Not to sound like a Debbie Downer, but grooving and especially parting operations can be a real pain in the neck. Built-up edge (BUE) is common, as is edge chipping, plastic deformation of the cutting tool, and thermal shock when the insert retracts from a deep, coolant-starved groove. If not caught in time, any of these can lead to catastrophic tool failure, potentially breaking the toolholder, scrapping the part, or both.
Fortunately, cutting tool manufacturers have worked hard to address these problems. Grooving and parting (aka cut-off) tools now last longer, cut faster, and are more versatile than ever. From through-the-tool, high-pressure coolant capabilities to tooling systems that are equally adept at turning as they are grooving, what were once indexable problem children have become rock stars of the CNC lathe and multitasker department.
John Winter, Eastern U.S. product manager for Sandvik Coromant, Mebane, N.C., suggested that success in grooving and parting applications often depends on proper tool selection. “I see a lot of customers using whatever’s available in the tool crib, but this is often the wrong geometry for the application,” said Winter. “For instance, customers will often grab a grooving or a groove-turn insert for a parting operation. Yes, this will work, but it’s quite likely that the tool will chip as the part breaks free.”
The problem here is one of insufficient cutting speed—as the cut-off tool nears the part centerline, SFM (surface feet per minute) drops to zero. That’s why Winter and others recommend using an insert with a geometry designed specifically for parting operations, one that’s resistant to chipping, folds the chip for efficient evacuation, and generates the best possible surface finish on the part’s backside.
Edwin Tonne agreed with each of these attributes. A senior engineer and training manager for Horn USA Inc., Franklin, Tenn., Tonne said tool breakage is quite common during parting operations. “It’s a demanding process. The tool is in continuous contact with the material, trapped on both sides until the workpiece breaks free. This builds tremendous heat, and the reduction in cutting speed raises forces significantly.”
Stability is crucial in these situations, he added. So is effective cutting fluid introduction. As such, Horn and a number of other cutting tool manufacturers have developed systems that facilitate the delivery of high-pressure coolant (HPC) directly to the material shear zone, keeping the area cool and well lubricated.
Material variability also must be addressed. This is a valid concern for job shops and other high-mix, low-volume manufacturers that might cut aluminum one day and a superalloy the next. In these situations, a “universal” or general-purpose geometry won’t perform as desired. Said Tonne, “This is precisely why it’s so important to find a material-specific grade and geometry whenever possible, especially when machining tough or hard materials like nickel-based superalloys and austenitic stainless steels.”
Steve Vanderink, sales manager for Iscar USA, Arlington, Texas, pointed out the need for process reliability and versatility during grooving and cut-off operations, which is good advice for any machining application. “That means the ability to adjust to any part or material that comes through the shop while maintaining a solid, predictable process,” he said. “In terms of parting, it’s a very abusive application, particularly when cutting off solid bar stock to center, so carbide substrates need to be both wear-resistant and strong.”
In today’s cutting tool market, customers want it all, Vanderink added, then listed several examples to support his argument. Among these are extreme rigidity for higher productivity and the stable processes just described, quick-change capabilities that help reduce downtime when changing tools and inserts, and coolant-through toolholders to deliver cutting fluid directly to the work zone for better chip control and increased insert life. Last but not least, customers demand economical tooling.
Tooling manufacturers have long supported this last goal by providing as many cutting edges per insert as possible. But as Vanderink pointed out, delivering coolant directly through the insert becomes increasingly tricky as edge count goes up. That’s why Iscar and some competitors have developed clamping devices that accomplish the same thing, or by plumbing coolant through internal channels within the toolholder. “This allows you to use lower-cost, non-coolant through blades, for example, while still introducing a generous supply of cutting fluid,” he said.
Just as efficient parting is crucial in many turning applications, so is grooving. Sandvik Coromant’s Winter noted that proper programming techniques can help assure success. For instance, when the groove is not much wider than the tool, the approach is straightforward—plunge to the groove’s finished diameter, then take a clean-up pass on each side, breaking the edge as you go. For wider grooves and large undercut areas, though, many shops will use a step-over method, taking a series of sequential roughing passes in the Z- direction, followed by the previously described finishing passes.
“This is a simple way to program a grooving routine, but since one corner of the insert is doing most of the work, it makes for poor chip control and tool life,” he said. “An alternating step-over method is preferred in this case; because you are always driving the chip with the center of the insert, it allows for better chip control. It also keeps the tool wear centered on the insert where it belongs. Plunge turning with a zig-zag approach is another method, but here you need to use an actual groove-turn insert, which has a geometry designed specifically for side turning in either direction and will provide much greater tool life and productivity than a plunge-only grooving tool.”
Walter USA LLC, Waukesha, Wis., is another cutting tool provider with plenty of grooving, parting, and groove-turn inserts to choose from, yet product manager Sarang Garud noted that there’s also copy turning. Historically, copy turning describes a lathe with a special attachment called a tracing arm. This follows the profile of a part template, “copying” that shape onto the workpiece. Thanks to CNC, however, such contraptions are no longer needed, making the original technology obsolete for all but woodworking hobbyists and a few specialty machine tool builders.
“In today’s machining world, the term ‘copy turning’ has come to represent various profiling operations,” said Garud. “These include recessing, producing undercuts, and so on, many of which can be performed using groove-turning inserts. And while we certainly provide these tools, it’s sometimes more effective to use traditional ISO V- or D- style diamond-shaped turning inserts, geometry permitting, as well as ISO R-style “full round” inserts. And in our case, we’ve introduced a copy turning system that combines the best attributes of each.” More on this shortly.
Garud suggested that whatever the turning tool, using a “towards the spindle” turning approach offers several advantages over the back-and-forth, zig-zag toolpaths mentioned earlier. Because groove-turn inserts are more susceptible to deflection from side loading, tool life can suffer. It can also be difficult to achieve the high feedrates possible with ISO-style inserts, so chip control is often less effective. “In addition, traveling away from the spindle means there’s a tendency to pull the workpiece out of the chuck, and the machine tool is sometimes not as rigid in this direction.”
It’s clear that each provider has its own tooling recommendations, but there are a few bits of application advice that each can agree on:
Always strive for the most rigid setup possible, with short overhangs and robust clamping.
For longer parts, use a tailstock and live center to support the end of the workpiece.
Most shops tend to back off on feeds and speeds when difficulties arise. Always follow the manufacturer’s recommendations and, when in doubt, ask.
Use the correct geometry—parting inserts for parting operations, groovers for grooving, etc.
Select a material-specific grade and geometry, especially for tougher, harder materials.
For parting operations, make certain the tool is on center (+/-.002"), and for the last 10 percent of the cut, reduce feedrate slightly and turn off the coolant.
An 8° parting insert will follow a straighter line than one with a 15° angle that allows the part to fall off sooner, leading to a larger burr. It’s important to balance the two.
On lathes with a sub-spindle, always use a parting tool with a 0° angle. It will cut the straightest and leave very little burr.
Each of the companies here has success stories they’d like to share, and rightfully so. HORN, for example, has recently released several new carbide grades and insert geometries for use with alloys such as titanium, Inconel, and PH stainless steel. Tonne said the 3V IK is designed with coolant through the insert for improved heat removal and lubrication of the cut, while the IG35 grade for stainless steels and the SG3 coating for high-temperature alloys are each noteworthy additions to any machine shop’s tooling arsenal.
Walter’s Garud listed several benefits of the company’s new W1011-P copy turning system. “Our patented seating technology provides a 50 percent improvement in indexing accuracy over conventional turning inserts, while the triangular design adds one more cutting edge over conventional V- or D- style diamond-shaped inserts and two-sided grooving inserts, thus reducing cost per edge,” he said. “And despite concerns over turning away from the spindle, the positive lead-angle on the insert’s trailing edge thins the chip, allowing up to 60 percent higher feedrates without compromising tool life.”
Iscar offers similar benefits. Vanderink said parting to centerline can be very difficult, especially on high-temp alloys or when you need to get it done quickly while maintaining part quality. Doing so is even more difficult when machining high-temp alloys. “With the Jet Crown System, we have seen cost savings of up to 75 percent in some cases. In one recent example, a customer was parting off 1.250" [31.75 mm] deep in titanium using a 3 mm wide insert. Compared to their legacy tool, they doubled feedrates from .002" to .004" [0.05 to 0.01 mm] per revolution, bumped up the cutting speed from 80 SFM to 90 SFM, and went from three parts per tool to fifteen.”
Lastly, Sandvik Coromant’s Winter highlighted the tooling provider’s Y-axis parting solution. “CoroCut QD allows cutting forces to be directed through the blade and into the machine turret, increasing bending stiffness by more than 600 percent” he said. “This means feedrates 30 percent or greater than with standard part-off tools and a corresponding improvement in tool life. We’ve also tested using the Y-axis for groove-turn operations and have seen better finishes, chip control, and tool life. For shops with a Y-axis CNC lathe or multitasking machine, we strongly recommend giving this innovative solution a try.”
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