Hard to Drill
- June 15, 2020
Holemaking is difficult enough as it is. Add tough materials into the mix and it can be a money pit. These tips can help
Chip packing. Drill walk. Premature wear. Tool breakage. These are just a few of our favourite issues faced by machinists when drilling—problems that can quickly escalate to “I’ll just go fishing” status when Inconel, titanium, cobalt-chrome or hardened steels are part of the equation. The good news is that most cutting tool manufacturers design tooling to tame even the wildest of alloys. It’s your job to find those tools and then apply them properly. It’s okay, we are here to help.
The good part about searching for the right tools is you won’t have to look far. David Vetrecin, Iscar Canada’s holemaking product manager, says the company’s Sumocham modular drill equipped with an ICM head is ideal for stainless steels and high-temp alloys. Walter USA LLC product manager Sarang Garud suggests his D4140 replaceable tip drill offers performance similar to that of solid carbide in a broad range of materials. Manfred Lenz, holemaking product manager for Seco Tools LLC, says solid carbide is preferred for smaller diameter holes, but the Crownloc Plus drill with a P or M-geometry head is a cost-effective and high-performing alternative. OSG applications engineer Alyssa Walther, fresh off a week of internal product testing, recommends the WHO-Ni (also called the WHO-55) for nickel-based alloys and hardened steel up to 55 Rc.
Those are just a few examples. All are excellent products. All have their share of sweet spots and success stories. While the advice on which one to purchase might vary depending on who you ask, each of the experts we interviewed offers similar recommendations. Walter’s Sarang Garud, for instance, points out that tool geometry is an important factor when drilling tougher materials, although the reasons might surprise you.
“The right geometry helps assure proper chip formation and evacuation, something that’s far more critical in drilling operations than it is on turning or milling operations,” he says. “Without good chip evacuation, the drill immediately seizes. Proper geometry goes a long way towards extending tool life.”
For austenitic stainless steels and superalloys like Hastelloy or Inconel, Garud says to look for drills with ‘easy-cutting’ geometries—typically a positive rake angle of 10° to 15° to effectively curl the chip. That’s the same reason spiral-flute drills tend to perform better than those with straight flutes. Finally, PVD coatings generally perform better in these materials, while CVD is more effective in hardened steels and cast irons.
Baking the cake
Garud also notes that hydraulic or shrink-fit toolholders are far better at minimizing runout, a crucial aspect of any high performance machining application. Iscar’s David Vetrecin agrees, pointing out that these toolholders are more rigid than traditional Weldon shank and ER collet holders, further improving tool life and hole quality when drilling hardened materials and tough, gummy, high-temp alloys.
He also suggests paying close attention to your cutting fluids. “I’ve walked into far too many shops that were running a general-purpose coolant, with a concentration that was way too low,” says Vetrecin. “They usually see tool life three to four times higher after switching to a properly mixed cutting fluid with the extreme pressure (EP) compounds needed for machining tough materials. In addition, we recommend using high pressure coolant whenever possible, as this reduces heat and helps evacuate chips.”
Underfeeding the drill is another common problem. Nickel-based alloys in particular tend to harden, making a bad situation even worse. Again, the setup must be very rigid, lest chipping occurs (something the ICM head with its reinforced cutting edge helps prevent). Steer clear of double margin drills, as holes made in high temp alloys tend to shrink behind the drill, sometimes trapping it. “Drilling in superalloys and hardened steels is a little like baking a cake,” he quips. “Without all the right ingredients, it’s not going to turn out very well.”
Heat is the enemy, explains Manfred Lenz of Seco Tools. He concurs with Vetrecin’s comments about keeping the feedrate up to reduce work hardening, but says that many shops run higher spindle speeds than they should, exacerbating the problem. “You can’t expect someone who drills steel and aluminum all the time to understand superalloys. They’re used to be running at 600 sfm, so they drop it down to 300 or so on their first Inconel job, but it’s still way too fast. That’s why I always tell customers to follow the cutting tool manufacturer’s recommendations. This is true for all materials, but doubly so with high temp or hardened materials.”
And doubling up on Garud’s coating recommendations, Lenz says there have been new developments in coating technology lately, with niobium nitride (NbN) among them. “It’s a game-changer in a variety of applications, but especially demanding materials. Shortly before its introduction, we wanted to do some additional field testing, so we sent a drill and some inserts to a well-known aircraft manufacturer. They emailed back a week later to say they weren’t going back to their old grade, and ‘when can we get some more?’” The product wasn’t even in the catalog yet, so Lenz ended up delivering specials until Seco could ramp up production. “It was that effective,” he says.
OSG's Alyssa Walther has a similar story to tell, this one on hardened materials. “The WHO-Ni is an up-sharp, coolant-fed drill designed specifically for demanding aerospace and medical applications,” she says. “Its WXS coating is optimized for high heat and high hardness applications, with a heavy core diameter for optimal stability and a low helix design that maximizes chip removal. But we also have drills for super hard materials. The WH-70 drill, for example, handles steels up to 70 Rc, while our XH-DRL is actually known as a tap extractor but can also be used to drill holes in hardened workpieces. Granted, it’s not very fast—you’re probably running a feedrate of around 0.012 mm/rev (0.0005 in.), with a generous supply of lubricating oil and pecking like crazy, but they’re very effective when applied properly.”
Walther makes a valid point for anyone who might be tapping or thread-milling holes in superalloys that backs up what others warned about: work hardening.
“A lot of times, people will come to us and say, ‘Hey, your catalog says the feeds and speeds are correct for Inconel 725, but our taps keep breaking.’ And after a little investigation, we usually find the problem isn’t the tap—it’s the drilling operation beforehand. They didn’t realize their drill was dull or they were running too fast or feeding too slow and the material around the drilled hole is hard as a result. That’s why we tell people to routinely inspect their drills—if the chisel edge has broken down, they’re probably overfeeding it, or they didn’t have the correct geometry to begin with. Either way, it’s important to look at the entire process because the problem might not be what you think it is. And if things aren’t going right, call us,” says Walther. SMT