By Kip Hanson
Are you guys crazy? Those words are often associated with a purchase requisition for solid carbide drills. I get it. They’re darned expensive. And once you get over the sticker shock and decide to buy one, then what? Too much runout, the wrong feeds and speeds, and the next thing you know there’s carbide shrapnel flying everywhere.
Still, would you ever go back to high-speed steel (HSS) end mills? How about hand-sharpened HSS tool bits for fly cutters and lathe tools? Given that HSS provides a fraction of the productivity and tool life of carbide, of course you wouldn’t (hobbyists and repair applications excepted).
Judging by the fact that Kennametal, Sandvik Coromant, Iscar, and other leading cutting tool manufacturers no longer offer HSS drills, it seems these tools are falling out of favor. That, or the lower profit margins and need to sustain two distinct manufacturing lines, have eliminated any desire to stay on the HSS bandwagon.
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And yet, there’s no shortage of HSS drills out there. A quick Google search returns names like Cleveland and Precision Twist Drills, companies with brand recognition and a long history of making fine drill bits. They’re relatively inexpensive, readily available, and come in screw machine, aircraft, and every length in between. HSS drill bits are also tougher than carbide, less susceptible to breakage from deflection or imperfect setups.
But they’re not carbide, and that means slow. So going back to the part about too much runout and the wrong feeds and speeds, coupled with a carbide drill’s significantly higher productivity, the message is clear: when shops use a high-quality toolholder (no side-locks, please) and follow the manufacturer’s cutting parameters, they will make more (and better) holes in less time, ultimately leading to a lower cost per part.
Did I mention high-pressure, through-the-tool coolant? Without that, forget it. (Feel free to stop reading and go back to work.)
It’s also important to recognize, however, that solid carbide drills are indeed expensive. My recommendation to mitigate this (aside from good toolholding, the right feeds and speeds, and coolant-thru HPC) is to use them for holes under 12 mm or so in diameter, then switch to modular drills with twist-on carbide heads for anything in the 12 mm to 32 mm hole range (give or take), followed by indexable insert drills for everything else.
For massive holes or where available spindle power is a constraint—say anything over 50 mm for most machines—take a look at hole saws or trepan tools. For the right application, they rock. There are also circuit board drills for very tiny holes, which again provide excellent results in some applications (which is softer materials, for the most part). They’re also great for relieving the throbbing pressure under your thumbnail after smacking it with a hammer.
The cutting tool providers mentioned earlier have dropped HSS for a reason. Sorry about that, but the good news is that the approach just listed—solid carbide, then modular, then indexable insert drills—provides the most cost-effective and productive solution for the majority of all holemaking operations. Get drilling. SMT
Technical Editor Kip Hanson has more than 40 years experience in the manufacturing industry. He is the author of Machining for Dummies and Fabricating for Dummies, and has written over 1500 articles (and counting) on a diverse range of topics, among them machining, sheet metal fabrication, 3D printing, automation, software systems, Industry 4.0, and the Industrial Internet of Things.
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