Going deep

Author Cutting Tool Engineering
January 01, 2010 - 11:00am

A growing number of machine shops are switching from gundrills to solid-carbide drills for deep-hole applications.

A new generation of solid-carbide twist drills capable of drilling 16 to 40 diameters deep is making headway in a market dominated by gundrills. 

Featuring chiseled edges and spiral flutes to enhance accuracy and chip evacuation—and densely composed carbide to ensure hardness—the tools can drill five to 10 times faster than gundrills. 

Guhring II.tif

Courtesy of Precision Dormer 

Click the image above to view Precision Dormer's R571 CDX-DH “One-hit" Deep-Hole Drill making holes 102mm deep at a speed of 157mm/min.

Advances in solid-carbide drills are increasingly attractive to machine shops that either gundrill on their lathes or subcontract the work to gundrilling shops, said carbide drill manufacturers interviewed for this story. 

But sources also pointed out that compared to gundrills—which typically feature a carbide or hardened HSS tip, a hardened steel shank and a straight flute—solid-carbide, deep-hole drills do have limitations. 

Guhring II.tif

Courtesy of Iscar 

Iscar uses a 5.4mm solid-carbide long drill to add oil way holes to a crankshaft. Click the image above to view brief demonstration.

For example, their drilling limit (to date) is 40 diameters deep, whereas gundrills easily burrow 50 diameters and farther. Tom Edler, national holemaking manager for Iscar Metals Inc., Arlington, Texas, which makes gundrills and solid-carbide drills, said the former can handle a wider variety of difficult-to-machine materials. 

“Because gundrilling is a slower process, evacuating chips is easier,” he said. “As you get into your titaniums and stainlesses, chip evacuation is a big problem and gundrills are more versatile in handling that. I still see an awful lot of gundrilling out there.”

Guhring II.tif

Courtesy of Guhring

The tip of Guhring’s coolant-fed RT 100T solid-carbide drill is coated with titanium nitride to help protect against abrasion and heat. 

Another obstacle for solid-carbide tools is machinists’ fears about its brittleness and, therefore, potential to break, as CNC lathe operator Ray Domingue will attest. 

Domingue (pronounced do’mang) recalled how he doubted whether a ¼ "-dia., solid-carbide drill would remain intact in material as tough as the alloy Nitronic 50 stainless steel—much less burrow a 9½ "-deep hole in less than 4 minutes, as a local salesman claimed. 

Yet he had to give it a try. Like most shops during the past year, his employer, Lafayette, La.-based Bosco Machine Shop, has placed a high priority on streamlining operations and trimming costs.

Last August, the company decided to bring a gundrilling operation in-house that it had been subcontracting to an outside shop. The application involved drilling a 9½ "-deep hole in each of 120 parts called gamma tandems, which are used in the oil industry, the source of more than 90 percent of Bosco’s business. Containing 22 percent chrome and nearly 12 percent nickel, Nitronic 50 is abrasive and a perpetual drilling challenge, according to Domingue.

The plan was to gundrill on the company’s Haas SL30 lathe. “We bought several lengths of ¼ "-dia. gundrills,” Domingue said. “I built a program, and we ran the machine roughly 24 minutes to go through 9½ " of the material.”

Domingue had drilled just a few parts when Marco Vazques, a sales rep for toolmaker Guhring Inc., Brookfield, Wis., dropped by. “He told me one of his solid-carbide drills can do the same thing in 3 to 4 minutes,” Domingue said. “I said, ‘Oh, really? Well, bring it in at your cost. We’ll run it, and if it works I’ll buy it.’ Honestly, I did not think [the solid-carbide drill] could do the job. Drilling is the hardest application on Nitronic. And carbide’s very brittle.” 

Domingue set up the lathe with a standard-length carbide drill to make a pilot hole 2 diameters deep (roughly ½ ") for Guhring’s coolant-fed, RT 100T 0.25 "-dia., solid-carbide drill. “Then I indexed to the longer [solid-carbide] drill, which was 30 diameters,” he said. “After I indexed to the Guhring drill, I programmed a M00 code. I then hooked up a high-pressure washer to the lathe’s coolant system [to provide coolant at] 2,000 psi, 3½ gpm. I used a sealed collet to hold the drill because of the high coolant pressure. 

“Once I entered the hole, I turned on the high-pressure coolant.” The cutting speed was 140 sfm at a 2,140-rpm spindle speed with a feed rate of 0.0035 ipr.” The drill finished the job, including the pilot hole, in about 3½ minutes. “I accomplished 65 holes before I had to have the drill resharpened and coated [with titanium aluminum nitride],” Domingue said.

He added that the drill produced “small, tight” chips, and the 2,000-psi coolant helped to ensure rapid chip evacuation. Also, the solid-carbide drill proved precise, with less than 0.010 " runout on the opposite end of the hole. 

Productionwise, the Guhring tool enabled Bosco Machine to drill holes eight times faster than a gundrill. Costwise, the company saved on outside labor and shipping costs. And while the solid-carbide drill cost $375 vs. $70 for the gundrill, its added productivity easily eclipsed the initial outlay. 

Tool Test

In a performance test conducted last September, Guhring compared its RT 100T solid-carbide drill to a gundrill. Each 0.25 "-dia. tool drilled 1,000 9.4 " deep holes in Nitronic 50. The Guhring tool completed the job in 22.59 hours vs. 323.48 hours for the gundrill. At a shop rate of $65 per hour, that translates to a cost per hole of $4.46 for the RT 100T vs. $37.06 for each hole made by the gundrill. 

In addition to the company’s 3-year-old line of solid-carbide drills for holes 20 and 30 diameters deep, Guhring in November introduced a tool capable of drilling 40 diameters deep. “Actually, you can go deeper, depending on the tool’s diameter,” Hellinger said. “For example, the ⅛ "-dia. drill has about 6 " of flute length, so it allows you to drill up to 45 times the diameter of the tool.”

As with Iscar’s Edler, Hellinger acknowledged the drills’ limits. “Stainless steel, heat-treated steel and steel alloys are easy applications,” Hellinger said. “We’ve also run it in cast irons, and we’ve been doing a lot of work in nickel alloys, where it’s also done well. The drill probably wouldn’t be the best [option] in chilled cast iron or aluminum. But we do have another design we’re working on for aluminum applications; it’s a slower helix design and it will be coming out in about 6 months as a standard product.”

Reduced Thrust Forces

Introduced 3 years ago, Iscar’s solid-carbide deep-hole drills can produce holes 22 diameters deep, the company reported. The drill tip has a 140° positive-point, four-margin design to minimize thrust forces, assist with centering and tracking and preserve the edge. The company provides post-coat polishing on the flutes, margins and cutting edges to reduce friction and torque requirements. The tools are composed of submicron carbide with 10 percent cobalt, which is a key feature for fast drilling, Edler said.

“To be exact [the grade], is a 0.8μm grain size,” he noted. “That dense composition makes the drill hard and tough.” Traditionally, he continued, hard and tough were the two opposite ends of the spectrum. “Typically, when you make drills this long, you do it with a tough grade of carbide, which gives them strength.” But tough doesn’t translate into good wear resistance, he added.

Seco drill.psd

Courtesy of Seco Tools

Seco Tools’ SD16 solid-carbide drill is capable of producing holes 16 diameters deep. 

“Drilling is an extremely hard application because it’s enclosed, there’s nowhere for the heat to go. In some materials, the heat can’t travel into the material; it goes right into the carbide. So by having the hard characteristic as well, you have the best of both worlds. And that’s what allows us to run these tools as aggressively as we do.”

Also key to achieving high feeds with solid-carbide drills is coolant, and the higher the better, Edler said. He recommended 1,000 to 1,400 psi. “The idea is to get the chips out. That’s always been the problem when you’re drilling deep.” He added that minimum-quantity lubricant also works well with the tools.

Enhanced Tip Design

Among the latest companies to enter the solid-carbide, deep-hole drilling market is Troy, Mich.-based Seco Tools Inc., which earlier this year launched as part of its Feedmax line the SD16 drill, capable of going 16 diameters deep. (Seco also makes specials for drilling holes up to 25 times the tool’s diameter.)

“We have chosen a drill-tip design that’s different from the standard conical drill point geometry,” said Tom Sandrud, Seco’s holemaking manager. “We have a four-facet geometry with both primary and secondary relief angles, to achieve more relief at the center of the drill. That generates a free-cutting chisel edge [and helps] positioning of the drill.”

He added that the SD216’s configuration eliminates the need for a pilot hole. In addition, its double land margins, combined with the wide secondary land margin, enables it “to get engaged at a very early stage of the drilling process,” Sandrud said. “It helps generate good hole geometry, hold tight tolerances and (ensure) a straight hole.” 

Though the company is a newcomer to the solid-carbide deep-hole drill market, Sandrud said sales of its new tool show steady growth—and he anticipates further advances in drilling technology. “Within the last 4 or 5 years, more companies like Seco have added longer drills to their lines. And our competitors continue to work in the same direction as we are.” That is, trying to make drills as long—and effective—as possible. CTE

About the Author: Daniel McCann is senior editor of Cutting Tool Engineering. He can be reached at dmccann@jwr.com or (847) 714-0177.

Guided TA.psd

Courtesy of Allied Machine and Engineering

Allied Machine and Engineering Corp.’s Guided T-A, which features a replaceable carbide or HSS tip and an HSS body with straight flutes, can be used on a mill or lathe. 

An alternative to gundrills and solid-carbide tools 

Drilling deep holes isn’t the sole province of solid-carbide drills and gundrills. Fifteen years ago, Allied Machine and Engineering Corp. (AMEC) began manufacturing its Guided T-A drill (to complement its T-A drilling system) for gundrill applications. The Guided T-A is a pre-engineered, made-to-order tool for customer-specific applications, be they 5, 10, 28 diameters deep or more. It consists of a high-strength steel body with a precision-ground bearing area. It accepts the company’s standard line of carbide or HSS inserts, which range from 0.374 " to 4.5 " in diameter, and specials can be ordered. The tool can be used on either a mill or lathe. 

Jake Miller, product manager at the Dover, Ohio-based company, said the drill incorporates features that give it advantages beyond both gundrills and solid-carbide drills. “Traditional gundrills have a kidney-shaped edge and use a single-effective cutting action,” he said. “The T-A insert in the Guided T-A has a double-effective cutting geometry, which, in most cases, allows you to feed the tool with twice the chip load.”

In addition, the Guided T-A’s design enables it to overcome problems that could throw a solid-carbide drill off course, Miller added. “The solid-carbide tool is less capable of dealing with interruptions, such as drilling through intersecting holes or exiting onto a curved or draft surface,” he said. “The general tendency of carbide drills encountering these situations is to lead off or break due to more force on one side of the tool. But the Guided T-A has a bearing area right behind the spade drill on the front. The bearing area stays engaged with the hole you are creating and does not allow the drill to push off.” AMEC recommends that the user establish the hole with a short-length tool at a depth of one to two times the diameter. This starter hole is the same diameter as the Guided T-A and helps to minimize walking and drill whip.

Three years ago, oil and gas industry supplier Smith International Inc., Houston, tried the Guided T-A for an application it had done previously with an HSS core drill. The job involved drilling holes 14 " deep at a 30° angle in 4140 steel pipe heat treated to 30 to 32 HRC.

Ray Stafford, Smith’s tooling supervisor, said workers had spent about 5 hours making each hole using the core drill procedure. “We’d begin with a starter drill to get a pilot hole, then we’d come behind with an HSS extended-taper shank drill until we almost broke though and then we’d use the core drill to break though. So we were looking for something faster.”

Smith asked AMEC to make a Guided T-A 1¾ " in diameter and 14 " long. “We got the specs we needed, and when we put it in the application it worked extremely well,” Stafford said. “We drilled the hole from start to finish.” And they did so in 1 hour and 15 minutes. Moreover, Stafford added, “We had to resharpen the core drill after every three holes; with the [Guided T-A] we could do as many as nine holes before we had to replace the HSS tip. And now we’re finding more places to use this tool throughout the shop.”

—D. McCann


Allied Machine & Engineering Corp.
(800) 321-5537

Bosco Machine Shop
(337) 857-1887

Guhring Inc.
(800) 776-6170

Iscar Metals Inc.
(877) 294-7227 

Seco Tools Inc. 
(800) 832-8326

Smith International Inc.
(800) 877-6484

Related Glossary Terms

  • abrasive


    Substance used for grinding, honing, lapping, superfinishing and polishing. Examples include garnet, emery, corundum, silicon carbide, cubic boron nitride and diamond in various grit sizes.

  • alloys


    Substances having metallic properties and being composed of two or more chemical elements of which at least one is a metal.

  • cast irons

    cast irons

    Cast ferrous alloys containing carbon in excess of solubility in austenite that exists in the alloy at the eutectic temperature. Cast irons include gray cast iron, white cast iron, malleable cast iron and ductile, or nodular, cast iron. The word “cast” is often left out.

  • centering


    1. Process of locating the center of a workpiece to be mounted on centers. 2. Process of mounting the workpiece concentric to the machine spindle. See centers.

  • collet


    Flexible-sided device that secures a tool or workpiece. Similar in function to a chuck, but can accommodate only a narrow size range. Typically provides greater gripping force and precision than a chuck. See chuck.

  • computer numerical control ( CNC)

    computer numerical control ( CNC)

    Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.

  • coolant


    Fluid that reduces temperature buildup at the tool/workpiece interface during machining. Normally takes the form of a liquid such as soluble or chemical mixtures (semisynthetic, synthetic) but can be pressurized air or other gas. Because of water’s ability to absorb great quantities of heat, it is widely used as a coolant and vehicle for various cutting compounds, with the water-to-compound ratio varying with the machining task. See cutting fluid; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.

  • cutting speed

    cutting speed

    Tangential velocity on the surface of the tool or workpiece at the cutting interface. The formula for cutting speed (sfm) is tool diameter 5 0.26 5 spindle speed (rpm). The formula for feed per tooth (fpt) is table feed (ipm)/number of flutes/spindle speed (rpm). The formula for spindle speed (rpm) is cutting speed (sfm) 5 3.82/tool diameter. The formula for table feed (ipm) is feed per tooth (ftp) 5 number of tool flutes 5 spindle speed (rpm).

  • feed


    Rate of change of position of the tool as a whole, relative to the workpiece while cutting.

  • flutes


    Grooves and spaces in the body of a tool that permit chip removal from, and cutting-fluid application to, the point of cut.

  • gundrill


    Self-guided drill for producing deep holes with good accuracy and fine surface finish. Has coolant passages that deliver coolant to the tool/workpiece interface at high pressure.

  • gundrilling


    Drilling process using a self-guiding tool to produce deep, precise holes. High-pressure coolant is fed to the cutting area, usually through the gundrill’s shank.

  • high-speed steels ( HSS)

    high-speed steels ( HSS)

    Available in two major types: tungsten high-speed steels (designated by letter T having tungsten as the principal alloying element) and molybdenum high-speed steels (designated by letter M having molybdenum as the principal alloying element). The type T high-speed steels containing cobalt have higher wear resistance and greater red (hot) hardness, withstanding cutting temperature up to 1,100º F (590º C). The type T steels are used to fabricate metalcutting tools (milling cutters, drills, reamers and taps), woodworking tools, various types of punches and dies, ball and roller bearings. The type M steels are used for cutting tools and various types of dies.

  • land


    Part of the tool body that remains after the flutes are cut.

  • lathe


    Turning machine capable of sawing, milling, grinding, gear-cutting, drilling, reaming, boring, threading, facing, chamfering, grooving, knurling, spinning, parting, necking, taper-cutting, and cam- and eccentric-cutting, as well as step- and straight-turning. Comes in a variety of forms, ranging from manual to semiautomatic to fully automatic, with major types being engine lathes, turning and contouring lathes, turret lathes and numerical-control lathes. The engine lathe consists of a headstock and spindle, tailstock, bed, carriage (complete with apron) and cross slides. Features include gear- (speed) and feed-selector levers, toolpost, compound rest, lead screw and reversing lead screw, threading dial and rapid-traverse lever. Special lathe types include through-the-spindle, camshaft and crankshaft, brake drum and rotor, spinning and gun-barrel machines. Toolroom and bench lathes are used for precision work; the former for tool-and-die work and similar tasks, the latter for small workpieces (instruments, watches), normally without a power feed. Models are typically designated according to their “swing,” or the largest-diameter workpiece that can be rotated; bed length, or the distance between centers; and horsepower generated. See turning machine.

  • milling machine ( mill)

    milling machine ( mill)

    Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.

  • polishing


    Abrasive process that improves surface finish and blends contours. Abrasive particles attached to a flexible backing abrade the workpiece.

  • relief


    Space provided behind the cutting edges to prevent rubbing. Sometimes called primary relief. Secondary relief provides additional space behind primary relief. Relief on end teeth is axial relief; relief on side teeth is peripheral relief.

  • shank


    Main body of a tool; the portion of a drill or similar end-held tool that fits into a collet, chuck or similar mounting device.

  • spade drill

    spade drill

    Flat end-cutting tool used to produce holes ranging from about 1" to 6" in diameter. Spade drills consist of an interchangeable cutting blade and a toolholder that has a slot into which the blade fits. In horizontal applications, universal spade drills can achieve extreme depth-to-diameter ratios, but, in vertical applications, the tools are limited by poor chip evacuation.

  • titanium nitride ( TiN)

    titanium nitride ( TiN)

    Added to titanium-carbide tooling to permit machining of hard metals at high speeds. Also used as a tool coating. See coated tools.

  • wear resistance

    wear resistance

    Ability of the tool to withstand stresses that cause it to wear during cutting; an attribute linked to alloy composition, base material, thermal conditions, type of tooling and operation and other variables.