Advanced Drilling

Chipbreaking and removing chips — important in milling and turning — also are critical in drilling. The greater the depth of the hole, the more difficult it is to control the process and to remove chips.

Originally, a deep hole was defined as one with a depth of five times its diameter or greater. Today, specialized deep-hole drilling systems easily handle large-diameter holes and lengths of more than 10 diameter. Many shops also produce holes with depths of more than 100 diameter. In addition, they demand high metal-removal rates along with accuracy regarding hole straightness, tolerances and surface finishes.

To satisfy these demands, it is important that chips be broken and removed without jamming or damaging drilled surfaces. For deep-hole drilling, there are systems that deliver cutting fluid to and transport chips away from the actual drilling site. Three of these are gun-drill systems, ejector systems, also known as double-tube systems (DTS) and single-tube systems (STS).

The gun-drill system is designed to drill deep holes with diameters less than 0.75 in. It uses an old principle to supply cutting fluid to the drill tip. A duct in the drill delivers coolant to the cutting edge, after which a V-shaped chip flute along the outside of the drill tube removes chips. The drill portion consists of a carbide tip brazed to a shank.

While gun drilling provides excellent tool life, smooth surface finishes and straight holes, its main disadvantage is that it has a slow feedrate because of to poor tube strength.

The ejector system works well for holes with depth-to-diameter ratios of 50:1 or less, and hole diameters of 0.75 in. to 7.25 in. Ejector systems have twin drill tubes, and cutting fluid is pumped between the inner and outer tubes. While drilling, a major portion of the cutting fluid is fed forward to the drill head and the remainder is forced through a groove in the rear section of the inner tube. Negative pressure arises in the front section of the inner tube, and forces cutting fluid at the drill head along with chips out through the inner tube. For ejector systems, either brazed or indexable insert drills can be used.

Self-contained systems adapt to most conventional machines, including both NC and CNC lathes. However, ejector systems produce short chips and require a rigid setup, a larger volume of coolant as compared to gun drilling, and special overload sensors.

Single-tube systems operate with an external cutting fluid supply and internal chip transport. With such a system, shops can drill holes smaller than what is possible with an ejector system. Cutting fluid travels between the drill tube and the drilled hole then exits, along with chips, through the drill tube. The velocity of the cutting fluid is so high that chips effortlessly pass through the tube. This makes the STS ideal for drilling workpiece materials that have poor chip-breaking qualities.

Because chip evacuation is internal, chip flutes are not required, and shanks are completely round. That provides higher rigidity than is possible with a gun-drill system. Tooling for an STS can include either brazed or indexable-insert drills.

High-performance drilling

Increasing productivity depends on boosting drilling speed, the penetration rate or both. The definition of high-speed drilling is somewhat arbitrary. Some drill manufacturers refer to “high-speed drilling” as drilling at spindle speeds fast enough for penetration rates three to ten times greater than those considered conventional. Others simply define high speed as drilling faster than usual.

Factors that affect drilling at lower speeds become more pronounced and critical as drilling speed or feedrates are increased. The build up of heat, chip removal as the tool feeds deeper into the hole and runout all have to be considered at high drilling speeds and feeds.

As drilling feeds and speeds increase, the challenge of providing sufficient coolant to remove the chips becomes more critical. The volume of chips produced, if not removed, can cause chip jams, recutting and high heat that shortens tool life. When reducing cycle times by 90 percent, there is simply not much time to remove chips, and a high pressure coolant flow is required. Many older machines can not supply sufficient volumes of coolant to remove chips but, in some cases, those machines can be retrofitted to increase coolant flow.

At lower speeds, high-speed steel (HSS) drills often are effective, and provide a tool with relatively high bending strength and toughness. However, at higher speeds it is necessary to use carbide or ceramic tools. These tool materials trade some of the toughness of HSS for greater wear and heat resistance.

Drill and toolholder balance also becomes critical at drilling speeds that exceed 10,000 rpm. Shrink-to-fit and hydraulic toolholders usually are the relied on to stay within acceptable limits for out-of-balance at those speeds.

Older machine tools often have can not provide the concentricity necessary to take full advantage of today's new, precision drills. Total runout should be held to 0.001 in. While lathe drilling, with both the turret and chuck contributing to hole concentricity, runout at the tool should be no greater than 0.0005 in.

Toolholders provide the link between the machine tool spindle and the cutting tool, and must meet the same rigidity and concentricity specifications as the spindle. For example, consider a coolant-fed, indexable drill with a 4-in. projection in a toolholding system that allows a maximum of 0.0002 in. within one inch of the toolholder face. The tip or cutting edges could runout over 0.001 in. Toolholders are available that can true the tip or cutting edge to within 0.0001 in. Such a toolholder can improve workpiece accuracy and tool life significantly at increased drilling speeds.

For high-speed drilling, toolmakers design products that prevent chips from coming into contact with a tool's cutting edges and flutes. Hot chips can soften and smear against the tool, filling in microscopic crevices in its edge surface. This built-up edge becomes the new cutting surface and may force the tool off center, leading to a failure. High-pressure coolant, delivered through the tool, can flush hot chips away from the tool and out of the hole.

Coolants are critical at high speeds, and the choice of coolant must match the material being drilled. For example, when drilling steel, the principal desired action of a coolant is to provide cooling to the workpiece, but when drilling aluminum the primary aspect desired in a coolant is lubricity.

High-speed drills typically are made from a high-temperature grade of solid tungsten carbide. In operations in which chatter may become a problem, a tool made from a finer grade of carbide could be used. Cutting tool manufacturers offer carbide grades with grain sizes of 0.0000197 in. (0.5 µm) or smaller, as compared to a more standard 0.0000984 in. (2.5 µm) grade. Carbide tools with smaller grains resist wear with less of a sacrifice in toughness.

Shops can also obtain solid-carbide drills with two grades of cemented carbide sintered together. The core is a tough, high-cobalt, high-strength grade for low-speed center performance, while the outside material is a harder, low-cobalt, wear-resistant grade for high-speeds. This design lets shops increase penetration rates substantially, especially for drilling stainless steel materials that normally require low feedrates. High temperatures usually rule out the use of tools with steel bodies and carbide tips that are brazed on because there of the danger of softening the braze that holds the tip to the tool body.

Although ceramic tools exhibit greater high-temperature hardness than carbide tools, they must be run on machine tools that can deliver sufficient speed and rigidity. Other tool materials that exhibit high wear resistance include polycrystalline diamond (PCD) and cubic boron nitride (CBN). However, these materials are expensive and, like ceramics, must run on high-performance machine tools.

Thin-film coatings also can contribute significantly to longer tool life, greater wear resistance and faster operating speeds and feeds. Coatings increase surface hardness, reduce friction and heat buildup and increase resistance to edge buildup, galling and fissure propagation. Common coatings for high-speed drilling include titanium nitride (TiN), titanium carbonitride (TiCN) and titanium aluminum nitride (TiAlN).

TiN is a cost-effective, universally applicable coating that increases tool hardness to over 80 Rc. TiCN is a multilayer structure that inhibits surface fractures from propagating to the tool or the wear part substrate. It works best for machining hard materials and in high-shock applications such as interrupted drilling cuts. TiAlN provides extra hardness and heat resistance for machining such abrasive materials as cast iron and high-silicon-content aluminum alloys. This coating performs well in high thermal-stress conditions, such as in dry and near-dry machining along with deep and small-hole drilling where cutting fluids have difficulty penetrating.

In addition, proprietary coatings are available that manufacturers claim provide all the advantages of these three standard coatings. For example, combination coatings with a hard and soft layer can enhance machining capabilities. The soft layer, a lubricity coating, optimizes chip evacuation along the flutes and from the hole by eliminating edge build-up.

Near-dry/dry high-speed drilling

High-speed drilling and dry drilling have a lot in common. For both processes, the key is protecting the tool from heat. For machining with exposed edges (turning and milling), dry cutting is more easily accomplished than in the enclosed confines of a drilled hole. When cutting edges are exposed, chips leave the cutting zone quickly and have little contact with the workpiece or the tool. Both remain relatively cool. But in drilling, cutting edges are subjected to high temperatures that arise from the cutting process and hot chips. The effectiveness of dry drilling varies considerably from metal to metal. Cast iron is the material most often dry machined. Carbide is the tool material typically used for cutting dry. Ceramic tooling also is appropriate because it retains its hardness at high temperature and runs without coolant or lubricant. However, it can be used only with materials that form small, easily managed chips. Ceramic tools are brittle and require close control of runout.

When eliminating a high-pressure coolant stream, some provision must be made for lubrication and chip removal. A soft lubricant coating, applied over the tool's hard coating, keeps hot chips from adhering to the tool and facilitates chip evacuation.

An alternative to machining completely dry is suspending an extremely small amount of coolant in a pressurized, air-coolant mist and directing it at the tool's cutting edge either as an external spray or through the tool. Flows of less than 1.7 oz/hr are typical, as compared to a flood-coolant flow of 1.6 gallon/min. Other, more expensive techniques for meeting dry-drilling requirements include chip suction systems, more efficient through-the-spindle minimum lubrication systems and drilling upward so gravity removes chips.