A strange fluid

[Billy MillsParticipant @billymills]

When you publish a circuit it should:-

a) work as stated without tinkering.

b)be reproducable with readily available components

c)be  readable immediatly

For a non-specialist readership, simplicity is also very important.

 

Having built a couple of sinker EDM’s for producing punch tooling, building a worthwhile machine is not difficult.  The electronics are very simple, for a sinker the aim is to keep the tool-workpiece gap constant as errosion continues. So you use the gap voltage as a too close/too far indication and drive the motor accordingly.  Capacitors are not a problem but the circuit produces HUGE amounts of electrical noise. If you don’t know -or are not  told- how to control  the noise then you might have all sorts of problems. The layout and wiring is very important. Flushing the active region is also very important for longer cuts.

 

A key mechanical element is to  minimise backlash. The working gap is around 0.1mm for some jobs so you need to position within -say- 0.01mm to get good gap stability and cutting rate. If there is too much backlash then you end up with “Hunting” as the gap opens and closes excesivly with only the odd spark instead of a steady stream.

 

It is absolutly silly to have 200Hz vibration. When working correctly you should have hundreds of thousands of sparks per second each cutting away at the workpiece in a controlled way. Opening up the gap at 200Hz rate totally messes up steady cutting.  ULTRASONIC drive is used – at around 200,000Hz by some to cause dielectric pumping- to get rid of the conductive particles out of the gap. There is loads of info for the interested on the web.

 

Regards,

Alan.

 

 

 

[Jeff DaymanParticipant @jeffdayman43397]

Versaboss/Hansrudolf,

 

No – I mean the Ben Fleming EDM How To Book. Link to it at Camden’s is below.

 

http://www.camdenmin.co.uk/engineering-practice-skills/the-edm-how-to-book-p-2112.html

[Howard JonesParticipant @howardjones35282]

can I just add to allans comments.

the design should be modular so that changing aspects of the design as availability of components changes doesnt mean going entirely through the bowl of spagetti to get it working again.

domestic use of the spark eroder is often quite different from industrial use. industrial savvy is a starting point for technique but isnt the whole banana.

 

some info from my rocket scientist friend.

because the spark craters the job, each time the electrode closes on the job the sparks occur in a different place on the workface. they occur on the nearest surface.

each spark displaces a piece of crud from the workface. as this builds up between the surfaces the eroding becomes less effective. if you dont clear this crud regularly you end up with tide marks along the side of the cut. every few pecks of the electrode it needs to be backed right out and the eroded surface flushed to remove all crud.

when the electrode contact is being made the working fluid needs to be still or it affects the evenness of the erosion.

an eroder really needs to be able to be manually traversed down to the job or up and away from it.

it needs to drive the stepper automatically during the pecking phase.

it needs to have a backout action at some settable interval.

it needs to be able to start and stop a coolant pump while backed out so that the workface can be flushed out.

it needs to be able to stop the fluid circulation while the eroding sequence is running.

ringing in the electrical circuit is the cause of electrode wear and probably the overloading of the chips. that’ll do for now.

[modeng2000Participant @modeng2000]

I’m not sure if this link to a spark eroder that appeared in ME July 1976 has been mentioned before so here it is:-

http://www.metalwebnews.com/manuals/spark-erosion.pdf

John

[Robert Miller 1Participant @robertmiller1]

Further to Alan Gray’s post at 08/10/2010 18:04:46:

Alan has raised an interesting point.  Increasingly the marriage between the mechanical and the electronic is becoming unavoidable.  Stepping motors and microprocessors are the gears of this century.

 

The electronics side of this marriage presents a barrier to construction by the amateur, and if that isn’t enough of a problem there is usually the issue of software as well.  If electronics is to be incorporated into an essentially mechanical project, there needs to be some requirements imposed on the presentation of the electronics in the article. 

 

 As electronics evolve away from parts with a 0.1″  lead pitch toward ever smaller packages with fine pitch 0.5 mm lead spacing or worse, no accessible leads at all such as ball grid array parts, it becomes difficult (but by no means impossible) and somewhat costly for the amateur to implement.  Sparkfun in the US have a number of tutorials on surface mount soldering on their website. ( I have no affiliation with Sparkfun.)

 

Sparkfun tutorials

At a minimum, the electronics in any project should be accompanied by computer readable copies of the following on the publisher’s website and released under some form of copyright copyleft.

1)  A clear and complete schematic diagram.

2)  A parts list consisting of currently available parts identifying the manufacturer and one (or preferably more) vendor(s).

3) A circuit board layout in file formats for the user and for the circuit board manufacturers.

4) A complete commented listing of any software required identifying the language, compiler used, and vendor thereof.

5) A binary file of the object code and instruction for loading it into the target microprocessor.

 

For the hardware, CADSoft’s Eagle provides schematic capture and circuit board layout and is available in a free version that is probably suitable for most hobby  projects. (I have no affiliation with CADSoft.) 

Eagle CAD

For software, things get a bit too complicated to suggest a vendor.  There are many.

 

 

 

 

 

[Billy MillsParticipant @billymills]

Howard

Think you have been slightly lead astray. The gap is small so that the fluid breaks down and allows current to flow into the workpiece. That detatches a small sphere of the job which is in the gap. If you don’t flush out the “swarf” it accumulates. You can’t have the flow on then off! the flow is continuous. When the spark passes through the dielectric it vaporises the fluid then the vapour channel collapses when the current stops. If you get it working nicely there are hundreds of  thousands of sparks per second.  Each spark is around a micro-second. It makes a nice fizzing sound.  There  is a fair bit of dielectric flow with some jobs however flushing removes the conductive material from the gap, it is more important with deeper jobs.

 

It is common to rough out with a high current then finish cut more slowly at a lower current. If the machine ( and the Operator) is up to it the finish can be  very good. Electrode wear is down to the material that the electrode is made of (- Copper , Copper -tungsten or graphite are common) and the type of pulsing used, Graphite can have much lower wear rates than metalic electrodes and machines beautifully. Don’t think ringing has anything to do with it. The physics happen when the channel ionises, not after. EDM is very inneficient in energy terms, the dielectric can get very hot however it does stuff that might otherwise be impossible.

Regards

Alan.

Edited By Alan Gray 1 on 11/10/2010 19:33:35

[Jeff DaymanParticipant @jeffdayman43397]

To Versaboss / Hansrudolf

 

Just checking to see if you received my private message sent a few days ago.

 

JD

[Jeff DaymanParticipant @jeffdayman43397]

To Versaboss / Hansrudolf

 

Just checking to see if you received my private message sent a few days ago.

 

JD

[VersabossParticipant @versaboss]

Jeff, thanks, the message is here, but I just saw it 2 days ago. I will need some time to digest it, and I don’t know when I will continue with my EDM. Anyway, the linear slides are what I also was thinking about..

Seems your system is almost an industrial one; well done that!

Greetings, Hansrudolf

[alan frostParticipant @alanfrost17805]

I hope I can settle the name of the mysterious fluid-opinions differ -a very few say ’tis “dielectic” , most say ’tis “dielectric” but oi say ’tis “dylectrick” (with apologies to Blackadder). Orwl agree– it don’t matter a lot.

[Author]

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