FUNDAMETLSChoosing EDM consumables
Photo courtesy of Mitsubishi EDM
by Greg Langenhorst
Product Manager
Mitsubishi EDM
Wood Dale,IL
It has been said that all men are created equal, but
this is certainly not the case with EDM consumables. .
Several types of EDM consumables are used on a daily basis in machine shops. This article
will focus on wire, filters, resin, carbide contacts, and guides items.
Wire
Two kinds of wire are used in EDM machining: coated and brass. Gisco, the US importer
for Cobra Cut, Bronco Cut, and Mega Cut coated wires, has set the standard for this type
of wire through years of working with the EDM machine manufactures. Many people think that
Cobra Cut and Mega Cut are the same products, sold under different names. This is far from
the truth.
Mega Cut was developed for Japanese-style machines that use round diamond guides and
water-jet threaders with heat-style cutters. This product has overcome dirt and zinc
flaking problems that plague some other wire types. In both brass and coated wires,
straightness is a major issue for the threaders. If the wire has too much curl, it will
not stay in the jet stream and thread properly.
Very small, or fine wire (0.0012´´ to 0.004´´ dia), can be problematic because wire
tensile strength weakens as the diameter decreases. Molly and tungsten have been used for
these sizes to gain strength. They are both very expensive and it is difficult to develop
good cutting technology for them. Micro Cut wire (steel-core brass coated) is a newly
developed fine wire that has the strength of steel and the surface of brass. For today's
machines, this is an excellent choice for wire in sizes from 0.0012´´ to 0.004´´ dia.
The EDM's guide type and auto threader type will dictate which wires to use. An open
V-type guide can use a plus or minus tolerance wire and is not as critical on dirt or zinc
flaking. A round guide machine can plug up with too much dirt or zinc flakes. An
auto-threader machine that uses heat for a cutter requires a coated wire where the coating
is not too thick because the heat can cause the zinc to bubble up on the surface and then
not fit through the guide to thread.
Wire type is chosen mainly by application. High tensile brass is the most common and
least expensive wire. It's applicable to most EDM work. If cutting higher degrees of
taper, a soft brass wire will bend easier. For very high tapers, the Mega T Cut is the
best. It is a soft brass zinc coated wire that bends easily. The zinc helps the wire slide
through the round guides without curling.
For cutting carbide or thicker parts with small wire, Mega Cut A is very good. It is a
high tensile wire with a zinc coating that reduces the spark temperature in the gap,
creating less damage to the part surface. If speed is the name of the game, Mega Cut D or
HS is the best. It is a medium tensile wire with a 50/50-zinc copper induction annealed
surface that aids in the flushing action in the gap along with the ability to carry more
current.
Operating cost is a major issue with coated wire. Cost includes wire purchase price and
machine wear and tear. Another cost is power feeder life.
The best way to justify the wire cost is performance. How many inches of cut are
performed to the amount of wear and tear? How much more work can be done in the same
number of hours? Does using the wire give you a higher quality part to sell and use? These
factors make the coated wire increasingly more popular. It's more expensive, but the end
result is worth it.
Other drawbacks on some of these wires is that they may not run on your machine, slip
in the driver rollers, cut, or thread. High-speed wires which are run hard on older
machines may cause the power supply to overheat and fail. Check with your manufacturer
before using these wires and ask for the technology to run it.
Filters
Filters in EDM machines remove sludge particles from the dielectric fluid through the
use of a medium. Essential to the EDM process, filters reduce DC arcing and tank sludge
and improve cutting efficiency. Filters are made of paper or polyester and have a micron
rating of either nominal or absolute.
Paper filters are normally used once, while polyester can be washed and reused many
times. A filter is rated as nominal once it is pre-coated and running. A filter is rated
absolute if it does not require pre-coating and provides that level of filtration from
start to end of life. Pre-coat time varies by the paper's starting size. When a new filter
starts at a 15-micron with a nominal rating of 3, it will take longer to precoat than a
paper that starts at a 10-micron with a nominal of 3.
The bonding material is glue that holds the filter together. If this glue is very hard
and brittle, the filter may leak sooner than if a more pliable glue was used. Choose a
filter that is approved and costs the least per hour to run, then test it for performance.
Keep in mind that the cheapest filter may ultimately cost more because of longer changing
time (which means down time).
Resin
Resin is used as a carrier to rinse the discharged, dissolved solids out of the EDM
work tank to avoid contamination of the workpiece. Four types of resins are available:
mixed bed, made of 40% cation and 60% antion; dual bed, where cation and antion are in
separate parts of bag or tank; virgin; and virgin regenerated.
Anion and cation are plastic-like beads with porous surfaces that collect the
positively and negatively charged particles in the EDM water. This controls the operating
conductivity of the water that is critical in keeping the cut consistent. Generally, a
mixed-bed resin canister is preferred over separate ones because it can provide better
water quality and last longer. Virgin resin is the most expensive and provides the highest
quality water. To maintain a lower conductivity level, virgin works best. Regenerated
costs less but does not last quite as long as virgin, and won't maintain conductivity as
low as virgin.
Carbide contacts
 Carbide contacts are critical in transferring the energy produced by
the EDM to the wire that actually cuts the parts. A wide range of carbide grades is
available. Some people believe that different grade carbides will run different amounts of
hours. The difference may be there, but it is very small. The main difference is the shape
of the carbide that contacts the wire.
A curved surface will perform better than a flat surface because of how the wire bends.
The grooves come mainly from the wire breaking contact with the carbide surface. Each time
this happens a small arc occurs, creating the wear mark. High-speed wire causes more wear
simply because it vibrates more.
Guides
Machines that use round diamond guides require that each size wire has a set of guides
the same size. Older machines use different guides for AWF and non-AWF types, and in some
cases different upper and lower guides. New machines still have different upper and lower
guides, but threader and non-threader machines use all the same guides.
Round diamond guides have a life expectancy of approximately 4000 hours. Handling and
cleaning them affect their performance.
When buying new guides, make sure they are designed to fit your machine. OEM guides are
the best; if you buy some after-market guides, you may not achieve the proper clearance,
proper diamond set height, or proper lead-in angles and radii.
Guides are the highest priced EDM consumable, but when you consider price per hour,
guides become much less expensive. Some machines use V guides, some use round guides,
which are made of sapphire and diamond. OEM guides are built to exact machine specs.
After-market guides may not have such exact tolerances on diamond position or guide
height.
Finally, for very high taper jobs, round guide systems require wide-angle guides. These
have a larger clearance and radius to allow easier bending of the wire. |
