I have been working on an Electro-Arc tap burner in the machine shop. The power feed mechanism only works partially, as the feed-rate potentiometer seems to have no effect on the actual speed of the motor. We confirmed the potentiometer to be in working order so I believe there is a fault somewhere on this PCB. Not knowing any better, I would label this circuit as a "variable relay". Essentially using a low current potentiometer to control an much higher current power circuit. What would you check first on this board?

 

I would first suspect the relay, also check the voltage out of the transformer, it looks a fairly simple board to reverse-engineer and draw up the schematic for it.
I rebuilt an Electro-arc tap buster, many moons ago and no longer have the prints.
IIRC it used a simple 50hz/60hz vibrator for the forward retract of the electrode.

 

The power feed mechanism only works partially, as the feed-rate potentiometer seems to have no effect on the actual speed of the motor.

Let me ask a question about that statement. Does it actually feed and burn out the tap, even if not as fast as you think it should? Things like this are slow working, they will only move as fast as the tap or what ever is getting burn ed away. Trying to "speed" it up will only short it out and make it work even slower.

Also what type of fluid are you using in the hole and what type of electrode?

 

Let me ask a question about that statement. Does it actually feed and burn out the tap, even if not as fast as you think it should? Things like this are slow working, they will only move as fast as the tap or what ever is getting burn ed away. Trying to "speed" it up will only short it out and make it work even slower.

Also what type of fluid are you using in the hole and what type of electrode?

I have been testing the feed mechanism with no arc. I just want to be able to vary the feed rate but from what we have seen, the potentiometer has no effect on voltage to the motor. We are using a molybdenum electrode and the electro arc fluid that resembles coffee creamer.

 

It has been quite a while, but if IRC the head advance is done in a semi-automatic method dependent on how much disintegration has taken place.

 

I have been testing the feed mechanism with no arc. I just want to be able to vary the feed rate but from what we have seen, the potentiometer has no effect on voltage to the motor. We are using a molybdenum electrode and the electro arc fluid that resembles coffee creamer.

Without a sparking taking place I don't think you will get movement. Usually water or something very thin is the fluid. I also never heard of using Moly as the electrode, carbon or graphite is the normal thing.
I did EDM work, the big brother to what your doing for many years.

 

Without a sparking taking place I don't think you will get movement. Usually water or something very thin is the fluid. I also never heard of using Moly as the electrode, carbon or graphite is the normal thing.
I did EDM work, the big brother to what your doing for many years.

The quill feed moves, we just are not able to change the feed rate. I don't see how the spark could effect the feed rate adjustment but then again I do not understand this overengineered power feed mechanism that has also been micky-moused into a partially working state.

 

So I learned that the cable going from the bottom of the machine to the power feed box is not just a power supply cable. Instead there are eight small gage colored wires carrying signals for god knows what. I tried my best to follow the traces for all the wires but they all eventually turn red and go to all different places. Before I waste a whole day tearing apart this system I thought I would post this update and see if anyone here knows better than I do and can solve this puzzle. 10 points for the first person who gets it!

I posted some photos with this message along with some notes I took about what wires go where. I figured some out (like the amp meter) but others are a mystery.

And in the likely chance that this mess is just not worth the hassle, can I just use a full bridge rectifier and a potentiometer on the DC output to get the variable feed rate back? I cannot begin to understand why it would've ever been done any other way. I'll attach a picture of my proposed circuit. Motor draws 0.2A.

 

there are two broken traces near top left corner of the PCB.
reverse engineering this board should be very simple (if needed) since it is only single sided TH design with very few parts...