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More of the Hole Story

 Ellen Kehoe








 By Ellen Kehoe
Senior Editor

As the saying goes, you can always build a better mousetrap. Likewise, you can make a good hole better, according to the author of SME Technical Paper TP79PUB215, in this case by ball microfinishing. SME papers cover a range of topics and developments in holemaking, such as location methods, coolant types and deliverydrill geometry, combo tools (such as the "Rotabroach"), hole quality and other techniques such as punching, oscillatory boring and helical milling.


Going Deep

Several papers report on process improvements across a sizable spread of hole types: deep holes, deep miniature holes, small holes and miniature holes. How deep is deep? “While many engineers categorize any hole depth of more than six diameters as ‘deep hole drilling,’ I consider a deep hole as one in which factors in any drilling tend to present problems of hole quality, cycle time, drill life or, as is most often the case, a combination of all three,” explains the author of TP78PUB149. “With the millions of possible combinations of materials, equipment, drill types, diameters, tolerances, etc., the depth at which problems develop will vary greatly.”Sketch of ball helical milling (D. Olvera et al., SME Technical Paper TP09PUB52)

The above paper states, “no other element has as much to do with effective deep hole drilling as getting the chips out of the hole.” Writers from GM’s Buick Motor Division and General Motors Institute (now Kettering University) agree that chip ejection from the hole is a key issue, if not the most crucial. The GM authors were investigating the drilling of crankshaft oil holes in pearlitic nodular cast iron, which was introduced for GM crankshafts beginning in 1967, replacing forged steel. The newness of the material in the auto industry meant there was little information available concerning optimum machining conditions, adding to the already-difficult operation of drilling the oil holes.

Among the conclusions reported: “Disposal of chips and dissipation of heat are the major controlling factors in deep-hole drilling. Material hardness has a great effect on tool life, chip disposal and heat dissipation. Conventional drilling with coolant-fed twist drills has a potential for deep-hole drilling of nodular iron crankshaft holes.” The GM authors also mention the use, at that time (1970), of gundrilling of forged crankshafts by rival automaker Chrysler Corporation at its Trenton, MI, engine plant, and the process being used by Toyo Kogyo, then the third largest vehicle builder in Japan, to adapt shell molding to the production of ductile iron crankshafts, allowing the oil holes to be “cast” into the crankshaft.

An analysis of drill deflection for deep miniature holes is presented by a Rockwell International scientist in a 1980 paper. The author offers conclusions related to thrust force, primary lip relief angle and drill grind concentricity to the blank, flute length, force limitations, hardness variations, bushing and gundrilling and the use of tungsten carbide drills.

Small to Miniature

Focusing on precision manufacturing rather than high production rates for microelectronics, array inkjet printers and the liquid droplet radiator array under a NASA contract, an overview of small-hole manufacturing is presented based on contacts with some 30 companies. The review deals with 10 fabrication processes for precision holes in the 10 to 250 μm (0.0004 to 0.0100″) range, including electroform, chemical milling, laser drilling, electron beam machining, electrodischarge machining, mechanical punching/broaching, mechanical drilling, soluble core glass fibers, ion milling and photosensitive glass. The conclusions reached emphasize that some processes require special materials or are restricted to certain materials and some require very thin substrates.

Moving down the scale, just what is classified as a miniature hole? A 1979 paper defines it as being made with a 3 mm drill or smaller, drilled to extremely close tolerances with no necessity of subsequent machining operations. “The common expedient of extrapolating the principles and practices of drilling larger holes to the drilling of miniature holes is, more often than not, unsuccessful…. The successful application of any equipment to the drilling of miniature holes requires a thorough understanding of the appropriate techniques and the correct selection and use of the appropriate cutting tools.”

From the vantage point of 1979, the author says, “until the comparatively recent development of the LASER” [note the original capitalization as an acronym], “the technique used to drill miniature holes in sapphire jewel bearings was virtually identical to that used by early man to drill similar holes in colorful rocks, a process used in the making of beads.”


Laser Precision

Speaking of lasers, a 1980 paper describes how manufacturing engineers at Solar Turbines International, then an operating group of International Harvester (now Case IH; Racine, WI) had been using lasers since 1976 for precision small cooling holes and cutouts in combustor liners made of heavy-gage, high-temperature alloy material. Technical details are given on the in-house investigation to find the most economical methods available and to justify purchase of a nine-axis laser (Nd:YAG) machining center.

Other papers also cover production laser hole drilling. Several specific cases of drilling holes in various materials—metals, ceramics and plastics—using the capabilities of CO2 lasers are discussed in TP74PUB279. The production of high-quality, cost-effective pierced holes with neodymium (Nd)-doped glass lasers showed how what was “once believed to be a tool of the future” became “a new tool which now has a place in industry.” Making better holes using plasma is discussed in TP13PUB19, presented at FABTECH 2012.

A ding or divot in the interior of a hole (D. McLenithan, SME Technical Paper TP13PUB19)

ECM Techniques

The drilling of deep, fine holes in hard materials, such as for film cooling gas turbine blades, is discussed in a paper on electrochemical machining (ECM) techniques presented at an SME Nontraditional Machining conference in 1989. Shaped tube electrolytic machining (STEM), capillary drilling (CD) [also called electrochemical fine drilling (EFD)], electrostream drilling (ESD) and electrojet machining (EJM)—the latter an addition in a similar 1990 paper by the same authors—can drill stress-free metallurgically undamaged holes of length-to-diameter ratios up to 300:1. The primary common feature of these methods is the use of strong acid electrolytes. The two papers include descriptions of the processes and the control technology developed to minimize the effects of certain problems associated with each method.

A feasibility study for the practical use of ECM for boring and rifling major-caliber gun barrels is presented in a paper by a U.S. Naval Ordnance engineer. The data were utilized to design and build an ECM machine with a stroke of 24′ (7.3 m) together with a cathode/tool capable of rotating two and a half revolutions.



A Rolls-Royce author reviews nontraditional drilling methods developed in response to the "limited capability of conventional twist drilling to both penetraShaped tube electrolytic machining (STEM) (M.S. Ahmed and A. Duffield, SME Technical Paper TP90PUB259)te the tough metal involved and accommodate the large numbers of holes needed" in jet engine manufacture. The new techniques of "electron beam, laser, EDM [electrical/electro discharge machining] and ECM are such that material toughness hardly comes into consideration," although each process has boundaries defined by technical and economic conditions. 

EDM multiple-hole machining, EDM through-hole applications, microhole drilling using automatic spark erosion, ultrafast EDM hole drilling and EDM methods of producing small
are among other related papers. The use of CNC EDMsmall-hole drilling is detailed for special applications such as holes in mold cavities for vacuum molding, air vanes for jet engines, wire nuts commonly used in the aircraft industry, fuel injectors, taps and stainless steel valves.

Two papers by SME Fellow K.P. Rajurkar (University of Nebraska-Lincoln) and colleagues, presented at North American Manufacturing Research Conferences (NAMRC), describe drilling of noncircular blind microholes by micro EDM and the study of dielectric flow in microhole drilling by EDM.

More than 100 papers in SME’s knowledge collection focus on aspects of holemaking, including many dealing with composites (but that’s a “hole” other story). SME Technical Papers (coded as TP…PUB…) and search options for the collection are available at

Published Date : 3/10/2015

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