July 2010 / Volume 62 / Issue 7|
By Alan Richter
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
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
“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.Beam Me Through
An alternative to laser drilling is electron beam drilling, which is able to achieve a depth-to-diameter ratio up to 25:1, according to Kenneth E. Norsworthy, manufacturing engineer for new product development at Owens Corning Ridgeview, Duncan, S.C. Initially developed for internal use, the company provides electron beam drilling as a service.
The process takes place in a vacuum chamber, where electron beams are accelerated to about two-thirds the speed of light. The process produces holes from 0.001 " to 0.040 " in diameter, with the typical range being from 0.004 " to 0.040 ", in metal, ceramic and composite materials up to 3⁄8 " thick, Norsworthy noted. The facility controls hole diameter by varying the power density, focus and pulse width, and reports that size accuracy is ±0.001 " and hole placement accuracy is ±0.0005 ".
Courtesy of Owens Corning
Workpieces up to 1m in diameter receive from 35,000 to six million holes, which can be angled up to 40°.
Norsworthy indicated that electron beam drilling is suitable for a host of industries, including pulp and paper, aerospace and food and beverage, to produce parts such as those for extrusion, filtration and air flow. According to the company, the process is well suited for high open-area screens and open areas can be up to 45 percent.
Because the process is performed in a vacuum, it’s usually not economically feasible to electron beam drill a small number of holes. The process is also not suitable for workpieces with a low melting temperature, such as many plastics. Compared to conventional drilling, however, electron beam drilling quickly produces a large number of holes. “Try putting six million holes in a part with any other process,” Norsworthy boasted.Vaporized Openings
When the workpiece material is electrically conductive, EDM holemaking is an option, whether sinking an electrode to produce a fine hole as small as 10µm or smaller or using a wire. Although a slower process than conventional drilling, EDMing offers a high level of predictability and repeatability when producing a high volume of small holes, according to John Bradford, micromachining R&D team leader for Makino Inc., who’s located at the Mason, Ohio-based machine tool builder’s technical headquarters in Auburn Hills, Mich. And EDMing may achieve a faster per hole cycle time when the time to correct an unstable drilling process is considered.
Bradford added that the EDM process can achieve accurate, virtually burr-free results with aspect ratios up to 100:1 whereas mechanical drilling is often only effective to a 15:1 aspect ratio. “EDMing, along with the possibility of laser machining, is a superior solution to that of mechanical drilling,” he said.
Many end users consider EDM holemaking to be the realm of fast-hole EDMs, or hole poppers, but that’s generally not the case for holes requiring precision because hole poppers use a higher input voltage than sinker EDMs and employ a water dielectric. “Those two in combination give you excellent machining speed, but inversely equate to less hole accuracy for straightness and roundness and a rougher surface finish,” Bradford said.
“I don’t think a hole popper is suitable for precision holemaking,” concurred Mark Kinder, president of Plastic Design Corp., Scottsdale, Ariz., a moldmaker and part molder. He added, though, that some users have tweaked to their hole poppers to go beyond a machine’s precision limitations.
Even so, those machines primarily create holes from 0.025 " to 0.040 " in diameter and PDC often needs to rough sinker EDM 0.006 "- to 0.007 "-dia. start holes in 400 series stainless steel. The company then finish wire EDMs holes as small as 0.010 " in diameter with a wire as small as 0.002 " in diameter to eliminate bellmouthing and achieve the final size. “We don’t concern ourselves with the fine-hole recast finish due to the clean up with the wire EDM,” Kinder added.
PDC uses a Makino Edge2 sinker EDM with a fine-hole option and typically applies a rotating copper-tungsten, hollow-pipe electrode and low-viscosity dielectric oil. “It won’t suspend the swarf as well as some of the more viscous fluids, but it allows us to do finer burns,” he said.
For one roughing experiment with low-viscosity oil and a 0.010 "-dia. pipe electrode, PDC achieved a 100:1 depth-to-diameter ratio, a 0.001 " bellmouth and a breakout location within ±0.001 " of the start position. However, Kinder noted that PDC tries to avoid aspect ratios higher than 50:1.
An option for achieving greater aspects is to sinker EDM a blind-hole, flip and relocate the part and EDM a second connecting hole to create a single, deep through-hole, explained Bradford, noting that higher aspect ratios require flushing through an electrode.
Although fine-hole EDMs are able to create holes smaller than 0.0005 " in diameter, Bradford noted that tube electrodes are generally not available below 0.003 " in diameter, which has about a 0.001 "-dia. through-hole. “You need a source pressure of 1,500 psi or more to get even a low-viscosity dielectric to flow through the tip of that electrode,” he said.
Courtesy of Makino
For the tightest tolerance holes, Bradford noted that an ultraprecision wire EDM tops a fine-hole sinker. For example, a user could feed a 15µm-dia. wire through a 30µm-dia. start hole to produce a hole with 500nm circularity. “But the wire’s primary advantage is you’re minimizing recast and also producing a surface finish that is two to three times better or more than what you could produce on our fine-hole sinkers,” he said. Bradford noted that the recast layer from a fine-hole sinker is about 2µm at a hole transition edge and in the double-digit nanometer range for a wire. A tungsten-carbide electrode could impart a 0.17µin. Ra finish.
A wire machine also doesn’t have an aspect ratio restriction and because managing electrode wear is not an issue, hole size consistency is only limited by the wire EDM’s mechanical and electrical capabilities. “A wire EDM is by far the best choice if you’re doing holes that require consistency from one to the next,” Bradford said. “And if you require a sharp edge transition, a wire EDM is by far the best choice.”
Bradford has recently noticed a substantial increase in EDM holemaking. “Four years ago, 10 percent of our activities were around fine-hole testing and manufacturing, and today it may be as high as 65 percent.” CTE
About the Author: Alan Richter is editor of Cutting Tool Engineering, having joined the publication in 2000. Contact him at (847) 714-0175 or firstname.lastname@example.org.
Using metrology to improve total hole quality
EDMed holes is challenging enough for manufacturers, but they must also be able to measure the holes with repeatability. That’s the challenge Plastic Design Corp. faces when EDMing holes smaller than 0.005 " and trying to achieve a 0.00005 " tolerance on diameter. “I say ‘trying’ because of our ability to measure that,” said Mark Kinder, company president. “We can distinguish less than a tenth, but we don’t know what our true distinction is.”
Off the EDM, PDC uses a Nikon VMR vision measurement system, which Kinder noted is “good down to about 1 micron,” but setup and wall effect light reflection issues can impact repeatability. “If we can’t establish repeatability to half of what we’re trying to discriminate, then we’re not going to hang our hat on it.”
More precise metrology equipment is available, but PDC determined that the vision system provided the best value in terms of cost vs. precision.
John Bradford, micromachining R&D team leader for Makino Inc., emphasized that when EDMing microholes, an end user must purchase its metrology equipment at the same time it purchases the EDM to validate hole quality. “As holes get smaller and tolerances get more stringent, that piece of inspection equipment is critical to support the internal manufacturing process,” he said.
Although PDC is EDMing micro ejector and core pin holes for molds it uses internally, achieving the required tolerances is necessary to precisely mold parts for customers. For example, Kinder said PDC molds a lot of PEEK (polyether ether ketone) and knows that the material flashes when the clearance between the mold and pin is from 0.0003 " to 0.0004. " However, a pin will start to bind when the clearance is less than 0.0002 ". Therefore, the company must hold all working clearances to less than 0.0003 " but greater than 0.0002 ".
It’s not enough to achieve a precise hole location and size tolerance at the start of a hole. “We look at the total quality of the hole though the bore,” Kinder said. “Otherwise a core pin will get canted or an ejector pin will wear and gall and finally seize up.”
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