Cutting Tool Applications, Chapter 9: Drilling Methods and Machines

Many types and sizes of drilling machines are used in manufacturing. They range in size from a simple bench mounted sensitive drill press to the large multiple-spindle machines able to drive many drills at the same time.

Simple drill press: A simple drill press, shown below, may be floor-mounted, or have a shorter main post and be mounted on a bench. The motions of this machine are very simple. The table on a floor model can be raised or lowered and rotated around the machine column. The spindle rotates and can be raised and lowered, with a stroke of 4 inches to 8 inches. Stops can be set to limit and regulate the depth.

Sensitive drill press: The name "sensitive" is used to indicate that the feed is hand operated and that the spindle and drilling head are counterbalanced so that the operator can "feel" the pressure needed for efficient cutting. A table-mounted sensitive drill press is shown below.

The drill press has the same motions as the previous one plus a telescoping screw for raising and lowering the table and a sliding drill head. These two features allow easier handling of parts of varying heights.

Radial drill: For handling medium to very large castings, weldments, or forgings, radial drills are ideal; The length of the arm along which the spindle housing rides specifies the size. This arm can be from 3 feet to 12 feet long. The column that holds the arm may be from 10 inches to 30 inches in diameter.

For very large work, the arm may be rotated 180 degrees and work placed on the shopfloor. Speeds and feeds are dialed in by the machine operator and are the same as for other drill presses. Drilling is either hand or power feed.

Drilling machine components Rigid and accurate construction of drilling machines is important to obtain proper results with the various cutting tools used. The sensitive drilling machine construction features are discussed in this section because its features are common to most other drilling machines.

Base: The base is the main supporting member of the machine. It is heavy gray iron or ductile iron casting with slots to support and hold work that is too large for the table.

Column: The round column may be made of gray cast iron or ductile iron for larger machines, or steel tubing for smaller bench drill presses. It supports the table and the head of the drilling machine. The outer surface is machined to function as a precision way of aligning the spindle with the table.

Table: The table can be adjusted up or down the column to the proper height. It can also be swiveled around the column to the desired working position. Most worktables have slots and holes for mounting vises and other workholding accessories. Some tables are semi-universal, meaning that they can be swiveled about the horizontal axis.

Head: The head houses the spindle, quill, pulleys, motor and feed mechanism. The V-belt from the motor drives a pulley in the front part of the head, which in turn drives the spindle. The spindle turns the drill. Speeds on a stepped V pulley drive are changed by changing the position of the V-belt. Speeds on a variable-speed drive mechanism are changed by a hand wheel on the head. The spindle must be revolving when this is done.

Quill assembly: The spindle rotates within the quill on bearings. The quill moves vertically by means of a rack and pinion. The quill assembly makes it possible to feed or withdraw the cutting tool from the work. Located on the lower end of the spindle is either a Morse tapered hole or a threaded stub where the drill chuck is mounted. For drilling larger holes, the drill chuck is removed and Morse tapered cutting tools are mounted.

Size classification: The size (capacity) of a drilling machine is determined by all the following features: - Twice the distance from the center of the spindle to the inner face of the column. - The maximum length of quill travel. - The size of the Morse taper in the spindle. - The horsepower of the motor.

Drilling systems Drilling systems are usually automated and computer controlled. Speeds, feeds and depth of cut are often pre-set. Such systems combine drilling operations with reaming, tapping, countersinking, etc.

Multi-spindle drilling: This type of drilling can be done on drill presses by using special attachments. The spindle locations are adjustable, and the number of spindles may be from two to eight. Drills, reamers, countersinks, etc., can be used in the spindles. The RPM and feedrate of all spindles in one drill head are the same, and the horsepower needed is the sum of the power for all cutting tools used. In this type of machine, a large number of holes may be drilled at one time. Several different diameters of drills may be used at the same time.

Gang drilling: An economical way to perform several different operations on one piece is by gang drilling, as shown below. This might include drilling two or more sizes of holes, reaming, tapping and countersinking. The work is held in a vise or special fixture and is easily moved along the steel table from one spindle to the next.

The drill presses usually run continuously so the operator merely lowers each spindle to its preset stop to perform the required machining operation.

Turret drill: Turret drills, with either six or eight spindles, enable the operator to use a wide variety of cutters and yet move the workpiece only a few inches, according to the hole spacing. The turret can be rotated (indexed) in either direction and then lowered, by hand or automatically, to make the cut.

Some turret drills have automatic, hydraulically controlled spindles. Speeds, feeds and depths of cut can be preset for fast production. These machines are also made with the entire operation computer controlled, (CNC turret drill), so that the operator merely has to load and unload the parts, as shown below.

Operation set-up In drilling operations the three most common workholding methods are: Vises. Vises are widely used for holding work of regular size and shape, such as flat, square and rectangular pieces. Parallels are generally used to support the work and protect the vise from being drilled. Vises should be clamped to the table of the drill press to prevent them from spinning during operation. Angular vises tilt the workpiece and provide a means of drilling a hole at an angle without tilting the table. Angle plates. An angle plate supports work on its edge. Angle plates accurately align the work perpendicular to the table surface, and they generally have holes and slots to permit clamping to the table and holding of the workpiece. Drill jigs. A drill jig is a production tool used when a hole, or several holes, must be drilled in a large number of identical parts. The drill jig has several functions. First, it is a workholding device, clamping the work firmly. Second, it locates work in the correct position for drilling. The third function of the drill jig is to guide the drill straight into the work. This is accomplished by use of drill bushings.

Toolholding devices Some cutting tools used in drilling can be held directly in the spindle hole of the machine. Others must be held with a drill chuck, collet, sleeve, socket, or one of the many toolholding devices.

Drill chucks: Cutting tools with straight shanks are generally held in a drill chuck. The most common drill chuck uses a key to lock the cutting tool.

Sleeves: Cutting tools with tapered shanks are available in many different sizes. When a cutting tool that has a smaller taper than the spindle taper is used, a sleeve must be fitted to the shank of the cutting tool.

Sockets: If the cutting tool has a tapered shank larger than the spindle taper, a socket is used to reduce it to the correct size.

Deep-hole drilling The term "deep holes" originally referred to hole depths of over five times the diameter. Today, deep-hole drilling is a collective name for methods for the machining of both short and deep holes.

Deep-hole drilling is the preferred method for drilling hole depths of more than 10 times the diameter, but because of the method's high metal-removal capacity and precision, it is also competitive for small holes down to two times the diameter.

During drilling, it is important that the chips be broken and that they can be transported away without jamming and affecting the drilled surface. In deephole drilling, cutting fluid supply and chip transport have been provided for by the development of three different systems that permit trouble-free machining of hole depths of more than 100 times the diameter. The three systems are called: the Gun drilling system, the ejector system (two-tube system) and the single tube system (STS).

Gun drilling systems The gun drill system uses the oldest principle for cutting fluid supply. The cutting fluid is supplied through a duct inside the drill and delivers coolant to the cutting edge, after which it removes the chips through a V-shaped chip flute along the outside of the drill. Due to the V-groove, the cross section of the tube occupies 3/4 of its circumference. Below is a gun drilling system and its component parts.

Gun drills: Gun drills belong to the pressurized coolant family of hole making tools. They are outstanding for fast, precision machining regardless of hole depth. As a rule, a gun drill can hold hole straightness within 0.001 inch per inch (IPI) of penetration, even when the tool is reasonably dull. For most jobs a gun drill can be used to cut from 500" to 1000" in alloy steel before re-sharpening is necessary. In aluminum, it might be 15,000", while in cast iron it is usually around 2000".

Depending on the tool's diameter, a gun drill is seldom run at feedrates exceeding 0.003" per revolution (IPR). This is extremely light compared to twist drill feeds, which typically range from 0.005 IPR to 0.010 IPR. But gun drilling does use a relatively high speed compared to high speed steel (HSS) twist drilling. This accounts for the high metal-removal rates associated with the process. In aluminum, speeds may be 600 surface feet per minute (SFPM), in steels from 400 SFPM to 450 SFPM.

Speeds and feeds for gun drilling are based on the workpiece material and shop floor conditions. Published charts only provide starting points. On-the-floor experimentation is critical to determine the right combination for maximum tool life.

Gun drill body: The body of a gun drill is typically constructed from 4120 aircraft quality steel tubing that is heat treated to between 35 to 40 Rc. A 4140 steel driver is brazed to one end of the tube and a carbide tool tip is brazed to the other end. Figure 9.18 shows five different tool tip geometries with various coolant hole placements.

There are two body styles for multiple flute tools: milled and crimped. The former is a thick wall tubular shaft with the flutes milled into the body. The latter is a thin wall tubular shaft that has the flutes swaged into it. The number of flutes depends on the material being cut. When drilling in a material that breaks easily into small chips, such as cast iron, a two-flute tool is the choice. On the other hand, for a material such as D2 tool steel, a single-flute design is preferred. In this case, chips tend to be stringy and a single-flute tool will minimize the chance of jamming as they are removed from the hole.

Consider a crimp-style gun drill body with two flutes produced by swaging and a conventional milled-style gun drill. The coolant holes in the crimped body have an irregular shape that permits carrying a much larger volume of coolant than comparable holes in a conventional equivalent diameter tool body. Also, the flutes that are formed are much deeper than milled tools because allowance does not have to be made for wall thickness between flute and coolant hole. These deeper flutes improve the chip-removal efficiency of the tool.

Gun drill tip: A conventional gun drill has a hole in its carbide tip underneath the cutting edge. Pressurized cutting fluid is pumped through the tool's body and out the hole. The fluid serves a three-fold purpose: it lubricates and cools the cutting edge; it forces the chips back along the flute in the tool body; and it helps to stiffen the shank of the tool.

A new design has one hole in the top of the tool tip that effectively directs fluid at the cutting edge. The other hole, which is in the conventional location, helps to provide the chip ejection function. Total flow of cutting fluid is doubled with this two-hole arrangement. More importantly, the design produces chips about half the size of a conventional gun drill of the same diameter using the same speed and feedrate, so that packing of chips along the tool's shank is avoided in most materials.

The most common tool tip material is C2 carbide, which is one of the harder grades and is generally associated with cast iron applications. Because excessive tool wear is a major problem when cutting steel, a hard grade such as C2 is recommended, even though C5 carbide is labeled as the steel machining grade in most text books. C5 carbide is a shock-resistant grade, not a wear-resistant grade, so that it is not as suitable for a gun drill tool tip. C3 carbide is harder than C2, and is used for certain applications; however, greater care must be taken when re-sharpening this material because it is easier to heat check the cutting edge.

Recently, coatings such as titanium nitride are being applied to gun drill tips to extend tool life. Physical Vapor Deposition (PVD) is the only practical process for depositing coatings on precision tools such as gun drills, but the results have not been encouraging. Unlike coating high-speed steel tools, PVD coating of a carbide gun drill tip does not seem to form a good metallurgical bond. The coating wipes off during the metal cutting process. Using Chemical Vapor Deposition (CVD) will form a metallurgical bond between the coating and carbide substrate, but the high heat required by the process distorts the tool. Hopefully these problems will be resolved in the near future.

The ejector system: The ejector system consists of drill head, outer tube, inner tube, connector, collet and sealing sleeve. The drill head is screwed to the drill tube by means of a four-start square thread. The inner tube is longer than the outer tube. The drill tube and the inner tube are attached to the connector by means of a collet and a sealing sleeve. The collet and sealing sleeve must be changed for different diameter ranges. Below is the ejector system and its components.

The single tube system (STS): The single tube system is based on external cutting fluid supply and internal chip transport. As a rule, the drill head is screwed onto the drill tube. The cutting fluid is supplied via the space between the drill tube and the drilled hole. The cutting fluid is then removed along with the chips through the drill tube. The velocity of the cutting fluid is so high that chip transport takes place through the tube without disturbances. Since chip evacuation is internal, no chip flute is required in the shank, so tip cross-section can be made completely round, which provides much higher rigidity than the gun drill system. Shown below is the single tube system and its components.

Comparison of STS and ejector systems: Both the single tube system and the ejector system have wide ranges of application, but there are times when one system is preferable to the other.

STS is preferable in materials with poor chip formation properties such as stainless steel, low-carbon steel, and materials with an uneven structure, when chip breaking problems exist. STS is also more advantageous for long production runs, uniform and extremely long workpieces and for hole diameters greater than 7.875 inches.

The ejector system requires no seal between the workpiece and the drill bushing. The system can therefore be adapted easily to existing machines and is preferable in NC lathes, turning centers, universal machines and machining centers. Since the cutting fluid is supplied between the outer and inner tubes, no space is required between the drill tube and the hole wall as in the case of STS drilling. The ejector system is therefore often used for machining in workpieces where sealing problems can arise. The ejector system can be used to advantage when it is possible to use a predrilled hole instead of a drill bushing for guidance, for example in machining centers.

George Schneider, Jr., is the author of Cutting Tool Applications, a handbook to machine tool materials, principles, and designs. He is the Professor Emeritus of Engineering Technology at Lawrence Technological University, and former Chairman of the Detroit Chapter of the Society of Manufacturing Engineers.