DMP Final

3. What is the relationship between cutting and tool life? What is the relationship between workpiece hardness and tool life?

-As cutting speed increases, tool life decreases.The relationship between cutting speed and tool life is VT^n = c, where T is tool life and V is cutting speed. The condition of the workpiece material has a strong influence on tool life, generally the harder the workpiece is, the lower tool life is.

2. Explain how different types of chips produced determine overall quality of the cutting operation.

-Continuous chips are associated with good surface finish and steady cutting forces. -Discontinuous chips result in poor surface finish and fluctuation of cutting forces. -Built-up edge chips (BUE) result in poor surface finish. -Serrated chips result in poor surface finish. This implies that the type of chip produced will determine the overall quality of the cutting operation

26. Negative rake angles generally are preferred for ceramic, diamond, and cubic boron-nitride tools. Why?

Although hard and strong in compression, these materials are brittle and relatively weak in tension. Consequently, negative rake angles (which indicate larger included angle of the tool tip; see, for example, Fig. 21.3 on p. 567) are preferred mainly because of the lower tendency to cause tensile stresses and chipping of the tools.

10. Are the locations of maximum temperature and crater wear related? If so, explain why.

Although various factors can affect crater wear, the most significant factors in crater wear are diffusion (a mechanism whereby material is removed from the rake face of the tool) and the degree of chemical affinity between the tool and the chip. Thus, the higher the temperature, the higher the wear. Referring collectively to all the figures on pp. 581 and 588, we note that temperature and crater wear indeed are related.

25. Describe the necessary conditions for optimal utilization of the capabilities of diamond and cubic-boron-nitride cutting tools.

Because diamond and cBN are brittle, impact due to factors such as cutting-force fluctuations and poor quality of the machine tools used must be minimized. Thus, interrupted cutting (such as milling or turning splines) should be avoided. Machine tools should have sufficient stiffness to avoid chatter and vibrations (see Chapter 25). Tool geometry and setting is also important to minimize stresses and possible chipping. The workpiece material must be suitable for diamond or cBN; for example, carbon is soluble in iron and steels at elevated temperatures as seen in cutting, and diamond would not be suitablefor these materials.

28. List and explain the considerations involved in determining whether a cutting tool should be reconditioned, recycled, or discarded after use.

By the student. This is largely a matter of economics. Reconditioning requires skilled labor, grinders, and possibly recoating equipment. Other considerations are the cost of new tools and possible recycling of tool materials, since many contain expensive materials of strategic importance such as tungsten and cobalt.

1. Briefly describe: what are the four types of chips produced in metal cutting?

Continuous chips: These are usually formed with ductile materials that are machined at high cutting speeds and/or high rake angles. -Discontinuous chips: They are produced by brittle materials, and consist of segments that may be attached firmly or loosely to each other. - Built-up edge chips (BUE): It is called BUE because layers of material of from workpiece gradually deposit on the tool tip, thereby changing the tool geometry. - Serrated or segmented or non-homogenous chips: They are semi-continuous chips with large zone of low shear strains and small zones of high shear strain. Metals with low thermal conductivity and strength that decrease sharply with temperature produce BUE, example titanium

5. List the prototyping processes that are best suited for the production of ceramic parts. Explain.

For direct production of ceramic parts, three-dimensional printing is likely the best option. With the proper binder, this can also be accomplished by fused-deposition modeling, and is also possible by selective laser sintering. However, the ceramic particles will abrade the tooling in FDM and require much heat to fuse in SLS. The 3D printing approach, where a binder is sprayed onto the ceramic particles, is the best approach for making green parts, which are then fired in a furnace to fuse the powder.

27. Do you think that there is a relationship between the cost of a cutting tool and its hot hardness? Explain.

Generally, as hot hardness increases, the cost of the tool material increases. For example, ceramics have high hot hardness and are generally made of inexpensive raw materials. However, their production into effective and reliable tool materials involves major steps (see Section 18.2 onp. 476) and, hence, expenses. Likewise, carbides utilize expensive raw materials as well as involving a number of processing steps. Diamond and cubic boron nitride are expensive as well

21. Which tool-material properties are suitable for interrupted cutting operations? Why?

In interrupted cutting operations, it is desirable to have tools with a high impact strength and toughness. From Tables 22.1 and 22.2 on p. 603, the tool materials which have the best impact strength are high speed steels, and to a lesser extent, cast alloys and carbides. Therefore, one would prefer to use high-speed steels and carbides in interrupted cutting operations. In addition, in these operations, the tool is constantly being heated and reheated. It is therefore desirable to utilize materials with low coefficients of thermal expansion and high thermal conductivity to minimize thermal stresses in the tool which could lead to tool failure.

14. The cutting force increases with the depth of cut and decreasing rake angle. Explain why.

It is logical that the cutting force increases as the depth of cut increases and rake angle decreases. Deeper cuts remove more material, thus requiring a higher cutting force. As the rake angle, α, decreases, the shear angle, φ , decreases [see Eqs. (21.4) and (21.5) on p. 570], and hence shear energy dissipation and cutting forces increase.

11. Is material ductility important for machinability? Explain.

Let's first note that the general definition of machinability (Section 21.7 on p. 583) involves workpiece surface finish and integrity, tool life, force and power required, and chip control. Ductility directly affects the type of chip produced which, in turn, affects surface finish, the nature of forces involved (less ductile materials may lead to tool chatter) and more ductile materials produce continues chips which may not be easy to control

19. Given your understanding of the basic metal-cutting process, what are the important physical and chemical properties of a cutting tool?

Physically, the important properties are hardness (especially hot hardness), toughness, thermal conductivity and thermal expansion coefficient. Chemically, it must be inert to the workpiece material at the cutting temperatures.

20. Explain why the power requirements in cutting depend on the cutting force but not the thrust force.

Power is the time rate of doing work. Work is the product of force and distance, so power is the product of force and velocity. There is a velocity in the cutting direction, so the product of force and velocity in the cutting direction leads to a power requirement. In the thrust direction, there is no velocity.

23. Why does temperature have such an important effect on tool life?

Temperature has a large effect on the life of a cutting tool for several reasons. First, all materials become weaker and less hard as they become hotter; therefore, higher temperatures will weaken and soften an otherwise ideal material. Second, chemical reactivity typically increases with increasing temperature, as does diffusion between the workpiece and the cutting tool. Third, the effectiveness of cutting fluids is compromised at excessive temperatures, meaning there is higher friction to overcome, and therefore more tool wear is expected. Finally, in interrupted cutting, there can be excessive thermal shock if the temperatures are high.

9. In making a prototype of a toy automobile, list the post-rapid-prototyping finishing operations that you think would be necessary. Explain.

The answer depends on the particular rapid-prototyping process used to create the toy. Consider, for example, fused-deposition modeling: It may be desirable to sand or finish the surface because of the surface texture that exists from the extruded filament. A base coat and paint then can be applied, followed by detailed decorative paint, if desired. Stereolithography may require (and generally it does so) post-curing, followed by roughening (such as by sanding) to allow paint to bond well, followed by painting, as above.

7. Careful analysis of a rapid-prototyped part indicates that it is made up of layers with a distinct filament outline visible on each layer. Is the material a thermoset or a thermoplastic? Explain.

The filament outline suggests that the material was produced in fused-deposition modeling. This process requires adjacent layers to fuse after being extruded. Extrusion and bonding is obviously possible with thermoplastics but very difficult for a thermoset.

4. How can a mold for sand casting be produced using rapid prototyping techniques? Explain

There are a few ways that a sand casting mold can be produced using rapid prototyping techniques. Case Study 20.6 on p. 558, where a sand is the powder used and a binder produces the desired shape. However, a pattern can be produced using any rapid prototyping technique.

8. Why are the metal parts in three-dimensional printing often infiltrated by another metal?

There are a number of reasons for this infiltration. Note that infiltration is also a common approach for PM parts, and the reasons for infiltrating parts produced in three dimensional printing are the same. There are obvious benefits to the mechanical properties that can be achieved in such materials by infiltrating the structure with another metal. Also, such materials cannot be contaminated, so that finishing operations such as electroplating can take place if the material is infiltrated.

16. What are the effects of performing a cutting operation with a dull tool? A very sharp tool?

There are many effects of performing a cutting operation with a dull tool. Note that a dull tool has an increased tip radius (see Fig. 21.22 on p. 590); as the tip radius increases (the tool dulls), the cutting force increases due to the fact that the effective rake angle is decreased. In addition, we can see that shallow depths of cut may not be possible because the tool may simply ride over the surface without producing chips. Another effect is inducing surface residual stresses, tearing, and cracking of the machined surface due to the heat generated by the dull tool tip rubbing against this surface. Dull tools also increase the tendency for BUE formation, which leads to poor surface finish.

17. Describe the effects that a dull tool can have on cutting operations

There are many effects of performing a cutting operation with a dull tool. Note that a dull tool has an increased tip radius (see Fig. 21.22 on p. 590); as the tip radius increases (the tool dulls), the cutting force increases due to the fact that the effective rake angle is decreased. In addition, we can see that shallow depths of cut may not be possible because the tool may simply ride over the surface without producing chips. Another effect is inducing surface residual stresses, tearing, and cracking of the machined surface due to the heat generated by the dull tool tip rubbing against this surface. Dull tools also increase the tendency for BUE formation, which leads to poor surface finish.

13. Explain the consequences of allowing temperatures to rise to high levels in cutting.

There are several consequences of allowing temperatures to rise to high levels in cutting (see also pp. 580-582), such as: (a) Tool wear will be accelerated due to high temperatures. (b) High temperatures will cause dimensional changes in the workpiece, thus reducing dimensional accuracy. (c) Excessively high temperatures in the cutting zone can induce thermal damage and metallurgical changes to the machined surface

15. What are the consequences if cutting tool chips?

Tool chipping has various effects, such as poor surface finish and dimensional control of the part being machined; possible temperature rise; and cutting force fluctuations and increases. Chipping is indicative of a harmful condition for the cutting tool material, and often is followed by more extreme failure.

12. Tool life can be almost infinite at low cutting speeds. Would you then recommend that all machining be done at low speeds?Explain.

Tool life can be almost infinite at very low cutting speeds (see Fig. 21.16 on p. 584) but this reason alone would not necessarily justify using low cutting speeds. Most obviously, low cutting speeds remove less material in a given time which, unless otherwise justified, would be economically undesirable. Lower cutting speeds also often also lead to the formation of a built-up edge and discontinuous chips, thus affecting surface finish. (See also Example 21.2 on p. 585.)

22. Make a list of the alloying elements used in high-speed steels. Explain what their functions are and why they are so effective in cutting tools.

Typical alloying elements for high-speed steel are chromium, vanadium, tungsten, and cobalt. These elements impart higher strength and higher hardness at elevated temperatures. See Section 5.5.1 on p. 135 for further details on the effects of various alloying elements in steels.

24. What precautions would you take in machining with brittle tool materials, especially ceramics? Explain

With brittle tool materials, we first want to prevent chipping, such as by using negative rake angles and reduce vibration and chatter. Also, brittleness of ceramic tools applies to thermal gradients, as well as to strains. To prevent tool failures due to thermal gradients, a steady supply of cutting fluid should be applied, as well as selecting tougher tool materials

18. Can high-speed machining be performed without the use of a cutting fluid?

Yes, high-speed machining can be done without a cutting fluid. The main purposes of a cutting fluid (see Section 22.12 on p. 616) is to lubricate and to remove heat, usually accomplished by flooding the tool and workpiece by the fluid. In high speed machining, most of the heat is conveyed from the cutting zone through the chip, so the need for a cutting fluid is less (see also Fig. 21.14 on p. 582).

6. Can rapid-prototyped parts be made of paper? Explain

Yes, rapid-prototyped parts can be made of paper. The laminated-object manufacturing process produced parts from paper or plastic.