Chapter 22 Flashcards
Carbide tools
Coated, exclusively by the chemical vapor deposition process (cc’d)
Things that influence tool material decisions
Work material characteristics (hardness), part characteristics (geometry, accuracy, finish), machine tool characteristics (work holders, sped and feed ranges), support systems (sensors, controls, chip removal).
Three main sources of heat
1) the shear front itself 2) the tool/chip interface contact region 3) the flank of the tool where the produced workpiece surface rubs the tool.
Rate of wear?
Directly related to temperature
Hot hardness
Tools that do not lose hardness at the high temperatures associated with high speeds
Common toughness tests?
Izod and Charpy impact tests.
Toughness
The ability to take impacts during interrupted cutting
Impact strength
It’s ability to withstand impacts, and not wear the tool down. This saves money on tools.
Cutting speed and feed are
Limited br the capability of the tools material
Physical vapor deposition
15 to 20 percent of all took steels are coated by this process
Cermets
New ceramic materials
Tool steels
Carbon steels and low medium alloy steels. Important properties are hardness hot hardness and toughness
High speed steels
First introduced by f w Taylor and Mandel white in 1900. Retains its cutting ability at high temperatures. Can opérateurs at about double or triple cutting speeds.
High speed steels are used in
Drills and many milling cutters and single point tools
Strengths of high speed steels?
Great toughness, easily fabricated, and best for sever applications where complex tool geometry is needed (gear cutters, tap, drills, framers, and dies)
Hss coated with TiN includes
Readers, taps, chasers, spade drill blades, broaches, bandsaw, and circular band saws.
Pvd processing
Tools are subjected to a vigorous cleaning process as a pretreatment.
Main advantage of tin coated hss tooling
Reduced tool wear. Less stock removal. Longer tool life. Low coefficient of friction
Carbide cutting tool I serfs are traditionally dived into two primary groups
Straight tungsten grades, use for machining cast irons, nonmetallic surfaces. Grades containing major amounts of titanium, tantalum, use for machining ferritin workpieces.
Carbides
Nonferrous alloys, called sintered carbides, because they are manufacture by powder metallurgy techniques.
Cemented carbide tools are available in insert form in many shapes such as:
Squares triangles diamonds and rounds. Mechanical clamping attached the insert.
Chip groove
Has a positive take angle at the tool too, may be used to reduce d cutting forces without reducing the overall strength of the insert significantly.
Purpose of multiple coatings?
Prolonged tool life better metallurgical bond.
Two most effective coasting processes:
Chemical vapor deposition and physical vapor deposition of titanium nitride and titanium carbide.
Chemical vapor deposition:
Clean tools are heated until they reach the coating temperature, the coating cycle is initiated by the introduction of gages into the reactor.
Forming titanium nitride for coating
Nitrogen hydrogen gas mixture is substituted for methane
Pvd
Coated by condensation of a metal vapor. The vapor is formed by a source material called the charge, it is heated to temps less than 1000 C.
Pvd process features
1 coating takes place inside a vacuum chamber, with the workpiece heated. 2 before coating, all parts are given a final cleaning inside the chamber to remove oxides and improve coating adhesion. 3 the coating temperature is relatively low for cutting tools. 4. The metal source is vaporize in an inert gas atmosphere. 5. These are ion assisted deposition processes.
CVD over PVD
CVD is done at higher temperatures, so the coating are superior. The application can only be used to loosely tolerances tools.
PVD over CVD
Can be used for all high speed steels, and carbide tools. Fine blanking punches. I
Sources of tool life variability
1) variation in work hardness 2 variability in cutting tool materials, geometry, and preparation. 3 vibrations in machine tool 4 changing surface characteristics of workpieces.
Criteria that defines tool death
When surface finish deteriorates, when the dimension is out of tolerance, power consumption increases to a limit, chip discoloration happens, and cutting time.
Other ways to express tool life
Volume of metal removed, number of pieces machined, number of holes drilled.
Machinability
Defined by the ease of difficulty with which the metal can be machined.
Machinability
Defined by the relative cutting speed for a given tool life when cutting some material.
Cutting speed
Measured by the maximum speed at which a tool can provide satisfactory performance.
Other definitions of machinability
The ease of removal of Chios, the quality of the surface finish, the dimensional stability, or cost to remove a given volume of metal.
Reconditioning cutting tools
1 resharpen to original tool geometry specifications. 2 grind cutting edges and surfaces to a fine finish. 3 remove all burrs in resharpened cutting edges. If a tool with a burr is coated, the uncleared surface will be exposed to wear. 4 avoid resharpening practices that overheat and burn of melt the tool surfaces.
Cutting fluids
Fredrick w Taylor demonstrated that cutting fluids increase cutting abilities. Cutting fluids act as a coolant and a lubricant. The reduction in temperature aids in retaining the hardness of the tool and it’s cutting life.
Cutting fluids should be
Stable in use and storage, non corrosive, and nontoxic.
