- Published: May 15, 2017
Micro drill manufacturers achieve Lilliputian dimensions, solving big problems for their customers
You just received the order for those parts you quoted last month, the ones with holes smaller than the hairs you’re now pulling out of your head. How are you going to drill them? It might not be as hard as you think. Toss out the bids for the hugely expensive laser drilling machine you were considering. Tell the shop across town you won’t need its EDM hole popping services. You’re going to drill those holes the old-fashioned way, but use a newfangled twist: a super tiny carbide drill.
Be careful handling them. These drills are fragile, especially those in the 0.5 mm and smaller neighbourhood. You should probably plan on hand loading the tools into your machining centre, as the speed of most toolchangers today is enough to shock these drills into oblivion. And you’re going to need all the spindle rpm you can muster. The good news is there are quite a few tool options to choose from, and plenty of good advice to get you started.
Luke Pollock, product manager for Walter Tools, says micro drills are used “all over the place” but one notable application is for the food industry, where stainless steel tubes drilled with thousands of holes are used to separate the non-edible parts of chickens and turkeys from the meat that goes into our food products. “It sounds a little gruesome, but it’s actually a pretty high-tech process,” he says. “The manufacturer has to drill maybe 40,000 holes in each screen, so their biggest concern is punching holes as fast as they can.”
Pollock defines micro as any drill smaller than 3 mm. Most carbide drills in this classification, regardless of manufacturer, have either a 3.0 mm (0.118 in.) or 3.17 mm (0.125 in.) shank. In terms of geometry, 140° point angles are fairly common, with multi-facet split points, helix angles, and cutting geometries not all that different than “macro” or traditional carbide drills. Multiple length-to-diameter (L:D) ratios are also available, with a few drills boasting 18xD and even 30xD, well into “deep hole” territory.
And while the thought of successfully grinding tools this small and this long might seem implausible, some micro drills are even eligible for regrinding; Walter Tools, for example, regularly reconditions drills up to three times, and more in some instances. “A lot of it depends on the customer’s part requirements and their willingness to just keep pushing the point back further and further. Eventually you’ll start to lose performance, but sometimes the customers don’t care about that if the tolerances and process reliability permits.”
So what are the key differences between micro and macro, or traditional carbide drills? According to Pollock, one key differentiator is the coatings used, which become thicker relative to the drill as its diameter decreases. This is why he recommends coating only the tip of the drill, while polishing the balance of the flute length. “This protects the tip from heat and wear, but provides a very smooth surface along the rest of the drill, promoting chip evacuation.”
Speed it up
As with the others interviewed for this article, Pollock suggests using the cutting tool manufacturer’s recommended feeds and speeds for the application, but admits this can be difficult to achieve on very small holes. For example, a 0.5 mm (0.019 in.) drill running at just 18 sq m/min (200 sfm) would require 40,000 rpm, faster than many machines are capable of. In this case, a speedier head might be in order. These can be driven via air, mechanical, or electric power, but Brian Sawicki, business development manager at Tungaloy America Inc., says the company’s SpinJet requires nothing more than clean, filtered cutting fluid, and is capable of achieving 60,000 rpm at 4MPa (580 psi) coolant pressure.
Aside from proper surface speeds, Sawicki recommends light feedrates, often just a “few tenth” per flute (roughly 0.008 mm), depending on the material and drill size. He says one of the largest consumers of micro drills is the aerospace industry, which uses them to drill holes in fuel filtration components made of A286 and MP35N, alloys that are high in nickel and chromium and a real bear to cut. “In this particular application, the holes have to be completely burr free, otherwise the fuel flow will be disrupted,” he says. “The good news is, that’s a tell-tale sign the drill is getting dull. Once you start kicking up even the slightest burr, it’s time to change the tool, otherwise it’s going to break off. There’s not much forgiveness with drills this small.”
Another area with little to no forgiveness is tool runout, something that’s especially important to control as drills edge into micro territory. This is why most cutting tool manufacturers recommend either shrinkfit or hydraulic toolholders for micro drills, although some suggest that a precision ER collet is likewise suitable. One such proponents is Joe Negron, micro industrial product and sales manager for Kyocera Precision Tools Inc., who says a high quality collet is often a better choice for the extreme spindle speeds common with micro drilling.
Negron says those shopping for micro drills for use in precision metalworking applications should steer clear of a lower cost option for making micro holes, the circuit board drill. “Many end users are confused by this,” he says. “PCB-style drills were designed for the printed circuit board industry during the 50s and 60s. They have a two-piece brazed construction, with a 3.17 mm (1/8-in.) shank diameter and a standard 38 mm (1 1/2 in.) length. Some are equipped with a plastic ring on the shank, for use as a locating surface in PCB drilling machines. The problem with PCB drills, however, is that because of the tremendous production volumes associated with this work, it’s all gone offshore and the cutting tool manufacturers have been forced to squeeze every penny possible from the cost of their products. The drills often separate during metal cutting operations, and you’ll likely have other quality problems besides. For general purpose machining, you really want to stick with solid carbide, high performance drills designed specifically for this kind of work.”
Like several others competing in this challenging micro arena, Kyocera offers coolant-fed drills, which in its case are as small as 1.5 mm (0.059 in.) diameter. Because of the extremely small internal holes in such drills, high pressure coolant is recommended, and proper filtration is even more important than with larger hole drilling and machining operations. Kyocera also offers one of the smallest drills available, just 0.04 mm (0.0015 in.) in diameter. But even at this miniscule size, they’re not the smallest.
OSG Tool Canada Ltd. offers the Max-Mini UVM 5D drill, which according to application engineer Sam Matsumoto, is available down to 0.02 mm (0.0008 in.) diameter, and is used to drill holes in ceramic green sheets and probe cards. “The majority of usage is in the semiconductor industry, mainly for smart phones,” he says. “This includes machinable work materials ranging from machinable ceramics, various stainless steels, and also hardened steel such STAVAX 53HRC (a pre-hardened stainless tool steel).”
Despite their elfin size, Max Mini drills receive a proprietary coating that Matsumoto refers to as super smooth. “The thickness of the UVM coating is 0.2 microns (0.000008 in.). With our high quality and high precision coating process, there are no droplets present on the coating surface, which assures great surface finish and minimum friction while drilling. Our standard offering goes to 20xD, along with pilots drills and diamond coated tools for nonferrous applications.”
The Max-Mini may enjoy the record as the smallest commercially available drill so far, but it’s also one of the most expensive. “Special care should be taken with drills this small.” SMT