In this article we will discuss about:- 1. Introduction to Transverse Rolling of Discs 2. Components of Transverse Rolling of Discs 3. Study of Deformation.
Introduction to Transverse Rolling of Discs:
The transverse rolling processes are used to roll shapes on cylindrical components.
The commonly known processes are as follows:
(i) Transverse rolling of discs.
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(ii) Roll piercing of solid bars to manufacture seamless tubes.
(iii) Asset mill for tube thickness reduction.
(iv) Transverse rolling of shafts.
(v) Thread rolling.
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(vi) Gear rolling.
(vii) Reeling process for straightening of circular bars and tubes.
A large variety of finished as well as semi-finished components are made by these processes. Examples are ball bearing cups for bicycles, components of free wheel, cycle rims, seamless tubes, gears, threaded components, pulley type components, stepped shafts, etc. One of the lathe processes-knurling is also an example of transverse rolling.
Components of Transverse Rolling of Discs:
There is increasing interest to use this process for manufacture of general axi-symmetric components. Generally such components are made on lathes. In turning we cut shapes on axis-symmetric components and waste lot of material in the form of chips. It is possible to roll these shapes, at least some of these without wasting any material in chips.
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In this regard the research work done by Kapoor and Juneja, Haider, Shailendera has shown that there are immense possibilities of transverse rolling process. They demonstrated by actually producing the components that the process is many times quicker as well as cheaper than turning the similar components on lathe provided sufficient numbers of components are required.
Figure 12.1 illustrates some components rolled by this process. As regards the speed of production it may be mentioned that a component shown in Fig. 12.2 which would take at least 10 to 15 minutes on a lathe if machined from a casting, can be produced in 10-15 seconds by rolling the disc.
The scheme of a transverse rolling machine developed in Production Engineering Laboratory of IIT Delhi by Kapoor and Juneja is illustrated in Fig. 12.3 (a, b). It has two disc type rolls which rotate at same r.p.m. and in the same sense. The center distance of rolls can be changed with the help of a hydraulic cylinder and four tie bars.
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The work piece is mounted on a mandrel on live (rotating) center placed symmetrically with respect to the rolls. The design is unique because only one hydraulic cylinder is used and rolls get centered automatically according to the size of job. This also ensures equal distribution of rolling load on the two rolls as well as the process exerts minimum load on the mandrel on which the component is mounted.
As the rotating rolls press on to the component, the work piece also starts rotating due to friction. Further in-feed motion of rolls deforms the disc work piece plastically as it rotates. The hydraulic cylinder as well as bearing blocks of disc rolls can slide freely on machine bed, therefore, the rolls get adjusted according to work piece diameter.
As the rolling proceeds, the material of disc spreads on sides. If the roll surface has a form machined on to it, the work piece will get the corresponding fitting form. Thus if the rolls are conical, the work piece would get rolled into a pulley like component.
The process can be used with advantage to roll pulley like components, to roll gears in hot and cold state, axles, ball bearing cups used in bicycles etc. Figure 12.4 shows the cross sections of some rolled axi-symmetric components. The component shown on extreme left is similar to ball bearing cup of bicycle. The middle one is like a flat pulley. The one shown on extreme right is the outer disc of a free-wheel. The teeth are punched after machining the other surfaces.
A large amount of experimental data on the shapes of products that are produced has been collected and analyzed by Kapoor, Haider and Shailander. With this data it is possible to design the size of raw material from the dimensions of finished product. Besides the machine load and torque can also be determined. The deformed shapes of disc when rolled on entire width are shown in Fig. 12.5.
The range of products may be increased immensely by multiple rolling sequence. The first rolling pass consists of rolling with a pair of angular rolls to split the disc into two limbs of equal or unequal thickness. In the second pass the angular limbs are rolled flat to other finished shapes or those desired for the third and subsequent rolling pass.
Even internal shapes may be produced by rolling on to a mandrel. However, for carrying out multiple rolling with different rolls the rolls are to be changed which is a time consuming process besides being laborious. If number of components are large, all the components may be rolled with first pair of rolls, then rolls are changed and second pass is given, otherwise more than one machines are required.
1. machine bed; 2. guide ways; 3. turret having; 4. slides; 5. bolts for fixing roll chokes; 6. guide pins; 7. roll chokes; 8. rolls; 9. mandrel for fixing work piece; 10. work piece; 11. telescopic shafts; 12. hydraulic cylinder for engaging and disengaging roll power system; 13. couplings; 14. main hydraulic cylinder for moving turrets; 15. link connecting cylinder with L.H. turret; 16. link for connecting R.H turret with piston of hydraulic cylinder; 17. slide for cylinder; 18. end stop for piston.
A new type of transverse rolling machine (Fig. 12.7 a, b) with a double turret has been suggested here in order to have an easier and quick change of the pairs of rolls just like the change of tools on a turret lathe. Here two turrets are needed because two rolls are needed for one roll pass. Such a machine could produce a much larger variety of axis-symmetric shapes.
Study of Deformation in Transverse Rolling of Discs:
Figure 12.8 shows the cross-section of a disc rolled by angular rolls. The two limbs formed are straight and are formed by simple shear.
Because of severe deformation, the grid printing technique generally used for such studies does not work here. Instead thin wires were imbedded across the width of disc and after the deformation the disc was cut through the wires.
The distortion of wires was studied. The process is illustrated in Fig. 12.9. The figure shows how the wires bend when rolled with rolls having small cone angle and big cone angle. The process of deformation is similar to that of initial chip formation that takes place in metal cutting.
Analysis for Load and Torque in Rolling with Flat Rolls:
With straight roll surfaces three different cases as shown in Fig. 12.10 may be analyzed separately because the deformation processes in the three cases are different.
These are as follows:
(i) Rolling with cylindrical rolls on the full width or part width of disc in Fig. 12.10(a, b).
(ii) Rolling with angular rolls. (Fig. 12.10c)
(iii) Rolling with rolls having small cylindrical width and inclined side faces. This case is a combination of first two cases. (Fig. 12.10d)
Out of the above three cases let us first analyze the case of rolling with cylindrical rolls, i.e. (Fig. 12.10 a, b). Upper-bound method is used in the following analysis. Figure 12.11 shows the arrangement of work piece and rolls during rolling when the disc is rolled on its entire width. The width changes from w1 to w2. Roll radius is Rr, and work piece radius is Rp. Angle of contact of either roll with the work piece is γ.
The rolls while rotating advance towards disc and push the material which suffers shear along a shear plane AB and goes into flange. Figure 12.12 also shows that roll has advanced through a distance FA and has displaced the volume ADEF into flange BCD. The two shaded areas are equal since there is no volume change in plastic deformation.
Consider a small sector of the contacting interface bounded by two radial lines inclined at a small angle dθ (Fig. 12.11) emanating from roll center. As the roll and the work piece rotate with interference between them, a relative radial velocity U pushes the work material. Let Pθ be the roll pressure at the contacting cylindrical interface at an angle θ with the rolls center line. Figure 12.12(a) shows that the material shears over the slip plane AB and passes into the flange. Figure 12.12(b) shows the velocity field.
The roll in-feed velocity U is represented by line OH in Fig. 12.12(b), the shear velocity along the shear plane AB is represented by OG. HG represents the sliding velocity of the material along the roll surface ADC.
In the velocity triangle HGO, the three velocities are related as given below-
Figure 12.13 shows the rolling load v/s roll penetration during rolling of disc with flat rolls. Figure 12.14 shows the rolling torque in transverse rolling of disc for different roll penetrations.
Analysis for Rolling Load with Conical Rolls:
The deformation study of rolling with angular rolls has shown that the disc periphery gets separated into two limbs by simple shear. Thin wires were imbedded into the disc before rolling. After rolling the disc was cut through the positions of wires and the distortion of wires was studied. In discs rolled with rolls of small cone angle the wires remain straight during rolling while those rolled with high angled conical rolls the deformation is non-uniform and the wires bend during rolling.
When the disc is very wide with respect to the roll penetration, it would be a case of indentation. For indentation in plane strain the slip line field is shown in (Fig. 12.15). However, we consider a disc which is thin and even a small penetration of roll results in change in width near the periphery.
From deformation studies it is evident that deformation takes place by shearing of material over the shear plane AB (Fig. 12.16). Let us take that over a small angular width of disc, the disc material is pressing against the roll with a velocity U in Fig. 12.16(b). It suffers a shear deformation along shear plane AB by shear velocity MN and changes the direction of flow from OM to ON which is parallel to AC.
In the velocity triangle OMN the three velocities are related as below-
With the help of above equation we can determine the roll pressure Pθ at an angle θ and for the roll separating force we have to determine the contact area of the roll with the work piece and integrate Pθ cos θ + µ Pθ sin θ over the area of contact.
Similarly the expression for torque may be obtained as given by Equations (12.9-12.10). Here the area of contact of work piece with the roll is along the surface which is inclined at angle α to the vertical plane.
The differential area dA enclosed between two radial lines inclined at angle dθ is as below-
Now h/sinα is the inclined side of groove. The radius Rr varies from the bottom of the V-groove to the periphery. As an approximation we take an average value of Rr. Figure 12.17 shows the roll load (experimental) in rolling the disc with rolls of 60° cone angle.
