CANADA'S LEADING INFORMATION SOURCE FOR THE METALWORKING INDUSTRY

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CANADA'S LEADING INFORMATION SOURCE FOR THE METALWORKING INDUSTRY

CANADA'S LEADING INFORMATION SOURCE FOR THE METALWORKING INDUSTRY

Parting Ways: Still using yesterday’s grooving and cutoff tools? Time for an update.

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Few machining events are more disheartening than spending 10 or 15 minutes turning a beautiful, accurate workpiece only to have the parting tool fail on the final operation; time to break out the hacksaw. Sadly, such occurrences are far too frequent. As with deep grooving, cutoff tools are prone to coolant starvation. Built-up edge (BUE) is common. So is tool chipping, thermal shock, and plastic deformation, any of which can lead to catastrophe if not caught in time.

The reasons are simple. Cutoff tools and groovers are often asked to plunge deep into a workpiece. Achieving efficient chip evacuation is difficult, and as mentioned, cutting fluids struggle to reach where needed most—the work zone. But most challenging of all is the fact that cutting speeds approach zero as the tool nears centreline. The good news? You can do something about it.

Tungaloy’s newly developed ADDFORCE clamping system is said to provide “higher tool rigidity, increased stability and reliability for deep grooving and parting-off.”  Image: Tungaloy

A trove of tips
So say machining experts from Iscar, Horn, Tungaloy, and Sandvik Coromant. Though their recommendations on cutting tool brands might differ, most agree on the following best practices:

When cutting off solid parts (those with no through-hole), program the tool to stop just shy of complete cutoff and allow the part’s weight and centrifugal force to complete the parting operation. Depending on the size of the workpiece, this can range from 0.05 mm (0.002 in.) to 0.25 mm (0.010 in.) or more away from X0.

For cutoff and grooving operations, the tool should be within 0.10 mm (0.004 in.) of the part’s centreline. Some recommend that external groovers (and, by extension, cutoff tools) work best when placed slightly above centre, while internal and face grooving tools are the exact opposite.

When parting off, reduce feedrates by at least 50 per cent and up to 75 per cent when the tool reaches a diameter of 2 to 3 mm (0.04 to 0.12 in.). At least one contributor suggested switching from constant surface speed (G96) to a constant spindle speed (G97) of 1000 RPM at this same diameter.

On groove widths greater than 4X depth, use a groove-turn method with a tool designed for multidirectional turning. Conversely, a groove depth exceeding 4X its width is best attacked using a traditional plunge grooving approach and a tool approximately 80 per cent the width of each radial pass.

If available, high-pressure coolant at 20 to 70 bar (300 to 1000 psi) will significantly extend tool life and improve chip control while permitting higher cutting speeds. This is especially true when applied through the tool and directly into the cutting zone.

Tungaloy Canada’s product manager Hartej Gill notes as well that machinists should keep the parting tool blade length to a minimum, and that tool deflection is equal to three times its overhang amount. Also, don’t hesitate to use custom inserts for high-volume production runs; tooling that he suggests offers excellent performance.

HORN says its EH geometry parting and grooving tools support feedrates up to 0.4 mm/rev (0.016 ipr). Image: Horn USADialing in
“There are general-purpose geometries available that can part-off, groove, groove turn, face groove, and internal groove,” he says. “These are perfectly fine for short-run production, but for higher volumes, a material-specific grade with a geometry designed for that particular width and depth is best. For example, dedicated part-off inserts like those offered by Tungaloy have cup-shaped chipbreakers that produce chips narrower than the insert width. These geometries also curl the chip into clockwork-like springs, making them easier to evacuate from confined spaces.”

Ashok Guruswamy, product manager for Grip/Turn products at Iscar Tools Canada, agrees on the need for efficient chip formation. He says selecting the proper tool geometry when grooving and parting has a significant influence on chip formation, chip shape, surface finish, and tool life. He also seconds what was said earlier about high-pressure coolant. “This is the idea behind our new Jet-Crown parting system, which is designed for use with square Iscar’s Tang-Grip adapter blades,” says Guruswamy. “It has two coolant holes for effective ultra-high pressure coolant flow up to 340 bar (4900 PSI) and a unique clamping system for vibration-free grooving and parting. And for higher productivity, we offer 3 mm (0.118 in.) and 4 mm (0.157 in.) wide High Feed inserts (HF) for the Tang-Grip.”

Though primarily designed for  high-efficiency turning operations, Sandvik Coromant’s PrimeTurning  can also be used to machine wide grooves and undercut areas.  Image: Sandvik CoromantFerocious feeding
Higher feedrates on any machining operation are a good thing but imagine grooving or parting up to 0.4 mm/rev (0.016 ipr)! If you have a Y-axis lathe and one of Horn USA Inc.’s EH-style inserts and System 100 toolholders, this is entirely possible. That’s according to the company’s training and technical specialist Edwin Tonne, who adds, “If your machine is rigid enough to support such feedrates, Y-axis parting is extremely productive.”

There is a caveat, notes Duane Drape, national sales manager at Horn. “The challenge here is that not all machine builders support constant surface speed mode, or G96, on the Y-axis. In some cases, you need to trick the machine by temporarily changing a parameter to make the machine think that the Y-axis is the X-axis, and then change it back when the part is complete.”

Another unconventional recommendation for mill-turn lathes, multitaskers, and multi-spindle machines is to use a live-tool attachment and rotary slotting cutter for parting and grooving operations. Tonne says such “groove milling” is very effective on smaller parts under 28 mm (1.10 in.) and for grooves up to half that amount in depth. “In most applications, the live tool and lathe spindle would be set to counter-rotate, although it’s also possible to lock the main spindle and perform a slitting operation to cut the part away.”

Y-not?
Sandvik Coromant product and industry specialist Kevin Burton has similar advice. Like Horn, Sandvik Coromant also offers a Y-axis parting system. Burton notes that, compared to conventional parting on the X-axis, Y-axis directs the cutting forces into the tool assembly and turret, hence its ability to support extreme feedrates like those just mentioned. However, since swapping the X and Y axes via manual parameter setting is inconvenient, he suggests using 3D coordinate rotation if available (G68.1 on FANUC controls) or by switching the axis assignment with a P-code on the G96 command. Warns Burton: “Be sure to check with your machine builder before attempting these programming techniques, and always verify them in dry run mode before use.”

Lastly, he notes that wider grooves and undercut sections can often be machined using Sandvik Coromant’s PrimeTurning solution. This is especially suitable for multitasking machines, where the B-axis head can be swiveled to the appropriate angle for the workpiece, although standard turret-style lathes are also a candidate. “Here again, the programming is a little different, so ask your CAM provider for support or download our CoroPlus ToolPath code generation tool,” he says. “Either way, PrimeTurning frequently offers productivity gains of 200 per cent or more, with triple the tool life.”  SMT

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