Printer Friendly

Getting to the bottom of deep holes.

If you are new to the machine tool industry, or just don't get out to the shop very often, you're probably wondering what constitutes a deep hole and why it deserves any special attention. "How deep is deep?"

The generally accepted industry definition of a deep hole is any hole whose depth measures at least four or five times more than its diameter. Historically, deep holes have been a special problem to drill for several reasons.

Chip disposal is the main problem. Using a traditional twist drill, the deeper you drill the further the chips must travel to clear the hole and the greater the chance of jamming or binding on the way out. Then there is the problem of getting cutting oil to the drill bit face--and the obvious consequences of failing to do so. Next, friction between the drill bit, chip, and hole wall can dramatically build up torsional forces and snap the drill bit.

Complicating matters even further is that these factors all affect the hole's straightness, roundness, diameter, and finish--the quality of the hole. Usually, the deeper the drill bit goes, the more difficult these four hole qualities are to maintain.

Finally, there is the business problem of yield and loss of added value. Usually, parts with deep holes are considered expensive, high-risk parts. Much of the value has been added by the time the drill bit reaches the bottom of the hole. If the hole is not to specifications, you've lost the part and, perhaps, the tool as well. Largely for these reasons, historically, deep-drilled holes have been a leading cause of part rejection and costly rework.

Some of the earliest and ongoing solutions to the deep-hole drilling challenge focus on more efficient ways to remove chips and keep the drill-bit face lubricated.

Deep-hole drilling traditionally has relied on gundrilling or BTA (Boring and Trepanning Association) methods. Generally, gun drills handle hole sizes down to about 0.050" dia and up to 1.500" dia. Gundrill performance overlaps that of BTA-type tools in the 7/16" to 3/4" dia range, but is comparatively slow on diameters greater than 3/4". BTA tools work from 7/16" dia up to almost any practical size required.

The gundrill shares one design concept with coolant-fed twist drills and other tools: the coolant is applied directly to the tool/workpiece interface. Coolant holes in the center of the tool shank supply fluid under pressure to the tool face, while coolant and chips return to the collection point by traveling through the space between the tool shank and the hole wall. Most BTA systems do just the opposite. They pump coolant through the space between the tool shank or boring bar and the hole wall, forcing chips and spent fluid to return through the hollow center of the shank and bar. Besides lubricating the wear pads and reducing friction and pad wear, this method also prevents chips from scoring the freshly cut workpiece surface.

BTA methods, such as Sandvik's Ejector drilling system, are based on a drill design that presents multi-bit cemented carbide to the drilling face and sucks chips out through the center of a hollow drill tube. Internal chip evacuation eliminates all the problems associated with trying to clear chips via the flutes of a solid section twist drill or the clearance segment in the shaft of a gun drill.

The basic principle is to press efficient carbide cutting edges into the work at a high feedrate; to support the working head with a very strong, stiff hollow tube; and to evacuate chips through the center of the tube.

The cutting heads typically contain an arrangement of several carbide inserts for cutting and two guide pads, which ride on the inside of the hole and balance cutting forces. Arrangement of the inserts is critical to making a straight, round cut. Mounted on a stiff machine (conventional lathe, boring mill, or machining center) with good spindle bearings, Ejector drilling will reliably hold +0.003/-0.000" on diameter, and |+ or -~0.002"/" of hole on straightness. Finish is around 60 micro.

Some limitations do apply to Ejector drilling. First, holes less than 3/4" dia require a dedicated deep-hole drilling machine because of the oil pressure requirements. Second, it requires machinable materials that generate easy-to-control chips, eliminating most nickel-based and many nonferrous metals.

According to Tony Yakamavich, application engineer, Sandvik, Ejector drilling users in automotive, military, fluid power, oilwell tooling, and other manufacturing industries report the prevailing margin of improvement in metal removal rate has ranged from 5-to-1 to better than 10-to-1, depending on the previous method used and the depth-to-diameter ratio of the hole.

One not-so-obvious method of producing deep holes is through the use of lasers. "Though laser drilling is often considered last for making deep holes, it should be one of the first processes considered because of its flexibility, low tooling costs, productivity advantages, and unique part processing features," says Ron Sanders, director of material processing technology, Laserdyne.

Laser hole drilling consists of two distinct hole-drilling processes: percussion drilling and contour cutting, each providing holes with different characteristics.

With percussion drilling, the laser system and beam remain stationary while a pulse or series of pulses penetrate the material. Typically, this is the fastest processing method and is used for drilling shallower holes where hole taper is not critical. Where hole taper is a concern, a technique called "pulse shaping" can be used to minimize the taper.

Contour cutting methods can produce holes as small as 0.004". With this method, hole shape is defined by the path of the laser motion system. Consequently, contour cutting can be used for producing holes of various shapes. With contour laser cutting, an assist gas jet coincident with the laser beam provides mechanical energy to remove molten material downward and out the rear side of the workpiece. The result is holes with straighter walls than produced by percussion drilling. As with percussion drilled holes, the cut is characterized by a narrow heat affected zone reflected in a narrow kerf (or cut width) and recast layer.

Hole shape and size are controlled through the laser system software to produce round, oval, rectangular, or any shape imaginable. Not only can the hole shape be varied, but each hole can have a different shape. With multi-axis positioning, complex hole cross sections can be created as well.

The laser machining process can be used to successfully drill holes in a wide range of materials. Generally speaking, the process is the same for most materials, though the rate of penetration varies greatly depending on the material being drilled.

Terry VanderWert, metallurgical engineer and director of marketing for Laserdyne Div of Lumonics Corp, points out that virtually all types of metals, non-metals, organic graphite reinforced composites, and metal matrix composites can be effectively laser drilled. Laser capability is not governed by the same properties as mechanical processes. Instead, thermal properties such as melting, vaporization temperatures, and thermal conductivity are the important considerations in laser drilling. The absorption of the laser beam at the wavelength of laser light is different for each material.

EDM methods

Another not-so-obvious method of producing deep holes is through the use of Electrical Discharge Machining (EDM). "Most people believe deep-hole drilling would never be possible by EDM, however, when you are looking at extremely small holes, or you're looking to machine rectangular or square holes with utmost precision, then you're talking EDM," says Dan Zeman, assistant manager, applications department, Mitsubishi EDM.

"When high-production, high-tolerance hole drilling is essential to an application, then the 0.020" dia range typically run by EDM can be the answer. Machining small holes is a very delicate process," Mr Zeman continued.

By means of an EDM, a small-hole drill jig is able to produce holes as small as 0.004". The electrode is positioned inside a precision ceramic guide, which assures accuracy by holding the electrodes firmly in place. The copper tubing or tungsten carbide electrode used in machining such small diameter holes is very common for all EDMs. Such tubing is readily available, going down to a size no thicker than a strand of hair.

EDMs work well for materials that are difficult to drill. You can achieve a high-precision hole on materials ranging from hardened tool steels to materials harder than cutters. For example, to deep-hole drill materials such as tungsten carbide, which is normally ground with diamond wheels, EDM provides one of few viable options.

Exotic materials also make hole drilling a problem for many people. According to Mr Zeman, some materials are difficult to machine with their gummy texture or surface finish. "This is especially true in the aerospace and experimental materials industries," Mr Zeman observed. "Conventional machining faces too many problems with these exotic materials."

Often when a hole is drilled conventionally, sidewall impact occurs. This is especially true when tight hole-center tolerances are involved. "With EDM, there is virtually no impact created," Mr Zeman said. "This lends itself well when drilling a hole with a very thin wall."

According to Mr Zeman, the better the flushing is, the faster the burn time will be. As with all sinker EDM applications, small-hole drilling demands good flushing. This reduces overall machining time, provides less recast, less heat, and improves surface integrity inside the hole.

One company manufacturing dedicated EDM drills is Castek Chicago. The drills are designed for use with hardened tool steels, carbides, and other conductive materials. They use brass or copper electrode tubes from 0.012" to 0.125" dia. Normal drilling depth-to-dia ratios of up to 200:1 can be exceeded under certain conditions. "These drills can be set to enter material at any angle," says Lloyd Stone, president, Chicago EDM. "They will drill into curved surfaces, irregular shapes, and even multiple layers if proper precautions are taken."

While all of the methods discussed in this article are able to create deep holes, they can also prove beneficial in many "not-so-deep" hole applications. That's a hole with a depth-to-diameter ratio close enough to 4-to-1 or 5-to-1 that the temptation is to get by with a conventional twist drill. T&P
COPYRIGHT 1993 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:techniques in boring deep holes
Author:Stovicek, Donald R.
Publication:Tooling & Production
Date:Jul 1, 1993
Previous Article:Thin-film diamond at the cutting edge.
Next Article:Valve maker cuts production time 40%.

Related Articles
Inside tips on gundrill geometry.
Drilling micro-size deep holes.
Drilling machines: features, functions and future.
Covering the boring machine bases.
Innovations in holemaking.
New machine combines accuracy, flexibility for deep hole boring.
Chapter 10: boring operations and machines.
New twist on a boring bar.
Boring bars.
Drilling depths for easy tapping.

Terms of use | Copyright © 2016 Farlex, Inc. | Feedback | For webmasters