The present invention relates to an area of machining of metal parts by surface plastic deformation with goals of surface strengthening, creating compressive stresses in a surface layer, and improving surface roughness, and modifying a surface layer structure.
Methods and devices for ultrasonic strengthening machining of parts of machine building, using balls, cylindrical rods and indenters of other shapes as deforming elements, fixed to the output end of an ultrasonic waveguide or moving freely along the axis of an ultrasonic tool are known.
Russian Patent No. 1,523,316 describes a method of forming a given microrelief of a surface treated by a means of surface plastic deformation using an indenter fixed to an output end of an ultrasonic waveguide. The method provides periodical variation of high-frequency signal amplitude of ultrasonic transducer excitation for matching the kinematics of a tool movement to the kinematics of a motion of the ultrasonic waveguide output end thereof to form a given microrelief of a surface.
The given strengthening machining scheme ensures stable work at the amplitude of ultrasonic oscillations of an indenter from fractions to units of microns, because of the ultimate rigidity of the tool structure and the features of operation of the ultrasonic oscillation systems under load. A depth of strengthening is not specified in the Russian Patent No. 1,523,316, but such a low amplitude of oscillation enables the strengthening of a surface in a depth not more than 20 to 30 microns.
The described method does not take into account the factor of tilt angle change of the tool axis concerning the tangent to a workpiece surface at a current point of contact of an indenter with a surface during machining curved shaped surfaces. This circumstance leads to a vector direction change of impulses acting on a surface, and the corresponding change of the efficiency of machining.
Russian Patent No. 841,942 describes a tool which includes free deforming elements as balls rolling on a workpiece surface, conical, barrel-shaped or roller-shaped indenters. A set of indenters is set simultaneously in the cartridge, strictly appropriate to a shape and size of a workpiece surface. Using free indenters ensure enhanced efficiency of surface strengthening in comparison with the variant described above. However, the tool design described in the patent allows the machining of parts of just one standard size. Change in the shape and the size of a workpiece surface demands not only replacement of a pin holder, but a waveguide as well. The tool requires improved accuracy of mounting of the tool on a lathe, and imposes enhanced requirements to the accuracy of the lathe.
In the described tool design, a contact of the waveguide with deforming elements is pin-point or linear and hence the efficiency of the transmission of acoustic power from the waveguide to a workpiece surface is low. This condition does not enable accomplishing a high degree of ultrasonic strengthening.
The circumstances described above, reducing the efficiency of ultrasonic strengthening machining, are missing in the tool described in U.S. Pat. No. 6,458,225. This patent describes a method of ultrasonic impact machining (UIM) of friction surfaces of brake parts using a tool having free indenters. The patent also discloses a mechanism of surface strengthening and its microrelief improvement during realization of this method. The described UIM scheme is intended for machining of cylindrical surfaces only (e.g., on brake drums), or flat ring surfaces (e.g., on brake rotors). UIM of conical surfaces is also available using this scheme.
The present invention relates to a method of ultrasonic impact machining (UIM) using free indenters which ensures effective enhancement of hardness, creation of compressive stresses, and modification of surface layer structure based upon diminishing of mosaic blocks up to an amorphous structure. As a consequence of these physical effects, UIM results in at least one of abrasive damage resistance improvement, contact injury resistance improvement, enhancement of corrosive damage resistance under stress, endurance improvement, enhancement of resistance to fatigue and dynamic fractures, creation of conditions for forming a surface with given physical and mechanical properties, and on this basis, abandonment of heat treatment of a material.
The necessity of strengthening surfaces of parts having shape, including a combination of different external and internal faces including cylindrical ones of different diameters, conical, front and curved faces of constant or varying radii, often occurs in the industry. UIM of surfaces formed by curved generatrices or straight generatrices with different tilt angles to the pivot pin, at constant orientation of the indenter axis concerning the workpiece pivot pin results in the changing of the machining efficiency with changing of the tangent tilt angle to the generatrix of a workpiece surface at the current point of treatment. The UIM method of the present invention provides uniform machining of these surfaces due to the tool axis orientation being substantially perpendicular to a workpiece surface at a current contact point independently of a form and its mechanical trajectory.
The efficiency of machining is preferably determined by UIM process variables such as: an amplitude of ultrasonic oscillations of the waveguide output end at a given ultrasonic oscillating frequency; a geometry and a mass of an indenter; a force of pressing the tool to a workpiece surface and a deflection rate of the pressing spring; a vector direction of deforming force impulses concerning a workpiece surface, caused by indenter impacts on this surface; a circumferential velocity at a contact point of an indenter with a workpiece surface; a feed rate of a lathe, and a number of working strokes. The parameters of strengthening treatment are preferably selected from mechanical properties of a workpiece material, workpiece surface geometry, machining process productivity and the main prescribed UIM results, determining a degree of workpiece life enhancement, which preferably includes a magnitude and a depth of induced compressive stresses propagation, a magnitude and a depth of enhanced microhardness propagation, a surface microrelief being formed, and a nature of treated material structural modification.
The present invention also comprises a method of selecting UIM parameters which provide a given efficiency of surface strengthening and forming of a prescribed relief of a strengthened surface. It is important in this method to take into account the curvature factor. The present invention also enables selection of tool design parameters, in particular clearances in the guide channel of an indenter, which provide a prescribed quality of surface strengthening machining.
The device provides realization of a method of machining at cutting machinery with numerical control (NC) by means of appropriate tool turning synchronous to its travel along the workpiece surface contour, providing the tool axis orientation being perpendicular to a workpiece surface at each point of its mechanical trajectory. In order to avoid the necessity of changing the machine control program, determining a mechanical trajectory of the tool during the UIM process, corresponding to a compound contour of a workpiece surface, the axis of a tool turn is required to pass through a current point of contact of an indenter with a workpiece surface.
Referring now to the drawings:
a-1c illustrate patterns of indenter indentations on a workpiece surface at different values of the lapping factor K.
a-3c show the influence of a clearance in a guide channel of an indenter on its parasitic oscillation when operating.
The present invention relates to a method for ultrasonic impact machining (UIM) of surfaces of different curvatures using a rod-shaped indenter which is free in a direction of an oscillating system axis, executing ultrasonic impacts initiated by an action of the natural oscillations of a tool and resonance oscillations of an ultrasonic transducer, thereby providing uniform machining of external and internal surfaces with straight and curved shapes, with a goal of strengthening a surface. This method ensures effective enhancement of hardness, creation of compressive stresses, modification of surface layer structure based upon diminishing of mosaic blocks up to an amorphous structure. As a consequence of these physical effects, UIM of the present invention results in at least one of abrasive damage resistance improvement, contact injury resistance improvement, corrosive damage resistance enhancement under stress, endurance improvement, enhancement of resistance to fatigue and dynamic fractures, creation of conditions for forming a surface with prescribed physical and mechanical properties, and on this basis, abandonment of heat treatment of a material.
Uniform surface strengthening during the UIM process of the present invention is provided on a condition that the vector of force impulses caused by the indenter impacts on a workpiece surface is oriented perpendicular to the workpiece surface at each point of contact along a motion path of the tool independently of any configuration of the motion path. The maximal efficiency of machining is preferably ensured subject to the vector of force impulses which is directed perpendicularly to a tangent to a surface at a point of contact of an indenter with the surface. Hence, during the process of machining surfaces formed with curved generatrices or straight generatrices with different tilt angles to the pivot pin, it is necessary to turn the tool appropriately so that at a current point of contact of an indenter and a surface, an indenter can be oriented perpendicularly to tangents to a surface contour, and passed through a contact point. More particularly, an axis of the tool is preferably perpendicular to tangents, passing through a current point of contact between the tool and the workpiece surface, and wherein a tool pivot pin passes through the point perpendicularly to a workpiece axial section, passing through the point.
The main characteristics which preferably determine the UIM quality and ultimately the life time of a part include, but are not limited to: magnitude and depth of induced compressive stresses propagation, magnitude and depth of enhanced microhardness propagation, surface microrelief, and treated material structure. Value for each of these characteristics achieved due to UIM is preferably determined by the following process variables of strengthening and finishing treatment, individually and in combination: amplitude of ultrasonic oscillations of the waveguide output end at a given ultrasonic oscillating frequency; geometry and mass of an indenter; force of pressing the tool to a workpiece surface and deflection rate of the pressing spring; vector direction of deforming force impulses concerning a workpiece surface, caused by indenter impacts on the surface; circumferential velocity at a contact point of an indenter with a workpiece surface; feed rate of a lathe and number of working strokes. The UIM parameters are preferably predetermined in a prescribed order from the specified UIM results and preferably take into account the following preconditions: mechanical properties of a workpiece material, workpiece surface geometry, and machining process productivity.
During a first stage, the following treatment process variables are preferably selected: amplitude of ultrasonic oscillations of the waveguide output end at a given ultrasonic oscillating frequency; indenter mass, based on the given outlet parameters of a value of induced compressive stresses, which level must be at a given depth, not lower than a yield strength of workpiece material or a magnitude stipulated for it; and a degree of microhardness enhancement in the surface layer at a given depth up to a level not lower than 1.1 of the initial material microhardness, and taking into account the mechanical characteristics of the workpiece material. An indenter geometry is preferably selected based on the desired surface microrelief.
During a second stage, the required UIM productivity P and the lathe operating mode are preferably determined based on a workpiece surface area and the prescribed time of its machining. Machining productivity is preferably defined by the expression:
P=S/t,
where:
The process variables selected at the first stage enable evaluating a size of an indenter indentation on the workpiece material. The ratio between a total area of indentations in whole and a workpiece surface area is preferably designated as the lapping factor K, which is defined by the expression:
K=Σsi/S,
where:
A required frequency of impacts fim determined at a third stage is defined by the expression:
fim.=PK/Si.
At predetermined parameters (a tool non-stationary mass, mass and geometry of an indenter, vibrational amplitude of the waveguide output end, elastic properties of workpiece material, etc.), the frequency of impacts is preferably adjusted by a force of pressing the tool to a workpiece surface and a deflection rate of the pressing spring.
a-1c illustrate examples of patterns formed by indentations of an indenter on a workpiece surface having different values of the lapping factor K. The parameter a is preferably defined by a lathe feed rate f, the parameter b is preferably defined by the ratio between a circumferential velocity at point of a contact of an indenter and a workpiece surface, and a frequency of impacts fim. A given microrelief of the workpiece surface is obtained by predetermining respective indenter radius, impact frequency and lapping factor as a function of the above-described treatment parameters including oscillating amplitude and tool speed.
The tool meeting requirements for a machine tool for a dimensional treatment is necessary for UIM realization. Accordingly, a tool design must be rigid enough to keep an accurate mounting under effect of static and dynamic work loads during the UIM process. Actuated parts of the tool must not have backlashes, decreasing the accuracy and quality of treatment. The tool must provide pressing of an indenter and an oscillating system to a workpiece surface with a prescribed force and adjustability. At the same time, damping of a dynamic load, occurring during treatment must be ensured. A tool design must provide quick and easy replacement of elements according to the service regulations, in particular indenters, a pin holder, and a waveguide.
The tool design meeting these requirements is shown in
An indenter 26 for striking directly at a workpiece surface is fixed to a holder 28, which is rigidly connected to the bowl 16 by a cup 30. The guide channel for an indenter 26 in the holder 28 is provided with a close tolerance, ensuring lack of backlash, which causes a reduction of accuracy and efficiency of machining. Pressing of the oscillating system and indenter 26 to a workpiece surface and dynamic load damping are performed by a spring 32. The pressing force is preferably adjusted by a female screw 34. The tool body 18 is preferably rigidly fixed to the lathe tool holder.
Strong requirements on accuracy are imposed in the production of machine actuated parts.
The extent to which the two last constructive parameters have an affect, at constant values of other parameters, is shown in
A circuit schematic for realization of the present UIM method with a lathe having NC is shown in
The tool 10 is preferably fixed on a swing mechanism 44. The mechanical turning of the tool, which is respective to a workpiece surface curvature and perpendicular to it at the axial section at a current point of treatment, is occurring synchronously with the travel of the tool 10 along the curved contour of the workpiece 36 by means of the swing mechanism 44. The center of curved arcs of the guides of the swing mechanism 44 is preferably located in the point of contact of the indenter with the workpiece surface 36. The pivot pin of the tool 10 passes through a current point of contact of the indenter and the workpiece surface 36 and the pivot pin is also substantially perpendicular to the axial section of the workpiece 36, passing through this point. Superpositioning of the pivot pin of the tool 10 with the current point of treatment allows application of the lathe control program for UIM, which is used for turning the surface and determining a tool mechanical trajectory corresponding to the contour of the workpiece 36.
To turn the tool 10 in the guides of the swing mechanism 44 in coordination with travel along the contour of the workpiece 36, a drive 46 is preferably used which is mechanically connected with the swing mechanism 44 and electronically connected with programming device controlling lathe motions (NC) 48. Control of the drive 46 is preferably carried out from the NC system 48 according to a workpiece profile approximation algorithm. Thus, the NC system 48 provides coordinated principal travels of the lathe 38, rotation of the spindle n with a workpiece 36 fixed on it, longitudinal motion of the tool 10 fz, transversal motion of the tool 10 fx and turning of the tool 10 fa.
The present scheme of UIM enables machining of both external and internal surfaces.
The exemplary embodiments herein disclosed are not intended to be exhaustive or to unnecessarily limit the scope of the invention. The exemplary embodiments were chosen and described in order to explain the principles of the present invention so that others skilled in the art may practice the invention. As will be apparent to one skilled in the art, various modifications can be made within the scope of the aforesaid description. Such modifications being within the ability of one skilled in the art form a part of the present invention and are embraced by the appended claims.