Problems with parting

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Dealing with the challenges of chip formation, evacuation, and tool life during grooving and part-off operations.

By Kip Hanson

In terms of machining difficulty, parting (a.k.a. cutoff) is a lot like drilling. The cutting tool is the same width/diameter as the feature being machined, making it tough to introduce coolant into the work zone and equally tough to get chips out. And as with the chisel edge on a drill bit, cutting speeds drop to zero as a parting tool reaches centerline. Depending on the material, the result is chipping or breakage, built-up edge, unexpected tool failures and inconsistent part quality.   

Parting (a.k.a. cutoff) is a tough nut to crack. Or at least, it used to be.

Seco Tool and others make grooving inserts that work equally well for turning and profiling, as shown with these -FT -RP geometries optimized specifically for these operations. Image: Seco Tools

Room to groove
Abhay Chaubal, product manager for the Americas at Seco Tools, seconds this viewpoint. “The biggest challenge when it comes to straight radial grooving and cutoff is that you’re creating a small opening in the workpiece that’s as wide as the insert itself,” he says. “So theoretically, the chip will be exactly as wide as the tool producing it, leading to binding.”

The solution, he explains, is to make sure that the chips produced during these operations have “room to groove,” meaning they don’t catch on the side walls and can easily move through the space just created. This is why Seco Tools and other cutting tool manufacturers focus their efforts on cupping the chip, making it narrower than the insert and allowing it to drop free.

“Ideally, it should form into a clock spring shape that will fall away once cutting is complete,” says Chaubal. 

Another example of unique grooving geometry is large copy-turning with wide grooves. Since the insert approaches the cut from multiple directions, the chip control geometry must follow suit. Horn’s new KR geometry for grooving and longitudinal turning/copy profiling was designed to reduce cycle time and produce manageable chips in multiple directions. Image: HORN

That said, there are trade-offs. In order to “cup” the chip and force it to be narrower than the groove, the top of the insert must be similarly cupped. This non-flat surface is not a problem during cutoff, but where grooves are concerned, what should be a perfectly even and flat longitudinal surface will exhibit a slight deviation from true at either end. “You won’t have a straight line,” he adds. 

Another concern is the “cutting speeds drop to zero” phenomenon touched on earlier. This problem doesn’t exist when cutting off tubes or parts with through-holes, but for parts where the tool must reach centerline, chip control and tool life alike suffer. Here, Chaubal recommends using a tougher grade of carbide, as well as a part-off tool with a slight angle (5° is not uncommon) on its cutting edge. 

If your CNC lathe is so-equipped, Y-axis parting might be your new best friend. Image: Sandvik Coromant

“Coolant starvation is also a challenge,” he adds. “For this, we’ve developed our Jetstream technology, which brings cutting fluids directly to the insert edge. We have a coolant port above the insert, which helps create a wedge between the insert and the chip while reducing the temperature in the cutting zone, and below, which cools the insert flank and mitigates wear. And by keeping everything cooler, you improve tool life and part quality.”

Under pressure
Edwin Tonne, marketing and training manager at Horn USA, suggests that there are several considerations when it comes to chip control and chip forming during grooving and part-off operations. “In general, the goal for a parting or grooving insert is the same as a CNMG or any other turning insert—to reduce the chip size to a manageable length so that it can be easily removed from the machining area,” he says. “You also want to avoid having it wrapping around the workpiece or toolholders, and avoid damaging the insert from chip hammering.”

For maximum machining productivity, look for parting tools that deliver cutting fluid above and below the workzone. Image: Sandvik Coromant

 For part-off operations, the most important factor is protection of the cutting edge, he adds. At the beginning and end of the cut, conditions are very unstable; if left unaddressed, these can damage or break the cutting edge and, in turn, destroy the tool. That’s because the tool and holder will deflect slightly when the tool enters the workpiece, causing uneven loading at the cutting edge and high pressure around the tool radii and flanks. And as noted earlier, the cutting speed approaches zero as the tool nears the end of the cut, so higher pressure is encountered at the cutting edge. “With part-off, these factors must be balanced in a way that reduces entry force but allows protection of the edge,” says Tonne.

Material is another consideration. For example, soft materials like aluminum require part-off tools with steeper top rakes and high edge sharpness, helping to reduce material adhesion that can lead to tool failure. It’s for this reason that HORN has developed more than 26 geometries across multiple platforms to address the many variations related to material, process stability, and cutting force requirements.

 In grooving, the balance of forces is equally important, but you should also consider other factors like feature size control, surface finish, heat, and chip width. If the chip width is not reduced during the grooving process, notes Tonne, surface finishes and dimensional tolerance can suffer. 

“In some applications, creasing of the chip helps to break them up, resulting in swarf that can be removed efficiently from the machine by the chip auger. Alternatively, if the chip is creased too much, unnecessary heat is introduced, resulting in poor tool life. Because of this, one universal geometry does not exist, even though many cutting tool suppliers have researched and attempted it,” he says.

Keep it cool and clear
Sandvik Coromant is another cutting tool provider addressing these challenges. Product manager Scott Lewis agrees with Chaubal on the importance of proper chip formation and the need for increased coolant flow during parting and grooving. Says Lewis, “Pretty much everyone has some form of coolant through the tool these days, and speaking for Sandvik Coromant, I can tell you we have it across all of our parting and grooving systems.”

This includes over and under coolant, either through the part-off blade or through the shank on grooving and shallow cutoff tools. For toolholders thus equipped, clean, filtered high-pressure coolant should be used if available, preferably upwards of 70 bar (1000 psi). “That’s a huge benefit, not only for tool life but also for chip control and flushing the chips out of the groove,” says Lewis. “This is especially true with deeper grooves, as it helps keep chips from binding up and reduces the risk for damage due to recutting.”

He elaborates on Seco’s clock spring statement, agreeing that on cutoff operations especially, a tightly-curled chip is better than the sixes and nines that machinists typically strive for, as these are less likely to jam. “If the chip former is working properly, it collapses the chip, making it a bit narrower than the groove,” he says. “It will then coil up and fall away, leaving less chance that it will whack the insert on the next rotation or score the side walls.”

Aside from high-pressure coolant and high-performance grooving and parting tools, Lewis is also a fan of Y-axis machining. Greater stiffness, higher stability, less chatter, and the ability to part off larger diameters and feed heavier while achieving better chip control—these are just a few of the advantages made possible on Y-axis equipped multitasking centers and some mill-turn machines. 

And yet, that advice comes with a word of warning. “On tools not specifically designed for Y-axis operations, the higher feedrates make it easy to overrun the chip breaker, leading to long stringy chips,” Lewis says. “But we’ve developed insert geometries to eliminate this problem, allowing shops to cut off parts faster than they ever thought possible. It’s pretty cool watching it for the first time.” SMT

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