Why Shops Are Replacing Gundrills with Solid-Carbide Drills

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.

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.

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.”

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.”