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****************** Recommended Reading: Manufacturing Engineering and Technology Marks' Standard Handbook for Mechanical Engineers ******************
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Discontinuous chips Continuous chips Built Up Edge Tool Wear Taylor's Tool Life Theory Chip Formation In Turning Suggested Reading The purpose of any milling and turning operation is to remove material to achieve the required dimensions of the finished part. Arguably, it could appear that milling and turning employ significantly different methods; however the fundamental principle of material removal by chip formation is identical for both. To illustrate the principle of chip formation the example of a planing machine is perhaps easiest to visualize. Planing will not be described in detail, suffice it to say that tool and workpiece move relative to each other along a linear path; the tool again forms chips in a manner fundamentally the same as milling and turning.
Click the image for full size picture Fig.1.1.1 shows a side view, relative to feed direction, of a planer – prior to engagement of the tool and workpiece. The rake and clearance angles are two of the most important variables to be considered in chip formation; note also that the wedge angle formed is a direct relationship of these two angles; i.e. the greater the sum of clearance and rake angle, the smaller the wedge angle.
The clearance angle ensures only the cutting edge of the tool contacts the workpiece and the back, or heel, of the tool does not rub the finished surface – degrading finish and consuming extra power. By examining the wedge angles in Fig. 1.1.1. b&c) it is apparent that the smaller the wedge angle the weaker, and less able to dissipate heat, is the tip of the tool, hence the ‘perfect’ rake and clearance angles for a given material are often a compromise between outright cutting ability and adequate strength. Under different cutting conditions, i.e. varying depths of cut, workpiece material, rake angles, cutting speed and machine rigidity etc, different types of chip can form. The two basic types of chip are continuous and discontinuous. Types Of Chip
This type of chip is usually formed when cutting
hard, brittle materials, partly because these materials cannot withstand
high shear forces and therefore the chips formed shear cleanly away.
However, the chips formed may be firmly or loosely attached to each other
or may leave the cutting area in a fine shower – as often encountered when
cutting hard Brass. When discontinuous chips are formed there is a greater
possibility of tool chatter; unless the tool, tool-holder and workpiece
are held very rigidly; due to pressure at the tool tip increasing during
chip formation and then releasing suddenly as the chip shears see
FIG.
1.1.4. a - Chip Formation with Tool Chatter) This type of chip is usually formed when cutting soft or ductile materials such as Aluminium or Copper (fig 1.1.4b). There is less likelihood of chatter and surface finish is usually better than when discontinuous chips are formed. A disadvantage of continuous chips is the fact that they can become very long and become entangled with the machine or pose a safety hazard. This problem can be overcome by the use of chip-breakers; a device clamped to the top of the tool that encourages the chip to curl more tightly, hitting the workpiece and breaking off. Fig. 1.1.5. shows a tool with a Built Up Edge (B.U.E.). A B.U.E. is formed when particles of the workpiece material weld to the rake face of the tool during cutting. Large B.U.E.s can be very detrimental to surface finish and integrity, they effectively change the geometry of the cutting edge and consequently shear plane angle, this can lead to residual stresses in the material below the depth of cut. As a large B.U.E. dislodges it can deposit work hardened particles, to become embedded in the finished surface. A thin, stable B.U.E. is generally considered desirable as this can tend to reduce frictional wear on the rake face of the tool (see sec. 1.1.b – tool wear).
Wedge shape cutting tools normally wear in two ways. Fig. 1.1.6. shows the typical wear pattern of a wedge shape cutting tool.
Crater wear occurs on the rake face of the tool just behind the cutting edge and is caused by the rubbing of the chip across the surface. Flank wear occurs on the clearance angle of the tool causing rubbing and degradation of the surface finish. Cutting tools are deemed to have failed and require regrinding or replacing when flank wear exceeds 0.25mm or when cratering appears, this allows regrinding with minimal removal of tool material. It can be seen from Fig.1.1.6. that when cratering appears the cutting edge becomes thinned and less able to dissipate the heat generated during cutting; leading to unpredictable, and possibly catastrophic, failure. Fig 1.1.7 shows the three distinct stages of tool wear, I) initial rapid wear, ii) uniform wear, iii) final wear – failure.
A number of variables including feed rate, depth of cut, cutting speed, coolant, tool material and workpiece material affect tool life. F.W. Taylor, an American engineer, developed a standardised test to determine the relationship between the surface speed (relative to the tool) of the workpiece material (cutting speed) and the time the tool remains useful. Taylor’s test examined two of the variables – cutting speed and the time in cut. The test was repeated for different combinations of tool and workpiece material. He decided to measure flank wear of the tool under test, as this is consistent and easy to measure. It was found that a practical amount of wear to measure before breakage was 0.75mm for solid and brazed tips or 1.25mm for ceramic tools (see section 2.1. – tool materials). Many tests were carried out to determine the time taken to reach this amount of wear at different cutting speeds, the results were plotted on a graph showing that a logarithmic relationship existed between cutting speed and cutting time (Fig 1.1.8.)
The relationship established, VTn = C, is known as Taylor’s Tool Life Equation where V = Cutting Speed, T = Time in cut (in minutes), n = the index for the particular combination of tool and workpiece material and C = the constant for the particular set up. The value of n can, in most cases, be found in suitable machining handbooks in table form. Example calculations using Taylor’s equation can be found here. 1.1.c) CHIP FORMATION IN TURNING If the fundamental principle of chip formation is now translated from the straightforward example of planing to that of turning, the similarities become apparent. Fig 1.1.8.
Click the image for full size picture By comparing Fig 1.1.8. and Fig 1.1.1. It can be seen that certain features of turning are analogous to planing i.e. depth of cut to width of cut; rotary motion to linear motion; feed per revolution to depth of cut. Click here to pop-up fig 1.1.1. to compare. As the workpiece rotates, the tool is fed along the workpiece at a constant rate (F) taking a cut of even thickness and forming chips in a manner identical to that illustrated in Fig 1.1.1.b). Formulae for calculating spindle speed and feedrate can be found here.
Manufacturing Engineering and Technology Kalpakjian, Schmid and Schmidt. Marks' Standard Handbook for Mechanical Engineers |
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FIG. 1.1.1. a) Chip Formation - Planer 
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