Chapter 22

What are alternative tool life criteria that are more convenient to use in production operations?

(1) complete failure of the cutting edge, although this criterion has disadvantages, as discussed earlier; (2) visual inspection of flank wear (or crater wear) by the machine opera-tor without a toolmaker's microscope; (3) changes in the sound emitting from the operation, as judged by the operator; (4) degradation of the surface finish on the work; (5) increased power consumption in the operation, as measured by a wattmeter connected to the machine tool; (6) workpiece count, in which the operator is instructed to change the tool after a certain specified number of parts have been machined; and (7) cumulative cutting time, which is similar to the previous workpiece count, except that the length of time the tool has been cutting is monitored.

What are the four categories of cutting fluids according to chemical formulation?

(1) cutting oils, (2) emulsified oils, (3) semichemical fluids, and (4) chemical fluids

What cutting-tool materials achieve this combination of properties in varying degrees?

(1) high-speed steel and its predecessors, plain carbon and low alloy steels; (2) cast cobalt alloys; (3) cemented carbides, cermets, and coated car-bides; (4) ceramics; (5) synthetic diamond and cubic boron nitride

Cutting Fluid Filtration Advantages

(1) prolonged cutting fluid life between changes—instead of replacing the fluid once or twice per month, coolant lives of 1 year have been reported; (2) reduced fluid disposal cost, since disposal is much less frequent when a filter is used; (3) cleaner cutting fluid results in a better working environment and reduced health hazards; (4) lower machine tool maintenance; and (5) longer tool life

What are the two principal forms of chip breaker design commonly used on sing point cutting tools?

(a) groove-type chip breaker designed into the cutting tool itself, and (b) obstruction-type chip breaker designed as an additional device on the rake face of the tool.

Cemented Carbides (Sintered Carbides)

A class of hard tool material formulated from tungsten carbide. Consequently, straight WC-Co tools cannot be used effectively to machine steel. It was discovered that additions of titanium carbide and tantalum carbide to the WC-Co mix significantly retarded the rate of crater wear when cutting steel Basic Classes: (1) nonsteel cutting grades, consisting of only WC-Co; and (2) steel-cutting grades, with combinations of TiC and TaC added to the WC-Co

Cutting Fluids

A cutting fluid is any liquid or gas that is applied directly to the machining operation to improve cutting performance (addresses friction and heat generation)

High Speed Steel

A highly alloyed tool steel capable of maintaining hardness at elevated temperatures better than high carbon and low alloy steels - Tungsten Type - Molybdenum Type

What are the mechanism that cause wear at the tool-chip and tool-work interfaces in machining?

Abrasion, Adhesion, Diffusion, Chemical Reactions, Plastic Deformation

End Milling Cutters

An end milling cutter looks like a drill bit, but close inspection indicates that it is designed for primary cutting with its peripheral teeth rather than its end

Notch Wear

An extreme condition of flank wear often appears on the cutting edge at the location corresponding to the original surface of the work part

Dry Machining

Avoids the problems of cutting fluid contamination, disposal, and filtration, but leads to problems of its own: (1) overheating the tool, References | 529 (2) operating at lower cutting speeds and production rates to prolong tool life, and (3) absence of chip removal benefits in grinding and milling.

Cutting Oils

Based on oil derived from petroleum, animal, marine, or vegetable origin. To achieve maximum lubricity, several types of oils are often combined in the same fluid.

What are the three regions can usually be identified in the typical wear growth curve?

Break-in period, steady-state wear region, failure region

Abrasion

Caused by hard particles in the work material gouging and removing small portions of the tool

Ceramics

Ceramic cutting tools are composed primarily of fine-grained aluminum oxide (Al2O3), pressed and sintered at high pressures and temperatures with no binder into insert form. Aluminum oxide cutting tools are most successful in high-speed turning of cast iron and steel. Ceramics are not recommended for heavy interrupted cut operations (e.g., rough milling) because of their low toughness. In addition to its use in conventional machining operations, Al2O3 is widely used as an abrasive in grinding and other abrasive processes

Chemical Fluids

Chemicals in a water solution rather than oils in emulsion. Chemical fluids provide good coolant qualities but their lubricating qualities are less than those of the other cutting fluid

Chip Breakers

Chip breakers are frequently used with single-point tools to force the chips to curl more tightly than they would naturally be inclined to do, thus causing them to fracture

Coated Carbides

Coated carbides are cemented carbide inserts coated with one or more thin layers of wear-resistant material, such as titanium carbide, titanium nitride, and/ or aluminum oxide. Coated carbides are used to machine cast irons and steels in turning and milling operations

Cermet

Composite of ceramic and metallic materials

Cast Cobalt Alloys

Consist of cobalt, around 40-50%; chromium, about 25-35%; and tungsten, usually 15-20%; with trace amounts of other elements Wear resistance of the cast cobalts is better than high-speed steel, but not as good as cemented carbide. Toughness of cast cobalt tools is better than carbides but not as good as HSS. Hot hardness also lies between these two materials.

Emulsified Oils

Consist of oil droplets suspended in water. Because they contain both oil and water, the emulsified oils combine cooling and lubricating qualities in one cutting fluid.

Crater Wear

Consists of a cavity in the rake face of the tool that forms and grows from the action of the chip sliding against the surface.

Failure Region

Cutting temperatures are higher, and the general efficiency of the machining process is reduced

Inserts

Cutting-tool inserts are widely used in machining because they are economical and adaptable to many different types of machining operations. They are available in a variety of shapes and sizes for the variety of cutting situations encountered in practice The effect of this edge preparation is to increase the strength of the cutting edge by providing a more gradual transition between the clearance edge and the rake face of the tool.

Synthetic Diamonds

Diamond is the hardest material known. Synthetic diamond cutting tools are made of sintered polycrys- talline diamond, which is fabricated by sintering fine-grained diamond crystals under high temperatures and pressures into the desired shape. Applications of diamond cutting tools include high-speed machining of nonferrous metals and abrasive nonmetals such as fiberglass, graphite, and wood. Machining of steel, other ferrous metals, and nickel-based alloys with SPD tools is not practical because of the chemical affinity that exists between these metals and carbon (a diamond, after all, is carbon).

What are lubricant cutting fluids operate by?

Extreme pressure lubrication - a special form of lubrication that involves formation of thin solid salt layers on the hot, clean metal surfaces through chemical reaction with the lubricant.

What are the three possible modes by which a cutting tool can fail in machining?

Fracture failure, temperature failure, gradual wear

Cermets

Generally reserved for combinations of TiC, TiN, and titanium carbonitride (TiCN)/ Applications of cermets include high-speed finishing and semifinishing of steels, stainless steels, and cast irons.

Semichemical Fluids

Have small amounts of emulsified oil added to increase the lubricating characteristics of the cutting fluid. In effect, they are a hybrid class between chemical fluids and emulsified oils.

Steady-state Wear Region

In the figure, this region is pictured as a linear function of time, although there are deviations from the straight line in actual machining

None Steel Cutting Grades

Nonsteel-cutting grades refer to those cemented carbides that are suitable for machining aluminum, brass, copper, magnesium, titanium, and other nonferrous metals; anomalously, gray cast iron is included in this group of work materials

What are certain features of flank wear that can be identified?

Notch wear and nose radius wear

Flank Wear

Occurs on the flank, or relief face, of the tool; it results from rubbing between the newly generated work surface and the flank face adjacent to the cutting edge

Nose Wear

Occurs on the nose radius leading into the end cutting edge

Form Milling Cutters

Peripheral milling cutters in which the cutting edges have a special profile that is to be imparted to the work

What are the classification for milling cutters?

Plain milling cutters, form milling cutters, face milling cutters, end milling cutters

Single Point Tool Geometry

Rake face is defined by two angles, back rake angle (αb) and side rake angle (αs ). Together, these angles are influential in determining the direction of chip flow across the rake face. The flank surface of the tool is defined by the end relief angle (ERA) and side relief angle (SRA). These angles determine the amount of clearance between the tool and the freshly cut work surface. The side cutting edge angle (SCEA) determines the entry of the tool into the work and can be used to reduce the sudden force the tool experiences as it enters a work part. The end cutting edge angle (ECEA) provides a clearance between the trailing edge of the tool and the newly generated work surface, thus reducing rubbing and friction against the surface. The nose radius (NR), which determines to a large degree the texture of the surface generated in the operation. (These are called the tool geometry signature)

Cubic Boron Nitride

Second hardest material known. Cubic boron nitride (symbolized cBN) does not react chemically with iron and nickel as SPD does; therefore, the applications of cBN-coated tools are for machining steel and nickel-based alloys (similarly to SPD, cBN is expensive).

Taylor Tool Life Equation

States that higher cutting speeds result in shorter tool life

Steel Cutting Grades

Steel-cutting grades are used for low carbon, stainless, and other alloy steels

Drills ( Twist Drill is Most Common)

The body of the drill has two spiral flutes (the spiral gives the twist drill its name). The angle of the spiral flutes is called the helix angle, a typical value of which is around 30°. The point of the twist drill has a conical shape. A typical value for the point angle is 118°. The relative velocity at the drill point is zero, so no cutting takes place. A twist drill is normally limited to a hole depth of about four times its diameter (4:1) Straight Flute Drills: Operate like twist drills except that the flutes for chip removal are straight along the length of the tool rather than spiraled (3:1) Gun Drill: A straight-flute drill designed for deep-hole drilling (125:1) (Requires drill bushing) Spade Drill: An alternative for large-diameter holes

Broaches

The broach consists of a series of discrete cutting teeth along its length (most broaches are made of high speed steel). The speed motion is accomplished by the linear travel of the tool past the work surface. The shape of the cut surface is determined by the contour of the cutting edges on the broach, particularly the final cutting edge

Plastic Deformation

The cutting forces acting on the cutting edge at high temperature cause the edge to deform plastically, making it more vulnerable to abrasion of the tool surface

Chemical Reactions

The high temperatures and clean surfaces at the tool-chip interface in machining at high speeds can result in chemical reactions, in particular oxidation, on the rake face of the tool. The oxidized layer, being softer than the parent tool material, is sheared away, exposing new material to sustain the reaction process.

Tool Life

The length of cutting time that the cutting tool can be used

Flooding Cooling (Application Method)

The most common method is flooding, sometimes called flood-cooling because it is generally used with coolant-type cutting fluids. In flooding, a steady stream of fluid is directed at the tool-work and/or tool-chip interface of the machining operation. A second method of delivery is mist application, primarily used for water-based cutting fluids.

Saw Blades

The saw blades possess certain common features, including tooth form, tooth spacing, and tooth set. Tooth form is concerned with the geometry of each cutting tooth. Tooth spacing is the distance between adjacent teeth on the saw blade. Tooth set causes the kerf cut by the saw blade to be wider than the width of the blade itself; otherwise, the blade would bind against the walls of the slit made by the saw.

Break-in Period

The sharp cutting edge wears rapidly at the beginning of its use

Rake Angle

The very hard materials must be designed with either negative rake or small positive angles. This change tends to load the tool more in compression and less in shear, thus favoring the high compressive strength of these harder materials.

Face Milling Cutters

These are designed with teeth that cut on both the periphery as well as the end of the cutter

Diffusion

This is a process in which an exchange of atoms takes place across a close contact boundary between two materials (causes the tool surface to become depleted of the atoms responsible for its hardness)

What are the two principle locations on a cutting tool that gradual wear can occur?

Top rake face and the flank (crater wear and flank wear)

What are the important properties required in a tool material?

Toughness, Hot Hardness, Wear Resistance

Plain Milling Cutters

Used for peripheral or slab milling

Lubricants

Usually oil-based fluids (because oils possess good lubricating qualities) formulated to reduce friction at the tool-chip and tool-work interfaces (are most effective at lower cutting speeds - so that they don't lose effectiveness)

Adhesion

When two metals are forced into contact under high pressure and temperature, adhesion (welding) occurs between them.

Coolants

cutting fluids designed to reduce the effects of heat in the machining operation