Safety in small numbers: Drilling Performance

Successful deep-hole drilling is hardly easy. Move it into the microscale, which generally is considered to be no more than 3 mm (0.12″) in diameter, and the process becomes even more challenging.

Deep hole is a relative term, with the consensus that it starts at three diameters deep, but cutting tool manufacturers offer microdrills for going deeper — sometimes considerably deeper. Sandvik Coromant Co. in Mebane, North Carolina, for example, offers the CoroDrill 462 and CoroDrill 862 microdrills for producing holes up to six and 16 diameters deep, respectively, said Cory Schwenke, product manager for solid round tools.

He added that there are variations to those product lines. The solid-carbide CoroDrill 462 microdrills are available uncoated in diameters from 0.03 mm (0.0012″) to 3 mm and coated with titanium aluminum nitride in diameters from 0.2 mm (0.0079″) to 3 mm. The toolmaker produces solid-carbide CoroDrill 862 microdrills for going up to 16 diameters deep and ones made from polycrystalline diamond for reaching 12 diameters deep. CoroDrill 862 microdrills are coated with titanium aluminum silicon nitride, with silicon enhancing the coating’s heat resistance.

“When you’re drilling, heat is the devil,” Schwenke said. “It can cause a multitude of problems.”

Therefore, coolant is required to bring down the temperature when deep-hole microdrilling. The CoroDrill 462 line requires external coolant to be applied while CoroDrill 862 carbide tools 1 mm (0.039″) and larger have coolant channels. The smaller carbide tools, as well as all the PCD ones, receive external coolant, he said. Hole depth also plays a role, with either external or internal coolant being suitable for holemaking up to nine diameters deep and internal coolant only for making holes 12 to 16 diameters deep.

Sandvik Coromant makes the carbide microdrills as off-the-shelf standards, Schwenke said, but not the PCD microdrills.

“For the PCD lines,” he said, “we do have standard article codes, but those are all made to order in our facility in Spanish Fork, Utah, so they’re nonstock standards.”

Diving Deeper

Ceratizit USA Inc. in Schaumburg, Illinois, also provides microdrills for going deep. Its WTX Micro drills are available in diameters from 0.8 to 2.9 mm (0.031″ to 0.114″) for going up to 30 times diameter, said Patrick Maigatter, product manager for cutting tools and holemaking. The through-coolant tools are suitable for drilling metals that generate relatively short chips that can be evacuated out of the tiny holes.

WTX Micro drills are available in diameters from 0.8 to 2.9 mm for going up to 30 times diameter. Image courtesy of Ceratizit USA

“Basically,” he said, “it’s used for universal materials, which would be in the case steel, cast iron and stainless. Under the right conditions, it could also be used in titanium and titanium alloys.”

Ceratizit coats the tools in-house with Dragonskin, which describes the coating and the coating
technology, Maigatter explained.

“Dragonskin is our way of designating our premium coating offering,” he said, “and it can be different coatings.”

For the WTX tools, Maigatter said Dragonskin is an aluminum chromium nitride multilayer hard coating. Like other coatings for microdrills, it is deposited via physical vapor deposition.

He added that, if desired, uncoated WTX tools can be ordered as specials.

“But in this case,” Maigatter said, “for micro-deep-hole drilling, I would not recommend that.”

One of the latest offerings for deep-hole microdrilling is the Ken-Drill Micro from Pittsburgh-based Kennametal Inc. Launched in April, the toolmaker developed the line after conducting a survey of more than 340 participants nearly three years ago, said Frank Martin, holemaking global product manager.

“Based on the survey,” he said, “we knew exactly what the market was. We wanted to have a verified product line that is capable of machining multiple materials.”

Martin said the result is a line of through-coolant microdrills from 1 to 2.9 mm in diameter that drill up to 50 diameters deep. The smallest diameter starts with 1 mm because Kennametal determined that is what most of the market requires.

“Once we have a foot in the door and have ramped up our portfolio and volume,” he said, “we might focus on going a little bit smaller, maybe going down to 0.8 or 0.7 mm.”

When going significantly deep, effective coolant flow is extremely critical, Martin added, and the toolmaker enhanced the flow with a triangle-shaped coolant channel while maintaining tool stability.

“With a triangle,” he said, “you will see that the coolant flow rate is 3.7 times higher compared to a round coolant channel.”

Martin said the increased coolant volume flow enables the microdrills to be used on machine tools with a low coolant pressure capability. For example, the triangular channel can achieve the same volume flow on a machine with 20 bar (290 psi) of pressure as a drill with a round channel on a machine with 69 bar (1,001 psi) of pressure.

“We can apply this tool on a weaker machine,” he said.

Geometries at Work

When microdrilling very deep holes, Martin said cutting forces must be reduced to avoid tool breakage. Kennametal accomplishes that on its KenDrill Micro with special gashing around the drill point and special edge preparation on the cutting edges. The gashing helps direct the chip into the flute while the edge prep provides a smooth chip flow.

KenDrill Micro through-coolant microdrills are for producing holes up to 50 diameters deep. Image courtesy of Kennametal

He explained that KenDrill Micro drills have a multilayer PVD coating but not on the entire tool.

“Just the head area where you have the margin land design on it,” Martin said.

Because the coating slightly increases surface roughness, he added that the highly polished flutes are uncoated to help evacuate chips.

“Chip evacuation is really key,” Martin said.

Another way to help the coolant flow evacuate chips out of the hole is to alter a microdrill’s helix angle, Schwenke explained.

“If you have a high helix on a drill design,” he said, “you’re going to have some chip packing. If you start with a high helix and slow down that helix as you go further down the drill, it’s going to allow a lot more space for those chips to evacuate.”

Schwenke said CoroDrill 862 tools have the -GM geometry to create a more optimized drill for a specific task than the CoroDrill 462.

“If you’re looking for an optimized product where you’re running large batches of the same application, the same material, and you really want to nail down your process,” he said, “that’s where you’re going to go with the 862.”