Dry drilling in cast iron with a Guhring solid-carbide RT 100 with FIREX and MOVIC coatings.
At trade shows and on shop floors, there’s a lot of talk about "going dry." Sophisticated coatings and the need to keep up with foreign manufacturing will supposedly push U.S. manufacturers to dry machining. But while dry milling and dry grinding may be growing in popularity, dry drilling is another matter.
Some people drill dry, but not very many. And certainly not high numbers of holes.
"It doesn’t really exist in high-production operations," said Dr. John Sutherland, professor of mechanical engineering and director of the Center of Environmentally Responsible Manufacturing at Michigan Technological University, Houghton, Mich. He said that dry drilling currently is limited to operations involving cast iron and the production of shallow holes in certain steels and aluminums.
Chuck Petrosky, staff engineer for Solid Carbide Round Tools at the Kennametal Technology Center, Latrobe, Pa., agreed, adding that drilling presents unique challenges compared to other machining operations.
"[Drilling is] a continuous cut, whereas milling is an interrupted cut," he said. So, when a milling tool exits the cut, it has time to cool.
Even in drilling operations where coolant floods the hole, heat and friction can prevent the fluid from reaching the point of cut. Petrosky said that with no coolant to remove heat from the cutting surface, holes are likelier to close up, increasing the chances of drill breakage.
Another problem with running dry is that the chances of galling are greater. Besides cooling the tool, a metalworking fluid provides lubricity, which results in a cleaner finish and helps prevent galling.
Coating Is Key
Petrosky thinks that if dry drilling gains greater acceptance, it will be due to the continued development of advanced tool coatings. He cited titanium diboride as being a particularly good coating for drilling aluminum.
"It’s a coating which aluminum doesn’t adhere to," Petrosky said.
Another option is MOVIC, a "glide" coating developed jointly by Guhring of Germany and Vilab of Switzerland. MOVIC acts as a lubricating agent between the cutting tool and chip.
"MOVIC-coated tools produce about one-sixth the friction of TiN-coated tools," said Mark Megal, marketing manager for Guhring Inc., Brookfield, Wis. MOVIC is a nonstick industrial coating made of molybdenum disulfide. Megal said that MOVIC is akin to a surface treatment in that it impregnates the tool substrate rather than just coating it.
Thomas Friemuth, chief engineer and head of the manufacturing processes division at the Institute of Production Engineering and Machine Tools at the University of Hanover (Germany), said that coatings with a low friction coefficient and low thermal conductivity work best at isolating a tool from heat. He recommends TiAlN-based coatings for the dry drilling of ferrous materials.
Another key to successful dry drilling is ensuring adequate chip evacuation. Sutherland suggested using wide-flute drills, which allow chips to readily exit the hole. He cautioned, though, that the flutes shouldn’t be too wide.
"The trick is to have plenty of room for the chips to evacuate without [undermining] the strength of the tool," said Sutherland. Dry drilling already subjects the tool to higher stresses than coolant-fed operations, and any compromise in strength could lead to breakage.
Kennametal Applications Engineer Mark Huston thinks that dry drilling may eventually move away from metal tools altogether. "There’s the potential down the road for ceramic drills," he said.
A ceramic drill can withstand extreme heat and high speeds and compression while maintaining its cutting edge. But, Huston said, there are limited applications for ceramic drills.
"You have to have absolute rigidity and almost zero runout, because ceramic isn’t very flexible," he said.
Have Europe and Asia Gone Dry?
Technical issues have clearly inhibited U.S. manufacturers from adopting, or even trying, dry drilling. That, apparently, is not the case overseas.
We’ve all heard that international manufacturers are leaving their U.S. counterparts in the dust when it comes to dry drilling. Do they really have a leg up on us? Probably. How did they do it? By learning to crawl before trying to run.
Petrosky said that his German colleagues have a different concept of "dry" than U.S. companies, and that they don’t expect to go totally dry anytime soon. "They look at it more as reducing the use of coolant," he said.
Petrosky said that European manufacturers began by employing minimal-mist coolant techniques coupled with cold air and coatings to reduce their coolant use.
Friemuth disagreed, saying, "Our experiences are essentially based on purely dry drilling." He acknowledged that the range of operations available for dry drilling was limited, but added that feed and speed rates for those operations were the same as when performed with coolant. He said that there was virtually no loss of productivity when dry drilling tempered steels and low-to-medium-tensile-strength gray cast irons.
Assuming that’s true, why aren’t U.S. manufacturers more interested in dry drilling? According to Sutherland, the lack of solid information regarding tool life, chip flushing and thermal stability keeps American metalcutters tied to coolant.
He also said that minimal-coolant applications, like those mentioned by Petrosky, would appeal to American manufacturers with high production rates and a strong desire to reduce their coolant use.
Sutherland said that some large U.S. automotive and aerospace manufacturers already use minimal-coolant techniques, but not as many as in Europe and Asia. He credited different political, economic and environmental controls, as well as the sheer size of the United States for the lukewarm interest in minimal-coolant techniques.
"We have a lot of room for waste products," said Sutherland. "If you’re in one of the lowland countries in Europe and you have wastewater from cutting fluids, you have a bigger problem than you do here."
Sutherland also said that he thought Japanese manufacturers were more environmentally conscious due to their country’s limited area, and that things might be different if the U.S. was a smaller country.
"I’m confident that if the entire population of the United States lived between Chicago, Tulsa and St. Louis, we’d figure out creative ways to do things, too," said Sutherland.
In the end, the real limiting factor for U.S. manufacturers going dry may be the nature of the industry itself. Some take an "if it ain’t broke, don’t fix it" attitude toward machining. Without environmental legislative pressures, some are hesitant to alter tried-and-true flood-coolant processes.
"Manufacturing is pretty slow to grasp certain things," Petrosky said. "Change doesn’t come very quickly."
Cold-Air Guns Blast Heat
Heat buildup and chips are drill killers. So what do you do when coolant can’t, or shouldn’t, be used in a drilling operation? Stick to your guns.
Cold-air guns deliver a stream of air that’s 50° to 80°F colder than standard compressed air. Most guns incorporate a vortex tube that converts air into two low-pressure streams: one hot and one cold. The hot air is directed out an exhaust port, while the cold air is sent down a flexible hose aimed at the point of cut.
Mike Rawlings, product engineer at air-gun manufacturer ARTX Ltd., Cincinnati, said that his company’s product will cool a 0.500"-dia. drill running at 3,000 rpm, regardless of the depth of cut.
"It will work as long as you have some kind of pecking sequence," said Rawlings, who added that pecking is also necessary with most flood-coolant operations. The difference when using an air gun, though, is that there is no mess.
ARTX’s basic system lists for $234 and includes 12" of flexible line, four generators to change the volume of air and a magnetic base.
Another cold-air gun manufacturer is Exair Corp., also in Cincinnati. Its basic unit costs $229. According to the company’s applications engineer, Joe Panfalone, air guns are a good alternative to expensive mist systems. He conceded, though, that they’re not the panacea for all applications.
"Water has 40 times more heat-carrying capacity than air," explained Panfalone. "But [using an air gun] is better than running dry." He added that a cold-air gun isn’t a "product that’s going to put the flood-coolant industry out of business. It’s for specific applications."
He recommended air guns for operations in which the workpiece material could be damaged if exposed to coolant and for toolroom machines that can’t be equipped with flood-coolant systems.
Panfalone said that cold-air guns could be used more than they currently are. He also believes that research on developing systems that combine cold-air technology and advanced tool coatings holds a lot of promise for improving metalcutting operations in the future.
- D. Phillips
Related Glossary Terms
- 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.
Workholding device that affixes to a mill, lathe or drill-press spindle. It holds a tool or workpiece by one end, allowing it to be rotated. May also be fitted to the machine table to hold a workpiece. Two or more adjustable jaws actually hold the tool or part. May be actuated manually, pneumatically, hydraulically or electrically. See collet.
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.
- depth of cut
depth of cut
Distance between the bottom of the cut and the uncut surface of the workpiece, measured in a direction at right angles to the machined surface of the workpiece.
Rate of change of position of the tool as a whole, relative to the workpiece while cutting.
Grooves and spaces in the body of a tool that permit chip removal from, and cutting-fluid application to, the point of cut.
Condition whereby excessive friction between high spots results in localized welding with subsequent spalling and further roughening of the rubbing surface(s) of one or both of two mating parts.
- gang cutting ( milling)
gang cutting ( milling)
Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.
- gray cast irons
gray cast irons
Alloys of iron, carbon and silicon in which more carbon is present than can be retained in austenite. The carbon in excess of austenite solubility in iron precipitates as graphite flakes. Approximate composition of gray irons is: 2.5 to 4.0 percent carbon, 0.5 to 1.0 percent manganese, 1.0 to 3.0 percent silicon, 0.05 to 0.15 percent sulfur and 0.05 to 0.8 percent phosphorus. Some Society of Automotive Engineer grades are G-1800, G-2500, G-3000, G-3500 and G-4000.
Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.
- interrupted cut
Cutting tool repeatedly enters and exits the work. Subjects tool to shock loading, making tool toughness, impact strength and flexibility vital. Closely associated with milling operations. See shock loading.
Measure of the relative efficiency with which a cutting fluid or lubricant reduces friction between surfaces.
- metalcutting ( material cutting)
metalcutting ( material cutting)
Any machining process used to part metal or other material or give a workpiece a new configuration. Conventionally applies to machining operations in which a cutting tool mechanically removes material in the form of chips; applies to any process in which metal or material is removed to create new shapes. See metalforming.
Any manufacturing process in which metal is processed or machined such that the workpiece is given a new shape. Broadly defined, the term includes processes such as design and layout, heat-treating, material handling and inspection.
Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.