Re: More on Encoders&Index

Hi Arne,

There are two approaches to EDM - the simple 'sinker' type uses a shaped
electrode in what might be described as a 'drill stand' type of machine
where the shaped form is driven down onto the work until a suitable
current passes and is then kept at that height above the workpiece by a
feedback system. The first simple one I made had a screw feed for the
'cutter' head driven by a stepper motor and, when the cutting electrode
touched the workpiece, it shorted out the 'direction' input to the
stepper control chip and reversed the motor. When the electrode came
clear of the workpiece again the motor started driving it down again -
very crude but it did work. More sophisticated machines control the
current accurately to get a predetermined rate of cut and to control the
surface finish. One practical example of the use of these machines was
at John Bedfords Engineering in Sheffield where they make spanners
amongst other tools. When a new spanner die (drop stamp tool) is made,
the first half dozen blanks pressed in it are copper. These are taken
away, carefully etched a little with acid to reduce their size by a
predetermined amount, and put away safely. When the dies start to wear
in use, one of these copper blanks is taken and used as the electrode on
a plunging EDM machine to cut the die deeper and make a new clean
impression.
The other type of EDM machine is the 'wire' EDM. In this a thin wire,
usually of tungsten, is used as the cutting electrode in a CNC
controlled machine - a bit like a hi-tech bandsaw. In fact, the machines
can look a bit like a bandsaw as the wire is usually fed from a large
spool at the top to another at the bottom and back again to equalise the
wear and keep the cut the same width throughout its length. This machine
can't just use current feedback directly to the power supply as the
speed at which the CNC part feeds the wire into the work has to be
adjusted to suit the speed at which the wire electrode is cutting. Where
the material being worked on is an irregular shape (i.e. not sheet) or
where it perhaps has 'tough spots', the speed must be adjusted
dynamically in response to the current feedback or the wire will be
broken. Again, the current has to be accurately controlled to achieve a
consistent and predictable width of cut (just like you need to know the
diameter of an end mill in a CNC mill). So, there has to be a direct
analog feedback path to the CNC control software to control the speed of
feed.
This technology is used extensively in industry now and an example of
its capabilities is in watchmaking where many of the parts, including
some gear wheels, are cut out by wire edm and need hardly any finishing.

Ian

"Arne Chr. Jorgensen" wrote:

> Jon, you would like to control the EDM current. I don't know what
> you mean with " sinker EDM ",
> - I have seen your "wire EDM". I have tried to understand this EDM
> thing, and I found something that
> looks like you could use a shaped tip. Say you used a circular end
> of a pipe, - could you "burn" a hole with
> this shape ?
>
> Okay, - I use the "wire EDM" in the following: You would like to
> control the height above the surface, - by using
> the current. If you are too high, then no current will flow. If you
> are too low, then you have a short. I would believe the
> distance you are talking about is in the micro area, - why would you
> control it with the EMC ?
> Well, I do understand that you would need to or want to follow some
> path, hence use the EMC, but I would think that the
> electrode would need a independent control ?
>
> The way I imagine it, would be something like this: use a power
> op-amp in a control loop with a voice coil actuator. You could read
> in the "position" with an A/D converter, - and run the Z axis, - but
> this would just move it when the value came above some threshold,
> the fine movements was up to the voice coil.
>
> Am I just stupid ?

Best wishes

Ian

--

Ian W. Wright LBHI
Sheffield Branch Chairman of the British Horological Institute.
Bandmaster and Euphonium player of the Hathersage Brass Band. UK.
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