ECM Revisited

Some may remember a little experiment I did with Electro Chemical
Machining a year or two ago:

http://www.indoor.flyer.co.uk/ecm.htm

The idea is you take a DC power supply some salt water and an
electrode and you make holes or dies, much like EDM.

Recently in the department they have been looking at machining a
motor casing in aluminium. This requires 240mm long holes at 5mm
diameter. These are pretty long holes. We have a sinker EDM but
the the travel is not nearly enough. I suggested a homebrew ECM
setup, well they were unconvinced to say the least so I had a go in
my lunch breaks.

Firstly you can do some clever tricks with ECM because of the way it
works. If you want to make long holes you can either use a long
metal tube as an electrode with insulated sides and control the
height using EDM type anti-short circuit methods (read complex). Or
better use a glass tube with a wire inside. The glass tube is the
insulator and also carries the salt water. You can pretty much let
the glass tube sit against the workpiece and just let the electrode
wire stop just short of the end of the tube. Add a simple gravity
feed and you have a hole driller.

This is what I did. First thing I learnt is that it would be better
to have a metal tube within the glass tube, it would put the
electrode closer to the edges of the glass tube and this should mean
that the progress of the glass tube through the material is speeded
up as usually the point where the glass tube hits the side of the
hole being machined is the point that machines the slowest as it is
furthest away from the electrode. This would also mean you could
pass the fluid down the metal tube and use the glass tube as a
sleeve. The second thing I learnt is that as the tube enters the
aluminium the tiny pump I am using really starts to struggle as it
pushes the fluid up the tiny gap.

My solution has been a new electrode design, I started with a EDM
sinker electrode, this is a brass tube but it has three smaller
brass tubes running up it, in my case I chose it as is is nice and
stiff and also straight. I then soldered a small copper washer to
the end of the tube ( actually I soldered a bit of sheet to it and
turned it in the lathe afterwards). This washer is larger than the
tube and defines the size of the hole, well nearly. To stop the
washer shorting with the workpiece a small plastic part was made,
this is basically just a little tube that extends past the washer to
prevent it bottoming out and also prevents shorts with the side
walls. It glues securely to the tube. The sides of the tube are
then laquered to prevent any ECM action on the sides of the hole as
it is cut.

The nice thing about this electrode design is that the insulating
tube can be very thin so that it does not impede the machining speed
too much, whats more because of the washer style electrode being
wider than the tube the flow is restricted over a much shorter
distance, this means that my windsreen washer pump can actually keep
working.

The rig at the moment is just a basic slide, the weight of the
carriage provides the gravity feed. The power supply is operating
from 20-60v upto about 5 amps. Everything gets a bit warm and this
caused problems when the plastic insulator came unglued, I have just
reglued it (with decent epoxy) and will try again tommorow. I had
probably made about 3mm progress in 5 min, not light speed perhaps
but pleasing.

A further idea I had is to return to glass tube and creat a slight
bell mouth in order to reduce flow restriction and also have a short
length of tube as the electrode inside the tube.

Sorry for the long and random post but this is exciting to me, as
you can ECM hardened materials as well as aluminium it is pretty
handy and very easy to do in theory. I suspect with a few brains
working on it something very capable could be knocked up for pennies.

Graham