5 Rules for Driving Deep-Hole Insert Drills

“As much as our customers want it to be a finisher, a drill is still a roughing tool,” says Salvatore DeLuca, product manager at Allied Machine and Engineering. “It’s always going to cut slightly oversized.”

Nonetheless, he says it is easy to understand high expectations for replaceable-tip drills like those manufactured by Allied, a specialist in holemaking tools. In the right application, the latest offerings can leave holes straight and smooth enough to make dedicated finishing tools unnecessary.

However, there is more to precise, efficient drilling than the tools themselves, particularly in holes with depth-to-diameter ratios of 9:1 or more. In a recent conversation, Mr. DeLuca outlined five recommendations for drilling deep with the company’s T-A system, a broad line of general-purpose, replaceable-tip drills designed to penetrate most steels, cast irons, high-temperature alloys and aluminum alloys.

1. Presetting Is a Prerequisite

The deeper the hole, the longer the drill. The longer the drill (the greater the distance between the tool tip and the spindle nose), the greater the extent to which runout can impact results. In some deep-hole applications, the slightest wobble can wear tools prematurely and compromise both straightness and surface finish.

However, the need for careful setup is no excuse to spend inordinate amounts of time identifying and correcting for runout with every tool or insert change. More specifically, there is no room in a modern production environment for climbing into workzones to mount indicators, nor for holding pieces of paper in place for tool touch-offs. Chances are, assembling tools offline with a presetter will save significant amounts of time in any deep-hole drilling operation. 

There is no room in a modern production environment for climbing into workzones to mount indicators, nor for holding pieces of paper in place for tool touch-offs.

In addition to saving time, making drill margins easier to find and measure prevents costly mistakes. “In my opinion, the ease of setup alone is worth the investment,” Mr. DeLuca says. “When you look at the reduction in setup cost and machine downtime due to incorrect setup measurements, it’s hard to imagine not having a presetter.”

At one customer’s facility, three of Allied Machine’s replaceable-tip, self-guided “Stealth” drills (so named for their quiet operation) are set up in the toolroom at the beginning of each shift. This process takes less than 10 minutes. In contrast, setting up three tools on the machining line could take as long as 30 minutes, amounting to 90 minutes of lost time over the course of three shifts. In a 24/7 production environment, that adds up to nearly 2,200 hours per year (a figure that does not account for machine downtime during tool changes). “A presetter is probably 5% of the cost of those lost hours,” Mr. DeLuca notes.

In this case, the Stealth Drill itself adds to the time-savings provided by offline presetting. This drill features an adjustable pin that moves the insert radially to mitigate the tolerance stack-up common to all replaceable-tip drills. As a result, dialing out runout no longer requires dismantling and cleaning the tool assembly, adjusting offsets, adding shims to tool turrets, or relying on experienced machinists to attempt an unorthodox solution. 

2. Coolant Makes the Chip

Barring some form of X-ray vision, chip formation is the only way to judge a drill’s performance during the cut, while there is still time to act. Curlicue shapes (that is, “sixes” and “nines”) are ideal, while longer, stringier chips carry heat away from the cutting zone less efficiently. Color matters, too. As the cutting zone over heats, chips become progressively gray, black and blue. Discolored chips can indicate the need to reduce heat by slowing spindle rpm, while stringy chips might indicate the need to increase the feed rate to a point in which machined material breaks rather than peels.

Other problems can be easier to overlook. “One of the most underestimated inputs in any machining process, even beyond drilling, is the coolant,” Mr. DeLuca says. Given the significant impact of coolant on how chips form, adjusting coolant concentration and flow can also help address problems with discolored “birds’ nests.”  

“One of the most underestimated inputs in any machining process, even beyond drilling, is the coolant.”

Water-soluble coolants are most common for general-purpose applications because they remove heat well. That said, semi-synthetic or fully oil-based formulations can be a better choice to improve lubricity and ensure short, segmented chips in certain cases. Although tools may wear more quickly and parameters may need to be less aggressive than with a water-based coolant, an insert coating that improves heat resistance might help compensate. It all depends on the application.

One non-negotiable factor is the use of through-coolant drills. Machine tools must also deliver sufficient coolant pressure. Small drills require significantly more pressure for chip evacuation to keep up with material removal because they must run at higher rpm to achieve the same surface speed as larger tools. “The greater the pressure, the faster the chips come out of the hole, and the faster you can spin the drill,” Mr. DeLuca says.

3. Pecking Means Problems

Allied’s T-A drills are available with 187 different combinations of standard insert geometries and coatings, and this is just one line of tools from one manufacturer. With so many application-specific choices available, strategies for insert drills that were once standard procedure might no longer apply.

One example is pecking. Periodically reversing the tool’s feed during the cut should not be required to ensure chips evacuate from the hole. “When you see pecking these days, it’s usually someone misapplying a tool or using the wrong tool entirely,” Mr. DeLuca says.

“When you see pecking these days, it’s usually someone misapplying a tool or using the wrong tool entirely.”

Pecking might be the only option if there is no time for new tools or other process adjustments. However, the result is always slower drilling. In addition, retracting the tool mid-cut can leave partially formed, undetached chips inside the hole. Carbide is wear-resistant but relatively brittle, a characteristic that can make inserts prone to premature or even catastrophic failure upon repeated contact with such a jagged, inconsistent surface. Even if inserts are not compromised, the workpiece could be. As mentioned above, most drills cut slightly oversized. “Maybe it’s only by a few thou (1 “thou” = 0.001 inch), but on some jobs, there might still be a risk of scarring the inside of the hole or cutting an oversized hole,” he explains. 

4. Inserts Are Self-Centered

Another outdated practice is the use of a 90-degree spot drill with a smaller diameter for the pilot hole. This makes sense intuitively, and, as is the case with pecking, can still be a useful rule of thumb for toolroom applications. After all, a smaller-diameter hole drilled slightly off-center is easier to correct than a larger one, while a sharp point angle helps the drill quickly and easily penetrate the part and settle into position. However, most CNC machine tools position precisely enough that the pilot drill can be the same diameter as the main drill. They are also powerful enough to drive duller points into virgin workpiece material.

In fact, most replaceable-tip drilling inserts feature self-centering tips with point angles shallower than 90 degrees. As a result, the insert can impact the hole’s major diameter before the self-centering point. “If you were to follow a standard spot drill with one of our tools, our drill will want to walk slightly, causing premature wear and potentially causing the tool to lead off,” Mr. DeLuca explains. “Using the same insert diameter and point angle ensures that the longer drill picks up perfectly where the spot drill left off.”

5. Caution Is a Virtue

In deep hole drilling, ramping immediately up to full speeds and feeds can create safety concerns and risk compromising the work. As such, many of Allied’s standard recommendations for general-purpose T-A insert drills can be summed up in one phrase: Approach the pilot hole with caution. For more specifics, see the images in the picture gallery above.

“Approaching a pre-existing hole cautiously helps avoid scarring or damage,” Mr. DeLuca says about these recommendations. “It allows the drill margins to engage with the material before you bring it up to speed. The hole begins to act like a bushing that keeps the drill centered and keeps parts and people safe.”