Wire guide for electric discharge machine

A wire guide directs a wire electrode from a refeed mechanism in an electrical discharge machine EDM to a workpiece. The wire guide has ceramic inserts defining a precision guide slot for the electrode and wherein one of the inserts is carried by a deflectable member spring biased to damp the electrode so as to maintain concentricity of holes formed by the electrode and thereby closely control the hole diameter.

FIELD OF THE INVENTION 
This invention relates to electric discharge machines (EDM); more 
particularly, it relates to a wire guide for use in apparatus for 
machining multiple parts or workpieces with substantially uniform 
accuracy. 
BACKGROUND OF THE INVENTION 
In many applications, electric discharge machining is used for the mass 
production of parts which must be machined to close tolerances. There are 
several factors which affect the repeatability of accuracy of machining by 
the EDM process including the process parameters of electrode size, spark 
energy, spark current, gap length, gap voltage and the wire guide used to 
direct the electrode to a workpiece. For example, when the EDM process is 
used to machine a small hole through a thin plate, the small dimensions of 
the wire electrode require a support system which is sufficiently accurate 
to support the electrode in all directions. Also, mechanical or electrical 
vibrations in the electrode system can result in variation in hole 
diameters. In the case of wire guides for directing electrodes with 
respect to an orifice plate for fuel injectors, for example, the 
electrode, even though closely guided, can vibrate in a manner so that 
accuracy in hole diameter cannot be achieved uniformly. 
Accordingly, there is a need for obtaining improved uniformity and accuracy 
in guiding an electrode wire in EDM apparatus. Further, it should be 
achieved without affecting the various other parameters involved in 
improving accuracy of the EDM process. 
A general object of this invention is to provide an improved wire guide for 
use in apparatus for electric discharge machining to obtain a high degree 
of repeatable accuracy in the machining of parts. 
SUMMARY OF THE INVENTION 
In accordance with this invention, apparatus is provided for electric 
discharge machining of workpieces on a mass production basis with greater 
workpiece feature formation repeatability than achieved heretofore. This 
is accomplished by dampening the electrode at precision guide slot inserts 
whereby the tolerance stack-up is compensated in the electrode support 
system so that uniformity of formation of hole dimensions and shape is 
achieved. 
Further, according to the invention, a wire guide is provided for use in 
electric discharge machines which includes means which hold a wire 
electrode position to produce workpiece features having a specified size 
within predetermined tolerances. Specifically, multiple guide inserts are 
machined to form a precision slot for guiding a wire electrode and one of 
the inserts is spring biased to impose controlled dampening on the wire 
electrode so that the effectiveness of the electrical discharge is 
maintained to cause workpieces to be machined so that the size and shape 
of the feature is within predetermined tolerances.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring now to the drawings, there is shown an illustrative embodiment of 
the invention in a method and an apparatus for electric discharge 
machining using an adaptive control system which adjusts a selected EDM 
parameter in response to a feedback signal representing workpiece 
measurement data. The adjustable EDM parameter is spark energy. A hole or 
set of holes is the workpiece feature to be machined to a specified size, 
as measured by its fluid flow capacity. It will be appreciated as the 
description proceeds that the invention may be embodied in many different 
forms and utilized for many different applications. 
In the illustrative embodiment of the invention to be described, an 
electric discharge machining method and apparatus is utilized for 
machining a set of small holes in a thin plate. In particular, the example 
workpiece is an orifice plate for a fuel injection nozzle for use in an 
automotive engine. Such workpieces or parts must be manufactured in high 
volume using mass production techniques and must be held to very close 
dimensional tolerances. As shown in FIG. 1, the workpiece or part 
comprises a circular plate 10 with a set of six small holes 12 in a 
circular array surrounding the center of the plate 10. In the example 
workpiece, each of the holes has a specified diameter of six thousandths 
of an inch with a tolerance of plus or minus one ten thousandth of an 
inch. The set of holes of such size, taken collectively, has a certain 
fluid flow capacity. Measured flow capacity is used as an index of size. 
The electric discharge machine embodying the invention is shown 
diagrammatically in FIG. 2. In general, it comprises a machining station 
14, a test station 16 and a workpiece transfer mechanism 18 is adapated to 
move each workpiece through the successive stations. 
If desired, the machine can include a workpiece feed station and a 
workpiece sorting station. Such stations and their operation are further 
discussed in copending U.S. Ser No. 781,115 filed Sept. 27, 1985, now U.S. 
Pat. No. 4,725,705, for Method and Apparatus for Electric Discharge 
Machining, the specification of which is incorporated herein by reference. 
The machining station 14 comprises a workpiece holder 20 which has a nest 
22 thereon adapted to receive the workpiece 10. The holder 20 is mounted 
on an index table or turret (not shown) which is adapted to rotate the 
holder 20 to position the workpiece 10 in six different angular positions, 
one for each hole to be machined. The head of the electric discharge 
machine comprises a carriage 24 for feeding a wire electrode 26 for 
machining the holes in the workpiece at the work station on the holder 20. 
The wire electrode 26 passes through a refeed mechanism 28 and a wire 
guide 30 to the workpiece. The refeed mechanism 28 is adapted to advance 
the electrode wire, which is supplied from a spool (not shown), relative 
to the carriage 24 upon retraction of the carriage after each machining 
operation. After each machining operation to form one of the holes 12, the 
downward limit of the carriage 24 is sensed by a limit switch 32 which 
causes the holder 20 to be indexed to place the next hole location at the 
work station on the holder. When the holder 20 is indexed to the position 
for machining the last hole in the workpiece, a limit switch 34 enables 
the actuation of a transfer drive 36 of the transfer mechanism 18. Upon 
completion of machining of the last hole in the workpiece the limit switch 
32 causes energization of the drive 36 and the transfer mechanism 18 is 
actuated to transfer the workpiece 10 from the machining station 14 to the 
test station 16. The test station 16 and its operation are more 
specifically set forth in copending U.S. Ser. No. 781,115. 
The workpiece 10 is electrically conductive so that when the proper gap is 
established between the wire electrode 26 and the workpiece 10 a spark is 
established and maintained during the electrical discharge machining 
process. A suitable process is fully described in copending U.S. Ser No. 
781,115, it being understood, however, that the present invention is 
suitable for use with a wide range of EDM machines. 
Referring now to FIGS. 3-9, the wire guide 30 of the present invention 
includes a housing 38 with spaced sidewalls 40, 42 each of which carry two 
spaced dowel pins 44, 46 that are located into a wire guide support 48 on 
the carriage 24. 
The housing 38 has a tapered end 50 with a slot 52 therein which supports a 
spring biased pressure foot 54. The pressure foot 54 has an inclined 
surface 56 generally complementary to an inclined surface 58 on tapered 
end 50 to provide necessary relief in vicinity of the spark gap. 
The pressure foot 54 more particularly includes a pad 60 that is bonded to 
one end of a deflectable sheet 62 of phenolic impregnated fiberglass. The 
sheet 62 has a first ceramic insert 64 bonded thereto on the opposed 
surface 66. The insert 64 has a slot 68 formed thereacross with a chamfer 
70 that allows insert segment 64a to freely move with respect to insert 
segment 64b as the sheet 62 deflects. 
The first ceramic insert 64 is associated with two other ceramic inserts 
72, 74 carried in side slots 76,78 in machined wire guide holders 80, 82. 
The inserts 72, 74 are bonded in place by suitable adhesive. The holders 
80, 82 are secured to the housing 38 by screws 84 so that the ceramic 
inserts 72, 74 will be located to aligned edge chamfers 86, 88 thereon to 
form a V-slot 90 for guiding the electrode wire 26 through the wire guide 
30. Side set-screws 89 engage the wire holder 82 as shown in FIG. 9 for 
presetting the lateral position of V-slot 90. While inserts 64, 72, 74 are 
disclosed as being formed of ceramic material, any other electrically 
insulated material would be suitable for use in practicing the invention. 
As best shown in FIG. 6, the V-slot 90 in configured so that the surface 92 
on the assembled inserts 72, 74 is maintained slightly below the upper 
tangency 94 of the diameter of electrode wire 26. 
A uniform spring biased dampening is imposed on the electrode wire 26 by 
means of spaced springs 96, 98 held in place between a cross-arm 100 of 
pressure foot 54 and set screws 102, 104 in holders 80, 82 as shown in 
FIG. 8. The springs 96, 98 bias against cross-arm 100 and cause pressure 
foot 54 to pivot counterclockwise about a pivot pin 105 located between 
walls 40, 42. 
The pressure foot 54 thereby deflects the sheet 62 and the insert segment 
64a against the wire electrode segment 26 in a manner to yieldingly grip 
the electrode 26 as it leaves the wire guide 30. One key aspect of this 
spring bias feature is that it compensates for tolerance stack-ups in the 
support/guide system for small diameter electrodes. Such electrodes make 
it difficult to machine guide grooves and slots with sufficient accuracy 
to support such wires in all directions. The problem is obviated by use of 
the present invention. 
Another result of the invention is to dampen operational vibrations in the 
wire electrode and as a result more repeatable hole features are formed in 
the workpiece 10. The ceramic insert segment 64b and the ceramic inserts 
72, 74 are relatively fixed with respect to the vibration dampening insert 
segment 64a so as to smoothly guide the wire electrode through the wire 
guide 30 during the refeed of wire electrode to compensate for electrode 
wear at the spark gap. While round holes are shown in the illustrated 
embodiment, the invention is equally applicable to guiding electrodes 
configured to produce other geometry holes resembling the geometry of the 
electrode. 
Although the disclosure describes and illustrates a preferred embodiment of 
the invention, it is to be understood that the invention is not limited to 
such embodiment. Many variations and modifications will be apparent to 
those skilled in the art. For a definition of the invention reference is 
made to the appended claims: