by Kip Hanson
Small parts spell big trouble without the right turning tools
Poor surface finish, built-up edge, chipped or broken tools—these are just a few of the problems lathe operators encounter as part sizes decrease. The primary culprit is spindle speed—or rather, lack thereof. Back in the good old days of cam-driven screw machines and high speed steel tooling, spindle speeds of a couple thousand rpm were plenty for the majority of turning work. But in today’s demanding world, where coated carbides operate best at surface speeds of 700 sfm or higher in carbon steel, cast iron, tool steel, and even stainless, higher spindle speeds can make the difference between a winner job and one you’d just as soon forget.
Despite fast traverse rates and accuracy, however, the typical slant bed CNC lathe found on most showroom floors tops out somewhere between 4000 to 5000 rpm. This often means poor tool life and problems with part tolerances when turning anything smaller than your pinky finger. For these reasons among others, many shops turn to Swiss-style sliding headstock lathes, which often boast spindle speeds of 10,000 rpm or more and are designed specifically for small diameters and long, precision workpieces. For those who don’t have the luxury of such equipment—and even those who do—this article offers advice from cutting tool suppliers on improving tool life and increasing productivity, even when the small part turning odds are against you.
Speed’s a wonderful thing, but there’s no reason to turn down a job just because you can’t achieve optimal cutting conditions. Indexable insert manufacturers provide coated carbides that tackle far wider ranges of cutting speeds compared to even a few years ago. Daniel LaFauce, application engineer for Sumitomo Electric Carbide, Inc., New Berlin, WI, says the company’s 500-series turning inserts have a proprietary nano-layered “super ZX” physical vapour deposition (PVD) coating applied to a ground micro-grain carbide substrate, making it well suited to turning operations where spindle speed is anemic.
“At lower surface speeds, you’re often not going fast enough to break the chip,” says LaFauce. “Cutting forces are higher, which in turn generates more heat, so you end up with a built-up edge (BUE) and the tool eventually chips or breaks. By using a tool with a very sharp edge and a strong, lubricious coating, most of these problems are avoided, even down in the 200 sfm range.”
The right chipbreaker is another rabbit in the small part turning hat. With its “MESI breaker,” Sumitomo has replicated the small, hand-ground groove many manual lathe operators place in the tops of turning tools to reduce tool pressure and control the chip. LaFauce says MESI’s high shear, double angle geometry complements the ZX coating on many of the company’s inserts, and is popular with shops turning small parts made of titanium, Inconel, and other difficult materials. Somewhat surprisingly, LaFauce also recommends cermet, a ceramic-metallic substrate traditionally reserved for high speed finishing operations. “Our T1500Z cermet grade uses a bright coating designed for low surface speeds. We see some customers running these tools without coolant, and achieving just as good a surface finish at 300 sfm as they do at 1000 sfm . It’s the lubricity of the tool coating that is often key to successful small parts turning.”
On the edge
Nathan Preiss, product manager for Ingersoll Cutting Tools, Rockford, IL, says proper edge preparation is another important factor when turning small parts. “The best edge prep we’ve found is one that leaves the tool as close to dead sharp as possible. Larger edge preps or hones will have a tendency to push or displace material, thus affecting surface finish and tool life. But a very sharp edge with just the slightest hone allows the material to be efficiently sheared while maintaining edge strength, and works best in difficult to machine materials such as 316 stainless steel, Waspaloy, Hastelloy, and other high temp alloys.”
Preiss agrees with LaFauce, in that a lubricious coating helps with tool life when surface speeds are lower than optimum, and recommends Ingersoll’s TT9030, an AlTiN coating that is effective for small diameter applications. He also recommends coolant through the tool, especially with internal operations such as boring, grooving, and threading, where chip removal is problematic and cutting tools are often starved of lubrication.
some machines, just fitting the tools into the turret can be problematic. Small parts means little clearance for wrenches and toolholders, so finding turning tools designed for cramped quarters can make the difference between efficient machine operation and a knuckle-busting, time-wasting day. Preiss points to Ingersoll’s T-Micro system as one possible solution. Although originally designed for Swiss machines, where toolholder real estate is at a premium, T-Micro is a viable alternative even for conventional and gang-style lathes. “One feature that stands out with T-Micro is the insert locking screws are tilted, or ‘cocked’ off-centre,” says Preiss. “The design allows users to index the insert with the sleeve and toolholder still in the machine. This helps reduce down time, and avoids mistakes that come with having to touch off the tool after a tool change.”
Cramming a bunch of cutting tools into a small machine is often the norm with this type of work. Because Seco-Carboloy’s Mini Shaft can perform OD and ID turning with the same tool, it eases space constraints on Swiss machines and reduces time spent indexing and traversing between tools on turret-style lathes, says Don Graham, manager of education and technical services at Seco Tools, Troy, MI. “The beauty of the Mini Shaft is that you can put different tips on it,” he explains. “It uses a carbide insert that is mounted perpendicular to the tool shaft, and is locked in place with a serrated joint. The tip of the insert protrudes past the shaft, and is available in a number of different geometries—turning, boring, grooving, threading, back-turning, and even cutoff is possible.”
Given the right part—a thin-walled bushing or sleeve, for example—the entire part might be completed with a single tool. In lieu of that ideal situation, the Mini Shaft is still a great space saver, Graham says, and is popular with users of older or very small Swiss machines, where tooling stations are limited.
Graham disagrees with his counterparts somewhat, suggesting that a relatively “thick” chemical vapour deposition (CVD) coating can still be a good choice for small part turning. “Everybody talks about the fact that you can’t get sharp edges with CVD coatings,” he says. “The reason they say that is because using conventional coating techniques, putting CVD on a dead sharp edge often compromises toughness and resistance to chipping. However, if you’re careful about the way in which you coat the tool, and if you provide a tough substrate underneath the coating, sharper edges are possible.”
This provides the best of both worlds, says Graham. CVD coatings are typically more crater resistant than PVD when machining steels, stainless steel and cast iron, while the sharpness of these tools is a prerequisite for any small parts turning operation. Still, he admits that PVD is a good choice for many applications. “The most important thing is to eliminate BUE. Any coating is better than none in this respect. If you have sufficient spindle speed, CVD will wear better, but PVD is probably a better choice overall when cutting speeds are low.”
Brian Hamil, global product manager at SGS Tool Company, Munroe Falls, OH, says quality can be a concern as cutting tools become smaller, especially with micro-sized drills and endmills. “The biggest challenge many shops face with small part machining is the edge integrity of their cutting tools. It’s hard for an end user to take any kind of physical measurement, because contact with a micrometer or other instrument can damage the tool. About the only inspection you can do is to check it under a microscope. Even then, it takes significant magnification to detect micro fractures, or other flaws created during the grinding process.”
As a producer of rotary tools such as endmills and drills, much of the SGS lineup is geared towards machining work. Yet Hamil points out that Swiss machines are typically equipped with live tooling, and turn-mill centres account for a growing segment of the market. Wherever they’re used, endmills and drills should be held in an ER-style collet chuck or hydraulic holder if possible, and for very high rpm rotary tools, balanced holders should be used as well. “Eliminating runout is essential to good tool life in any machining application,” he says.
As parts and tools decrease in size, it’s important to avoid skimping on the quality of cutting tools and toolholders. Ultra fine grain carbide cutters made with grinding equipment designed for such work is necessary for maintaining predictable processes. SGS is supporting these efforts with a new line of micro-tools, scheduled for an early 2016 release. “The existing line is our MK series of endmills, which at this point only accounts for a small percentage of our tool lineup, but this will be expanded to 2000-plus line items,” Hamil says. “We’re seeing great interest from Swiss shops, obviously, but there’s also a lot of activity in mould and die, aerospace, the computer industry, and anyone doing medical work. Everything there is just getting tinier and more intricate, so there’s definitely a need for small parts and the cutting tools needed to produce them.” SMT