# Electric discharge machine - spark eroder - design



## picclock (Jun 14, 2012)

I've been clumsy enough to break another M3 tap and need to remove its remains. Most of the spark eroders I've seen use a wire which is fed into the work and a current limit, usually light bulbs. This leads to continual wire adjustment. However on you tube I saw a spark eroder using a door bell coil burning a hole through a razor blade. The nice thing was that the wire automatically advanced and as soon as contact was made a coil in series with the electrode pulled the electrode away, generating a continuous spark with no gap adjustment needed.

I thought I might lash up a solenoid or old relay and see if it would be viable. 
I plan on using a piece of piano wire for the electrode. Not sure if it would need oil or liquid immersion as the erosion is done by the very high voltage spark generated from the coil.

Has anyone else used this relatively crude technique for tap removal ?

Will post pictures if it works/doesn't work/explodes ;D

Best Regards

picclock

Link to youtube video
http://www.youtube.com/watch?feature=player_embedded&v=uUN4_-xp1Wc#!


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## Herbiev (Jun 14, 2012)

I dont think oil would work. They normally use deionized water (cannot conduct electricity) for the liquid


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## jonesie (Jun 14, 2012)

on an edm sinker the oil is used. it is used as the diaelectric and to flush the particles away to prevent a dc arc. water is used on wire feed edms. i have run a sinker for years building injection molds. will be watching to see how it works. jonesie


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## crankshafter (Jun 14, 2012)

Hi picclock.
In Model Engine Builder Magazine, issue #26 there is an article on how to build a simple EDM.
One day, when time comes ;D I will build one myself.
Best....
CS


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## Lew Hartswick (Jun 14, 2012)

crankshafter  said:
			
		

> Hi picclock.
> In Model Engine Builder Magazine, issue #26 there is an article on how to build a simple EDM.
> One day, when time comes ;D I will build one myself.
> Best....
> CS


Want to bet on that?  I retired 15 years ago and drug home a couple power supplies
to build an EDM and guess what, they are still sitting in the storage shed. Probably 
still be there when I'm gone. 
  ...lew...


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## Don1966 (Jun 14, 2012)

Yup! On that, I bought the book and the motor last year and have done nothing yet.

Don


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## picclock (Jun 17, 2012)

I built it and it seems to work. I found an old junked solenoid in the shed and stripped it down removing former, and milled off the encapsulated mains voltage coil. Rewound with wire I had lying around, just had enough to fill the bobbin.


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## picclock (Jun 17, 2012)

Made up an electrode holder, 1/2" brass, M6 tapped, piece of M6 studding (threaded rod) and M6 tapped nylon insulator. Electrode is currently some brazing wire, held in place by 2 M3 grub screws. Connection to electrode by crimped spade terminal.






Moving post support drilled and reamed 6mm for a free fit. Completed unit held in an old retort stand. Note: Never mount in an old retort stand - you leave it for 20 mins and the holder moves and you cut a slot - ask me how I know :-[






100W 12v lamp fitted in series with the supply to act as a current limit.






Overall assembly


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## picclock (Jun 17, 2012)

Does it work - very well I think. 






test with paraffin seemed to work best so far. Only problem is solenoid coil gets quite hot if pushed. Erosion rate in BDMS seems quite good, around 0.3mm per 5 mins. Will post video of operation when they have finished uploading. 

Almost seems to good to be true. I reckon it could be done with an old car starter solenoid if you have one.

Best Regards

picclock


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## picclock (Jun 17, 2012)

Video Links

http://player.vimeo.com/video/44200887

With paraffin bath :-

http://player.vimeo.com/video/44200886


Best Regards

picclock


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## Herbiev (Jun 17, 2012)

Thanks for sharing Picclock. I bet it will be years before you break a tap now ;D


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## Blogwitch (Jun 18, 2012)

Very nice indeed, and it works great!!

I uploaded a build article a couple of months back I think, basically the same mechanics, just a little more refined on the wiggly amp stuff.

http://www.homemodelenginemachinist.com/index.php?action=tpmod;dl=item381

Mine is on the drawing board to be made, as my friend who gets any eroding done for me retires soon.

They are so cheap and easy to make, every shop should have one gathering dust, even if it only used to 'drill' square or triangular holes now and again.


John


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## picclock (Jun 18, 2012)

@ Herbiev
Built it to remove M3 broken tap which I will do today. 

Operating current best set by adjusting the insertion distance of the core in the solenoid. The further in the less current. Best operation on 12V high setting.

After 5 mins on BDMS. White dot at bottom of hole is where I scraped out the crud with a toothpick. Hole is around 1.8mm diameter, depth at a guess around 0.5 mm.







Best Regards

picclock


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