Know how to scale down

Multiple methods are available to make a miniature hole. In the past, laser micromachining and electrical discharge machining were frequently the default options because of technical limitations with the grinding machines used to produce the required geometries on solid-carbide microdrills, said Jake Rutherford, research and development engineer for Kyocera SGS Precision Tools Inc. in Munroe Falls, Ohio.

“The new grinding machines have a tremendous amount to do with making accurate geometries that small,” he said.

Nonetheless, a cutting tool manufacturer designs microdrill point geometries to suit not just the intended application but the production of the drill, said Sarang Garud, product manager for Walter USA LLC in Waukesha, Wisconsin.

“The extremely small size of the drills makes it pretty hard to manufacture the drills,” he said, “and design for manufacturing governs some of the drill design decisions as well.”

Garud said two important factors when designing microdrills are cutting force reduction and secure chip evacuation.

Rutherford said he considers any drill with a diameter smaller than 3.175 mm (0.125″) to be “micro.” Kyocera SGS Precision Tools offers standard microdrills down to 0.041 mm (0.0016″) and ones with through-coolant capability as small as 1 mm (0.039″) for producing holes up to 15 diameters deep.

The CrazyDrill Flex microdrill is made of flexible tungsten carbide and features integrated cooling. Image courtesy of Mikron

For Mikron Tool SA, micro starts with diameters smaller than 1 mm, said Alberto Gotti, head of the Technology and Customer Project Center, who’s based at the toolmaker’s headquarters in Agno, Switzerland. (Mikron Corp. Monroe is in Monroe, Connecticut.) Mikron’s microdrill line starts at 0.1 mm (0.0039″) in diameter, and drills for cutting difficult-to-machine materials, such as stainless steel, titanium and heat-resistant superalloys, start at 0.2 mm (0.0079″). Tools are available for drilling up to 50 diameters deep.

“Microcutting tools are not simply a reduction in size,” he said. “With each downsizing step, the degree of difficulty increases exponentially. The smaller the surface area of the tool, the more difficult it is to manufacture the geometry while maintaining the required quality and tolerances.”

Gotti said the variety of microholemaking applications continues to increase as a result of the miniaturization trend, especially in the medical industry, where more minimally invasive operations are performed with ever-smaller instruments, such as stainless steel endoscopic forceps and titanium bone plates. Other parts include automotive fuel injectors, stainless steel watch cases, dental abutments and nickel-based turbine blades.

Garud said microdrills also are used to make holes in nozzles, perforated discs and pneumatic components.

Cool Down

Similar to macroscale applications, applying a through-coolant drill is required, if possible, when deep-hole drilling with micro​tools. In addition, it’s critical to have proper coolant concentration, quality and pressure to effectively evacuate chips out of the hole, said Jacob “Pete” Rak, applications engineer for Kyocera SGS Precision Tools.

“If possible, internal coolant should always be used for reliable operation with efficient chip removal,” Garud said, noting that Walter USA offers microdrills as small as 0.7 mm (0.0276″) with internal coolant.

However, some applications, such as those for the food and medical industries, may not allow the use of coolant because of customer specifications.

When coolant application is suitable, Garud said it’s extremely critical to use a tool manufacturer-recommended coolant filtration system so the delicate tools do not re-cut the tiny chips.

Gotti said when microdrilling with high cutting parameters, which requires a spindle speed of at least 24,000 rpm, internal coolant is essential. Mikron offers tools with integrated cooling channels from 0.2 mm in diameter. In addition, the company provides tools with both round coolant channels and ones with a teardrop shape, which reportedly provide up to four times the coolant volume.

“Efficient machining fails due to low thermal conductivity of the materials,” he said, “which increases the risk of overheating the cutting edges and thus their breakage.”

Effectively cooling the cutting edges also prevents built-up edge.

Kyocera SGS Precision Tools offers a variety of solid-carbide microdrills. Image courtesy of Kyocera SGS Precision Tools

For microdrills without coolant ports, Rak said end users must target the flood coolant along the flute length and not just the tip so coolant reaches down into the hole where the tool is buried in the workpiece. The fragile nature of a microdrill means that too much coolant pressure can break the tool.

“You have to have that perfect balance,” he said.

Poking Away

At a certain depth, depending on the application, Rutherford said a peck cycle becomes unavoidable with a non-through-coolant drill to clear chips and help direct coolant into the hole.

“When you do peck,” Rak said, “you don’t want to retract all the way out of the hole so the entry is not misaligned.”

Rutherford said the most strenuous time for any cutting tool is during entry into and exit from a workpiece.

“These little tools can’t afford any unnecessary abuse,” he said. “The more you can avoid loading and unloading a tool, the better tool life and performance you will get.”

Even improper handling can be detrimental to microdrill life.

“I have been guilty of breaking one or two off with my finger,” Rutherford said, “and it is terrible to get it out of there. It’s like a little razor blade splinter.”