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WIRETYPESAND
THEIR SELECTION
BY E.P. “BUD” GUITRAU
Introduction
In the early years of wire-cut
EDM, choosing wire was very
simple- copper wire was the only
choice. Within a very short time however, the limitations of straight copper
wire were discovered and other alloys
were tried. Brass proved to be
adequate for a long time, and is still
the most widely used wire material
today. More recently, the increasingly
difficult-to-machine alloys and the
demands for faster cutting speeds
brought about new wire materials that
could support the high-performance
power-supplies available today.
Wire selection now, can be just
as difficult as choosing the right electrode material for vertical EDM operations. With so many choices available
in the selection of the correct wire,
there are just as many possibilities of
selecting the wrong one.
We must first consider several
basic properties of EDM wire. After
learning this criteria, the selection
process can begin. We examine each
type of wire, its properties, and a
few applications.
1) Tensile Strength -This is the rating
of load-bearing capability given to
materials based upon their ability to
resist stretching or breaking. This is
determined by the maximum load in
pounds per square inch divided by the
cross sectional area of the wire. Small
diameter wires, tall or tapered parts
will require a wire of higher tensile
strength to maintain accuracy. Often,
this is the main criteria used in wire
selection, although this is not always
the most important.
Note: The tensile strengths listed on
the wire spools are based upon figures
for a 1 inch diameter bar of that material. Your .010 diameter brass wire will
obviously not withstand a load of
60,000 pounds. However, if we convert
that into a “diameter proportionate”
rating, it calculates out to be approximately 1700 grams for a .010 diameter
wire.
2) Fracture Resistance-To prevent
wire-breakage, high tensile strength
26
alone is not the total answer. Since
most wire breaks are encountered well
below their prescribed maximum
tensile strength due to poor flushing,
excessive current, hard servos, etc.,
the wire must have the ability to resist
breakage. High tensile strength
coupled with fracture toughness or
“resilience” is very important, otherwise excessive wire breakage will
defeat the purpose of high-speed
wire-cutting.
3) Conductivity-This is the measure
of a material’s ability to carry electrical
current. The higher the conductivity,
the more power can be presented in
the gap. In most cases, the end results
of higher power delivered to the gap is
increased speed. Copper wire offers
the highest conductivity of existing
wires, but it has limitations due to its
relatively low tensile strength (see
stratified wire).
4) Vaporization Poin t-In wire-cut
operations, a low melting/vaporization
temperature of the wire is usually the
best choice, exactly opposite the conditions desired in vertical EDM applications. This melting/vaporizationtemperature is good for several reasons;
a. The initial spark impulse vaporizes
both workpiece and wire. The quicker
the wire vaporizes, the faster heat
will be transferred to the workpiece
removing more material. The rapid
erosion (within reason) of the wire is
not a problem because it is continually
renewed by fresh wire from the spool.
b. When the surface of the wire can
vaporize rapidly, it “protects” the core
of the wire by not transferring heat to
it, which would anneal or weaken it
enough to break.
c. When the wire vaporizes instead of
melts, it will create more gasses
instead of “chips.” This results in much
smaller and fewer “chips” and contaminants to interfere with flushing.
Copper Wire
When wire-cut EDM was in
its infancy, copper wire was a natural
choice. The most obvious reasons
seemed to be its natural ability to conduct electricity, and the fine wire sizes
required were already available for
winding electric motors. Pure copper
wire however, has a relatively low
tensile strength (34,000 to 60,000 psi),
and is subject to excessive breakage.
m1
Also, if the most important aspect of
wire-cut EDM is flushing, then copper
wire’s greatest shortcoming is it offers
poor “flushability” due to its high thermal conductivity. A large amount of
heat is transferred to the wire instead
of the workpiece and is then carried
away with the spent wire instead of
vaporizing the workpiece material.
Brass Wire
Brass is made by alloying
copper with zinc. This affords higher
tensile strengths and better flushability because zinc has a much lower
melting point than copper. The most
common type is “yellow brass” with a
copperlzinc ratio of 63/37. This type
of brass wire is available in tensile
strengths from 50,000 to 145,000 psi.
By alloying brass with aluminum
or titanium, tensile strengths can be
increased to 140,000 to 160,000 psi,
but at the expense of flushability.
Coated Or Stratified Wire
Stratified or coated wires came
into being with the logic of, “If I need
the tensile strength of one material,
and the conductivity or flushability of
another, why not coat or plate one with
the other?” That is exactly what wire
manufacturers have done.
These wires are not “solid” wires
but have a wire core that has been
plated or coated with one or more layers of different materials. These coatings are usually made by an electrolytical galvanizing process using special
equipment to insure a homogeneous
coating of uniform thickness that is
concentric to the core diameter.
Stratified wires usually consist of an
alloy with a low vaporization temperature, plated to a higher tensile strength
core. Other types are “clad” with a
thicker external layer of alloy and then
heat treated to fuse them together.
Both types can provide high tensile
strength with good flushability and
conductivity.
Zinc has become a popular wire
coating because it provides faster cutting speeds and reduced wire breakage. This is because zinc has a much
lower melting temperature than brass
(419°C vs. 93OOC). In fact, the zinc
coating will actually vaporize (906OC),
before the brass can melt (see chart
1.O on the next page). This means that
EDM Today, MarchIApril Issue
aD
,
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the brass core can never get hotter
than the vaporizationtemperature of
the zinc coating. So as long as there is
a layer of zinc to “boil away,” the brass
core can continue to deliver current
without melting through. Wire breakage is reduced and cutting speeds
90 UP.
I
TEMPERATURE CHART OF WIRE COMPONENTS
MELT1NG TEMP
MATERIAL
C
ALUM1 NUM
(e‘
(i 4
CHART 1.0
/
660” /
930” /
BRASS
I
qualities).
A few years ago, an interesting
twist in wire development borrowed
existing technology from vertical EDM
machines by applying a thin layer of
graphite to a core of molybdenum
wire. Moly wire is incredibly strong and
conducts very well, but due to its high
F
1220”
YAPOREATION TEMP
C
/
F
2450” / 4442”
1706”
I
One analogy of the way stratified
wire works is similar to the way a
double boiler cooking pan works. No
matter how hot the flame (spark) is,
the water (zinc) in the bottom pan can
never exceed its boiling point. With the
external (or lower) temperature thus
“controlled,” the food (brass core) in
the upper pan can never reach a temperature high enough to scorch or
burn. The zinc coating, in effect, “protects” the brass core. This applies to
any of a large variety of coated wires
with a lower-temp external coating.
There is a zinc-coated brass
wire. It has a core of 63/37 brass with
a 2pm coating of pure zinc for better
flushability. The brass-on-brass wire
has a core of 80/20brass, with a 50/50
alloy coating. Note the higher percentage of zinc on the “cutting” surface to
lower the vaporization point and aid
flushability. Another type of stratified
wire has a copper core for conductivity
and a heavy zinc coating. This wire
then undergoes a heat treating process that alloys the two metals, forming a thick brass layer upon the copper
core. This type of wire offers high
conductivity with good flushability,
but with a slight reduction of tensile
strength. There is also a wire available
with a steel core (for strength), coated
with copper (for conductivity), and
then coated with brass (for cutting
EDM Today, March/Aprii issue
I
melting temperature, it provides very
poor flushability. The graphite coating
offers a much hotter spark within the
gap, producing more work. In addition,
graphite, being a “metalloid” does not
melt. In a water dielectric it will oxidize.
The resulting gasses will condense
back into solids within the cold water,
but these particles will be much finer.
This improves flushing conditions
because there is much less debris
remaining to contaminate the gap.
Molybdenum
This is a very high-strength wire,
with tensile strengths in excess of
275,000 psi, almost doubling the
strengths of any brass alloy. While it
offers a very high tensile strength and
high resistance to breakage, moly wire
cuts very slowly because of its poor
flushability. This is due primarily to its
very high melting temperature of
2620°C (4748OF). Moly wire is generally used for skim cutting in the smaller
diameters when corner radii requirements are extremely small and the
additional strength is needed to prevent breakage. Most other times its
use is not cost effective because of its
high price and slow machining times.
Wire Qualities
The wire itself must meet certain
standards to insure success. They are
as follows:
1) Material Quality -Wire must be
made of the highest quality alloy(s)
available for this purpose. The drawing
process must produce wire surfaces
that are smooth and free from nicks,
scratches or cracks that can propogate under tension and cause
wire breakage.
2) Diameter-The wire must have a
precision uniform diameter. Besides
the obvious problems this would produce with accuracy, this can effect the
tensile strength of the wire, which is
mathematically related to the crosssection of the wire. The standard tolerances for brass wire size is +_.000040”,
and +.000060” for plated wires.
3) Tensile Strength-The tensile
strength must be at least what the wire
is rated, or inaccuracies and excessive
wire breakage can occur, This is closely related to the quality of the alloy and
its uniform diameter, but is seldom a
problem because quality wires have a
generous safety-margin in their ratings.
4) Ductility-The ductility of the wire
must be uniform and consistent. While
tensile strength is important, if the wire
is too brittle it cannot withstand shock
and will break. Different hardnesses
are used for different applications. For
example: when cutting more severe
tapers (7”+), use of a “soft” brass wire
is advised, as pressure across the
guides will stress one side of the wire
more than the other and result in a
“curling” effect. Machines using automatic wire-threaders will prefer a
“hard” wire with no “memory,” for the
most reliable threading repeatability.
5) Spooling-The winding of the wire,
or spooling, must be precise and uniform. The quality of the wire is negated
if the spool is wound with snags, overlapping winds or variations in tension.
All of these adverse conditions will
further increase wire breakage.
6) Plating or coating (where applicable)
Must be uniform in depth and density.
If the wire is improperly plated, it can
have “thin spots” resulting in unnecessary breakage, not to mention dimensional inaccuracies. If the plating or
coating of the wire has not been
applied correctly, the plating can flake
off as it passes through the rollers and
continued on page 30
27
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I
continued from page 2 7
guides causing additional problems of
wire-breakage and inaccuracies.
7) Packaging-While not actually a
quality of the wire itself, packaging is
important in assuring that all the above
characteristics are used to their best
advantages. All spools should be
sealed in plastic bags to avoid oxidation and corrosion, and unwrapped
only when ready t o use. Most wire is
shipped "nested" in its own Styrofoam
boxes for protection. When purchased
in bulk, wire stock should be dated
and rotated to assure quality.
Troubleshooting
Now that we have all these
different types of wire, what do we
do with them? What wire with what
material? What do I use when I want
to go fast? What do I use when I keep
experiencing excessive wire breaks?
The chart to the right shows
some general guidelines for wire
troubleshooting. The chart also reflects
corrective action concerning wire
selection only. Situations where adjustments of servo speeds, flushing pressure, cutting conditions, etc., are not
noted here, but may be necessary to
realize the full potential of the troubleshooting process.
Increasingthe fracture toughness of the wire is perhaps the single
largest "fix" for successful wire-cut
operations. For increased speed, use
REMEDY
PROBLEM
WIRE
To Reduce Wire Breakage
Increase Tensile Strength
Increase Fracture Toughness
Al or Ti/Brass Alloy
Steel Core
To Increase Cutting Speed
Better Flushing
More Power
Zinc Coated Brass
Brass Coated Copper
To Cut Steep Tapers
Increase Tensile Strength
Inaease Ductility
Zinc Coated Brass
"Soft" Brass
To Cut Thicker Workpieces
Better Flushing
Increase Tensile Strength
Zinc Clad Brass
Brass On Brass
To lncrease Accuracy
Increase Tensile Strength
Increase Fracture Toughness
AI or Ti/Brass Alloy
Steel Core
The finest
€Dm Grcrphitet
In stock
coated or stratified wire, although zinccoated wires will show their limitations
in thicker workpieces because the zinc
coating is very thin and will be consumed before it exits the workpiece.
For taper cutting, use softer wire to
prevent the wire from curling as it
passes through the offset guides.
Automatic wire threaders prefer harder
wires to aid in threading through small
start holes. For thick workpieces
- for immediate shipment!
Pracoet liquid
PiIt rcIt ion Sy/tem/
8elmont offers a wide range of standard and customized filtration
systems by National Standard S. A - ideal for EDM, SCREW
MACHINES, ROLLING MILLS, HONING and GRINDING Video available
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Braided stainless steel flextubes
provide maximum filtering surface.
Submicron size particles are
removed at flow rates up to 1500
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Minimum maintenance, low
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Single units or central systems.
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RINGSOORFF:
CG (AIRCO):
UNION CARBIDE:
EELMONT:
EK-82, EK-85. EK-88
KK-8. KK-10
J-225 (EDA)
BEC-4, BEC-70
Available in plates, rounds, squares
or cut to order.
\*/
"factured
to your prints.
For more information contact.
For more information contact
BELMONT EQUIPMENT CO.
BELMONT EQUIPMENT CO.
32035 Edward Avenue
Madison Heights. MI 48071
DETROIT CHICAGO BOSTON
800/356-4811
LOS ANGELES 714/996-6400
32035 Edward Avenue
Madison Heights, MI 48071
DETROIT CHICAGO BOSTON
800/356-4811
LOS ANGELES 714/996-6400
BEimonT-
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For more information, circle 201 o n reader service card
30
For more information, circle 220 o n reader service card
EDM Today, MarchIApril
Issue
choose wires that provide good flushability. For high-accuracy applications,
increasing the wire tension will be a
big help, using steel-cored wire or
high-tensile brass,
Here is a chart showing the
maximum wire tension in grams that
different wire diameters can withstand.
Summary
Selecting the correct wire for
the job comes from basic knowledge
(which we have tried to establish here),
might help you. Aside from their actual
formal presentations, most of these
individuals welcome the opportunity to
speak directly to others during breaks
in these seminars or after-hours. They
would be glad to discuss any questions or particular problems you may
be having.
One final item before closing that
is often overlooked- Maintenance. If
your equipment is in disrepair or operating at less than optimum conditions,
then any objective evaluation of a par-
MAXIMUM WIRE TENSION IN GRAMS
PSI
60,000
70,000
.004
.006
.008
.010
.012
270
600
1100
1700
2400
300
650
1200
105,000
130,000
140,000
N/A
450
1000
1850
N/A
1650
2550
2700
3700
650
1500
2650
4200
6000
750
1750
N/A
N/A
N/A
90,000
N/A
N/A
N/A
1
‘Tension spxificationscourtesy of Cisco Equipment, Inc., 1990
@
8
experience and the recommendations
of the wire manufacturersand distributors themselves. Experience alone
used to suffice, but in the last few
years, wire technology has improved
so much and so fast, that we must rely
in part, on the advice and suggestions
of the wire manufacturers’themselves.
This is especially true with the newer
wire-cut machines as their power supplies, flushing capabilities, tapering
and tensioning devices and threading
mechanisms can be significantly different, especially between machines of
Swiss and Japanese origin.
Another excellent source of
valuable information can be the applications engineers employed by the
builders of your equipment. Many
times they will be sent samples of new
types of wire for “beta” testing before
it’s release to the public. In this manner, applications engineers can learn
each wire’s cutting characteristics and
develop efficient conditions and settings for that particular power supply.
These same engineers should also be
able to recommend wires and cutting
conditions for their older machines
that are still in the field.
In addition, there are also many
EDM seminars and technical conferences held throughout the country,
with speakers and specialists who
possess information or literature that
EDM Today, MarcWApril Issue
ticular wire’s performance will be difficult, if not impossible.
Wire machines require regular
preventive maintenance. Without this
important attention, performance will
drop off rapidly no matter what kind
of wire you are using. Cleanliness is
mandatory. Check the power feed
contacts, guides, bushings, rollers,
etc., daily. Don’t forget to monitor the
cleanliness and condition of the water
and check the condition of the filters
and resin regularly. With wire machines
running untended longer and longer
these days, we can easily take for
granted just how many hours they
actually are working. Charting these
hours and scheduling regular preventive maintenance will go a long way in
assuring optimum, or at least predictable cutting peformance, not to
mention prolong the life of other consumables and the machine tool itself.
Good luck!
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