Topic of the month –
December 2007
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Wire EDM - Cutting Speed A lot is written and said about cutting speed in
connection with wire EDM machines very regularly, and the result is often
more confusion than anything else! How is it specified? Strangely enough, cutting speed in wire cutting
is generally specified in mm˛/min. If a machine for instance
cuts at 300 mm˛/min, and the work piece is 50mm
thick, the effective feed rate will be 3mm/min. A “cutting speed” of 300 mm˛/min means that per
minute a surface of 300 mm˛ will be machined. Looking at the spark gap, you
realize that there are two machined sides, one on either side of the wire.
Their combined surface after one minute of cutting will be 300 mm˛. There is
of course also the radius in front of the wire, but since that surface is of
an insignificant size, we can for practical purposes neglect it. We have
therefore a machine surface of 150 mm˛ on either side of the wire, and with
the work piece height being 50 mm, the cutting progress per minute has to be
150 (mm˛) / 50 (mm) = 3 mm. The value everybody is really interested in is
therefore 3 mm/min. How fast is wire cutting really? Compared to traditional machining processes, wire
cutting is still pretty slow. The cutting speed of standard wire cutting machine
is approximately 300 mm˛/min. Values published by machine manufacturers
always state cutting speeds achieved under ideal conditions. In cases where
flushing is not ideal, i.e. top or bottom nozzle not in contact with the work
piece surface, the cutting speed will be compromised. How much loss in
performance one has to accept depends largely on the quality of the
generator. In a top of the range machine, erosion pulses are automatically
adapted to the cutting conditions through feed back from the cutting zone
(wire – work piece). This requires powerful electronic processing circuits,
which in turn are expensive. Entry level machines are not equipped with such features;
the loss of performance is therefore higher. Cutting speeds for entry level machines range
from 80 mm˛/min for fast wire machines to approximately 220 mm˛/min for a low
cost wire cutting system with a conventional wire system, using brass wire. From a standard machine one can expect cutting
speeds between 220 and 330 mm˛/min, while top end machines will achieve up to
550 mm˛/min. One has to always bear in mind that those are top
speeds, achievable only under ideal cutting conditions as earlier mentioned. How fast will wire cutting be in the future? R&D engineers predict that cutting speeds of
up to 800mm˛/min can be achieved in the foreseeable future. The availability of
faster electronic components has in the recent past lead to the development
of generators with much higher efficiency, and there is clearly still room
for improvement. There are however only a few manufacturers of EDM equipment
on the market who have the necessary R&D capacities and the funds to
facilitate the development of such high performance generators. As a result,
the gap between standard / entry level machines and high end state of the art
equipment will become bigger in the near future. How fast does Wire Cutting have to be? “Speed is not an issue” is a statement often made
by customers when machine specifications are discussed. When discussed in
more detail, speed generally does become an issue! Wire cutting is by default
a slow process, compared to conventional machining processes such as milling
and turning. Machining times of 10 or 20 hours per job are not unusual. A difference
in performance of 20% would mean between 2 and 4 hours difference in
machining time. Where machines are utilized daily for a full shift, such
performance differences can quickly add up to a couple of days! Does speed cost money? The question is asked regularly, the better
question however would be “does speed earn money?” The answer is YES to
both!! A fancy generator does cost more money that a
basic one, that’s pretty obvious. The initial investment however should not
be the only or the most important consideration when purchasing a wire
cutting system. The most important issue should really be “how much money is
the machine going to make?” To answer that question, the requirements of each
customer have to be evaluated individually. In order to do such an
evaluation, the machine supplier has to be willing and competent enough to become
a partner to the customer, forming a long time relationship. Anything else is
likely to result in frustration for the customer! |