Construction of an EDM was a four part series in MEW. Issues 168 to 172. I have only read it in passing, and never constructed one…. yet! Just have to finish a couple of other projects first!

Plus one for not using the vibrating electrodes. I made my living in a toolroom for many years operating EDM machines. Some had the vibration setting but was a gimmick that upset the stability. Getting an EDM machine working well is all about balance of servo/flushing of debri and the correct pulse on pulse off times. I suspect a home built machine will just work on relaxation principle and not pulse?

I am building Ben Fleming's second EDM machine. It is described in his book "Build a Pulse EDM machine". The book is not cheap and the electronics is even more expensive. I choose the pulse design because sensible capacitors for the resistor-capacitor set up used in his first book are hard to find.

Ben's second machine uses a voltage of around 100vdc which is switched by a MOSFET (a bank of 6 in his case) at frequencies up to about 20kHz. There is also provision to lift the electrode every ten or so seconds to flush out the debris. Everything is adjustable in the design, the front panel of the electronics box is very intimidating.

I can provide photographs etc if anyone is interested.

As requested, 3 photographs of the electronics box. The names of the controls are not stickers but an annotated photograph as a reminder to me (dither is the period of withdrawal of the electrode to allow escape of the debris).

Ben suggested using an old PC box. I had one but found its construction so complex that it became easier to build my own. The box is possibly too small but the use of 12" long metal stock allowed a quick and easy construction.

The circuitry is not complex. The power is provided by 80vAC through a full bridge rectifier. The maximum power is 650VA and is controlled by a bank of forced air cooled resistors. The switching of the power is by a MOSFET controlled by a 555 timer. Another 555 timer is used for the dither period. The electrode position is determined by comparing the discharge voltage with a required operating voltage and a minimum voltage required to prevent the electrode hitting the work. The electrode position is driven by a simple servo motor. I know there are other ways of controlling the discharge and position of the electrode but as an electronics simpleton I am happy with most of Ben's circuitry.

I have to say that the project is very much "work in progress". I have just started on the electrode head.

Yes, very good JA. Nicely done, thanks for sharing.

Dave

There is also some information on Ron's Model Engineering web site:

**LINK**

If you choose 'Resources' on the left hand side followed by 'How to?' You will see a list of topics. 'Remove a Broken Tap' takes you to the pages on 'Making and Using a Basic EDM'

Best regards

Roger

Much as I'd like to build something as sophisticated (and impressive) as the EDM machine JA has built – I'm afraid it's never going to happen at this stage in life. However – whatever their limitations – there are much simpler designs around that I might consider building – especially for the removal of broken taps and making small 'oles-in-ard' things…

I saw this simple EDM set-up at Guildford MES in 2016 – and had a chat to the builder (sorry I did take his details but cannot find them att). As I understand it, an electromagnet pulls the anvil up until a capacitor fully charges, where upon it falls back and touches the work, thereby discharging the capacitor – and causing the required spark – the cycle then repeating ad-infinitum. A very simple concept and no complex electronics required.

I'm not sure if this is the same as one of the other EDM systems already mentioned and there may well be all sorts of limitations associated with this simplicity – but it would certainly cover most of my (admittedly) limited needs…and the work samples shown were quite impressive…

Regards,

IanT

PS Sorry – image started off right way up but seems to have done a 90 degree rotation on up-load…

Hi, Bernard – sent you a PM.

There was(?) a very simple version that used 2 light bulbs & a bridge rectifier published in an old Popular Science book (Google on the web), March 1968. Intended, mainly, I believe, as a proof of concept although it could do real work. That said, this thing was *dangerous* with line voltages (110-120 volts or dc equivalents) "all over the place." I guess if I were desperate to remove a broken tap – where all else failed – I might give it a go. I have not reached that point – yet (not to say that I have had my share of broken taps). Reprints float around the web but I have no direct links.

The US magazines – Home Shop Machinist/Machinist's Workshop – (uncertain which one) had some updates to the original Popular Science article.

You do not need to rotate or oscillate the electrode for successful EDM. In fact doing so will reduce accuracy of cut, because the motion will need clearance to work, and that clearance will be multiplied by the distance from electrode tip to rotate / oscillate mechanism , cutting the hole much larger than the electrode.

An accurate linear slide to hold the electrode, and a DC gearhead motor or servo to operate a leadscrew to drive the slide up and down are all that is needed, if you use the Ben Fleming design. If you are going to spend some cash, get a decent linear slide and forget about rotating the electrode with the apparatus in your picture.

I built a Ben Fleming Mk.1 conventional EDM (sink) machine in 2009-2011 with such a slide (from Deltron in the USA) and some other variations and it has worked great since. If I make copper electrodes .003" smaller than the desired finished hole dimension the holes (whatever shape) come out VERY close to dead nuts on size.