| Reference Notes and Resource Articles on Precision Machining | Technical Information on Chip Formation | Mechanism of Grinding Process | Fatigue in Metals |

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Chip Formation during Machining Process

Mechanism of the Grinding Process

Fatigue in Metals

Fatigue in Metals

Crack nucleation during machining of metals

During the production of alloy metals or fabrication of structural components from metals, voids and brittle inclusions may remain. These serve as potential sites for crack initiation prior to or shortly after the component is used. If these cracks are located in a surface layer, then they can act as stress raisers and slip initiation sites.

The properties of the resultant surface layers after machining depend on the type of machining operation used. These operations can produce disturbed surface layers which introduce residual stresses.

As residual compressive stresses can inhibit slip and delay the crack nucleation, grinding and shot peening operations which introduce residual compressive stresses are recommended after the prior machining processes. These two processes can result in fatigue endurance limits which are substantially higher then those for polished, stress-free surfaces.

Nitriding, carburizing and flame and induction hardening also create high strength surface layers which have increased resistance to slip and hence crack nucleation.

For more information on fatigue in metals, please refer to "An Introduction to Fatigue in Metals and Composites" by R.L. Carlson and G.A. Kardomateas (Chapman & Hall)

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Chip Formation during Machining Process

Importance of Studying Chip Formation during Machining

As every one knows, chips are formed during the machining of workpieces. The side of the chip in contact with the cutting tool is normally shiny, flat and smooth while the other side, which is the free workpiece surface, is jagged due to shear.

It is important to study the formation of chips during the machining process as the former affects the surface finish, cutting forces, temperature, tool life and dimensional tolerance. Understanding the chip formation during the machining process for the specific materials will allow us to determine the machining speeds, feed rates and depth of cuts for efficient machining and increased tool life in the specific actual machining operation.

• Discontinuous
• Continuous
• Continuous with Built-up Edge
• Serrated chip formation

Discontinuous Chip Formation

Discontinuous chip formation normally occurs during the machining of brittle work material such as glass and silicon. This type of chips also occurs when machining using cutting tools with small rake angles, coarse machining feeds (large depth of cut), low cutting speeds and lack of lubricant or cutting fluid. Discontinuous chip formation leads to continuously changing forces, resultant vibration and chattering in the machine tools and thus results in a final workpiece with poor surface finish and loose tolerance.

Continuous Chip Formation

Continuous chip formation is normally considered to be the ideal condition for efficient cutting action as it gives excellent finish and occurs usually for ductile metals. The chip consists of a continuous "ribbon" of metal which flows up the chip-tool zone. It normally occurs at high cutting speed and rake angle, and a narrow shear zone. Use chip breakers during the machining to prevent the chips from entangling with the tool holder.

Continuous with Built-up Edge Chip Formation

Continuous chips with built-up edge is basically the same as continuous chips. However, during the former chip formation, as the metal flows up the chip-tool zone, small particles of the metal begin to adhere or weld themselves to the edge of the cutting tool. As the particles continue to weld to the tool, it affects the cutting action of the tool. This type of chip formation is common in machining of softer non-ferrous metals and low carbon steels. Common problems are the built-up edges breaking off and being embedded in the workpiece during machining, decrease in tool-life and final poor surface finish of the workpiece.

Built-up-edge (BUE) forms when there is a chemical affinity between workpiece and the tool, such as in cases of high strain-hardening, low feed speed, large depth of cut, low rake angle and high temperature. Here, the chip becomes unstable, breaks up and then forms again. The process is repeated continuously. BUE chip formation during machining would degrade the surface finish and changes the tool geometry.

Studies on the built-up edges have shown that the chip material is welded, deformed and then deposited onto the rake face of the tool layer by layer. It is thus possible to observe the presence of built-up edges by studying the back face of the chip during the machining process. This is normally used in micro or ultra precision machining operation.

To reduce built-up edges, improve the lubrication conditions, use sharp tools and better surface finish tool and also apply ultrasonic vibration during the machining process.

Serrated Chip Formation

Serrated chips are formed during the machining of semicontinuous material with zones of high and low shear strains. It normally occurs in metals where the strength decreases sharply with temperature. An example would be titanium.

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Technical Notes on the Mechanism of the Grinding Process

It has been assumed that during the grinding process, the material removal occurs by a shearing process of chip formation similar to that found with other machining methods such as turning or milling. However, while the grinding chips are similar to metal-cutting chips, the rake angles typical within the grinding process are much more negative. The velocities in grinding are also much higher, which would result in the deformation during chip formation being more nearly adiabatic. One consequence of this is the round and hollow shape of the chips which is evidence of surface tension effects acting on the molten curled chip.

The specific energy involved in grinding is defined as the energy per unit volume of material removal (specific power). Typically, the specific energies involved in grinding are much larger than in other metal-cutting operation. In other metal-cutting, shearing accounts for about 75% of the total chip formation energy, and chip-tool friction the remaining 25%. But in grinding, virtually, all the energy expended is converted into heat. Since the chip-formation process in grinding is extremely rapid, owing to the high cutting velocities and large strains, the process should be nearly adiabatic. This means that there is not sufficient time for any significant amount of the heat generated by plastic flow to be conducted away during deformation.

In an experiment using 32 (30,46,80,120) (G,I,K) S,V,B,E grinding wheels to machine AISI 1095 HR workpiece, we found that at slow removal rates, the specific cutting energy is extremely large, but it decreases with faster removal rates tending towards a minimum limiting value U_min of approximately 13.8J/mm₃ regardless of grit size effect. It seems that the wheel material influence is insensitive to the size effect too and that the plowing energy is expended by deformation of the workpiece material without removal.

The plowing deformation occurs as the abrasive initially cuts into the workpiece. Initially, the grit makes elastic contact, which is assumed to make negligible contribution (plowing) of the workpiece. As the cutting point on the abrasive grain passes through the grinding zone, its depth of cut increase from zero to a maximum value h_m at the end of the cut. On the average, chip formation commences only after the cutting point has penetrated to some critical depth of cut h'.

As mentioned above, relatively less plowing at higher removal rates would decrease the specific plowing energy U_pl. Thus, the limiting cutting energy U_min above is the sum of specific plowing energy U_pl and the specific energy for chip formation U_ch which is assumed to be constant.

The use of down-grinding in place of up-grinding should reduce or even eliminate initial plowing, since each cutting point would now initially engaged the workpiece at its maximum depth of cut.

The total specific grinding energy can be considered to consist of chip formation, plowing and sliding components:
U = U_ch + U_pl + U_sl

Experiments were conducted with sharp grinding wheels under well lubricated conditions using a heavy-duty straight oil in order to minimize any friction contribution. The results showed a correlation between grinding U_min and melting energy U_melt for a wide range of metals.

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