Where should the cutting tools be placed?

Chapter 4: Turning Tools and Operations - American Machinist

According to various sources on Google regarding the placement of cutting tools, it's crucial to position them accurately to ensure high efficiency and optimal results. Proper placement not only enhances the surface finish but also prolongs tool life, reduces cutting forces, and minimizes vibrations during machining processes.

Turning is a metalcutting process employed to create cylindrical surfaces. Generally, the workpiece is rotated on a spindle while the cutting tool is fed into it either radially, axially, or simultaneously in both directions to achieve the desired surface finish. The concept of turning broadly refers to generating cylindrical surfaces using a single-point tool; in many instances, it is specifically associated with the crafting of external cylindrical surfaces that are aligned parallel to the workpiece axis. When producing surfaces that are primarily perpendicular to the workpiece axis, the process is termed facing. In turning operations, the predominant feeding motion is axial relative to the machine spindle, whereas facing involves a dominant radial feed. For tapered and contoured surfaces, both modes of tool feed are typically required, a process often referred to as profiling.

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Generally, turning operations exhibit consistent cutting characteristics. For any given surface, only one cutting tool is utilized, and this tool must extend beyond the holder slightly to ensure clearance from the rotating workpiece. Once engaged, the tool maintains contact with the workpiece until the desired surface is fully produced. During this interaction, both the cutting speed and dimensions remain consistent for cylindrical surfaces being turned. In facing operations, however, the cutting speed varies in proportion to the work diameter, decreasing as the center is approached. Some setups employ mechanisms that adjust spindle speed to elevate the workpiece’s rotation rate as the tool moves towards the center.

Turning is typically characterized by stable conditions throughout the metal cutting process. Except at the beginning and end of any cut, the forces exerted on the cutting tool and the temperature at the tool tip remain effectively constant. During facing, however, variations in cutting speed can affect temperature at the tool tip, as higher temperatures are generally observed at larger diameters. Still, as cutting speed only minimally influences cutting forces, the forces applied on a facing tool can be anticipated to remain nearly constant during the operation.

Related Turning Operations

Machines can perform a wide array of additional machining processes on a lathe besides turning and facing.

Additional Lathe Operations

A brief overview of six other lathe operations follows:

• Chamfering: This tool is used to create angular cuts on the edges of a cylinder.
• Parting: The tool is fed radially into rotating work at a specific location along its length to sever the end of a component.
• Threading: A pointed tool traverses either the outer or inner surface of rotation to form external or internal threads.
• Boring: This involves enlarging a pre-existing hole, with a single-point tool moving linearly and parallel to the rotational axis.
• Drilling: Achieved by driving the drill axially into the rotating workpiece, drilling can be followed by reaming or boring to enhance accuracy and surface finish.
• Knurling: This is a metal forming operation that creates a regular cross-hatched pattern on the work surfaces.

Turning Toolholders

Previous chapters have introduced mechanical toolholders and the ANSI Identification System for turning toolholders and indexable inserts.

Toolholder Styles

The ANSI numbering system assigned specific letters to different geometries concerning lead angle and end cutting edge angle for turning toolholders. Primary lathe machining operations such as turning, facing, grooving, threading, and cutoff are covered by seven basic tool styles recognized by the ANSI system, encompassing designations A through G.

Designations of Tool Styles

• A Style: Straight shank with 0° side-cutting edge angle for turning operations.
• B Style: Straight shank with a 15° side-cutting edge angle for turning operations.
• C Style: Straight shank with 0° end-cutting edge angle, suitable for cutoff and grooving operations.
• D Style: Straight shank with a 45° side-cutting edge angle for turning operations.
• E Style: Straight shank with a 30° side-cutting edge angle designated for threading operations.
• F Style: Offset shank with 0° end-cutting edge angle employed for facing operations.
• G Style: Offset shank with a 0° side-cutting edge angle; classified as an 'A' style tool but with additional clearance for turning close to the lathe chuck.

Turning Insert Shapes

Indexable turning inserts come in various shapes, sizes, and thicknesses, featuring straight holes, countersunk holes, or chipbreakers. Selecting the right turning toolholder geometry along with appropriate insert shape and chipbreaker design significantly affects productivity and tool longevity in any turning task.

One critical factor in choosing the correct geometry is insert strength concerning the workpiece material or hardness. Triangular inserts are commonly favored due to their versatility across the seven basic turning holders previously described. Diamond-shaped inserts find use in profiling, whereas square inserts are favored for lead angle tools. A valuable guideline for assessing an insert's strength states that larger included angles at the insert corner yield greater strength.

Operating Conditions

Operating conditions influence three key variables in metal cutting: metal removal rate, tool life, and surface finish. Proper selection of these conditions is vital in balancing these factors to achieve optimal machining costs, production rates, and surface finishes, depending on specific operation requirements.

The proficiency of any machining operation hinges on the correct setup of both the workpiece and the cutting tool. This setup is particularly crucial when working with non-rigid materials or tooling needing extension to reach the machining area.

Deflection is an inherent challenge affecting the workpiece, cutting tool, and machine that cannot be entirely eliminated. Usually, this deflection is so minor that it goes unnoticed and has no bearing on the operation. However, it can create issues such as chatter, vibration, or distortion. Hence, meticulous attention must be paid to ensuring a robust setup for the specific operation, particularly for heavy or interrupted cuts.

Challenges with Unbalance

When machining uniquely shaped workpieces, especially those with uneven weight distribution or off-center loads, balancing becomes vital. Unbalanced conditions pose safety risks and can lead to inaccuracies, chatter, and damage to the machining apparatus. While these issues might be subtle, their severity escalates with increased operational speed, commonly manifesting during the use of turntables or lathe face plates.

As material is removed from a workpiece, its balance can shift, leading to severe unbalance conditions that become evident only when moving towards finishing cuts. This inconspicuous evolution may complicate achieving the required precision and surface finish.

Workholding Methods

In lathe operations, three principal work holding methods include:

• Using a chuck.
• Utilizing centers.
• Employing collets.

Chucks

The chuck, often with three or four jaws, is the most prevalent workholding method and attaches to the main spindle's end. A three-jaw chuck grips cylindrical workpieces effectively when ensuring the machined surfaces are concentric with the work surfaces.

Conversely, a four-jaw chuck allows individual jaw adjustments via radially mounted screws. While this adjustment can consume more time, it becomes essential for correctly mounting irregularly shaped workpieces.

Between Centers

For precise turning, particularly with non-cylindrical surfaces, workpieces can be secured between centers. Each end of the workpiece must be drilled with a conical center hole for accurate alignment with the lathe centers. A clamping device, or dog, secures the workpiece and engages with the drive plate on the main spindle, ensuring unified rotation.

Lathe centers provide pivotal support for the workpiece positioning between the headstock and tailstock. The headstock's center, known as the live center, rotates alongside the spindle, while the tailstock center (dead center) remains stationary. This static center must remain hardened and lubricated to sustain wear from the revolving workpiece.

It is crucial to maintain cleanliness around the taper of the spindle hole and the dead center, as debris or burrs can disrupt precision. Properly aligned centers are essential in ensuring accurate lathe operations, necessitating precisely drilled and countersunk holes in the workpiece.

Collets

Collets prove advantageous for gripping smoothly machined bar stock or workpieces requiring precise accuracy beyond standard three or four jaw chuck capabilities. These thin steel bushings, which are split longitudinally, fit the workpiece and, when drawn inward using a draw bar, securely grip the stock through internal taper engagement.

Toolholding Devices

The simplest toolholder post secures a single-point tool, typically using a curved block on a concave surface that facilitates tool inclination. The tool post is mounted on a compound rest, allowing angular positioning for various machining operations, such as screw-threading or taper creation.

A more sophisticated tool post, known as the square turret, also mounts on the compound rest and can hold up to four cutting tools. This design enables rapid tool access by unlocking and rotating the tool post as required.

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Standard toolholders are generally crafted to cut along the machine's and workpiece’s centerline. If the cutting point strays from this line, the clearance angle decreases, leading to decreased tool longevity and a poor surface finish. Such misalignment also risks pushing the workpiece away from the cutting tool during operations involving smaller diameters.

In contrast, if the cutting edge sits below the centerline, the rake angle turns excessively negative, generating heightened cutting forces, and resulting in chip curling that can lead to insert fractures or the workpiece overrunning the tool.

In some scenarios, adjusting the cutting point above the centerline by 2% to 4% of the workpiece diameter can alleviate issues during machining delicate components or when addressing ongoing chatter during deep grooving operations. This adjustment subtly alters the rake angle to lessen cutting forces.

Interrupting cuts poses unique challenges, particularly with larger diameter workpieces. Hence, positioning the cutting point marginally below the centerline can optimize the insert's cutting strength, supplemented by an appropriate lead angle that ensures engagement on stronger sections of the tool insert.

Cutting Conditions

After selecting the appropriate machine tool and cutting tool, the following cutting conditions require consideration:

• Cutting speed: The cutting speed defines the relative surface speed between the tool and workpiece, measured in surface feet per minute. Movement can occur in the workpiece, tool, or both.
• Depth of cut: This factor indicates how deeply the tool engages with the material, determining one dimension of the cut area.
• Feedrate: Defined as the axial advancement of the tool per workpiece revolution, this is measured in inches per revolution (IPR) or inches per minute (IPM).

Careful selection of feed rates, cutting speeds, and depth of cut directly influences productivity, tool longevity, and machine requirements, necessitating thoughtful consideration for each operation’s goal, whether roughing or finishing.

Hard Turning

With increased hardness of the work material, machinability declines markedly, which can complicate tool wear and fracture rates alongside surface finish quality. Although numerous mechanical and non-mechanical methods exist for material removal from hardened metals, traditional cutting processes can still effectively apply to hard metals by utilizing suitable tool materials and robust, high-speed machine tools.

Finish machining of heat-treated steel components for machines and automotive parts using polycrystalline cubic boron nitride (PCBN) tools exemplifies this approach, achieving excellent dimensional accuracy and surface finish. In many cases, hard turning can be a cost-efficient alternative to grinding; reports indicate that grinding is over ten times more expensive than hard turning.

Innovative cutting tool materials like PCBN and ceramics have facilitated the economical turning of hardened steel, prompting many machine shops to discontinue cylindrical grinders in preference for CNC lathes that provide versatility and cost-effectiveness.

When compared to grinding, hard turning offers various advantages, including:

• Faster metal removal rates and reduced cycle times.
• The possibility for dry machining (eliminating coolant use).
• Decreased setup time, allowing multiple operations with a single workpiece holding.

Modern CNC lathes deliver comparable accuracy and surface finishes to traditional grinding methods.

Moreover, hard turning requires less energy, reduces thermal and other forms of damage risks to workpieces, minimizes the use of cutting fluids, and presents lower machine tool costs. Eliminating the need for material handling, the process facilitates finishing while the part remains chucked in the lathe. Nonetheless, issues arise in holding large or slender workpieces given the higher cutting forces involved compared to grinding.

In addition, managing tool wear poses a challenge, especially relative to the automatic dressing routine for grinding wheels. Thus, determining the competitive edge of hard turning over grinding should be approached on a case-by-case basis, focusing on product surface integrity, quality, and overall economics.

Dry vs. Wet Machining

Only a couple of decades ago, cutting fluids contributed less than 3% to the expenses of most machining processes, primarily because of their low cost. However, that scenario has shifted significantly.

Currently, cutting fluids can comprise up to 15% of a machine shop's production costs, resulting in heightened concern amongst shop owners regarding these fluids.

Particularly those containing oil have turned into significant liabilities. While the Environmental Protection Agency (EPA) regulates their disposal, numerous states impose strict controls due to classifications as hazardous waste when they contain oil and certain alloys.

Due to airborne mists generated by several high-speed machining operations and fluid nozzles, governmental regulations also limit the allowable mist levels in the atmosphere. The EPA has proposed even stricter control measures for such airborne particulates, and OSHA is contemplating lowering acceptable exposure limits to fluid mist.

Increased maintenance, record-keeping efforts, and compliance with both existing and proposed regulations continue to inflate cutting fluids' costs. Many machine shops are thus contemplating a shift to dry machining to eliminate the costs and jurisdictional challenges linked with cutting fluids.

The choice between dry and wet machining should be made individually for each application. A lubricating fluid can prove beneficial in lower-speed operations, machining of difficult materials, or when demanding surface finish standards apply. Conversely, fluids with superior cooling capabilities come into play during high-speed tasks or when working with easily machined materials and jobs susceptible to edge buildup or tight tolerances.

However, the performance benefits of certain cutting fluids can be overshadowed by their costs, leading a growing number of applications where cutting fluids may be unnecessary or even counterproductive. Furthermore, modern cutting tools can function at elevated temperatures while compressed air may be applied for removing hot chips from the cutting zone.

George Schneider, Jr. authored Cutting Tool Applications, a comprehensive handbook on machine tool materials, principles, and designs, and now serves as Professor Emeritus of Engineering Technology at Lawrence Technological University, also having chaired the Detroit Chapter of the Society of Manufacturing Engineers.