AUTOMATIC STOP FOR RETRACTION IN AN EDM MACHINE

BUTEE AUTOMATIQUE DE COURSE DE RETRAIT SUR MACHINE A USINER PAR ELECTROEROSION

English Abstract

ABSTRACT OF THE DISCLOSURE
The disclosure teaches an EDM machine hav-
ing a base supporting a workpiece, a ram reciprocable
on the base and a plurality of electrodes carried by
the ram to engage and drill the workpiece. In
accordance with the invention, a limit switch limits
the retraction movement of the ram in combination
with an actuator magnetically held on and movable
on the ram and an automatic stop device. The stop
device includes a member on the base in a position
to engage the actuator during the drilling movement
of the ram to adjust the position of the actuator
on the ram.

Claims

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The embodiments of the invention in which an exclusive
property or privilege is claimed are defined as follows:-
1. An EDM machine having: a base supporting a work-
piece; a ram reciprocable on the base; and a plurality
of electrodes carried by said ram to engage and
drill the workpiece; characterized by a limit switch
to limit the retraction movement of the ram in
combination with;
an actuator magnetically held on and movable
on the ram and an automatic stop device including
a member on said base in a position to engage the
actuator during the drilling movement of the ram to
adjust the position of the actuator on the ram.
2. A machine of claim 1 further characterized by
adjusting means for changing the operative position
of the stop deivce.
3. A machine of claim 1 further characterized by
the automatic stop device having a threaded rod
mounted on the base and extending through the actuator,
said rod being parallel to the ram movement, and a
stop nut on the threaded rod to engage the actuator.
4. The machine of claim 1 further characterized
by the ram having a surface parallel to the ram
movement on which the actuator is positioned, and
the stop device causing sliding movement of the
actuator along said surface.

Description

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Description
An Automatic Stop For Retraction
In An EDM Machine
Technical Field
This invention relates to machines for performing
EDM (electrical discharge machining) and particularly
to an automatic retraction stop for the machine.
Background Art
In the EDM machine and process particularly in
drilling small diameter holes one or more small
diameter electrodes are advanced toward the workpiece
and an electrical differential between the electrode
and the workpiece results in the machining of a
small diameter hole for each rod being formed in the
workpiece. An electrolyte submerge~ the workpiece
and electrode to provide an electrical insulation
therebetween. The rod is eroded away as the hole is
- being drilled at a rate as much as four times the
machining of the hole. When a short circuit or other
malfunction occurs with any one electrode the ram
carr~ving the electrode is automatically retracted
to correct the malfunction. A minimum distance for
retraction is desirable in order that the electrode
will not be withdrawn from the holes which could
cause the part to be scrapped for elongated holes
and also to minimize the nondrilling time for each
operation.
Disclosure of Invention
A feature of the invention is the device to
minimize the retraction distance and to maintain this
distance substantially~constant regardless of electrode
wear and depth of drill hole in the workpiece.
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Anothex feature is an adjustment by which to change
the retraction distance if desired.
In accordance with a particular embodiment of the
invention, there is provided an EDM,machine. The
machine has a base supporting a workpiece and a ram
reciprocable on the base. A plurality of electrodes are
carried by the ram to engage and drill the workpiece.
A limit switch limits the retraction movement of the
ram and operates in combination with an actuator
magnetically held on and movable on the ram and an
automatic stop device including a member on the base
in a position to engage the actuator during the drilling
movement of the ram to adjust the position of the actua-
tor on the ram.
According to the invention the ram carrying the
electrodes has an actuator for the upper limit stop
for t.he retraction and the actuator is magnetically
held to a surface on the ram parallel to the direction
of ram movement. Cooperating with the actuator is an
automatic stop device that is positioned on the base
of the machine in a location to be engaged by the
actuator so as to reposition the actuator on the ram
as the latter moves toward the workpiece. This stop
preferably includes a threaded rod on the base extend-
ing parallel to the ram movement and sliding through ahole in the actuator. Stop nuts on the rod permit
- adjustment of the retraction distance and are set to
position the actuator from the limit switch the desired
retraction distance. ~s the ram moves toward the work-
piece this adjustment will slide the actuator along thesurface on the ram and maintain the actuator so as to
control the desired retraction distance at all times.
The foregoing and other objects, features and
advantages of the present invention will become more
apparent in the light of the following detailed des-
cription of the preferred embodiments thereof as shown

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in the accompanying drawing.
Brief Description of the Drawings -
Fig~ 1 is an elevation of a machine embodying
the invention.
Figs. 2, 3 and 4 are diagrams showing on an
enlarged scale the step by step movement of the stop.
Best Mode For Carrying Out l'he Invention
As shown in Fig. 1 the machine has a base 2 that
has a supporting surface 4 for a plurality of fixtures
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6 in each of which is releasably mounted a workpiece 8.
The base supports a ver~ically movable ram 10 that
carries at its lower end a die set 11 which supports
a plurality of holders 12 one above each fixture 6.
Each holder 12 carries a plural:ity of parallely positioned
rods or electrodes 14 for pexforming the drilling
operations on the workpieces. ~hese electrodes are
clamped in vertical position in the holders and may
be adjustably positioned therein being held as by
clamps 16.
Directly above the fixture 6 is a guide 18 mounted
on the base and through which the drilling rods or
electrodes extend and by which these electrodes are
yuided into the precise desired relation to the
workpiece directly below the guide. ~ertical movement
of the ram 10 carries all of the rods in unison
against the several workpieces and by electrical
discharge machining provide holes therein.
When any malfunction occurs as for example a
short circuit between one of the electrodes and the
associated workpiece, suitable controls reverse the
movement of the ram and retract it a small distance
to eliminate the short circuit and then again feed
the electrode against the workpieces. One type of
malfunction is a shorting between cne of the wires
and the associated workpiece and this is corrected
by a retraction o~ the electrode a vexy short distance.
The retraction movement of the ram on the machine,
if not limited, could permit the rods, that is the
electrodes to be withdrawn from the holes being drilled
and possibly resulting in a scrapped part if the
electrode warps away from the hole being drilled.
It is desirable to limit the retraction distance
to a speci~ic small dimension that will not vary
with each retraction or with the depth of the holes
or the amount of erosion of the electrodes. To
accomplish this the base carries a limit switch 20
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that serves to stop the upward retractive movement
of the ram~ The ram has an actuator 22 in a position
to engage this limit switch and the actuator is
- magnetically held in position on a vertical surface 24
on the ram. The base also carries a support 26 for
a threaded rod 28 that extends through a vertical hole
30 in the actua~tor and carries adjusting nuts 32
in a position to engage the underside of the actuator.
These nuts are set during operation so as to position
'0 the actuator the desired retraction distance from the
limit switch.
As the operation of the machine begins and a
retraction occurs, the upper movement of the ram
will carry the actuator against the switch to stop
the retracting movement. As the holes are drilled
deeper and as the rods are eroded, the ram moves
further down in the machine and the adjusting rod
stop nuts being fixed with respect to the base cause
a corresponding upward movement of the actuator
vertically along the surface 24 on the ram as shown
- in Fig. 3. When a retraction now occurs, the actuator
engages the limit switch after only the set retracting
- movement has occurred because of the higher position
of the actuator on the ram. The result is that the
same length of retraction movement occurs regardless
of the position of the ram. Since the erosion of
electrodes and the depth of hole being formed is thus
compensated for, the operative ends of the electrodes
are in fact withdxawn the same distance from the
bottom of the hole for each retraction. The net
eff~ct is to foolproof the machine and assure only
the necessary limited retraction of electrodes.
In this way, possible damage to the workpieces is
avoided and incidentally the nondrilling portion of
the cycle of the machine is minimized~

Although the extent of the rnovement of the
actuator in Figs. 3 and 4 is shown as a substantial
amount it will be understood that in fact the
actual movement of the actuator along the ram between
retractions may be extremely small on ~he order of
one-thousandths of an inch or even less depending upon
the frequency that retractio~ occurs. Where the
drilling operation involves a plurality of workpieces
as shown in Fig. 1, and as many as seventy-five
or more electrodes operating on each workpiece any
malfunction such as a short circuit on one electrode
may occur very frequentlyO The conventional machines
are electronically controlled so the malfunctions are
sensed electronically and cause retraction and
immediate return to feeding motion of the ram. Thus
a retraction stroke that can be kept to a minimum
uniform length throughout the machine operation is
desirable and shortens the length of the total
drilling operation.
It will be understood that the machine is built
so that the workpieces and operative ends of the
electrodes are immersed in a dielectric of a suitable
- type. However this is well known in the art and has
not been described and it is not in itself a part
of the invention in any way.
Although the invention has been shown and
described with respect to a preferred embodiment
threof, it should be understood by those skilled in
the art that othar various changes and omissions in
- 30 the form and detail thereof may be made therein
without departing from the spirit and the scope of
the invention.