Patent Application
20080283504
This invention is related to an electrical discharge machine (EDM) apparatus and method for providing a reverse taper bore in a stationary workpiece by wobbling an electrode holder while an electrode acts upon the workpiece.
EDM is a well-known and widely used machining technique. It operates through electrical discharges from an electrode. The process takes place in the presence of a liquid dielectric fluid such as de-ionized water. An electrode in the shape of an elongated rod, thin wire, or shaped article is put into close contact with the workpiece. Through an electrical potential difference, arcing occurs between the workpiece and the electrode which causes erosion of the workpiece material in a desired manner. EDM processes are used in numerous machining applications and are especially desirable for applications involving high accuracy, extremely hard workpieces, low applied loading of the workpiece, or deep bores in a workpiece.
Some machining projects pose difficulties for existing EDM technology. One example relates to fuel injection nozzles of the type used in certain internal combustion engines, such as diesel engines. In a typical diesel fuel injection tip, multiple fuel injection holes are bored near the end and around the circumference of the fuel injection tip to atomize highly pressurized fuel as it is introduced into a combustion chamber. The bores are conventionally formed as cylindrical bores of generally constant diameter. It is thought, however, that bores having other shapes may be better suited to some applications. In particular, it is believed that a reverse-taper of the fuel injection tip holes, where the interior orifice of the fuel injection bores (inside the fuel injector tip) have a larger diameter than the orifice diameter at the exterior of the injection tip, would provide superior results. Such reverse taper designs appear to increase fuel economy, increase the life of the injectors, and address problems relating to fuel cavitations and emissions.
Due to the size and configuration of many workpieces, including fuel injection tips, fuel injection borings must be made from the outside of the workpiece. A certain amount of deviation from a constant bore diameter can be provided with a wire EDM by controlling spark voltage and other parameters. Due to edge effects, however, this technique tends to produce a barrel-shaped internal bore. A need exists for an EDM machine capable of producing precise reverse-taper or cone-shaped bores in articles such as fuel injection tips.
An EDM apparatus having a wire electrode and a wobble guide system for machining a bore in a workpiece is disclosed. The guide system comprises a guide body near the workpiece which stabilizes a first portion of an electrode, and a second portion of the electrode which extends from a guide nose at the end of the guide body. A working tip at the end of the second portion of the electrode interacts with the workpiece to erode and remove workpiece material at a desired location. As the electrode is fed through the guide body during the EDM machining process, the guide body is wobbled relative to the longitudinal axis of a desired workpiece bore such that the working tip of the electrode traces a continuous closed curve, such as a circle, across a plane perpendicular to the longitudinal axis. As the electrode advances into the workpiece, the second portion of the electrode sweeps out a bore having planar cross sections with substantially similar shape and progressively larger size. This apparatus permits reverse taper bores having funnel or cone shapes.
Other features and advantages of the invention will be apparent from the following detailed description and the accompanying drawings.
The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description serve to explain the principals of the invention. In the drawings:
The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention or its application or uses.
The present invention provides a detailed specification of an apparatus and a method for producing reverse-taper bores in a workpiece using an electrical discharge machine. In particular, an apparatus and method for wobbling a guide body positioned near a workpiece is disclosed, whereby as the guide body is wobbled about a longitudinal axis of a desired workpiece bore, a working tip of the electrode erodes material from the workpiece. As the guide body continues to wobble, the electrode tip is advanced into the workpiece bore where it traces out successively larger closed curves, resulting in a reverse taper bore.
One of the ways injection tip holes 16 may be bored is by using an electrical discharge machine (EDM) apparatus. Advancing an electrode tip along a axis into the workpiece would create a generally cylindrical bore. One way to create a non-cylindrical bore is to wobble the electrode as the hole is bored.
Guide body 30 may be wobbled in any pattern. It need not even have an apex point 38, but could be wobbled in conjunction with non-longitudinal translation, thereby permitting compound bore shapes which have no fixed apex point.
Numerous means could be employed to wobble guide body 30. One embodiment is shown in
In one embodiment, the aforementioned moment force is applied to the guide support by means of two control rods 64 and 65 independently attached to first guide support portion 54 and second guide support portion 56 respectively. Force is applied to the control rods by means of two servo motors 66 and 67, each having respective torque arms 68 and 69 attached to servo motors 66 and 67 and to respective control rods 64 and 65. Referring to
The axis defined by first hinge 60 and the axis defined by second hinge 62 are perpendicular and preferably intersect at a defined point on the longitudinal axis of the desired workpiece bore 40, which point constitutes apex 38 of, e.g., a cone-shaped bore. As first guide support portion 54 and second guide support portion 56 are flexed, guide body platform 58 wobbles about apex 38. By precisely coordinating the distance traveled by control rods 64 and 65 and the flexing of guide body mount 52, guide body platform 58 can be wobbled in a desired pattern, for example, an elliptical pattern substantially centered about longitudinal axis 40. The term “ellipse” in this specification and the claims includes not only the common understanding of that term but also the special case where an ellipse is a circle.
Apex point 38 is defined by the intersection of the axis defined by the first hinge 60, the axis defined by second hinge 62, and the longitudinal axis of the desired workpiece bore 40. In operation, a workpiece would be attached to workpiece bracket 70 such that the outer orifice 18 of a desired workpiece bore 16 is at or near apex point 38. In the claims and this specification, an apex point described as being between the terminal end of guide body nose 37 and the external surface of workpiece 10 includes an apex point that is substantially coplanar with outer orifice 18.
As the EDM process commences, guide body 30 begins wobbling in a pattern, such as an elliptical pattern, or an ellipse that is a circle. As the working tip of the electrode 42 comes into proximity with the workpiece, it erodes the material to create outer orifice 18. Feeder mechanism 72 precisely feeds electrode 32 into guide body 30, which stabilizes a first portion of the electrode 34. As the second portion 36 of the electrode extends outward from guide body nose 37, the wobbling motion of guide body 30 causes it to sweep out a reverse taper elliptical cone shape according to the wobble pattern of guide body 30. As electrode 32 is progressively advanced deeper into the workpiece, the size of the closed curve generated by the movement of the working tip of the electrode 42 becomes progressively larger, thereby creating a reverse taper bore in the workpiece.
There are many other means of wobbling guide body 30 relative to the workpiece.
The design of the wobble mechanism within eccentric housing 82 could take many forms. A rotor could rotate about longitudinal axis 40 fixed to frame 80, with EDM chassis 50 being linkably attached to an eccentric on the rotor, thereby causing EDM chassis 50 to wobble about longitudinal axis 40 as the rotor rotated. Optionally, a variety of couplings, offset axles, or cams could be attached to frame 80 and connected to EDM chassis 50 to generate the wobbling motion described above.
As a person skilled in the art will readily appreciate, the above description is meant as an illustration of implementation of the principals of this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from the spirit of this invention, as defined in the following claims.
