Holemaking alternatives: Drilling Performance

Several alternatives to mechanical drilling make sense when the application is right.

Conventional drilling is the most common metalworking operation, with 92 percent of CTE readers performing it, according to a recent survey by market researchers ReadexResearch. But conventional drilling isn’t the only way to make a hole. Depending on the application and a manufacturer’s available equipment, alternative holemaking methods may be a better choice. Presented here are the pros and cons of using trepanning tools, thermal drills, lasers, electron beams and EDMs when holemaking.

For example, compared to drilling with a solid or indexable-insert drill and circular interpolating with an endmill, trepanning requires less machine horsepower, produces a slug, or core, of material that can be used to make other parts and can create holes with a larger depth-to-diameter ratio. “If people are trying to produce large-diameter holes and their machines don’t necessarily have the horsepower to push a solid drill, a good alternative is a trepanning tool,” said Mark Sollich, president of BTA Heller Inc., Troy, Mich.

Another alternative, thermal drilling, offers the advantage of reducing cycle time compared to conventional drilling, according to Mitch Ray, president of Flowdrill Inc., St. Louis, a manufacturer of tool bits for thermal drilling and roll-form threading. Also known as friction or flow drilling, the process’ main cost-saving improvement comes from reducing the number of operational steps. For example, an end user may be placing a fastening device, such as a threaded insert or ribbed nut, into a drilled hole and can flow drill and thread the same features without having to pay for the fastening devices, and the labor to position them and remove the generated debris.

Trepanning Deep Holes

Similar to holemaking itself, many methods exist for deep-hole drilling, such as trepanning, which uses a hollow rotary or nonrotary tool to cut a hole in a workpiece.

BTA Heller offers standard indexable-insert trepanning tools with replaceable carbide pads and cartridges from 2½ ” to 30 ” in diameter, with the majority from 3 ” to 8 ” in diameter, Sollich noted. The pads help guide the tool, maintain hole straightness and semiburnish the ID. The tools are rated to produce holes 100 diameters deep, “but it’s possible to go deeper,” Sollich said. “The main limitation is your machine tool and not necessarily the tooling.”

Courtesy of BTA Heller

BTA Heller offers indexable-insert trepanning tools with replaceable carbide pads and cartridges from 2½ ” to 30 ” in diameter.

In general, trepanning requires 7 to 10 hp per 1 ” of cutter diameter, with difficult-to-machine materials requiring more horsepower and thrust. Trepanning requires 15 to 30 percent less horsepower than solid drilling and is 10 to 15 percent faster, according to Sollich.

He noted that end users can apply trepanning tools on standard machines, such as lathes with OD chip exhausts, as well as dedicated deep-hole drilling machines for performing BTA (named after the Boring and Trepanning Association) drilling, where coolant is introduced around the tool and chip evacuation occurs through the tool’s center and the tube drill that drives it.

As the required hole diameter increases, the amount of coolant flow (in gpm) also increases. The rule of thumb is a machine needs to deliver 25 to 30 gpm per inch in diameter, meaning a 3 “-dia. hole requires 75 to 90 gpm to properly flush the chips, according to Sollich.

In addition, the coolant must be filtered so the coolant system isn’t pumping chips and fines back to the cutting edge for recutting. “When we design coolant systems, they’re filtered to 10 microns,” Sollich said, adding that the coolant tank needs to be large enough so oil or coolant has time to dwell and cool off. If coolant is constantly recirculating, it can become extremely hot and shorten tool life and cause unacceptable part growth. “You might possibly incorporate refrigeration, or chiller, units to cool the oil,” he added.

The majority of trepanning applications, whether producing drill collars and pipes for the oil field industry, aircraft landing gears or assorted parts at a job shop, are horizontal. Removing a large, heavy core can be a challenge. To pull out such a core, end users often drill and tap a hole in one end and insert an I-bolt, Sollich noted.

He added that producing through-holes is more common than blind-hole trepanning, which requires a core-cropping tool to enter the approximate ¾ “- to 1½ “-wide kerf, actuate a knife-style cutter at the bottom of the hole and cutoff the core during a secondary operation.

Regardless of the type of application, Sollich recommends a load meter for monitoring down-hole conditions when trepanning. A rising load meter reading indicates tool wear and the onset of potential problems. “A load meter can be set to a level where if an insert wears out or potentially breaks, the machine will automatically shut down before a catastrophic failure occurs,” he said.

According to Sollich, trepanning can achieve a ±0.002 ” tolerance on hole diameter and impart surface finishes from 83 to 125 rms, depending on the machine setup and material specifications.

Flowing a Hole

Unlike trepanning, thermal drilling uses a tool without a hollow core and doesn’t produce any chips. When flow drilling, a tungsten-carbide tool bit with a four-lobe geometry contacts the workpiece material using an axial pressure from 180 to 900 thust lbs., depending on tool bit diameter and the material type and thickness, and heats the workpiece to be soft and malleable enough to be formed and perforated, according to Flowdrill (see photo on page 30). “Those four lobes generate the friction, or heat, as the tool spins against the material,” Ray said. He added that the typical spindle speed is from about 900 to 3,000 rpm.

The process is suitable for malleable materials, including mild and stainless steels, aluminum, copper and some brass alloys, whereas high-porosity metals, such as cast iron, as well as some tool steels, Inconel and other hard and difficult-to-machine metals are not appropriate. In addition to being malleable, the workpiece thickness must be relatively thin, from ½ ” to 0.020 “. “If it’s thinner then that, the material will deform too much,” Ray said, adding that the process produces a bushing about three times the thickness of the workpiece.

An end user can then thread the bushing. “If it’s a threading application, we exclusively use a roll-form tap to keep the process clean and chipless,” Ray said. “There is no debris through either the drilling or tapping cycles.”

A shorter bushing can be formed by using a pilot hole, with a larger pilot hole used for generating a shorter bushing length. Where there’s a need to reduce the amount of restriction for the flow of air, liquid or gas through a pipe, for example, a shorter bushing would be required. But that’s an exception.

Courtesy of BTA Heller

Trepanning performed in a dedicated deep-hole drilling machine.

“The idea behind flow drilling is to get as much material in the bushing as possible,” Ray said. Producing the maximum bushing length can enable a manufacturer to switch from, say, a 3⁄8 “-thick workpiece to a 1⁄8 “-thick one and still have a 3⁄8 ” of thread integrity.

The company’s drill bits range from 1⁄16 ” to 2½ ” in diameter and can be coated to extend tool life in severe conditions. Typical tool life ranges from 3,000 to 5,000 holes when drilling stainless steel, 8,000 to 10,000 holes in mild steel and 10,000 or more holes in softer alloys, such as aluminum and copper, according to Ray, who estimates that 30 to 35 percent of sales are for specials, such as ones for also creating a chamfer or an oil groove. He noted that the cost per hole is 0.01 to 0.03 cents.

To achieve maximum tool life, Ray recommends applying a paste lubricant for manual operations and a liquid lubricant for automated ones every two to four holes to prevent bits from gathering metal particles from the workpiece material and provide some lubricity. He also suggests wiping a bit in use with an emery cloth to dislodge any debris on a daily basis. Flowdrill doesn’t regrind bits because of the inevitable tiny cracks that form in the transition zone between the conical and cylindrical sections, where torque is the greatest.

The bits are available in two styles: one for creating a collar around the hole and one that incorporates a cutter to spot face the workpiece surface.

Hole Ablation

Like mechanical trepanning, laser trepanning produces a core when holemaking. To cut those holes, the laser is turned on and moves in a circular path until the hole is completed, explained Stuart Woods, director and general manager of direct diode and fiber systems for Coherent Inc., Santa Clara, Calif.

The process is also suitable for percussive laser drilling, where the laser is quickly turned on and off to ablate a hole through the workpiece material. “The laser basically pounds its way through the metal,” Woods said. “You’re trying to knock atoms off.”

Woods pointed out that laser drilling and holemaking offers the advantage of lower cost compared to mechanical holemaking. Typically, this is about a 10 to 50 percent cost reduction, depending on the type and thickness of workpiece material. While the initial hardware cost of a laser is higher, the savings come from lower maintenance and the elimination of the need for post-processing. Lasers also reduce maintenance downtime and don’t produce any warping or punching effects. The latter occurs because lasers are a noncontact and focused thermal source, as opposed to conventional drilling, which exerts pressure on the workpiece and can negatively impact material shape.

In addition, lasers yield higher processing speeds, especially when drilling hard and brittle materials. “You blink your eye and it’s done,” Woods noted, adding that lasers are typically two to five times faster, depending on material type and thickness.

Woods noted that various types of lasers are available for producing holes in metal, including sealed CO2 and fiber lasers. “Typically, if you’re looking at metals, CO2 does a very good job,” he said. “It gives you the most functionality.”

High-power CO2 lasers are suitable for drilling holes from about 100µm to millimeter size in metal foils, such as stainless steel, with a thickness down to 0.1mm, whereas fiber lasers are better suited for thicker materials, according to Woods. That’s because a CO2 laser tends to form a kerf when cutting thicker materials.

When matching a type of laser with a work material, Woods noted that it’s important to know the material’s absorption profile so it will absorb the laser and have an interaction. Coherent’s applications laboratory regularly evaluates customers’ workpiece samples and provides feedback about the laser wavelength that will work best based on the application.

In his latest Laser Points column on MICROmanufacturing.com, the Web site for CTE’s sister publication, Ronald D. Schaeffer, CEO of laser job shop and systems integrator PhotoMachining Inc., Pelham, N.H., stated that the three most common laser holemaking methods are single shot, percussion and trepanning. All can be performed with a fixed-beam system or one in which mirror movement is controlled by a galvanometer. In practice, though, single shot and percussion are done more often with fixed-beam systems while trepanning tends to be performed with a galvo system.

Single-shot drilling is the fastest method and produces a hole approximately the diameter of the incoming beam. Taper tends to be more pronounced with single shot because there are no subsequent pulses to “clean” the hole. Percussion drilling involves application of multiple pulses to pierce the material. It’s a slower process, but it produces a rounder hole with less taper than a single-shot hole. Trepanning, when performed on a galvo system, yields holes with the best circularity and smallest taper, according to Schaeffer.

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