Metal Cutting
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Machining using edges with definite geometry
Parameters
The removal of matter per cutting tool is defined by characteristics of cut (cutting speed, speed in advance, depth of cut, choice of the tool, fragmentation of the chip,Ð) for which the principal parameters to be taken into account are :
the material of cut (tool);
matter of the part;
geometry of cut;
cutting conditions;
the stability and the rigidity of the chucking devices of the tool and the part;
the stability and the rigidity of the system of machining.
Materials of cut
The cut is dependent on the choice of a couple adapted tool/matter and on selected conditions of use in an optimal way. For that, the material of cut must have a certain number of characteristics:
Elasticity, flexural strength;
Great hot hardness, good heat strength, approximately 1000 ÐoC (figure 1);
Chemical stability;
Weak tendency to the matter welding;
It must be harder than material to be machined: high speed steel, tungsten carbides and cermets;
The high speed steel is a high alloy steel with chromium, molybdenum, tungsten and vanadium. This one can be machined then hardened by heat treatment and rectified with the grinding stone.
The metal carbide not being able to be run, the tool is obtained by sintering: by compacting different powders carburize with a binder, Cobalt for example. One obtains various nuances of carbide while varying the percentage of the hard and flexible substances. Compressed at very high temperature, the part takes its final form in the mould and becomes very hard. The tool obtained can then undergo various treatments: coating, sharpening of the edges, Ð… The coatings improve the lifespan of the tools. The carbide insert tooling obtained by sintering can then be brazed or attached on the tool holder.
Ceramics is also appeared as insert tooling sintered. They allow high cutting speeds but resist the shocks badly.
Figure 1 – Graph of heat strength
Designation of the tools
The tools are indicated in the language running by the operations which they can carry out. The standard gives a very precise designation for the tools with removable insert tooling.
Designation of the tools “high-speed steel”
One distinguishes the active part of the tool and the part of setting in position of the tool on the machine. The active part is made up of the unit: edge of cut, generating point of the tool, face of cut and face of skin; while the part of setting in position is characterized by the section of the body of the tool.
Designation of the carbide inserts tools
The removable insert toolings are intended to be fixed mechanically on the shanks by a support or via a screw. The designation of the “carbide-tipped tools” is defined by a code adapted to the definition of the insert tooling and a code adapted to the designation of the insert tooling holder.
Formation of the chip
Principle
The formation of the chip results from:
compression of the matter on the face of cut;
shearing of the chip following the plan of shearing;
flow of the chip.
Brought back edge
Definition: there is formation of a new edge of cut when a cluster of particles remains stuck to the edge of cut. The cut is then moved at the end of this new edge: the brought back edge.
This phenomenon is frequent at the low cutting speeds and chip low thickness. It is also necessary to take account of chemical phenomena between materials of cut and the work piece (diffusion, oxidation).
Minimum chip
The edge of the tool is not always as acute as one would wish it. The chip can be driven back (wrenching) when the thickness is the weaker than ray of rounded edge, cut is not more possible and the tool rubs on the surface which was work-hardened and warms up. There is a minimum chip of which the thickness corresponds to a value with the lower part of which it is not possible to go down.
Control of the chip
The shape of the chips
The problem of the shape of the chips in the material machining with long chips takes importance more and more. Today, manufacture requires short chips which are not gotten mixed up in the devices of measurement or the rotating chuck or on the tool even, and which can be evacuated easily of the machine. There is a classification of the various shapes of chips: helicoids slanted, helicoids cylindrical shorts, in spiralÐ…
The break chip diagram
When the chips are too long, or when the edge of cut does not resist (chipping), it is possible to act by changing the geometry of the insert tool used. One provides the cutting insert tools with grooves chip breakers. The shape of these grooves is studied according to the workability of material to machine. Each manufacturer of insert tools proposes a complete range of chip breakers which covers all the applications, of the strong outline to the completion. For a model of insert tool which is adapted to a material and a cutting speed, there is a range of values of the advance and depth of cut for which one will obtain a correct chip.
Wear and lifespan
Criteria selected to characterize wear
The phenomenon of wear of the faces of the tool causing the erosion of the sharp edge is the consequence various phenomena:
mechanical and thermal shocks,
abrasion.
Frontal wear determines the surface quality and dimensional accuracy. Wear in crater appears as for it on the face of cut. It is prevalent for work of outline and severe cutting conditions.
Lifespan
It is the effective time of cut of a new tool so