Electrical Discharge Machine Version 2 (edmv2)

[brino]

@rwm

Thanks for pointing out something I left out of my recent posts.

Here I had not mentioned the series resistor I used.
In the "version one" thread I mentioned the 10-ohm 50 watt resistor that is in series.
Like this:



This is also a great opportunity to point out:

1. Do this only at your own risk!
2. I am (and this site! are) not responsible if you hurt yourself, someone else or any property by doing this.
3. Electricity and fluids can be a bad combination.
4. This is from the internet -do your own research, understand what your doing, etc. etc.

Note: Since my workshop is a two-car garage attached to my house, I chose to use a non-flammable dielectric fluid. Distilled water is my dielectric rather than the typical kerosene, varsol, etc.
This build should also allow me to experiment with various fluids for wetting, cooling, etc

With all that said, I should actually be able to try this machine in auto mode this weekend.

-brino

!!!!EDIT!!!!!
Please see post #37 of this thread for an updated drawing!
Above I forgot the discharge or spark capacitor!

 

[rwm]

OK. That makes sense. So at max it pulls about 14 Amps? Must be a big transformer. Can't wait to see it run! I have some questions:
I see you have a constant power supply. Does the EDM power need to cycle on and off to work? Does it make a spark when operating? Is this process kind of like super-electrolysis? Or is there some other destructive mechanism as work?
Robert

 

[brino]

Today was full of software frustration!!

After having the USB-serial port work okay up 'til know I ran into a blue screen of death (BSOD) whenever I let the arduino monitor window run for very long. This put a big wrinkle into my debug plans.

The USB-to-serial converter I was using (link above somewhere) uses a "Prolific" device so I had installed version 1.12 of the prolific driver, initially it worked okay....but that was only for short periods. When I started seeing the repeated BSODs I went searching for a solution and found people recommending an older version 1.10 driver:
http://superuser.com/questions/680737/bluescreen-when-putty-reads-from-serial-port

I could not find the older driver, so I tried a newer one version 1.14. That one showed BSODs as well. CRAP!

I never did find an old version of the Prolific driver online, but luckily stumbled across a copy I used years ago.
I found version 1.1.0 from 2009 (the one I expect they meant in the link above).
I installed it and I no longer see the BSODs. I have attached it here.

However, now after a short time of using the monitor window, Win7 sees the string of data on the serial port and assumes it's some kind of mouse and installs a driver for a "Microsoft Serial Ballpoint Mouse". This seems to disable my real mouse and really screw with the system. Sigh.

After searching, I tried a number of things from here:
http://answers.microsoft.com/en-us/...e/e0e03b9b-e9ae-4645-8b3c-5754f06ec3b5?auth=1

So far the only work-around is to let it start the "Ballpoint" install and then in Win7 Device Manager disable that device.

What a pain!

However, I did eventually have some progress. I will try to post it later.....

-brino

 

[brino]

EDM_V2 is alive!

I had picked up a cheap, used drill-press vise for this project. I drilled and tapped a hole for a stainless bolt to connect the positive lead to the vise.
So far I am just re-using the original electrode from version 1, it is just a piece of 1/4" copper tube.
I had a 1/4" thick, by 1" wide by 3" long piece of mild steel as my first test piece.
I started with clean distilled water as the dielectric fluid.

Here are a few short video clips showing the machine in action.
You can hear the stepper motor, it is sort of a tapping noise.

movies1-3 are just different views of it running.
movie4 is a close-up
movie 5 is a view of the "spindle", I added some red and black sharpie lines so you can tell which way it's moving.

Here the machine is running in auto mode. One step in my code is equal to 10 stepper motor steps. I am using a threshold voltage of 50V.....if the discharge voltage is above 50V the arduino drives the head down, if it is below 50V the head is driven up.

-brino

 

[brino]

Here's the dimple left after a few hours of running

Just out of the tank:



and after wire brushing:


I measured it at about 50 thou. deep.

-brino

I have added thumbnail views below.

 

[brino]

It occurred to me that this machine was not working very fast.

I have a finite-state machine running on the arduino that samples the three switch, debounces them to eliminate mechanical noise, eventually gets to the "Auto" state, measures the discharge voltage, outputs some debug info on the serial port (state transitions and voltage readings) and then finally drives the head accordingly.

I decided to make a new arduino sketch with just the bare minimum code.
It simply measures the voltage and drives the head; no switch reads, no debug info, no other state changes.
I also made the step size equal to one motor step.

That sure changed things......see movie6 below.

-brino

 

[brino]

I still have a number of system improvements to make:

1. I need to make my temporary machine stand much more sturdy. When the motor is moving fast it sets up a little wobble in the machine. I expected this, I have seen twisting in the upright spine. Currently I believe the cut is wide and removing more metal than necessary. Perhaps I need to bite the bullet and start construction of the final, adjustable stand.
   
2. I need some kind of filter and pump system. I am amazed at the amount of reddish, magnetic debris left in the water. I believe it would also help the cutting progress to wash this debris from the cutting area. Currently I am flushing it using a syringe every once in a while.

In the code I already have ideas about:

1. re-writing the state machine such that the "auto" state is a single tight loop.
2. the switch debounce time could likely be shortened significantly
   
3. I still want to add limit switches to disable the motor at end of stroke.

I still have many variables to look at:

1. I can adjust the number of motor steps per machine step taken in auto mode
2. I can adjust the motor speed (separately)
   
3. I can adjust the voltage threshold
4. I can play with various high-voltage power supply options, including adding more discharge capacitance.

That's all for tonight.

-brino

 

[John Hasler]

Use interrupts to handle the switches.

 

[brino]

@rwm , sorry Robert I missed your questions......'til now......

Does the EDM power need to cycle on and off to work?

The EDM high-voltage supply does not need to cycle on/off. Basically when the electrode gets close enough to the work that the arc jumps the discharge capacitor dumps it's entire load and erodes a little bit off the work piece. There is a high-wattage series that drops the entire voltage if the electrode contacts the work piece. Either way, the power supply will need some time to recover the output voltage for the next spark.

Does it make a spark when operating?

Yes! These things are also called "spark erosion machines". All the action takes place under a dielectric fluid....basically an insulating fluid but with a polar molecule. Many systems use parafin. I am using distilled water, since parafin is flammable. (and my workshop is attached to my house!)

Is this process kind of like super-electrolysis? Or is there some other destructive mechanism as work?

It is really just the jump of electrons from the negatively charged electrode to the positively charged work piece that erodes off the "high-spot", then the next high-spot, etc.

-brino

 

[brino]

Use interrupts to handle the switches.

Hi John,
I believe from other posts that you have done many microcontroller projects.
But your comment confuses me.

An interrupt is for something that needs immediate service.
I WANT to debounce the switches as mechanical contacts are notoriously "noisy", there could be dozens of contact bounce events per switch operation.....that would drive an interrupt handler crazy!

-brino