New Tools and Tactics to Tame Drilling’s Trickiest Tasks

Alternatives to Carbide: Modular and Indexable Drills

Drilling is used in a broad range of jobs, however, and what’s good for one holemaking application might be less effective for others. For instance, while solid carbide offers many benefits, particularly when making smaller holes, what about larger ones where its cost becomes prohibitive?

Here again, cutting tool manufacturers have come to the rescue with various cost-effective solutions. For example, modular drills boasting quick-change carbide heads ease into solid drilling territory at diameters of around ½ inch and continue up to 1½ inches and beyond, depending on the manufacturer.

Beyond this are the indexable drills that machinists have been using for the past few decades, albeit versions with better carbide, coatings and geometries, followed by annular cutters and trepanning tools, which are technically not drills at all.

Then there’s gun drilling. Like the methods just mentioned, it’s also a specialty application and is beyond the purview of “regular” holemaking, but machine shops are likely to benefit from understanding its capabilities as an alternative (but probably outsourced) holemaking technique.

Part of that understanding is the realization that gun drilling might be unnecessary—Sandvik Coromant, Guhring and other cutting tool manufacturers offer coolant-fed, solid carbide drills in lengths 30xD or greater. Holes of that depth were, at one time, gun-drilling-only candidates.

“Just be sure that you slow these long drills way down—say 100 rpm or so—until they’re inside the pilot hole,” Schwenke says. “And if you do need to clear the chips, set the G83 clearance plane to just below the part’s upper surface, keeping the drill contained.”

Back to Basics with Solid Carbide

Returning to solid carbide drills, which can (or should) make up the lion’s share of all holemaking tools, some basic ground rules apply if you want to maximize tool life and get the most parts out the door in the least amount of time:

• Minimize runout: Whether using a drill, end mill or reamer, rotary tool runout must be kept to a minimum. Larson and Schwenke suggest that high-quality, well-maintained hydraulic toolholders are among the best ways to achieve this but agree that shrink-fit and mechanical holders have their place. Whatever the toolholding solutions, double-check the assembly with a dial indicator or, better yet, an optical or vision-equipped offline tool presetter.
• Spotting success: While “spot drilling” or drilling of a pilot hole is often unnecessary with a solid carbide or modular drill, that perk vanishes beyond a certain length-to-diameter ratio, which most experts say starts at 10:1, less in tough materials like Inconel and austenitic (300-series) stainless steels. Here, drilling to 1 diameter deep with a stubby drill is advisable, followed by progressively longer drills as needed to reach final hole depth.

It’s also important to adhere to the manufacturer’s recommended feeds and speeds, and when possible, apply at least 60 bar (870 psi) of filtered, properly mixed cutting fluid.

Read More: From Carbide to Coolant-Through: Deep-Hole Drilling, Simplified

Oil is another alternative, but it does not offer the same cooling capabilities as water-based alternatives (but boasts greater lubricity). “Coolant pressure is an important consideration, especially with smaller drills,” Schwenke says. “Good filtration for your coolant system is also needed and should not be overlooked.”

When Ideal Cutting Speeds Are Out of Reach

All this good advice becomes even more crucial with “micro” drills, or those under 1 millimeter in diameter. Here, even the smallest amounts of tool runout or failing to follow feed and speed recommendations are deal breakers (and quite possibly, drill breakers).   

The catch is that achieving recommended rates can be tough. Do the math on a 0.02-inch microdrill like those offered by Sandvik Coromant, Guhring and many other cutting tool providers and you’ll quickly see that your machine tool might be inadequate.

For instance, a cutting speed of only 200 surface feet per minute (SFM) requires a spindle speed of 38,200 rpm, far faster than most machining centers can achieve. Most modern cutting tools can run at surface speeds much higher than that, even in superalloys and hardened steels.

Larson’s advice is to do the best you can with your equipment. “A speeder head might help in this situation, but in most cases, you’re going to be under the recommended cutting speed. About the only thing you can do here is adjust the feed rate downward to maintain the proper chip load.”