Chapter 21

4. How is feed related to speed in the machining operations called turning?

In turning, the feed rate is the speed of the cutting tool along the work piece in the longitudinal (axial) direction. If the lathe carriage is driven independently of the spindle, there is no necessary relation between feed and cutting speed. There are practical considerations such as cutting forces, machine vibration, etc. In many lathes one motor drives both the spindle and the carriage. The carriage is usually driven through a gearbox and so feed rate is in inches or millimeters per revolution of the spindle. In this case the linear feed and the feed rate in units of distance/revolution are related as: In other machining operations there may be no relationship between cutting speed and feed rate When end milling, the spindle and table are driven independently. If the feed rate is the table speed past the spindle, it is set by the table feed motor-drive system. The spindle is driven by the spindle motor and the cutting speed V is the speed of the cutting edge through the work piece. For spindle rotational speed N rpm the cutting speed at the cutter periphery is:

2. What variables must be considered in understanding a machining process?

Independent variables that must be considered to understand the machining process are: • The cutting speed, • Feed, • Depth of out, • The cutting tool geometry, • Cutting tool material and the cutting fluid. Dependent variables and process performance are determined by the independent (input variables), and include: • Material removal rate, • Machining time, tool wear, • Finished surface roughness, • Surface integrity (finished surface and subsurface deformation state) • Cutting zone temperature • Cutting forces, • Chip formation and machine tool dynamics Other process variables may be constrained but not completely defined or fixed. There may or may not be complete control over such process characteristics as: • Work material, • The machine tool or the work holding device.

15. Why is metal cutting shear stress such an important parameter?

Shear stress is a constant of the material in terms of metal cutting. This means that it is not sensitive to changes in cutting parameters or process variations. Once this value is known for a metal, it can be used in basic engineering calculations for machining statics (forces and deflections) and dynamics (vibrations and chatter).

1. Why has the metal-cutting process resisted theoretical solution for so many years?

The deformation zone in which the chip is produced is not completely bounded. Bounded processes lend themselves to theoretical analysis, while processes with one or more unbounded surfaces must be solved iteratively if at all by theoretical means. In metal cutting the work piece has free surfaces. In lathe turning there are free surfaces at the work piece diameter and on the top of the forming chip. The boundaries that do exist (the tool-chip interface and the poorly- defined deformation zone boundary in the work material) are difficult to characterize. The metal cutting process generates strains that are very large and the strain rate is very high. Material properties at such conditions are not usually known. There are also a large number of process variables. Theoretical solutions or process models have to be validated experimentally. It is difficult to obtain reliable, consistent experimental results that quantitatively describe the local deformation in the chip formation zone. The chip formation zone extends into the work piece below what will become the finished surface.

18. Where does the energy consumed in metal cutting ultimately go?

The energy is converted to heat as at three zones • Primary Heat Zone: This is the Shear zone at which the shear occurs in the work piece which to be machined. This zone is called primary heat zone. At this region due to shear action the atomic bond between atoms of material getting broken at this instant the atoms releases the energy in the form of heat. So the heat generation takes place. • Secondary Heat Zone: To overcome the frictional force between the tool and the chip interface forms heat energy • Tertiary Heat Zone: To overcome the frictional force between the tool and the work piece interface forms heat energy.

7. What is the fundamental mechanism of chip formation?

The fundamental mechanism of chip formation is developed out of compression deformation which precedes shear. A shear-front lamella structure is developed by very narrow shear fronts which segment the chip material into very narrow lamellae. The shear fronts have micron-spaced periodicity and are the result of many dislocations moving at the same time. The onset of shear begins at the shear plane (defined by ø) and moves at the angle ø to form the chip. If the material is already cold worked, very little additional compression deformation is needed to activate the shear process. If the material is annealed (or as-cast), the compression deformation is extensive, causing the work piece to bulge and upset prior to shearing. The primary dislocation mechanism appears to be one of dislocation pileups against the cell structure produced by compression deformation or prior work hardening of the work piece material.

14. Explain why you get segmented or discontinuous chips when you machine cast iron.

The grain structure of cast iron is filled with flake graphite. These flakes produce regions that act like sharp-cornered flaws or voids, and concentrate the compression stresses. The shear fronts cannot cross these regions. Under the large strains, the metal fractures through the flake and the chips come out segmented or in fractured chunks.

17. How is the energy in a machining process typically consumed?

The machining process can utilized only 75% of the overall work done (Fc*V). Here ø is the only parameter which influencing the total machining process. This is divided into shear (actually compression and shear) to form the chips with a shear stress and the balance is consumed by secondary shear stress by the tangential force and sliding friction at the tool-chip interface.

12. How do the magnitude of the strain and strain rate values of metal cutting compare to those of tensile testing?

The magnitude of the strain and strain rates are very large for metal cutting compared to tensile testing. Metal cutting strain is on the order of (1 to 2) compared to tensile testing's (0.20 to 0.40), and Metal culting strain rates are [a*] compared to tensile testing's [b*].

5. Before you select speed and feed for a machining operation, what did you have to decide? (Hint: See Figure 21.4.)

The requirements of selection for speed and feed are: • Type of material to be machined. • Selecting the right machining process for the given material. • The type of tool and tool material is to be used for machining the given material, • The condition of the material such as hot rolled, normalized, cold drawn or quenched and tempered, etc. • The depth of cut.

3. Which of the seven basic chip formation processes are single point, and which are multiple point?

The single point chip formation processes are: • Turning, • Shaping, • Planing The multiple point chip formation processes are: • Drilling, • Milling, • Broaching, • Sawing, • Grinding.

6. Milling has two feeds. What are they, and which one is an input parameter to the machine tool?

The two feeds in milling are: •The table feed (in/min or mm/min) and •The feed per tooth (in/tooth or mm/tooth) The table feed is the direct input to the machine. Spindle speed is a machine input that is used to set the cutting speed.

13. Why is titanium such a difficult metal to machine? (Note its high value of HPs.)

Titanium is very strain rate sensitive. The faster it is deformed, the stronger it behaves. This causes problems because of the high strain rates in metal cutting, which rely on high strain rates to soften the metal for removal.