Mechanical feed EDM machine for making threaded and nonthreaded bores

An electrical discharge machining device that is used for both rotational EDM, such as tapping, and the like, and for conventional or non-rotational EDM applications. The device includes a base support to which a carriage is slidably mounted, with a motor driven, rotating electrode assembly mounted thereon. A threaded lead screw is removably coupled to the upper end of the rotating electrode assembly and a mating threaded collar is removably mounted on the support so as to receive the lead screw and thereby advance the carriage as the lead screw is rotated. The lead screw and threaded collar are interchangeable with other lead screws and collars having different thread pitches, so as to regulate the advancement of electrodes having different thread pitches. An electrode is mountable both on the rotating electrode assembly and on the carriage itself.

BACKGROUND OF THE INVENTION 
The present invention relates to electrical discharge machining (EDM) 
devices, and more particularly to heads and advancement mechanisms for EDM 
devices. 
Electrical discharge machining is conventionally used to machine very hard, 
electrically conductive workpieces, and the like, that are difficult or 
impossible to machine by conventional methods. In normal applications of 
EDM, the workpiece is immersed in a bath of dielectric fluid, and a power 
supply is applied across the cutting tool and the workpiece. When the 
cutting electrode is brought sufficiently proximite to the workpiece, the 
dielectric fluid breaks down, and an electric discharge or spark is 
generated across the space or gap between the electrode and the workpiece. 
With the electrical discharge, a minute amount of material is removed from 
the workpiece. 
It will be appreciated that in EDM processes, the spacing between the two 
electrodes, or the "spark gap", is critical. If the spark gap becomes too 
small, or actual physical contact is made, a short circuit occurs between 
the cutting electrode and the workpiece. Such a short circuit results in 
either the fusing of the cutting electrode to the workpiece, or a melting 
or embrittling of the effected area of the workpiece. A fused electrode is 
very difficult and time consuming to remove, and if sufficient damage 
occurs, the workpiece must be scrapped. 
The problem of maintaining a proper spark gap is further complicated when 
an EDM device is used to tap holes in or through a workpiece. In such 
tapping operations the cutting electrode must be rotated as it is advanced 
into the workpiece. Heretofore, due to the difficulty of maintaining the 
proper spark gap, thread tapping EDM devices were often forced to rely 
upon hand advancement of the cutting tool, in order to properly control 
the machining process. However, an operator is not always capable of 
reacting quickly enough when the spark gap becomes too small in order to 
reverse the advancement of the cutting electrode. Further, hand advanced 
machines are relatively slow, and difficult to accurately control. Such 
hand advanced machines require an operator's constant attendance, which 
greatly increases the labor costs of machining the workpiece. 
Some EDM devices have utilized automatic advancement of the thread tapping 
electrode. However, the advance mechanism for the electrode is relatively 
complex, and cannot be readily converted to cut different thread styles 
and sizes. The set-up time for such prior machines is therefore 
substantial. Further, multiple EDM devices are required in order to 
produce a full range of machining operations. Thread tapping EDM devices, 
that are designed for rotational advancement of the electrode, are not 
capable of being used for conventional EDM applications, in which the 
cutting electrode does not rotate. Similarly, devices designed for 
conventional EDM applications cannot be used for thread tapping. 
SUMMARY OF THE INVENTION 
The present invention solves the problems noted above by the provision of 
an EDM device that performs both rotational machining and conventional, 
non-rotational machining. The EDM device includes a moving carriage on 
which an electrode rotating assembly is mounted. The device regulates the 
advancement of the entire carriage relative to the device's support, so 
that an EDM electrode may be advanced either by the rotating assembly for 
thread tapping applications, or by the carriage itself for conventional, 
non-rotational EDM machining applications. 
Additionally, the EDM device permits rapid and easy set up for a wide 
variety of different machining operations. The device includes a threaded 
guide rod or lead screw that is removably and interchangeably coupled to 
the motor-driven, rotating drive shaft for the cutting electrode. A mating 
threaded sleeve is removably and interchangeably mounted on the device 
support, so that as the drive shaft rotates the mating guide rod and 
sleeve automatically advance the drive shaft. The guide rod and sleeve 
have a thread pitch that corresponds to the thread pitch of the cutting 
electrode, so that when the tapping electrode is changed, the threaded rod 
and sleeve are simply interchanged with a guide rod and sleeve having a 
corresponding thread pitch. 
These and other objects, features, and results of the invention will be 
apparent to one skilled in the art from the written specification, claims, 
and drawings herewith.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the preferred embodiment, as shown in FIGS. 1-4, an EDM device is 
referenced generally by the numeral 10. EDM device 10 includes a normally 
stationary support 12 on which is mounted a slidable carriage 14. A 
rotating electrode drive assembly 16 is mounted on carriage 14 and 
translates therewith. Carried on the lower end of carriage 14 is an 
electrode mounting platen 18 (FIGS. 1-3) that is used in non-rotational or 
conventional EDM applications. At the top of drive assembly 16 is coupled 
a threaded guide rod or lead screw 20, which is matingly received in a 
threaded guide collar 22 mounted on support 12. Both guide rod 20 and 
guide collar 22 have a thread pitch that matches the pitch of the thread 
to be machined in the workpiece. As drive assembly 16 rotates, lead screw 
20 and guide collar 22 cooperate to advance carriage 14, and thus 
simultaneously advance drive assembly 16. Lead screw 20 and guide collar 
22 are removably, interchangeably coupled to EDM device 10, so as to be 
readily accessible for interchanging with other lead screws and collars 
that correspond to various other thread pitches. 
More specifically, as shown in FIG. 2, support 12 is a flat, generally 
vertical base plate that is mounted upon a suitable normally stationary 
stand 30. As shown in FIG. 2, stationary stand 30 is the support stand for 
a conventional, non-rotating EDM device. On the rear surface of support 
plate 12 are a number of mounting channels 32 in which a mounting key or 
the like may be inserted for mounting on stationary stand 30. 
Alternatively, support plate 12 may be secured to stand 30 by bolts or 
other conventional fasteners. Extending along the forward face of support 
plate 12 is a pair of vertical ways 34 (FIG. 3) on which carriage 14 is 
slidably mounted. 
Carriage 14 is a rectangular, generally flat plate having a pair of 
vertical ways 36 extending along the rearward surface, and rearwardly 
extending side edges that accommodate the sliding engagement of ways 36 
with ways 34. As shown in FIG. 2, on the forward face of carriage 14 is 
secured a drive assembly housing 40. As shown in FIG. 1, housing 40 
includes a rectangular top wall 41 and a rectangular bottom wall 42, to 
which removable sidewalls are secured in order to permit access to drive 
assembly 16. 
Drive assembly 16 is mounted on the forward face of carriage plate 14 
(FIGS. 1-3). Drive assembly 16 includes an axially vertically oriented 
drive shaft 50 that is rotationally mounted in an upper bearing 52 and a 
lower bearing 54. Bearings 52 and 54 are preferably conventional ball 
bearings. Shaft 50 is rotated by means of a driven pulley 56 keyed to the 
upper end of drive shaft 50. A direct current, servomotor 58 is mounted on 
carriage 14, and translates with it. Motor 58 is reversible, and is 
capable of rotating in very small increments or arcs in response to an 
electrical controller (not shown). In this example, motor 58 is mounted on 
the outer face of upper bearing 52. Motor 58 is operably connected to 
drive shaft pulley 56 by an endless drive belt 60. Drive belt 60 is a 
syncronous belt, with cogs along the inner surface to positively engage 
belt 60 with pulley 56 and prevent any slippage of belt 60 as motor 58 
reverses direction. Motor 58 is used to rotate shaft 50 for both the 
rotation of the cutting electrode, and the advancement of carriage 14, as 
described below. Preferably, motor 58 is electrically insulated from the 
EDM device to prevent damage thereto by the direct current cutting 
current. 
Mounted on shaft intermediate bearings 52 and 54 is a rotating couple or 
commutator ring 70. Commutator 70 is secured to shaft 50 by set screws or 
other conventional fasteners and is electrically conductive with shaft 50. 
An electrically conductive brushing 72 (FIGS. 1 and 2) contacts commutator 
70 and is mounted on carriage plate 14 by a mounting bracket 74. Current 
is applied to brushing 72 through cable 73 from a power source (not shown) 
of conventional design, in order to apply operational power to the cutting 
electrode as discussed below. Brushing 72 is removably mounted in bracket 
74 so that brushing 72 may be replaced as excessive wear occurs, in order 
to maintain proper registry with commutator 70. Commutator 70 insures that 
the electrical power flows directly through drive shaft 50 to the 
electrode to prevent damage to bearings 52 and 54. 
Secured to the lower end of shaft 50 is an alignment head 80 of the type 
used in the mounting of cutting bits on conventional machining tools. 
Alignment head 80 includes a lower mounting aperture in which a cutting 
electrode 82 is removably secured. Alignment head 80 is adjustable in 
order to maintain electrode 82 in substantial vertical alignment as 
electrode 82 is rotated. 
Cutting electrode 82, as shown in FIGS. 1-3, is a threaded, tapping EDM 
electrode. Electrode 82 has an elongated shaft of sufficient length to 
pass through the particular workpiece being machined. Tapping electrodes 
82 have a variety of thread patterns that vary in pitch, depth, direction 
of threading and the like, as determined by the particular threaded 
aperture intended to be produced. Alternatively, other types of cutting 
electrodes 82, such as cylindrically shaped electrodes or single point 
electrodes, may be used to machine non-threaded circular bores through the 
workpiece. Such electrodes 82 will produce a smooth bored aperture, and 
the rotation of electrode 82 insures that the aperture produced will be 
correctly aligned through the workpiece. 
Protruding above pulley 56 is an upper coupling end 90 of drive shaft 50. 
As shown in FIGS. 2 and 3, upper coupling end 90 includes a threaded, 
upstanding projection 92 onto which lead screw 20 is secured. Lead screw 
20 is therefore axially aligned with drive shaft 50, and thereby is 
axially aligned and commonly rotating with electrode 82. Located at the 
lower end of guide rod 20 is a pair of opposed flats 94 that provide a 
seat for a conventional wrench. Flats 94 provide a means for holding onto 
lead screw 20 as lead screw 20 and shaft 50 are screwed together. Lead 
screw 20 may therefore be quickly removed from shaft 50, since coupling 
end 90 and flats 94 are readily accessible to the operator from outside of 
housing 40. 
Lead screw 20 is threaded with a pitch that is substantially identical to 
the thread pitch of electrode 82. Various alternative lead screws 20 are 
therefore interchangeable with lead screw 20 on device 10, with each 
particular lead screw complementing a particular electrode 82. 
Fixed to the upper surface of support plate 12 is collar mounting bar 100. 
Mounting bar 100 is a rectangular metal block that is bolted to support 
plate 12 and which extends out over shaft 50. As best shown in FIG. 2, 
located through the forward end of mounting bar 100 is an aperture 102 
through which lead screw 20 is slidably received. The diameter of aperture 
102 is slightly greater than the outer diameter of threaded lead screw 20 
so that lead screw 20 does not engage mounting bar 100. Guide collar 22 is 
clamped to the upper surface of mounting bar 100 by a pair of L-shaped 
clamps 104. Guide collar 22 is located to be communicative with aperture 
102 and matingly receive lead screw 20. Clamps 104 are bolted into 
mounting bar 100 so that guide collar 22 may be quickly and easily removed 
with the use of a wrench. However, clamps 104 tightly secure collar 22 to 
mounting bar 100 so that collar 22 remains fixed in position during 
operation of device 10. 
Collar 22 is threaded in order to mate with the thread of lead screw 20, 
thereby acting as a lead screw guide. A variety of collars 22 are 
therefore interchangeable with guide collar 22 on device 10 in order to 
accommodate a variety of electrodes 82. As shown in FIGS. 1, 2 and 4, 
collar 22 is a split 106 ring that is closed by a pair of tightening bolts 
108. The diameter of guide collar 22 may therefore be reduced by 
tightening bolts 108. Collar 22 is tightened in order to maintain intimate 
contact between the threads of collar 22 and lead screw 20 and therefore 
collar 22 insures the accuracy of the threads cut by electrode 82. 
On one side of support plate 12 is secured a vertical stop-mounting bar 
110. Stop-mounting bar 110 is a relatively thin metal bar on which an 
upper stop 112 and a lower stop 114 are adjustably mounted. Upper and 
lower stops 112 and 114 are adjustably affixed to bar 110 by hand-turned 
clamp bolts so as to be selectively positionable along the length of bar 
110. Stops 112 and 114 form adjustable contact surfaces for an upper limit 
switch 116 and a lower limit switch 118, respectively, that are fixedly 
mounted on sliding carriage 14. Limit switches 116 and 118 are of a 
conventional type and are used to signal the EDM device control (not 
shown) when carriage 14 has reached the preselected maximum upper and 
lower travel positions. The travel or stroke of electrode 82 may therefore 
be selected by adjusting the position of upper and lower stops 112 and 
114. Also mounted on lower stop 114 is a switch 120. Switch 120 is 
positioned on lower stop 114 so as to be contacted by limit switch 118 
when carriage 14 reaches the lower limit of its travel. Switch 120 signals 
the device control (not shown) in order to shut off power to brushing 72, 
reverse the direction of rotation of motor 58 and to increase the speed at 
which motor 58 rotates shaft 50. By means of switch 120, when machining of 
the workpiece has been completed, device 10 retracts electrode 82 at an 
increased speed, and thereby reduces the down time of EDM device 10. 
Secured to the side of support plate 12 opposite stop mounting bar 110 is 
another stop mounting bar 130. Mounting bar 130 extends along support 
plate 12 and includes a stop 132 that is adjustably mounted thereon so as 
to be slidable along the length of bar 130. Stop 132 is fixed in position 
on mounting bar 130 by a hand turned clamp bolt so as to be selectively 
positioned anywhere along the length of mounting bar 130. Secured to the 
upper edge of carriage 14 is a displacement gauge 134. Displacement gauge 
134 is a conventional dial-type displacement gauge that engages mounting 
bar 130 and measures the relative displacement of gauge 134 along the 
length of bar 130. Stop 132 is used as a zero-point positioned at the 
maximum lower limit of travel of electrode 82. After gauge 134 has been 
zeroed at stop 132, during operation of device 10 gauge 134 provides the 
operator with information on the amount of machining that remains. 
Platen 18 (FIGS. 1-3) is secured to the lower end of carriage 14 so as to 
extend at right angles from the forward face of carriage 14. Platen 18 
extends underneath shaft 50 and alignment head 80 and a vertically 
oriented aperture 140 extends through platen 18 in alignment with shaft 
50. Electrode 82 passes through aperture 50 and protrudes beneath platen 
18 in order to access the workpiece beneath platen 18. Aperture 140 has a 
diameter greater than the greatest diameter of electrode accommodated by 
alignment head 80. Electrode 82 therefore does not engage platen 18 during 
operation of EDM device 10. 
On the lower surface of platen 18 are two T-shaped channels 142 (FIGS. 2, 
6), which are used to mount an electrode plate 144 (FIG. 6) for 
non-rotational, or conventional machining. Electrode plate 144 includes 
two mating T-shaped projections 146 which are received and secured in 
channels 142 in a suitable manner. On the lower surface of electrode plate 
144 is a depending electrode 148, FIGS. 6 and 7. Electrode 148 has a 
configuration other than a regular cylinder, thus requiring that electrode 
148 be advanced non-rotationally in order to produce a properly configured 
cavity. Electrode 148 includes a pair of conventional electrical leads 149 
that are connected to a conventional EDM power supply. As motor 58 
rotates, shaft 50 causing thread lead screw 20 to advance carriage 14, and 
electrode 148 is advanced non-rotationally toward the workpiece. 
In operation, to tap an aperture through a workpiece 150, workpiece 150 is 
connected to the EDM control by leads 152 in a conventional manner and 
workpiece 150 is immersed in a bath 154 of dielectric fluid. An 
appropriate threaded electrode 82 is selected and coupled in alignment 
head 80. A corresponding threaded lead screw 20 and mating guide collar 22 
are selected, with lead screw 20 telescopingly slid through aperture 102 
and screwed onto shaft projection 92. A wrench is fitted onto flats 94 in 
order to fully tighten lead screw 20 onto drive shaft 50. Collar 22 is 
threaded over the upper end of lead screw 20 and is fixedly clamped onto 
mounting bar 100 by collar clamps 104. Bolts 108 are tightened until the 
threads of collar 22 intimately contact the threads of lead screw 20. 
Displacement gauge stop 132, and limit switch stops 112 and 114 are set at 
the required level with electrode 82 positioned adjacent the workpiece. 
The electrical control unit (not shown) for EDM device 10 applies current 
to brushing 72 in a conventional fashion to provide current to electrode 
82. Motor 58 rotates drive shaft 50 in a direction of advancement along 
with lead screw 20. The rotation of lead screw 20 in guide collar 22 
causes carriage 14 to lower, and therefore downwardly advances electrode 
82 toward workpiece 150. Included in the control unit is a mechanism for 
determining the spark gap between electrode 82 and the workpiece. When the 
spark gap has become too small, the control unit automatically reverses 
motor 58. The reversal of motor 58 causes electrode 82 to be raised, or 
backed off of the workpiece 150, until the current drain falls within 
predetermined acceptable limits. The operational direction of motor 58 is 
then again reversed in order to recommence advancement and machining of 
electrode 82 through the workpiece 150. When lower limit switch 118 
reaches lower stop 114 at its maximum downward travel, motor 58 is 
reversed to withdraw electrode 82. Switch 120 simultaneously increases the 
speed of motor 58, so that down time for device 10 during the raising of 
carriage 14 is reduced. When upper limit switch 118 contacts upper stop 
112, electric motor 58 is shut off. 
In applications where EDM device 10 is used for conventional or 
non-rotational machining, the process followed is similar to that set 
forth above with the exception that electrode plate 144 with non-rotating 
electrode 148 is mounted onto platen 18. Motor 58 still provides the 
advancement function for carriage 14 and thus for electrode 148, but 
alignment head 80 rotates above platen 18. As described above, the control 
unit senses the current applied to electrode 148, so that motor 58 is 
reversed in the event that the spark gap between electrode 148 and the 
workpiece 150 becomes too small. 
Alternatively, lead screw 20 and threaded electrode 82 may be coupled to 
the same end of shaft 50 (not shown). In such an alternative embodiment, 
lead screw 20 is connected to the lower end of shaft 58 by a coupling 
similar to coupling end 90 described above. Guide collar 22 is mounted on 
a lower depending end of support plate 12. Electrode 82 is then coupled by 
a similar alignment head 80 directly to the lower end of lead screw 20, so 
that electrode 82 is coupled to shaft 50 by means of lead screw 20. 
Essentially, EDM device 10 is inverted, and electrode 82 is then coupled 
to the lower end of lead screw 20. 
Further, non-rotating, or conventional electrodes 148 may also be mounted 
on the same side of carriage 14 as lead screw 20. EDM device 10 is 
inverted, and a portion of the resulting lower end of support plate 12 is 
extended to depend beneath the lower end of lead screw 20. Electrode 148 
is then mounted on the depending portion of support plate 12 for 
non-rotational machining as described above. 
It will be understood by one skilled in the art from the above description 
that various modifications and improvements may be made without departing 
from the spirit of the invention disclosed herein. The scope of the 
protection afforded is to be determined by the claims which follow and the 
breadth of interpretation that the law allows.