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Part of the Hole Story

  Ellen Kehoe

 

 

 

 

 

 

 

By Ellen Kehoe
Senior Editor

It couldn’t have been long after the first tools were hacked into shape that the first holes were made in some rock or tree. Fast forward many centuries and the first machine tool used by mechanically minded folks is likely a drill press. Everyone feels like an expert after making a perfect hole in something, but precision holemaking involves “many factors that must be considered regardless of how simple making holes might seem,” says the author of SME Technical Paper TP62PUB6.

“To qualify as a good machinist is to be able to consistently make holes in exactly the right place—exactly to size, straight, and with good finish. … After most of the usual miseries have been experienced, then there comes a time when a hole driller gets involved in what is known as ‘precision holes.’” The paper describes some of the general techniques of making precision holes, such as boring and reaming, and discusses the various aspects of the processes. For example, bushings guide reamers to locate holes in the correct position, but due to the close fit needed, ample coolant must be provided to the assembly or the reamer will seize in the bushing.


Gundrilling

Another accepted way to make a precision hole is by gundrilling, which as noted in TP52PUB39 is still used most commonly by firearms manufacturers (hence its name) but also for oil and gas, engine and plastic injection mold components. By virtue of a single cutting edge (“…basically a one-sided, single-lip boring tool”) and guidepads that burnish the drilled surface, gundrills produce deep, straight, precise holes. A similar method, trepanning, makes a hole with a center core, which can be removed or falls away when the hole is through.“For gundrilling, the start of the hole is the most important issue,” the author explains. With, for example, a ½" (1.3 mm) diameter rod 20" (51 cm) long with a tool bit on the end, a starting or guiding device is needed—a bushing. “After the cutting edge or the head in the gundrill has passed in the work past the depth of the bushing, then the drill produces its own guide bushing and it is continued indefinitely.” Carbide drill tips helped produce even better holes.

Conventional external-chip-removal gundrill (from SME’s Tool and Manufacturing Engineers Handbook, Vol. 1, p. 9-55)

A gundrill tends to break up chips into smaller pieces, aiding their removal but necessitating the need to get rid of them quickly. Coolant is applied under pressure through a hole in the drill, and chips are flushed away by a groove (flute) along the tool. In deep-hole drilling, coolant enters through a hole running the length of the drill; in short holes, coolant is added through the bushing into a restricted-length flute just above the tool-workpiece interface. The restricted flute length in short-hole drilling makes it possible to pump greater volumes of cutting fluid to the tip at much lower pressures.
Internal-chip-removal gundrilling tip (from SME’s Tool and Manufacturing Engineers Handbook, Vol. 1, p. 9-61)
Among several other papers gundrilling are eight detailed case studies on production drilling and reaming of precision holes with gun-type tools. Types of drills and reamers, setup for gundrilling, coolant flow, drill sharpening and tool life are covered. Gundrilling equipment for exotic materials, including a machine with synchronized rotation of the drill and feed, is discussed in another work. Beryllium is among the most difficult exotic materials to handle but is an important material in atomic reactors. Its chips are small, extremely abrasive particles that float in coolant, so filtration is an issue.

A trio of papers from 1969 presents perspectives on the manufacturing of holes from the administrative, engineering and shop-floor points of view. The authors, all from the representative departments at Zagar Inc. (www.zagar.com; Cleveland), explore techniques in the holemaking process, inspection and tolerance problems and solutions, design specifications and equipment instructions, and equipment building.

 

Coolant Pressure

Two papers offer insights into the effects of pulsating coolant pressures in oil-hole (coolant-fed) drills. There was a substantial increase in the metal removal rate compared with steady flow, dammed-up chips were eliminated and higher feed rates helped chip removal. A "jet pulser" pump system is described as installed on machines processing a variety of materials.

Advances in coolant techniques such as the application of vapor coolant and pulsating coolant contributed to the wider use of the coolant-hole drill. A prototype oil-hole (coolant-fed) drill press for experimental drilling with drills up to ½" (1.3 mm) diameter is described. Instrumentation, drill point configuration, coolant, drilling technique and hole condition are covered.

More than 100 papers in SME's knowledge collection focus on aspects of holemaking, including applications of EDM/ECM, laser and ultrasonic techniques. SME Technical Papers (coded as TP…PUB…) and search options for the collection are available at http://tinyurl.com/SearchTPs.



Published Date : 2/26/2015

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