Method and apparatus for cutting insulating material

#The present invention relates to a method and an apparatus for cutting insulating materials. In particular, the present invention is directed to cutting quartz crystal plates for use in electromechanical filters and resonators.

#Wire electro-erosion machining is a known process for cutting conducting materials by means of a wire electrode. In such a known method and apparatus, a continuously moving copper wire is positioned proximate to the conducting material to be cut. A difference in potential is imposed between the wire and the material while a liquid solution, e.g., deionized water, is fed coaxially to the wire. A multitude of electric discharges occurs between the wire and the material causing a cut in such material by electro-erosion. This known technique results in a contact-free machining process without exerting force on the conducting material. Simple as well as complex patterns can be achieved in accordance with this prior art arrangement. However, the latter is limited in its applicability to conducting workpieces made, for example, of steel or copper. This known method is inapplicable for cutting insulating materials such as quartz plates.

#A known method for cutting piezoelectric crystals is described by W. P. Mason in "Piezoelectric Crystals and their Application to Ultrasonics", 1950, pages 167-168. In accordance with this method, cutting is performed by a wet-string type saw consisting of a plurality of endless strings running over pulleys. After cutting a crystal, the edges of such formed crystal plates must be ground to give a flat and straight edge finish. This method is adequate for cutting crystal plates. However, the frequency of such plates must be further adjusted by modifying the plates' dimensions. In other words, this prior art method and apparatus provides rough or uneven cuts and may cause undesirable stresses in the crystal. Although such a known method is acceptable for certain applications, it cannot be easily used where a very thin plate of quartz has to be cut in accordance with a predetermined pattern.

#FIG. 1 shows an illustrative embodiment of an apparatus made according to the present invention;

#FIGS. 2A and 2B, respectively, illustrate a top and side enlarged view of a portion of the apparatus of FIG. 1; and

#FIG. 3 is a side view of a portion of an apparatus made in accordance with another illustrative embodiment of the present invention.

#Referring now to an illustrative embodiment of the invention, as shown in FIG. 1, an apparatus for cutting a workpiece 1 made of insulating material comprises a base 3 coupled to a displacement-control unit 4 for selectively moving the base in an X-Y plane. The displacement-control unit 4 is mechanically coupled through linkage 5 to the base or table 3, and controls the movements of the table in accordance with a predetermined set of displacement patterns. Such predetermined patterns are fed to the displacement-control unit 4 by any suitable means such as computer tapes or cards. The workpiece 1 is coated on one of its surfaces with a conducting layer 2. Such a layer can be deposited on the insulating material 1 by any suitable and convenient technique such as vapor deposition, plating, or sputtering. Alternatively, such a conducting layer can be positioned proximate to the surface of workpiece 1 by means, for example, a double-faced tape.

#In the illustrative embodiment of FIG. 1, a cutting member 6 is positioned substantially perpendicular to the base 3. By way of example only, the cutting member 6 is preferably made of a thin copper wire having diamond particles imbedded therein. But other types of metallic cutting members can be used to replace the copper wire 6. The cutting wire 6 is moved in a vertical direction in FIG. 1 by means of a feeding reel 7 and a receiving reel 8. Pairs of wire guiding rollers 9 and 10 guide the cutting wire 6 in a direction substantially perpendicular to the major surface of insulating material 1. By way of example only, insulating material 1 will be hereafter referred to as a quartz plate. However, the principles of the present invention are also applicable to other insulating materials, such as mica, glass, etc.

#A potential source 11 has one terminal coupled to the cutting wire 6 and the other terminal coupled to the conducting layer 2. In accordance with the known wire electro-erosion machining process described above, a gap detector 12 is coupled between the cutting wire 6 and the conducting layer 2. The gap detector has an output lead 13 coupled to the X-Y displacement-control unit 4. The main function of the gap detector 12 is to monitor the distance between the cutting wire 6 and the conducting layer 2 at the cutting site and to generate a control signal on its output lead 13 for controlling the displacement unit 4. Such a gap detector can, for example, measure the capacitance between the cutting wire 6 and the conductive layer 2 at the cutting site, and generate an electrical control signal on output lead 13 as long as the capacitance is below a predetermined threshold value. Alternatively, gap detector 12 can be inserted in the electrical circuit formed by potential source 11, cutting wire 6 and conductive layer 2 to measure current variations between the wire and the layer at the cutting site, and to generate control signals dependent thereof. A liquid solution, for example, deionized water, is provided at the cutting site coaxially with the cutting wire 6 by means of an arrangement schematically shown by reference numeral 14 in FIG. 1.