The present invention relates to an electric discharge machine for forming holes of desired shapes in a workpiece while repeatedly causing electric discharges to occur between a tool electrode and a conductive workpiece, while moving the tool electrode towards the workpiece.  More particularly, the present invention relates to an electric discharge machine having a balance device for balancing gravitational force acting on a vertically moveable quill. 
Electric discharge machines are widely used to accurately process solid conductive workpieces into molds and dies.  The workpiece is fixed to a table arranged in a work tank, and a tool electrode of copper or graphite is attached to a quill, which is movable in a vertical direction, using a suitable electrode mounting device.  The work tank is filled with dielectric fluid, such as kerosene, and the tool electrode is positioned extremely close to the workpiece.  The space between the workpiece and the tool electrode is referred to as a xe2x80x9cgapxe2x80x9d, also known as the xe2x80x9cmachining gapxe2x80x9d; the size of this gap ranges from a few xcexcm to a few tens of xcexcm.  By applying a power pulse across the workpiece and the tool electrode during a controlled pulse ON time, the insulation characteristics of the dielectric fluid in the gap are broken down and a discharge is produced.  Microscopic amounts of the workpiece material are evaporated or made molten by the heat caused by this discharge, and flow into the dielectric fluid.  When the pulse ON time is completed, i.e., during the pulse OFF time, the insulation characteristics of the dielectric in the gap are restored. 
As a result of the discharges produced during the pulse ON time, microscopic crater shaped holes remain on the surface of the workpiece.  At the completion of the pulse ON time, a controlled OFF time starts during which application of a power pulse to the workpiece is interrupted and the insulation characteristics of the dielectric fluid in the gap may be restored.  Electric discharge machines usually have the ON time and OFF time controlled in a range of between about 1 xcexcsec and a few tens of msecs, to thereby apply a train of power pulses to the gap. 
During machining the electric discharge machine causes the tool electrode to move toward the workpiece, along the Z axis, in order to maintain the gap at a relatively fixed size.  As tool electrode advances, removing microscopic amounts of material from the workpiece without contacting the workpiece, cavaties.  complementary in shape to the tool electrode are accurately formed in the workpiece.  The cavities have good surface roughness characteristics.  This type of electric discharge machine, known as a sinker EDM is classified differently than a wire EDM, i.e., a machine which uses a moving wire electrode. 
Use of an appropriate flushing procedure to produce a flow of dielectric fluid through the gap to wash fragments removed from the workpiece away from the gap is important in electric discharge machines.  The flushing procedure serves to prevent undesired secondary discharges between the tool electrode and fragments that have been removed from the workpiece, and to restore reliable insulation during the pulse OFF time.  A skilled operator may form holes at appropriate positions in the sinker tool electrode and the workpiece (before processing) in order to supply fresh dielectric into the gap and to suck contaminated dielectric fluid out of the gap.  When the shape of these machining fluid circulation holes is limited due to the size and shape of the tool electrode, the operator may arrange an injection apparatus for injecting dielectric fluid towards the gap at an appropriate position.  Flushing is crucial to carrying out good discharge machining faster and more accurately, but skill is necessary in order to produce a uniform flow throughout the entire gap.  Causing the tool electrode to move periodically along the Z axis, first to rapidly rise up and then to rapidly fall, so that the contaminated dielectric fluid in the gap is almost completely removed from the cavities in the workpiece, is known as a xe2x80x9cjumpxe2x80x9d operation.  During a conventional jump operation, the tool electrode moves at a rate of a few hundred mm/min.  As the reciprocating distance of the tool electrode is made larger, more fresh fluid flows into the gap and more contaminated fluid is ejected from the gap.  Therefore, the tool electrode is preferably caused to rise up during a jump operation to at least the depth of a cavity being formed in the workpiece.  However, since there is no removal of material from the workpiece during a jump operation, if jump operations are carried out too frequently, it results in a lowering of the stock removal rate.  In order to perform a jump operation having a sufficiently large amount of movement without lowering the stock removal rate, the tool electrode is preferably moved with an acceleration and deceleration in excess of 1 G.
An object of the present invention is to provide an electric discharge machine that has, in addition to a quill movable in the vertical direction with high acceleration, a balancing device for balancing gravitational forces acting on the vertically moveable quill. 
Another object of the present invention is to provide an electric discharge machine that causes a quill, having a tool mounting electrode attached to a lower end thereof, to move accurately in the vertical direction with high acceleration. 
Additional objects, advantages and novel features of the invention will be set forth in the description that follows, and will become apparent to those skilled in the art upon reading this description or practicing the invention.  The objects and advantages of the invention may be realized and attained by practicing the invention as recited in the appended claims. 
In order to achieve the above and other objects, there is provided an electric discharge machine (EDM) according to the present invention for machining a workpiece by moving a tool electrode vertically towards a workpiece while repeatedly producing electric discharges between the workpiece and the tool electrode.  The EDM comprises a vertically movable quill, an electrode mounting device attached to a lower end of the quill for mounting the tool electrode, at least two linear motor movers attached to the quill symmetrically about the central axis of the quill, at least two linear motor stators, respectively opposite the at least two movers, and a balance device for balancing a gravitational force acting on the quill so that a line of action of a balance force can be aligned with the central axis of the quill. 
Preferably, the balance device comprises an air cylinder arranged coaxially with the quill. 
More preferably, the quill has a hole extending vertically in the center and the air cylinder is located in the hole.