Friction is an important process parameter in all most all metal forming processes. Some of the processes such as rolling of metals depend on friction for its successful execution. Friction is as important to rolling as it is for walking by humans and animals. Nevertheless, the evaluation of friction has so far been a difficult task and determination of exact value of co-efficient of friction or friction factor is illusive.
Another difficulty is to establish a similarity between the experimental conditions and the actual processes so that the data collected in experiments can be used with confidence and reliability to actual processes. The frictional stress depends on tool- work piece pair, the surface conditions, lubrication, the interface pressure which varies from point to point on the contacting surfaces, hardness values and lay of surfaces etc.
In most of the measuring methods, the contact stresses are measured over a certain area and at best we can get average values of frictional stress. In order that the values determined in the tests in the laboratory be applicable to actual processes the tests should be simulated to represent the actual conditions of the processes such as the work metal-tool pair, their surface conditions, methods of preparation of the surfaces, hardness values, interface pressures, lubricant used etc.
The evaluation of coefficient of friction or friction factor (for upper-bound solutions) is important for following three reasons:
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(a) The co-efficient of friction is an important parameter in all realistic analyses of metal forming processes. So for determining numerical values of forces and torques, it is necessary to have a correct value of co-efficient of friction or friction factor.
(b) When suitability of a lubricant for a particular process is to be evaluated. In such cases it is not necessary to measure exact value of friction, only relative values of a parameter which depends on friction can serve the purpose.
(c) In order to find the efficiency of a forming process it is necessary to access the frictional losses so that the tooling can be modified to have a better efficiency.
There are many excellent texts available which deal with the nature of friction.
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Methods of Measuring Friction in Metal Forming:
In metal forming, the following three types of situations may occur:
(i) There is simple sliding between work piece and tool surfaces under stresses less than the yield strength. Examples are- (a) drawing of flange in deep drawing wherein the sheet metal slides under blank holding stress which is much less than the yield strength, (b) sliding of slug in die in the blanking operation, etc.
(ii) The sliding between the tool and work piece surface occurs under stresses approaching the yield strength or at the yield strength. Examples are- (a) sliding of wire or strip in the land portion of die, (b) sliding of extrude in the die land.
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(iii) The sliding of work material occurs under severe plastic deformation. Examples are- (a) sliding of material in the conical portion of wire drawing die, (b) sliding of material in forging dies (c) sliding of material against the roll surface in rolling.
For the first case the test illustrated in Fig. 13.1(a) is often used. In this test, strip of the test material is pulled through two flat die blocks which are pressed together thus simulating the flange drawing. It would be better if the dies are made of same materials as those of actual die and blank holding plate.
For other cases of sliding the test illustrated in Fig. 13.1(b) may be used. In this test a pin is dragged on the surface under load after the application of the lubricant. Knowing the normal force and the frictional force one can determine the coefficient of friction which is equal to the ratio of frictional force/normal force.
In Fig. 13.1(c) an annular cylinder is pressed and twisted on the surface after applying the lubricant. The lubricant is applied only once and twisting is continued. The time taken for the lubricant film to break down is studied. The results of this test relate well to extrusion process in which case also the lubricant is applied only once.
The braking beads are often used in drawing sheet metal into rectangular cups and similar shapes, so that the sheet which forms the long side is drawn in tension. The sheet bends and unbends while passing over the bead under blank holder plate force. For simulation of this process and to determine the braking effect of bead and to determine the co-efficient of friction a test of type shown in Fig. 13.2 is devised.
The compressive force of hydraulic cylinder is measured by load cells and frictional stress is measured by the pull exerted on the strip. Two types of tests are carried out. In the first test fixed beads are used. In this case the pull (Pf + d) includes the frictional effect as well as due to plastic deformation due to bending and unbending.
Let the normal force be Nf + d. In the second test the fixed beads are replaced by rollers which are mounted on frictionless bearings. In this case the pull (Pd) only includes the effect of plastic deformation due to bending and unbending. By subtracting this value of pull from the previous value we can determine the pull required by friction only. Thus the average co-efficient of friction is given as-
Figure 13.3(a) illustrates the method employed for determination of co-efficient of friction in cases wherein the specimen is compressed to yield strength and is the dragged on a plane surface. In these tests the material does not undergo plastic deformation.
The co-efficient of friction during the plastic deformation depends on the degree of deformation as well as on the rate of deformation, so this case is different from the other two cases described above. For measuring friction for sliding during plastic deformation a number of different tests have been used.
In order to simulate the conditions of wire drawing or strip drawing, the actual strip drawing test (Fig. 13.3 b) is carried out. The two forces, i.e. the drawing force and die separating force are measured.
The normal and shear forces may be determined from these two forces. Also with help of analysis of strip drawing the values of these two forces may be determined and compared with the values obtained experimentally. The coefficient of friction may be estimated from the ratio of tangential and normal forces.
Another simpler case is illustrated in Fig. 13.3(c). In this experiment a strip of material is pressed down the inclined dies. The pushing force and the die separating force are measured. From the measured forces the components normal to die surface and tangential to die surface may be calculated and co-efficient of friction determined.
1. Measurement of Friction in Rolling:
In case of rolling the forward slip is an indicator of the friction or effectiveness of lubricant. If the interface friction between the strip and rolls is low the neutral section will advance towards exit side and hence the forward slip will also will be low.
With this simple observation one can determine the relative effectiveness of different lubricants. For measurement of forward slip, two fine lines may be scratched on the roll surface. Let the circumferential distance between them be equal to L1. After rolling the distance between their impressions is measured. Let it be L2.
Use of Imbedded Pins:
This method has been used both in forging as well as in rolling. In this method two pins mounted on force measuring units are imbedded in the roll in case of rolling or in the die in case of forging. Of the two pins one is normal to the surface and the other is inclined.
Thus two components of force, i.e. normal and tangential to the surface may be determined from the measured forces and the co-efficient of friction may be calculated. The locations of pins are chosen so that there is similarity of normal and tangential stresses on the two locations. Table 13.1 lists the average values of friction.
2. Measurement of Friction in Forging:
Compression of an Annular Ring:
Any factor that varies with the coefficient of friction may be used for measurement of friction provided there is no interaction of friction with other parameters which may alter the value of the measured parameter.
Measurement of load or barreling in simple compression of cylindrical specimen may also be used for measurement of friction, but that also involves the precise measurement of yield strength of material. In case of ring compression, however, the measurement of yield strength is not necessary.
The test involves compression of a short ring between two flat dies. The test was originated by Kunogi. In case of zero friction the inside diameter as well as the outer diameter of ring increase and ring deforms as a solid disc. As the friction is increased the outward flow of material decreases. At a critical value of friction the inner diameter starts decreasing while the outer diameter increases at a reduced rate.
Thus the change in the inner diameter during reduction in height becomes an indicator of coefficient of friction. Male and Cockcroft conducted a large number of tests on compression of rings made out of many materials. The dimensions of rings used by them were in the ratio 6:3:2 (outer diameter: inner diameter: thickness). Many researchers have taken these proportions as standard.
The ring compression test has been used by Yhu-Jen Hwu et.al. for determining the coefficient of friction as well as the yield strength of metals and alloys at room temperature and at elevated temperatures. The theoretical treatment of compression of rings has been carried out by Kudo and Avitzer.
The applicability of the analysis is limited by its assumptions which are as below:
(a) Plane sections before the deformation remain plane during the deformation, i.e. the deformation is uniformly distributed throughout the thickness of disc.
(b) The material of disc is rigid perfectly plastic solid which obeys von Mises’ yield condition. There is no work hardening.
(c) There is constant frictional stress at the interface between the work piece and the tool. The shear stress is equal to m.K. Here K is equal to the yield strength of material in shear and m is the friction factor which has the values between 0 and 1. The value 0 corresponds to frictionless case and 1 corresponds to sticking friction condition.
In actual practice the deformation is non-uniform along the thickness and there is barrelling on the inner as well as outer surface (Fig. 13.4), which can affect the calibration. Also the assumption of constant value of friction factor on the entire surface is also questionable. Nevertheless, the method has been used by several researchers. The evaluation of friction factor needs the calibration of compression curves.
