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Geometry, Parameters, and Strong Toolholders Vanquish Drilling Problems

Ilene Wolff
By Ilene Wolff Contributing Editor, SME Media

Drill manufacturers add new features for improved performance.

In addition to drilling, the CoroDrill 880 can be used for helical interpolation, boring, plunging, and turning. (Provided by Sandvik Coromant)

Proper drill selection, the geometry built into the drills themselves, applying proper drilling parameters, and a few tips and tricks from the pros can address nagging drilling problems such as drill breakage, unbroken chips, tool runout, poor hole edges, and poor tool life.

Paramount, though, is using the right tool. “Drills break for many reasons, ranging from using improper cutting data to just using the wrong drill for the job at hand,” said Martin Hobbs, product specialist for drilling and milling, USA-West, for Sandvik Coromant, Fair Lawn, N.J. “New technologies will help; however, nothing extends tool life more than using the proper tooling setup and application.”

Manufacturers have taken the guesswork out of many of the processes that shops used to have to figure out for themselves when selecting the right drill for cutting different materials—a carbon steel, for example, versus aluminum or stainless steel.

Manufacturers built their formulas into carbide drills, according to Steve Pilger, senior application engineer for threading and holemaking at YG-1 USA, Vernon Hills, Ill. “Let’s say we’re cutting a carbon steel; we have a carbon steel drill that has a fixed geometry, flute and chip removal system, coolant holes of a specific size, and a specific grade of carbide with a nano-thickness, multi-layer coating on it that works best for that specific material. So, it’s not like one drill does all.”

YG-1 has built drill selection based on the material to be processed into its 3-catalogs. “We say ‘hey, what are you drilling?’” Pilger said. “Are you drilling steel? You pick the steel drill. Are you drilling a stainless steel? You go with our Inox drill.”

Walter USA LLC, Waukesha, Wis., has different technologies to combat tool breakage, including newer
grades of coatings, including its own TiSiAlCrN/AlTiN coating to resist wear, said Sarang Garud, marketing product manager. Walter offers full coating and point coating on its drills.

Edwin Tonne, training and technical specialist for Horn USA Inc., Franklin, Tenn., said selecting the drill geometry that matches the material and application is critical.

“Horn’s DDM and DDP system are an example of tailoring the drill point to the application,” said Tonne. “The DDM system works well in difficult alloys like titanium and nickel, but may not be the best choice for iron. Consult the drill supplier for the best recommendation.”

Horn’s DDM drill system is also intended for machining stainless and acid-resistant steels along with the titanium and nickel alloys Tonne mentioned. They’re designed with internal cooling and have new coatings that aid in longer tool life.

The company’s DDP type tools are most suited for unalloyed steels, cast steel and alloyed steels with a tensile strength of up to 1,000 N/mm2. They, too, have internal cooling, although there is a variant suitable for flood coolant.

The Emuge EF Step Drill is a high-penetration rate, solid-carbide drill that drills and chamfers in one operation, saving time and providing a more accurate hole-to-chamfer location and optimal hole preparation for tapping or thread milling. (Provided by Emuge)

Marlon Blandon, product manager for thread milling, Emuge Corp., West Boylston, Mass., said shops can leverage the life of their tools by picking correctly.

“Tool selection will greatly influence tool life,” he said. “High-speed steel drills are a low-cost solution but rarely the best option. High-speed cobalt drills will operate more effectively in elevated temperature applications, but tool life and performance cannot compete with carbide drills. While carbide drills will normally be the best option, many tool geometry and surface treatment options should be considered.”

Blandon went on to suggest a checklist of possible reasons for problems if the right tool is already on the job. Surface coatings, geometry, and application parameters are on his list, but so are other influential factors.

“Assuming that the cutting tool is manufactured to the highest quality standards, the tool life should be consistent from one tool to the next when it is used in the same material and application,” Blandon said. “If tool life is inconsistent, then the issue lies with either the consistency of the machined part material, the wear within the machine tool spindle, toolholder rigidity, the piece part fixturing or the coolant application.”

Drill Geometry: Overachiever

Drill makers also use geometry to make their products material specific, and to aid in chip evacuation, improving surface finish, extending tool life and preventing drill breakage.

Blandon said manufacturers vary their drills’ flute construction and point geometry to match specific materials.

“High-performance carbide drills are the best option for high production lot sizes or higher quality hole requirements,” he said. “They are more expensive but typically have flute and point construction designed to curl and snap chips into manageable sizes that can be efficiently evacuated away from the cutting edge.”

The web geometry of a drill is also very important when it comes to chip evacuation, said Blandon. The web or core of the drill should be constant or parallel from the drill point to the rear of the flute. High-speed steel drills typically have an increasing web taper which reduces the flute space towards the shank. This restricts chip flow out of the flute.

“Some newer carbide HP drills that are designed for deeper hole applications now have reverse web taper where the web is actually smaller in the rear than it is at the point,” he said. “This provides the maximum amount of chip space within the flute.”

Drill makers also use the geometry of the cutting edge to their advantage. A traditional chip breaker on a high-speed steel drill has a series of small notches ground on the cutting edge, which creates discontinuous chip formations, Blandon said. This, however, is not a practical design for a drill with a carbide substrate due to the brittleness of the material in comparison to high-speed steel.

Instead of cutting-edge chip breaker grinds, said Blandon, high-performance carbide drills such as the Emuge EF-Series incorporate various flute shapes that roll the material within the flute and cause it to fracture. When operated at the proper speed and feed rates (which influence the chip thickness), these drills eliminate long, stringy chip formations that lead to flute clogging and drill fracturing. The other benefit of a high-performance carbide drill design is that the elimination of long chips allows the drill to operate at a constant feed rate without peck cycles.

“Using a double-margin flute geometry will help improve surface finish,” Blandon said. “The second trailing margin will help burnish the interior surface of the drilled hole during the drilling cycle.”

YG-1’s Pilger pointed out there are several different ways to drill today, and probably where drill breakage is most common is with solid-carbide drills used for certain materials and for certain hole diameters. Shops use them for drilling holes that are fairly accurate, fast and repeatable, given that their diameter ranges fit the diameters offered.

Selecting the right drill helps prolong tool life. (Provided by Horn)

“For carbon steel, we recommend our carbon steel drill, or our Dream Drill general; for 304 stainless, we recommend the Inox version of our Dream Drill,” he said. “Each one of these drills has the proper geometry and coatings to best deal with those materials and those applications.”

A solid-carbide drill, the Dream Drill, has a strong geometry for making a chip in steel. “It’s going to be completely different when you drill that same hole in stainless steel,” Pilger said. “So it has a geometry to give a better chip. It has a little hook design to help get that chip into the flute a little more precisely.”

Sandvik Coromant’s Hobbs said inserted core drills such as his company’s CoroDrill 880 and CoroDrill 881 offer the ability to change the geometry and coating grades that help to break chips. By their nature, core drills produce a harmonic vibration that is helpful in breaking chips. These harmonic vibrations are at times not enough, and long or unbroken chips can still occur.

With longer drills, said Hobbs, predrilling the hole is an unavoidable and necessary process to prevent breaking the longer drill. The process for longer drills also requires special attention to the longer drill entering the hole at slower speeds and feeds, as well as not turning the through-coolant on until the drill is in the hole and ready to drill. At this time, the proper cutting data and coolant can then be applied.

“The further the drill tip is from the toolholder, the more damage to the drill tip and the greater the effect on hole quality is had by a drill that runs out of round,” Hobbs said.

Garud said Walter’s DC170, with its unique combination of radial margins, coolant channels and solid-carbide mass directly behind the cutting edge, helps extend its life by effectively dissipating even extreme temperatures.

When to Go Fast or Slow Down

Each drill from YG-1 has material-specific and application-specific geometry and coatings for best results. (Provided by YG-1)

Proper drilling parameters, including speed and feed rates, can make a difference in drill breakage, chip evacuation, hole burrs, and surface finish.

Horn’s Tonne said in many cases end users use speed and feed parameters that are too low for the drilling system they are using.

“The cutting speed must be high enough to avoid material build up or built-up edge [BUE],” he added. “When BUE occurs, the drill behaves like a dull tool and thus creates pressures too high for the drill point.”

For most solid drilling operations, a ballpark thrust (Z) force can be 3,000-4,000 N (674–900 lb/f), and a dull cutting edge can easily double that value. Consult the tool manufacturer for recommended speed parameters and stay within the range given, Tonne advised. For feed, always run more than the width of the edge prep and less than the maximum feed per revolution.

To control burrs, reduce the feed by 50 percent until the margins are engaged. Exit burrs are a little trickier but can also be improved by reducing the feed by 50 percent when the drill is 0.5 mm from the exit.

“Material-specific geometries play a huge role in reducing exit burr,” said Garud. “For example, Walter-Titex brand Xtreme-CI drills are designed for reducing exit burrs in cast irons.” The drills have an “edge-break” chamfer on the corners of the cutting edges that greatly reduces the burrs, he said.

“Similar technology exists for aluminum drills as well,” Garud said. “Our DC170 drills have radial margins that help produce excellent finish on the holes. Adding extra ground margins (four instead of two) can also have significant improvement effect on the hole finish.”

Pilger at YG-1 has noticed that when customers start using carbide drills, they often look at the speeds and feeds in the catalog, which seem really high, so they reduce the speed. As a result, the drill runs too slow and does not generate enough heat. Then the material adheres to the drill and flakes off, taking off the coating and carbide with it. Eventually, the drill loses size and breaks.

Where it is appropriate to decrease speeds and feeds is with cross-holes, Pilger said. When the drill is approaching the second solid portion, reduce the speed and feed by 50 percent and re-engage the hole. This gives the drill a moment to make a footprint and get engaged. Then crank the speed and feed back up.

“Otherwise, you might hear a chatter, or break a drill, or you’ll see it’s walking,” he said. “What’s happening is you’re getting a mismatch because you’re hitting an uneven surface and you’re trying to minimize that mismatch.”

There is a time when you want to speed up. “In drilling, a very common way to improve your chip is to adjust one of the parameters on your machine,” Pilger said. “So, when you’re having chip issues and you want to get a better chip, you slightly increase the feed in steel [0.008-0.014 ipr or 0.2-0.36 mm/rev].”

Sandvik Coromant’s Hobbs agreed that it’s very important to know what a drill’s proper cutting data range is for the material being drilled. “High feed rates may produce too thick of a chip or even overload the drill’s web, causing the drill to break,” he said. “Low feeds can cause long, stringy and undesirable chips.”

Higher feed rates are often used to break a chip by making the chip thicker and more prone to break. There are cases where it can be better to increase the surface footage and decrease the feed rate in order to break a chip, Hobbs said.

At times, program interruptions in the feed motion can help break up chips or, in extreme cases, a pecking cycle may solve the problem, said Walter’s Garud.

Know How to Hold ‘Em

The Walter DC160 Advance solid-carbide drill can be used in a wide range of materials and applications. Margins are located in an advanced forward position to ensure rapid guidance in the hole. A steep gash angle provides space for chip flow, resulting in improved hole quality and reduced feed force. (Provided by Walter)

Picking the right toolholding system and inspecting it periodically to ensure it’s in good working order can extend tool life and help produce better holes.

“Your holder choice is really important when you get into high production environments like the auto industry,” said Pilger. “In most applications our customers typically hold our drills with collet holders with no issues. However, when drilling in high volume with high productivity, ridged holders with stronger gripping forces are needed.”

But if you want to step it up a little bit, eliminate runout, and hold the drill better, there are a couple of options. One is a shrink fit holder and the other is a collet chuck with hydraulic pressure. These two systems grip much more of the shank. In fact, university studies have shown that holding this way improves the runout by about 50 percent and improves tool life by 20-25 percent.

“In a high production environment, we highly recommend a shrink fit system or a hydraulic chuck for our solid-carbide drills,” Pilger said.

Sandvik Coromant’s Hobbs agreed the method used to hold drills is critical to the performance of the drill and to tool life. How securely a drill is held has a direct effect on hole quality, such as roundness and surface finish, and on tool life. Weaker toolholding methods allow for more runout at the tip of the drill.

This runout results in one side of the drill cutting more than the other. When this occurs, not only does the drill cut and wear more on that side, but hole quality diminishes as a result of the drill wanting to walk off center and scar the side walls of the hole.

“Heat shrink and hydraulic toolholders, such as Sandvik Coromant’s CoroChuck 930, provide the most stable drill holding and when compared to methods such as ER collets, may even double tool life,” he said.

The method used for holding a drill becomes more critical the longer the drill becomes. Long drills that are 10, 15, 20, 30 and 40 times diameter should never be held in any type of holder that is not absolutely rigid, Hobbs added.

“Heat shrink and hydraulic holders hold drills on center better than Weldon or collet chucks can,” he said. “This rigidity helps to prevent the drill from running out of round at the drill tip.”

Tonne advised using a high-performance collet system like Horn’s Fahrion Centro P, which can increase tool life by more than 50 percent due to the extremely tight tolerances of the taper and collet runout.

Emuge’s Blandon urged regular maintenance. “Machining center spindles must be properly maintained to reduce or eliminate runout at the drill point,” he said. “If the spindle is dirty or worn, the drill will rotate unevenly. This can lead to uneven loading at the drill point, which can cause chipping on the cutting edges and drill failure. The best holder technology for drilling today is normally a hydraulic chuck or an Emuge FPC mechanical sleeve design,” Blandon stated.

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