Beyond Coring

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by Kip Hanson

Three-flute drills assume high performance holemaking responsibilities 

Many shops use three-flute drills for “coring out” and straightening existing holes in castings or stampings, or for enlarging previously drilled holes in machined parts. In these instances, the drill works like a reamer–there’s no cutting at the tip of the tool, only along the margins. But there’s a new breed of three-flute drills available that’s designed specifically for holemaking in solid materials. If you haven’t taken a look recently, you might be missing out on higher penetration rates, improved hole quality, and a significant boost to the bottom line. 


Home run product
OSG is one cutting tool supplier with such an offering. Applications engineer Alyssa Walther says the ExoPro Mega Muscle is available in 3xD and 5xD flute lengths, with standard diameters ranging from 4 mm (0.157 in.) to 20 mm (0.787 in.). All have three flutes, are solid carbide with coolant through the tool, and are designed for high performance drilling of steel and cast iron. 

“The starting feedrate for a typical two-flute drilling application in alloy steel would be around two to three per cent of the drill diameter per revolution,” says Walther. “By comparison, the low end recommendation for our three-flute drill is four per cent, and we’ve seen up to sixteen per cent under the right conditions. One time, we accidentally rapid traveled a Mega Muscle drill into a cast iron  workpiece and it didn’t break. That’s a testament to how strong these tools are.”

Like the other tooling suppliers interviewed for this article, Walther says coolant through the tool should be used to aid in chip evacuation, preferably at 20 bar (300 psi) or higher–OSG uses 1000 psi (80 bar) in its testing labs. Most also agree that a hydraulic or shrinkfit toolholder would be the first choice to keep the drill secure and to minimize runout, although an ER collet might suffice. 

Three-flute drills are popular in automotive work, where hole quantities often number in the millions. Image: OSG“There’s a greater chance that the drill will spin in an ER collet, especially at the higher penetration rates these tools are capable of,” she notes. “So while we don’t say ‘no ER collets,’ we definitely recommend being cautious when using one, and starting out at the low end of your cutting parameters.”

Better holes, faster feeds
Nicholas Lieffers, cutting tool engineer at Nachi America Inc., says higher penetration rates are a definite advantage of three-flute drills, but adds that hole quality is just as important. “Quite often you’ll find that a hole made with a two-flute drill isn’t actually round, but slightly oval,” he says. “The additional cutting edge on a three-flute drill eliminates that–not only can you achieve much higher feedrates, but improve hole roundness and surface finish as well.”

According to Lieffers, Nachi’s Aqua line of solid carbide three-flute drills does just that. They are ground to a JS6 tolerance, which he says is more accurate than the H6 or H7 seen on competing drills. Together with the added stability of the third flute, this allows some customers to eliminate secondary reaming operations, saving additional time and money. 

It also provides longer tool life. Says Lieffers, “we see success in multiple applications, including stainless steel and hardened materials. For example, I worked with a customer recently that was drilling 46 Rockwell steel. They were obviously pleased with the higher production rates made possible with the Aqua drill, but when they inspected the tool after 1,000 holes and it still looked brand new, they were completely blown away. It’s a really versatile drill.”


Stepped holes like the one shown here are an excellent application for engineered three-flute step drills. image: Sandvik CoromantThird flute’s the charm
For those shops that prefer indexable tooling over solid carbide, there’s the Logiq3Cham three-flute drill from Iscar Tools Inc., a steel body, indexable head drill designed for steels and cast irons that’s currently available in size ranges from 6 mm (0.236 in.) to 25 mm (0.984 in.) diameters, with 4 mm (0.192 in.) versions being released later this year. 

Product manager for holemaking David Vetrecin says this will make the Logiq3Cham the smallest indexable drill on the market, something that required the company to develop special packaging to make the tool easier to setup. “The installation key needed to locate the insert onto the body is built into the head,” he explains. “That way you don’t have to actually touch the tip of the insert.”

Considering the feedrates made possible by three-flute drills–in some cases, 25 to 50 per cent higher, according to Vetrecin–it’s surprising that three flute drilling hasn’t caught on more. “The biggest challenge for any cutting tool manufacturer with these tools is chip evacuation. In our case, we completely redesigned the tip geometry and flutes to accommodate that. Given the challenges faced by shops that do a lot of holemaking, I see this is an increasingly common solution.”


Something special
Aaron Howcroft, global manager of product management and R&D at Sandvik Coromant, agrees. “Push a three-flute too fast and there’s a greater chance of it getting jammed with chips, especially as you approach 8xD. As a result, the cutting tool needs to have good coolant flow and sufficient flute clearance. These are reasons why our CoroDrill 430 series of three flute drill solid carbide drill is very popular with automotive suppliers making engine blocks, transmission casings, gear boxes, and so on.”

Holes made with a three-flute drill are both rounder and straighter than those drilled with a two-flute tool.image: NACHIMany of these components are made of high silicon aluminum. In this application, Howcroft recommends an engineered tooling solution using veined polycrystalline diamond (PCD) brazed to the flutes and cutting edges. This provides far greater tool life than carbide in this highly abrasive material, as well as less “sticking” thanks to PCD’s low coefficient of friction, leading to improved surface finish and hole accuracy. 

Howcroft points to another common use of three flute tools: step drilling and porting, operations that are prevalent in the automotive and fluid power industries. The CoroDrill 430 series drill ranges from 3 to 25 mm in diameter (0.118 – 0.984 in.), and can be ordered with up to three steps or chamfers for high volume production of multi-diameter holes. 

Whatever the drilling solution, three-flute drills cost more than their two-flute counterparts, and way more than the high speed jobber drills many shops are familiar with. Once you’ve gotten over the price difference, you’ll want to speak with your tooling representative about a reconditioning program–because of the complex (and proprietary) geometries used on these tools, each of the suppliers listed recommends sending them back to the manufacturer for sharpening. 

Says Howcroft, “three-flute drills, particularly those with brazed PCD edges, can mean a substantial financial commitment for some shops, although that’s easily offset by higher productivity, improved quality, and extended tool life. They make a lot of sense for a large number of applications.” SMT

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