Chapter 2
Metal Removal Methods

2.1 Introduction

Even with all of the sophisticated equipment and techniques used in today's modern industry, the basic mechanics of forming a chip remain the same. As the cutting tool engages the workpiece, the material directly ahead of the tool is sheared and deformed under tremendous pressure. The deformed material then seeks to relieve its stressed condition by fracturing and flowing into the space above the tool in the form of a chip.

2.2 Cutting Tool Forces

A general discussion of the forces acting in metalcutting is presented by using the example of a typical turning operation. When a solid bar is turned, there are three forces acting on the cutting tool:

Tangential Force: This acts in a direction tangential to the revolving workpiece and represents the resistance to the rotation of the workpiece. In a normal operation, tangential force is the highest of the three forces and accounts for about 98 percent of the total power required by the operation.

Longitudinal Force: Longitudinal force acts in the direction parallel to the axis of the work and represents the resistance to the longitudinal feed of the tool. Longitudinal force is usually about 50 percent as great as tangential force. Since feed velocity is usually very low in relation to the velocity of the rotating workpiece, longitudinal force accounts for only about 1 percent of total power required.

Radial Force: Radial force acts in a radial direction from the center line of the workpiece. The radial force is generally the smallest of the three, often about 50 percent as large as longitudinal force. Its effect on power requirements is very small because velocity in the radial direction is negligible.

2.3 Chip Formation and Tool Wear

Regardless of the tool being used or the metal being cut, the chip forming process occurs by a mechanism called plastic deformation. This deformation can be visualized as shearing. That is when a metal is subjected to a load exceeding its elastic limit. The crystals of the metal elongate through an action of slipping or shearing, which takes place within the crystals and between adjacent crystals.

Typical turning operation showing the forces acting on the cutting tool.

Most practical cutting operations, such as turning and milling, involve two or more cutting edges inclined at various angles to the direction of the cut. However, the basic mechanism of cutting can be explained by analyzing cutting done with a single cutting edge.

Chip formation is simplest when a continuous chip is formed in orthogonal cutting. In oblique cutting, a single, straight cutting edge is inclined in the direction of tool travel. This inclination causes changes in the direction of chip flow up the face of the tool. When the cutting edge is inclined, the chip flows across the tool face with a sideways movement that produces a helical form of chip.

2.3.1 Chip Formation

Metalcutting chips have been classified into three basic types:

Discontinuous Chip - Type 1: Discontinuous or segmented chips are produced when brittle metal such as cast iron and hard bronze are cut or when some ductile metals are cut under poor cutting conditions. As the point of the cutting tool contacts the metal, some compression occurs, and the chip begins flowing along the chip-tool interface. As more stress is applied to brittle metal by the cutting action, the metal compresses until it reaches a point where rupture occurs and the chip separates from the unmachined portion. This cycle is repeated indefinitely during the cutting operation, with the rupture of each segment occurring on the shear angle or plane. Generally, as a result of these successive ruptures, a poor surface is produced on the workpiece.

Continuous Chip - Type 2: The Type 2 chip is a continuous ribbon produced when the flow of metal next to the tool face is not greatly restricted by a built-up edge or friction at the chip tool interface. The continuous ribbon chip is considered ideal for efficient cutting action because it results in better finishes. Unlike the Type 1 chip, fractures or ruptures do not occur here, because of the ductile nature of the metal.

Continuous Chip with a Built-up Edge (BUE) - Type 3: The metal ahead of the cutting tool is compressed and forms a chip which begins to flow along the chip-tool interface. As a result of the high temperature, the high pressure, and the high frictional resistance against the flow of the chip along the chip-tool interface, small particles of metal begin adhering to the edge of the cutting tool while the chip shears away. As the cutting process continues, more particles adhere to the cutting tool and a larger build-up results, which affects the cutting action. The built-up edge increases in size and becomes more unstable. Eventually a point is reached where fragments are torn off. Portions of these fragments which break off stick to both the chip and the workpiece. The build-up and breakdown of the built-up edge occur rapidly during a cutting action and cover the machined surface with a multitude of built-up fragments. These fragments adhere to and score the machined surface, resulting in a poor surface finish.

Shear Angle: Certain characteristics of continuous chips are determined by the shear angle. The shear angle is the plane where slip occurs to begin chip formation.

Regardless of the shear angle, the compressive deformation caused by the tool force against the chip will cause the chip to be thicker and shorter than the layer of workpiece material removed. The work or energy required to deform the material usually accounts for the largest portion of forces and power involved in a metal removing operation. For a layer of work material of given dimensions, the thicker the chip, the greater the force required to produce it.

Heat in Metalcutting: The mechanical energy consumed in the cutting area is converted into heat. The main sources of heat are: the shear zone, the interface between the tool and the chip where the friction force generates heat, and the lower portion of the tool tip which rubs against the machined surface. The interaction of these heat sources, combined with the geometry of the cutting area, results in a complex temperature distribution.

2.3.2 Cutting Tool Wear

Cutting tool life is one of the most important economic considerations in metalcutting. In roughing operations, the tool material, the various tool angles, cutting speeds, and feed rates are usually chosen to give an economical tool life. Conditions giving a very short tool life will not be economical because tool-grinding, indexing, and tool replacement costs will be high. On the other hand, the use of very low speeds and feeds to give long tool life will not be economical because of the low production rate. Clearly any tool or work material improvements that increase tool life without causing unacceptable drops in production will be beneficial. To form a basis for such improvements, efforts have been made to understand the behavior of the tool, how it physically wears, the wear mechanisms, and forms of tool failure.

The different wear mechanisms, as well as the different phenomena contributing to the attritious wear of the cutting tool, are dependent on the multitude of cutting conditions and especially on the cutting speeds and cutting fluids.

Aside from the sudden premature breakage of the cutting edge (tool failure), there are several indicators of the progression of physical wear. The machine operator can observe these factors prior to total rupture of the edge. The indicators are:

• Increase in the flank wear size above a predetermined value.
• Increase in the crater depth, width or other parameter of the crater, in the rake face.
• Increase in the power consumption, or cutting forces required to perform the cut.
• Failure to maintain the dimensional quality of the machined part within a specified tolerance limit.
• Significant increase in the surface roughness of the machined part.
• Change in the chip formation due to increased crater wear or excessive heat generation.

2.4 Single Point Cutting Tools

The metalcutting tool separates chips from the workpiece in order to cut the part to the desired shape and size. There is a great variety of metal cutting tools, each of which is designed to perform a particular job or a group of metal cutting operations in an efficient manner.

2.4.1 Cutting Tool Geometry

The shape and position of the tool, relative to the workpiece, have an important effect on metalcutting. The most important geometric elements, relative to chip formation, are the location of the cutting edge and the orientation of the tool face with respect to the workpiece and the direction of cut. Other shape considerations are concerned primarily with relief or clearance, i.e., taper applied to tool surfaces to prevent rubbing or dragging against the work.

Terminology used to designate the surfaces, angles and radii of single point tools, is shown below. The tool shown here is a brazed-tip type, but the same definitions apply to indexable tools.

The Rake Angle: The basic tool geometry is determined by the rake angle of the tool. The rake angle is always at the top side of the tool. With the tool tip at the center line of the workpiece, the rake angle is determined by the angle of the tool as it goes away from the workpiece center line location. The neutral, positive, and negative rakes are seen below. The angle for these geometries is set by the position of the insert pocket in the toolholder. The positive/negative (d) and double positive (e) rake angles are set by a combination of the insert pocket in the tool holder and the insert shape itself.

There are two rake angles: back rake and side rake. In most turning and boring operations, it is the side rake that is the most influential. This is because the side rake is in the direction of the cut.

Rake angle has two major effects during the metalcutting process. One major effect of rake angle is its influence on tool strength. An insert with negative rake will withstand far more loading than an insert with positive rake. The cutting force and heat are absorbed by a greater mass of tool material, and the compressive strength of carbide is about two and one half times greater than its transverse rupture strength.

The other major effect of rake angle is its influence on cutting pressure. An insert with a positive rake angle reduces cutting forces by allowing the chips to flow more freely across the rake surface.

Negative Rake: Negative rake tools should be selected whenever workpiece and machine tool stiffness and rigidity allow. Negative rake, because of its strength, offers greater advantage during roughing, interrupted, scaly and hard-spot cuts. Negative rake also offers more cutting edges for economy and often eliminates the need for a chipbreaker. Negative rakes are recommended on insert grades which do not possess good toughness (low transverse rupture strength).

Terminology used to designate the surfaces, angles, and radii of single-point tools.

Negative rake is not, however, without some disadvantages. Negative rake requires more horsepower and maximum machine rigidity. It is more difficult to achieve good surface finishes with negative rake. Negative rake forces the chip into the workpiece, generates more heat into the tool and workpiece, and is generally limited to boring on larger diameters because of chip jamming.

Positive Rake: Positive rake tools should be selected only when negative rake tools can't get the job done. Some areas of cutting where positive rake may prove more effective are, when cutting tough, alloyed materials that tend to "work-harden," such as certain stainless steels, when cutting soft or gummy metals, or when low rigidity of workpiece, tooling, machine tool, or fixture allows chatter to occur. The shearing action and free cutting of positive rake tools will often eliminate problems in these areas.

One exception that should be noted when experiencing chatter with a positive rake is, that at times the preload effect of the higher cutting forces of a negative rake tool will often dampen out chatter in a marginal situation. This may be especially true during lighter cuts when tooling is extended or when the machine tool has excessive backlash.

Neutral Rake: Neutral rake tools are seldom used or encountered. When a negative rake insert is used in a neutral rake position, the end relief (between tool and workpiece) is usually inadequate. On the other hand, when a positive insert is used at a neutral rake, the tip of the insert is less supported, making the insert extremely vulnerable to breakage.

Positive/Negative Rake: The positive/negative rake is generally applied using the same guidelines as a positive rake. The major advantages of a positive/negative insert are that it can be used in a negative holder, it offers greater strength than a positive rake, and it doubles the number of cutting edges when using a two-sided insert.

With the cutting tool on center, various back rake angles are shown: (a) neutral, (b) positive, (c) negative, (d) positive/negative, (e) double positive.

The positive/negative insert has a ten-degree positive rake. It is mounted in the normal five-degree negative pocket which gives it an effective five-degree positive rake when cutting. The positive/negative rake still maintains a cutting attitude which keeps the carbide under compression and offers more mass for heat dissipation. The positive/negative insert also aids in chip breaking on many occasions, as it tends to curl the chip.

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Originally published in the February 2001 issue
of Tooling & Production.
Please Note: some pictures or diagrams are only available through the printed media.