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Level 39

23 - CUTTING TOOL TECHNOLOGY


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Tool technology aspects
The two main aspects of cutting tool technology are (1) tool material and (2) tool geometry.
Modes of tool failure
The three tool failure modes are (1) fracture failure, (2) temperature failure, and (3) gradual wear.
Locations of tool wear
Wear occurs on the top face of the cutting tool as crater wear and on the side or flank of the tool, called flank wear. Portions of flank wear are often identified separately as notch wear, corresponding to the surface of the work; and nose radius wear, corresponding to the tool point.
Mechanisms of tool wear
The important tool wear mechanisms are (1) abrasion, (2) adhesion, (3) diffusion, and (4) plastic deformation of the cutting edge.
Taylor tool life equation
The parameter C in Taylor tool lie equation is the cutting speed corresponding to a one-minute tool life. C is the speed-axis intercept on the log-log plot of the tool life data.
Expanded tool life equation
The expanded version of the Taylor equation can include any of the following: feed, depth of cut, and/or work material hardness.
Tool life criteria
As identified in the text, tool life criteria used in production include (1) complete failure of the tool, (2) visual observation of flank or crater wear, (3) fingernail test to feel flank wear, (4) sound of the tool, (5) chip disposal problems, (6) degradation of finish, (7) power increase, (8) workpiece count, and (9) length of cutting time for the tool.
Cutting tool material properties
Three desirable properties are (1) toughness to resist fracture failure, (2) hot hardness to resist temperature failure, and (3) wear resistance to prolong the life of the tool during gradual wear.
Alloying ingredients in HSS
Principal alloying ingredients in HSS are (1) either tungsten or a combination of tungsten and molybdenum, (2) chromium, (3) vanadium, and (4) carbon. Some grades of HSS also contain cobalt.
Cemented carbide cutting blades
In general, non-steel cutting grades of cemented carbides contain only Tungsten Carbide and Cobalt. Steel cutting grades contain TiC (Titanium carbide) and/or TaC (Tantalum Carbide) in addition to Tungsten Carbide and Cobalt.
Coatings of carbide inserts
The common coatings for coated carbide inserts are: TiN - Titanium Nitrate, TiC - Titanium Carbide, and Al2, O3 - Aluminum oxide.
Tool geometry elements
The seven elements of single-point tool geometry are (1) back rake angle, (2) side rake angle, (3) end relief angle, (4) side relief angle, (5) end cutting edge angle, (6) side cutting edge angle, and (7) nose radius.
Ceramic tool negative rake angles
Ceramics possess low shear and tensile strength but good compressive strength. During cutting, this combination of properties is best exploited by giving the tool a negative rake angle to load the tool in compression.
Tool holding methods
There are three principal ways: (1) solid shank, in which the cutting edge is an integral part of the tool shank, an example being high speed steel tooling; (2) brazed inserts, used for some cemented carbides; and (3) mechanically clamped inserts, used for most hard tool materials including cemented carbides, coated carbides, cermets, ceramics, SPD, and CBN.
Functional categories of cutting fluid
The two functional categories of cutting fluids are: (1) coolants and (2) lubricants.
Chemical categories of cutting fluid
The four categories of cutting fluids according to chemistry are (1) cutting oils, (2) emulsified oils, (3) chemical fluids, and (4) semi-chemical fluids.
Lubricating mechanisms of cutting fluids
There are two lubricating mechanisms that are believed to be effective in metal cutting: (1) boundary lubrication, which involves the formation of a thin fluid film to help separate and protect the contacting surfaces; and (2) extreme pressure lubrication, in which a thin solid layer of a salt such as iron sulfide is formed on the tool surface to provide lubrication.
Application of cutting fluids
The most common method of application is flooding, in which a steady stream of fluid is direct at the operation. Other methods include mist application, fluid-hole delivery through the tool, and manual application (e.g., using a paint brush).
Filter systems for cutting fluids
Cutting fluid filter systems are becoming more common due to the environmental protection laws and the need to prolong the life of the fluid before disposal. Advantages of filter systems include longer fluid life, reduced disposal costs, better hygiene, lower machine tool maintenance, and longer cutting tool life.
Problems of cutting fluids
Cutting fluids become contaminated over time with a variety of contaminants, including tramp oil, garbage, small chips, molds, fungi, and bacteria. In addition to causing odors and health hazards, contaminated cutting fluids do not perform their lubricating function as well as when they are fresh and clean.
Problems of dry machining
Problems with dry machining include (1) overheating the tool, (2) operating at lower cutting speeds and production rates to prolong tool life, and (3) absence of chip removal benefits that are provided by cutting fluids in grinding and milling.
Categories of cutting tools
The two principal categories of cutting tools are (1) single-point cutting tools (used on lathes) and (2) multi point cutting tools (used on mills, drills, reamers, and taps).
Selection of cutting tool
The objective when choosing a cutting tool is to safely machine a workpiece in the shortest amount of time while meeting the part’s quality requirement. Furthermore, the tooling should be the least costly and least complex to meet the production demands.
Factors for tool selection
The factors a machinist must know in order to select the proper tooling are (1) workpiece starting and finished shape, (2) workpiece hardness, (3) workpiece tensile strength, (4) material abrasiveness, (5) whether the material breaks into short chips or long stringy chips, (6) workholding setup, and (6) power and speed capacity of the machine tool.
Tool material characteristics
The characteristics of a good tool material are the following: (1) it is harder than the workpiece, (2) it retains hardness at high temperatures, (3) it resists wear and thermal shock, (4) it has impact resistant, and (5) it is chemically inert.