Diminutive drills

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by Kip Hanson

Manufacturing challenges increase as holes get smaller


It’s a simple fact: if you’re a machinist, you make holes. Lots of them. Unfortunately, drilling is also one of the operations most likely to turn machinists prematurely grey. Poor chip evacuation, heat buildup and drill runout are just a few of the problems common to would-be holemakers.

Taking it down a notch
Drilling gets even more challenging when venturing into hole sizes smaller than 3 mm. Toolmakers classify drills in this region as microdrills, tools capable of punching holes the size of a fat pencil lead, to ones smaller than the period at the end of this sentence. “With some of these drills, you only have to look at them wrong and they’ll break,” says Nika Alex, drilling product specialist at Mitsubishi Materials U.S.A. “Microdrills are very, very sensitive.”

Alex knows drills. The California-based toolmaker offers some of the smallest microdrills around, with solid carbide drills down to 0.1 mm, and coolant-fed versions to 0.5 mm. He recommends 1000 psi coolant pressure or higher for tools this small, using a very low viscosity coolant. “At these diameters, the right cutting fluid is critical. If it’s not possible to run water-based coolant, some of the newer synthetic oils are so light they’re almost like water. Those work fine with our drills.”

In the microdrill world, shanks measure either 3 mm or 1/8 in., depending on which side of the US/Canada border you live. This makes toolholding a relatively straightforward affair. It’s a good thing, too—it’s tough enough drilling a hole the size of a human hair, let alone gripping a shank that small. That doesn’t mean, however, that any old drill chuck will do.

“Runout is everything at this scale,” explains Alex. “Smart shops go with hydraulic holders, or even shrink fit. You have to aim for zero runout.” Mike Hafke, vice president of sales for Guhring Inc., agrees. “It’s an important consideration, to be sure. Optimum results come from toolholders specifically designed for these high performance micro drills.”

If drill makers had their druthers, every shop would have an HSK spindle and be willing to buy top of the line tooling. Unfortunately, that’s not reality. “There’s a whole bunch of little drilling machines out there that use BT holders. That’s why we offer hydraulic and shrinkfit chucks set up specifically to hold these 3 mm and 4.0 mm shanks, in whatever spindle configuration the customer is using.”

Don’t cheap out
Cost is a big concern with small-hole drilling. Not only are feed rates held very low to prevent breakage—.002 ipr is not uncommon—but attaining sufficient spindle rpm is always problematic. Both mean lower productivity and increased costs for machine shops. Adding insult to injury is the high price tag on microdrills, which cost between $15 to $40 or more, for a tool no more robust than a puff of wind.

Hafke suggests that, given the right application, there’s a lower cost alternative. “Cobalt and powdered metal drills are available down to 0.05 mm, in lengths up to 7 X diameter. Compared to carbide, you can figure on a rule of three—meaning carbide runs three times the speed, three times the life, and three times the price.”

The choice comes down to what you’re doing, he explains. “If you’re a job shop, and the customer is beating down your door to deliver a dozen parts, you can find a cobalt tool at a decent price with enough flute length to peck away at it. Maybe it won’t be the most efficient way to drill the hole, but you’ll get the job done.”

Hafke points out that production drilling is a whole different animal. The cost of the tool is inversely proportional to productivity—cheap drills mean fewer parts on the bench. “It’s all about how many holes you’ve drilled at the end of the day. In a production environment, spending a little more on the tooling delivers more parts out the door, with less machine downtime.”

Another way to save big bucks—given the right application—is circuit board drills. These little wonders, similar in size and shape to their high-performance cousins, are suitable for many materials—aluminum, brass, plastic and composites—and cost little more than a cappuccino.

Buyer beware, however. Joe Negron, sales manager for Kyocera Micro Tools, a division of Tycom Corp., Costa Mesa, CA, says that cheap imports selling for less than a dollar are available online, but these are constructed of two pieces then epoxied together—not exactly high-performance technology.

If you have the production volumes to justify it, Negron suggests that custom drills are a viable solution in this Lilliputian world. Since hole sizes under 3 mm are often drilled to their finished size without boring or other secondary operations, picking the perfect drill is a must. Granted, there are at least 169 standard drills in this range, but if you’re in between sizes, call Negron. “If you need five custom drills, that’s going to be a problem—figure a fairly high cost for each. But if you can use a few hundred drills over a six-month period, the price should be competitive with comparable catalog items.”

The benefit, says Negron, is a drill made to exactly the right diameter and length for the application. And, like many other high performance microdrills, Kyocera promises to deliver a common shank size, polished flutes and tight tolerances, in this case +.00000/-.00006 on the drill diameter. “Microdrills are super precise.”

There’s more to a good microdrill than precision, however. Sandvik Coromant Canada’s holemaking and tooling systems specialist Randy McEachern explains that the right geometry is equally important. “The chisel point and cutting edges must be designed to assure free cutting action and good chip evacuation. Both reduce the amount of axial thrust, a key factor when microdrilling. This is especially true in gummy materials such as stainless steel, where the chips can pack up and break the drill.”

Off to a good start
Getting the chips out of the hole is critical with any drilling operation, but even more so with fragile microdrills. Shops might be inclined to fix poor chip evacuation problems by pecking, yet McEachern recommends against this unless absolutely necessary. Pecking not only wastes time, but there’s always the chance that a chip will get stuck to the tip of the drill, or fall down the hole. Both can be disastrous.

The solution is coolant through the tool. “With deep holes, meaning 6X diameter or more, you won’t be very successful without a coolant-fed drill,” says McEachern. Another success factor is the starter hole. Stubby drills are available from most tool manufacturers, and Sandvik is no exception, “Normally if your solid carbide drill has a 140-degree point, I would recommend a pilot hole 1-2 diameters deep, with a point angle of 150 degrees. This prevents the follow-on drill from impacting along its full cutting edge at the start of the cut.”

If you’re ready to try your hand at microdrilling, there are plenty of high-quality products and a wealth of technical information available to get you there. You’ll need a very fast spindle—20,000 rpm is a good starting point, but 50,000 or more might be necessary—with coolant through the tool capabilities. Minimizing runout is a must, so high quality toolholders are strongly advised. And obviously you need a good cutting tool—microdrills can be had for a couple bucks, but you’ll likely have limited success with these. It’s better to buy the best, keep it cool, and feed it slow and true. Happy drilling. SMT

Kip Hanson is a contributing editor. [email protected]


Kyocera Micro Tools

Sandvik Coromant

Mitsubishi Materials


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