The present invention relates in general to electro-chemical milling machines, and in particular, relates to an improved drilling depth and drilling rate monitor for use in electro-chemical machining processes.
In recent years, extruded ceramic materials have been increasingly used as substrates for automotive catalytic converter products. During the course of their manufacture, these extruded ceramic substrates are forced through highly precise extrusion dies which are formed of ultra-hard materials.
The manufacture of extrusion dies from these ultra-hard materials is an extremely tedious process. The extrusion dies are formed with multiple apertures through which the extrudate is forced under high pressures. In one method of forming the extrusion die, mechanical drills are used to form the extrusion apertures. If the extrusion dies are formed of ultra-hard materials such as, for example, 17-4PH stainless steel or inconel 718, the drilling rate used for aperture formation is very slow and a great deal of time and effort is expended in extrusion die formation. If softer die materials are used the drilling rate is increased, but the lifespan of the resulting extrusion die is correspondingly shorter.
Because of these difficulties, extrusion dies are now provided with extrusion apertures which are formed by electro-chemical machining techniques rather than mechanical drilling. With an electro-chemical machining process for the manufacture of an extrusion die, the workpiece from which the die is to be formed is situated in a fixed position relative to a movable manifold. The manifold supports a plurality of cathode drilling tubes, each of which are utilized to form an aperture in the workpiece. The drilling tubes operate as cathodes in the electro-chemical machining process, whereas the workpiece comprises the anode in that process. The workpiece is then flooded with an acid electrolyte which is supplied via the manifold and material is selectively deplated from the workpiece in the vicinity of the drilling tubes to form the requisite aperture pattern.
In the manufacture of extrusion dies using an electro-chemical milling process, the rate of movement of the drilling tubes with respect to the workpiece must be precisely controlled. If the drilling rate varies the diameter of the apertures increases as the drilling rate slows. The aperture diameter decreases as the drilling rate increases. Since it is important that the aperture diameter be precisely controlled in ceramic extrusion dies, it is paramount that drilling rate be precisely controlled.
The precise control of the drilling rate in an electro-chemical machining process is not a simple problem. For example, the drilling rate which is achieved in the formation of extrusion dies is less than one-thousandth (0.001) of an inch per second.
Such extremely slow rates makes the design of circuitry for a drill rate monitor difficult. In the manufacture of some precision components such as extrusion dies for ceramic materials, it is also important, not only that the very slow rate of drilling be precisely controlled but also that the depth of the aperture formed in an electro-chemical machining process also be precisely controlled.
These and other objects are achieved by the drilling depth and drilling rate monitor of the present invention.