Global Tool Data Analysis approach – a service offered by Seco Tools to help its customers achieve manufacturing success – results in a complete overview of what it will take to obtain a fully optimal process.

To learn more, visit the Seco Tools website or Seco Consultancy and find out how Global Tool Deterioration Analysis fits into a wider understanding of manufacturing best practices.