Electrical Discharge Machine Version 2 (edmv2)

brino

Confirmed Tool User
H-M Lifetime Diamond Member
Joined
Jan 2, 2014
Messages
8,847
Hi All!

My first attempt at a home-built Electrical-Discharge Machine was posted here:
http://www.hobby-machinist.com/threads/my-edm-project.41481/

It obviously did work, but was really just meant as a "proof of concept".

The short comings:
1) it had no vertical feed. Whenever the vibrating head had "cut" as deep as it could, I had to adjust the two pieces of angle iron that were clamped together to move the head down. Of course in doing so the entire thing also moved sideways and changed angles. So not only was it a two person job, it took multiple attempts to get the spindle realigned with the first hole.
2) the entire machine was energized to the discharge voltage
3) there was no control for angling the head to get into more awkward locations.

I considered just adding a lead-screw feed mechanism to the original design, but that would mean that I still need to "babysit" the machine and turn a feed handle when it stopped cutting. I would rather have one I could ignore for a while while it worked away.

I am currently working on EDMv2, it should overcome the problems above by:
1) using a stepper motor and lead screw arrangement for vertical feed.
2) having an insulator in the head so that only the chuck and tool are energized
3) using some extruded aluminum supports with t-slots to give better adjustment

All the stepper motor designs I found in my initial research (see part one linked above) used large single-purpose driver boards. I did not want to get into building a large circuit either on a bin-board or making my own PCB. Instead, I decided to use a small arduino board and pre-built motor driver board both from ebay. In the arduino code it is easy to instantiate a stepper motor and send up/down step commands.

link to arduino "pro-mini":
http://www.ebay.ca/itm/130977194597?_trksid=p2060353.m2749.l2649&ssPageName=STRK:MEBIDX:IT

a little USB programming dongle for it:
http://www.ebay.ca/itm/141150349286?_trksid=p2060353.m2749.l2649&ssPageName=STRK:MEBIDX:IT

link to stepper motor driver:
http://www.ebay.ca/itm/281551893660?_trksid=p2060353.m2749.l2649&ssPageName=STRK:MEBIDX:IT

The machine support arm will be built such that gross positioning is done manually with handles to lock parts in the t-slots.

When using the machine to remove a broken drill bit or tap only around 1 inch of total vertical feed is needed. Making a hex recess for an Allan key would need even less, maybe only 1/4 or 3/8 inch. Therefore an automatic feed range of only a few inches is required.

Using a stepper motor with 200 steps per revolution means I can have rotational steps as small as 1.8 deg.
Combine that with a lead-screw and we're talking about minuscule feed steps.
For a 1/2-13 lead screw one full rotation is 0.077 linear inch, so one motor step is 0.000382 inch!
For a 5/16-18 lead screw one full rotation is 0.056 linear inch, so one motor step is 0.000278 inch!

However, since the lead-screw mechanism is simply a piece of all-thread rod and a nut from the bin, I am under no delusions that the amount of slop would allow such tiny steps. In fact since the feed may be constantly switching direction(see below), backlash will likely be a major limit to minimum step size.

In the code for the arduino I will easily be able to control the minimum step size. It may turn out that I use say 10 stepper motor steps as the minimum step size for my vertical feed.

For controls I am planning to have one switch that chooses between automatic and manual mode.

In auto mode the processor will measure the discharge voltage and extend the head one step if the voltage is above a threshold. If the discharge voltage is below the threshold, then the head will be withdrawn one step from the work. The discharge voltage will be reduced thru a resistor divider, limited by some diode/zener circuit, and then finally capacitor filtered before being run into an arduino analog input for sensing.

In manual mode, another "centre-off" toggle switch will allow me to extend or retract the head for positioning the tool and work.

In either mode limit switches will set the end of travel in both directions. When these microswitches are operated, further movement in that direction will be disabled. If I have spare digital output pins of the arduino, I might hang an LED near each switch to indicate when it has been tripped.

All switches will be "debounced" in the arduino code to limit motor chatter due to mechanical contact bounce.

The power supply for the discharge voltage will initially be the same one I used before. However, having automatic vertical feed will allow me to run better A/B comparisons to try different voltages, discharge capacitors and even reverse polarity.

For the dielectric fluid I see no reason to move away from the distilled water that worked well in the first installment. I may however, add a filter and recirculating pump as I get further down the road.

Some parts are already complete: the insulator, a plastic slider block for the spindle, motor mount and lead screw. I have basically just started writing code for the arduino, and have a bunch of assembly and debug ahead.

-brino
 
The stepper motor I am using was from the junk bin, likely removed from a printer, scanner, or photocopier.

It is marked:
Step-Syn stepping motor
type 103-718-0340
DC 4.2V 1.5A
1.8 deg. per step
Sanyo Denki Co.
Made in Japan

The motor originally had the gold-coloured mount you see below.
That mount did not suit my purpose, so I sketched, cut and bent a new one and welded the corners.
motor_mount1.jpg motor_mount2.jpg

more to come......
-brino
 
I decided to re-use the 1/4 inch spindle and drill chuck from my version 1 machine, but since I did not want the entire machine at the discharge voltage, I needed to put an insulator in the spindle.
I found some clear plastic rod kicking around and made this:

insulator1.jpg insulator_and_chuck.jpg insulator_and_chuck2.jpg

-brino
 
In auto mode the processor will measure the discharge voltage and extend the head one step if the voltage is above a threshold. If the discharge voltage is below the threshold, then the head will be withdrawn one step from the work. The discharge voltage will be reduced thru a resistor divider, limited by some diode/zener circuit, and then finally capacitor filtered before being run into an arduino analog input for sensing.

I love it! :encourage:
 
The lead screw is a 4" piece of 5/16-18 all thread I found in my bolt bin.
The black tube was (I think) from one of those folding camp chairs.

I found some steel bar stock and made a shouldered bushing; the big end will go on the motor shaft with a set-screw, the small end is a tight fit into the tube.
From the same stock I also made a 5/16-18 nut with a plain cylindrical outside. It is also a tight fit into the other end of the tube.

The motor will spin the tube, the tube will drive the nut, extending or retracting the screw.

Both friction fits will get a spot of thread-lock on final assembly.
The hex nut will be used as a jam nut to lock the lead screw to the next part of the spindle.

20160818_222858.jpg 20160818_222937.jpg

-brino
 
brino, it sounds like an interesting project. I am looking forward to seeing the completed project and a report on its use.:encourage:
 
In the first version I made two brass bushings to hold the "spindle"*. Due to the welded frame of that version I ended up with some misalignment between the two bushings causing the spindle to bind if I tightly mounted both bushings. At that time, I simply let one bushing "float" to test the EDM concept. The spindle was guided by the one bushing and the solenoid. (two points enough to define a straight line!)

In V2 I decided that to avoid some misalignment issues I would do two things:
  1. make a single long bearing block with a drilled and reamed hole for the spindle, and
  2. make all the mounts as adjustable as I could.
The spindle was reused from the first version. It is just a 1/4-inch steel rod. It was cut to insert the clear insulator rod to isolate the rest of the machine from the discharge voltage at the head. The flat that was used for the solenoid on V1 is now used for a set screw to lock on a collar. This collar is threaded in the other end for the lead-screw.
spindle1.jpg spindle2.jpg

I used a block of white plastic for the bearing/slider block. Note that I do not need the insulating plastic here, it was just the material I found that looked suitable. The slider block is mounted on a aluminum angle with slots in two directions for adjustability.

Milling the mount slots:
slider_bracket.jpg

Here's how the plastic bearing/slider block mounts:
slider_block1.jpg slider_block2.jpg
slider_block3.jpg slider_block4.jpg

The long slot on the "front" of the bearing block will be used for a small (#4-40) pin to stop the spindle from rotating.
I have yet to drill and tap that hole in the spindle.

-brino

*-Really the word "spindle" is wrong here as it will not "spin". Instead it will have an oscillatory motion.
 
For the sake of maximum adjustability, I decided to use some aluminum t-slot extrusions that I found at my local used tool store.

These will be used for the EDM motor and head mounting:
20160818_214929.jpg


So here is everything except the lead-screw test fit on the extrusion:
top_half.jpg bottom_half.jpg
entire.jpg

The lead-screw will go in the gap between the motor and the spindle. It's a good thing that gap is adjustable!

I will try to get some shots with the lead screw installed uploaded soon.....and perhaps some video of the head moving in manual mode.

-brino
 
Sorry for the delay, I got distracted by a couple auto repairs; exhaust on my wife's vehicle and coolant leak on mine. Now after a replacement coupler & clamps and a new radiator I had time to get back to this.
Also, up 'til now the programming was being done at the computer desk in the basement office space. Now with getting ready for higher voltage and a bucket of water I had to make room in the shop.
Currently it's on my welding bench, but I need to move it over to the bench where the shop PC is for program debug. So I need to move my sons project over to the welding bench. I need a bigger shop!

Progress since last update:
-I got the spindle drilled and tapped for the anti-rotation pin that rides in the slot of the white plastic slider block
(it is tapped #4-40 and of course I broke a tap on the first hole....if only I had some kinda machine to deal with that.........)
-I threw together a temporary stand, welded out of an old bed frame (the final frame will be manually adjustable)
-I used another piece of aluminum angle for a temporary switch mount (the top switch is auto/manual, the bottom switch is manual up/down)


So here's the latest:
top.jpg
middle.jpg
bottom.jpg

more to come....

-brino
 
I am currently using a two-output power supply. Each half running at 5V, but I wanted to separate any electrical noise of the motor pulses from the arduino supply.
power_supply.jpg

okay...it is still a bit of a rats nest......the arduino pro mini is plugged into a bin board for testing. That is mounted to a piece of mechano from an earlier robot project. I made up a couple supports that are repositionable within the t-track slots to mount the mechano and the motor driver:

arduino.jpg


full1.jpg

full2.jpg

full3.jpg

-brino
 
Back
Top