Surface Roughness:
The quality of surface is often of utmost importance for the correct functioning of many machine parts. For moving surfaces (running or sliding fit) the finish affects both friction and wear whether they are lubricated or not. In the case of locating surfaces, the finish also affects transition and interference fits, for, if the surfaces are rough, the area of ‘engagement’ may be reduced and the fit correspondingly weakened.
Surface roughness is caused by:
(i) The feed marked or ridges left by the cutting tool and
(ii) The fragments of built- up edge shed on the work surface in the process of chip formation.
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Surface finish can be improved by reducing the height of the feed ridges and the size of the built up edge.
Nature of Surface Quality:
While referring to the surface quality there are two aspects to be considered. One, the physical aspect, which refers to the deviations in physical properties of the superficial outer layer of metal from that of the interior and the other to the geometrical deviations of the real surface from the ideal one.
The most important parameter for the measurement of the geometrical deviations is the roughness or the micro irregularities. These are relatively finely spaced surface irregularities and depend on the machining conditions such as rate of metal removal, characteristics of tool and so on.
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There are other parameters like waviness and macro error which are taken into account only in some special cases. These arise due to lack of rigidity of the system, machine and tool non-uniformity of the tool advance, vibrations and so on.
Surface roughness is specified by referring to the centre line average or the root mean square average of the micro irregularities. These could be accurately measured by a stylus instrument like the profilometer, designed to give a direct reading of CLA and RMS values.
The average deviation of the irregularities (Ra) represents the average value of the deviations of the points of the effective profile (y1,y2,….. ,yn in Fig. 24.19) from the median line, measured over a length /. This is expressed in microns CLA.
The root mean square average (Ra) is the square root of the average of a series of measurements of surface deviation from the mean line and is expressed in microns r.m.s.
Expressions for Ra and Rq are given as below:
,
where Ra = Surface roughness in microns CLA<
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Rq = Surface roughness in microns
y = Ordinate of curve of profile,
l = Length over which average is taken
Effect of Surface Roughness on Function of Machine Parts:
(a) Roughness and Function:
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Fig. 24.20 (a) refers to the ideal case of boundary lubrication in which the lubricating fluid is said to be absorbed on the two plane metallic sliding surfaces and the thin unabsorbed layer keeps the two surfaces apart. Here, the friction during sliding is entirely owing to the shearing of liquid layers.
In the actual case, surfaces consist of small ridges and valleys [Fig. 24.20 (b)]. Here penetration of lubricant occurs and there is contact between surface at various areas, which at the high temperature and pressure encountered in practice, may lead to local welding at these points.
It this case, an additional force is required for shearing of the welded junctions. Further, due to the shear at welded junctions, metal particles may be broken away leading to greater wear due to abrasion.
Hence, in order to have fluid friction between two sliding surfaces, the following condition is given:
Where hmin is the minimum thickness of oil film, calculated on the basis of hydrodynamic lubrication theory; Rz1, Rz2 being the height of micro-irregularities from the two sliding surfaces.
(b) Roughness and Wear:
Another important characteristic influenced by surface roughness is the resistance to wear. Initial wear rate may be enhanced between sliding surfaces because of reduced area of contact due to bed surface finish. This may result in less concentration at the peaks of the surface and a considerable reduction in the load carrying capacity.
(c) Roughness and Dimensional Tolerance:
From the point of pieces execution, the roughness and tolerance may be considered as independent from each other, because it is possible to obtain smaller or bigger asperities in the work for a particular tolerance. But from the functional point of view, it is stated that there must be a certain ratio Rz/T which is characteristic for each functional case, where Rz is the height of asperities and T the tolerance.
Factors Affecting Accuracy of Machined Surfaces:
There are several factors which affect accuracy of machined surfaces.
In general, the accuracy is affected by:
(i) Static alignment (levelling of machine, alignment of slide ways and spindles),
(ii) Steady-State effects due to the elastic forces set up in the machine structure and work piece during cutting, and
(iii) Dynamic machining considerations (forced vibrations or induced vibrations).
Proper levelling of machine tool and alignment of various parts needs to be tested properly. The rigidity of machine tool and work-piece combination should be increased by proper selection and setting of machine tool to minimize deflection due to overhang and by proper clamping and support of the work piece. Forced vibrations are caused by unbalanced rotating masses or by periodic force vibrations caused by the teeth of milling cutter while engaging work piece.
If these cyclical force vibrations match with resonant frequencies of machine structure, these can be troublesome. These can be taken care of by reducing/increasing the spindle speed. Self-induced vibrations (chattering) occur due to problems in chip removal and are generally difficult to cure till sufficient care is taken.
Methods to Reduce Chatter:
Various methods to reduce chatter are given below:
(i) Rigidity of Machine Tool and Work piece:
The aim should be to increase the natural frequencies to values which are unlikely to cause chatter. Rigidity can be increased by effective clamping and support of cutting tool and work piece.
(ii) Cutting Machine Tool Geometry:
(a) Nose Radius:
Though large nose radius on cutting machine tool improves surface finish, but it promotes chatter and hence is limited to 1 mm when large quantities of metal have to be removed.
(b) Side Cutting Edge Angle:
Large value of side cutting edge angle is desirable with reference to improvement of tool life but it encourages chatter hence is limited to 30°.
(c) Sharpness of Cutting Edge:
It is observed that sharp tools chatter more than blunt ones because bluntness increases the resistance of the tool to penetration of the work-piece, and thereby damps vibration normal to the work piece surface.
(d) Rake Angle:
It is observed that high rake angles increase the shear angle, giving a more efficient cutting action and machining stability is marginally improved by increasing rake angle.
(iii) Width of Cut:
For stable cutting width of cut (depth of cut in turning) should be low, though it increases cost of metal removal.
(iv) Cutting Speed:
Cutting speed is normally selected to give a reasonable machine tool life or to restrict cutting power. If chatter is observed, it can be reduced by increasing speed and suitably altering other cutting parameters.
(v) Feed:
A small feed is always likely to promote chatter, which in low feed ranges can be improved by increasing the feed.
(vi) Tool Contact Length:
Cutting forces can be reduced by artificially restricting the chip-tool contact length by relieving the rake face. For short contact length, the feed force is not affected by change in feed. Restricted contact tools have a marked stabilizing effect in machining steels with contact lengths between 1.5 to 3.0 times the feed.