US2753429A - Electrical discharge machining - Google Patents

Description

u y 3, 1956 c. MCKECHNIE 2,753,429

ELECTRICAL DISCHARGE MACHINING Filed Feb. 25, 1955 1 i r U INVENTOR.

iTTOF/VZ'E United States Patent ELECTRICAL DISCHARGE MACHINING Ian C. McKechnie, Pontiac, Mich., assiguor to Elox Corporation of Michigan, Clawson, Mich.

Application February 25, 1953, Serial No. 338,670

4 Claims. (Cl. 219-) This invention pertains to a method and apparatus for removing material from electrically conductive stock by electrical-discharge-machining More specifically it relates to a self-contained electricaldischarge-machining apparatus which is simple in design, economical in manufacture when compared to apparatus now available, and which can be operated ofi a standard 60 cycle supply without use of rectifiers or other auxiliary a paratus.

The object of the invention is to provide an apparatus of the type above described.

For a description and illustration of a preferred embodiment of the invention, reference is made to the accompanying drawings in which reference characters have been used in the several views to designate parts referred t0.

In the drawings,

Fig. 1 is a side elevational view of the dischargemachining apparatus with certain parts broken away to show details thereof and the electrical connections shown diagrammatically;

Fig. 2 is an enlarged sectional view of a portion of the Fig. 1 apparatus; and

Fig. 3 is an enlarged sectional View of the electrode entering the work.

Referring now to the views and to Fig. 1 in particular, it may be seen that the apparatus comprises a supporting structure or frame it having an upward extension 12 on which is mounted an adjustable drill press head or other suitable support 14. The vibrating head 16 of the tool is mounted on the rack-bar 1S ordinarily provided and is therefore adjustable vertically by manipulation of a handle 24 An adjustable shelf 22 is provided for supporting a workpiece such as the one shown designated by numeral 24.

The vibratory head 16 of the apparatus is mounted in a casing 26 which consists of a pair of cylindrical housings having mating flanges adapted to be secured together by bolts 28. The casing 26 is secured to the rack-bar 18 by suitable fastening means and has an aperture 30 for accommodating the vibratory rod 32.

The rod 32 carries an armature 34 at its upper end. The latter comprises a pair of permanent magnets 36 of opposite polarity spaced a predetermined distance apart. A field structure 38 is disposed in surrounding relation to the armature 34 and includes a field coil 40 adapted for connection to an A. C. supply by means of leads 42.

The field structure has a bell-shaped lower extension 44 which mates with a second bell-shaped member 46, the latter having a cylindrical extension 48 which slidably guides the rod 32. Each of the bell-shaped memhers is provided with a flange 50, 52, which flanges are disposed in mating relationship as shown.

A flexible metallic diaphragm 54 is secured between the flanges 50, 52 at its marginal portion and at its center has an opening which receives the rod 32. A collar 56 and a nut 58 carried by the rod constitute means for secur- "ice ing the rod to the diaphragm such that the rod and armature are supported for vertical vibratory motion.

At its lower end the rod 32 carries a fluid connection block 60 which is provided with a fluid passage through which fluid may flow from a suitable supply 62. Below the connection block, the rod carries a chuck 64 which is adapted to mount an electrode 66. The electrode may be of any desired size and has a hollow passageway 68 through which fluid may flow from the supply 62.

Figs. 1 and 3 show a typical workpiece which consists of a fabricated part 24 in which a broken tap or screw 68 has become imbedded.

In the operation of the machine, the coolant supply 62 is turned on, the switch (not shown) energizing the field coil is closed and a source of E. M. F. 70 is connected across the work and the electrode by means of leads 72 and 74. The E. M. F. source '76 may be the same source as used to energize the field coil or a separate source may be provided. The coolant may be tap water, kerosene or a mixture. The coolant serves to cool the electrode and the area of the work adjacent the discharge area and expels the particles removed from the work by the action of the electrical discharge.

After the apparatus has been activated as above described, the electrode 66 is moved into proximity with the work by manipulation of the handle 29. The armature along with the rod 32 and the electrode 66 will vibrate at power line frequency which, in most cases, wil be 60 cycles per secon By properly adjusting the handle 20 with regard for the length of the vibratory stroke as determined by the constants of the flexible diaphragm 54, the electrode 66 may be positioned such that it will approach the work closely enough to permit an electrical discharge between the electrode and the work at the bottom of its stroke and recede sufficiently at the top of its stroke to prevent a discharge taking place when the electrode is positive relative to the workpiece.

Thus it will be seen that an electric arc or spark discharge will occur between the electrode and work once each cycle and the work will be eroded, the eroded particles being flushed away by the coolant flow as indicated diagrammatically in Fig. 3.

For maximum cutting efliciency it is desirable that the work be positive and the electrode negative when the discharge takes place. In the apparatus being described, A. C. current is used in the interest of economy and simplicity. It is therefore necessary to properly phase the mechanical motion of the tool with the power line frequency such that the work will be positive and the tool negative when the tool is at the bottom of its stroke, or at a define point with relation to the bottom of the stroke. This can be accomplished in several ways. The resonant frequency of the vibratory system can be fixed by carefully choosing the constants of the diaphragm 54. Under some conditions, it may be desirable to employ a phase-- shifting network having the required amount of inductancc, resistance or capacity to effect the desired result. My issued Patent No. 2,501,954, dated March 28, 1950, illustrates and describes the use of springs and networks for this purpose and reference is made thereto. Of course it is necessary to connect the leads to the power supply 7 or suitable alternate phased supply in a polarized manner so as to result in the desired displacement of the electrode relative to the power pulse within electrical degrees.

Instead of a pair of spaced permanent magnets 36, a single magnet may be used with its north and south poles properly oriented. Similarly, i may use a pair of diaphragms instead of the single diaphragm shown.

While I have described but one of the many forms the invention may take, it will be understood that this 3 has been done for illustrative purposes and that it is not desired or intended to limit the invention in its details or application except as set forth in the appended claims.

Lhave described the use of my method and apparatus as a tap buster or salvage machine. Obviously, however, the method and apparatus are useful in many other applications such as machining hard alloys, carbides, etc.

I claim:

1. In an apparatus for machining a conductive workpiece, an electrode, a head for reciprocating said electrode comprising an annular fixed coil, an armature having spaced permanent magnets disposed in the flux path of said coil, a yieldable member mounting said armature, an extension member connecting said electrode to said armature, means connecting said electrode and the workpiece to an A. C. source, and means connecting said coil to'said A. C. source.

2. In an apparatus for machining a conductive work piece by intermittent electrical discharges, a source of A. C. power, an electrode; means connecting said electrode and the workpiece to said A. C. source, means for reciprocating said electrode at A. C. power supply frequency comprising a fixed coil of ring form, means connecting said coil to said A. C. source, a rod adapted to mount said electrode, an armature carried on said rod and disposed Within said coil, said armature comprising a pair of permanent magnets of opposite polarity; and a yieldable member mounting said rod for axial reciprocation.

3. An apparatus for machining a conductive workpiece comprising, an electrode, means connecting said electrode to one side of an A. C. source, means for connecting a workpiece to the other side of said source, a fixed coil of annular shape, means connecting said coil to said source,

. an armature comprising a pair of permanent magnets of References Cited in the file of this patent UNITED STATES PATENTS 458,872 Depoele Sept. 1, 1891 2,167,078 Lakatos July 25, 1939 2,399,977 Bardos May 7, 1946 2,501,954 McKechni'e Mar. 28, 1950 -lul

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