Let me start with a small disclaimer. I'm not an EDM tech, don't really know all that much about EDM machines, but do have a background in circuit troubleshooting. So bear with me if I don't get all the terminology correct.

I recently inherited support duties for our EDM machines. The audible alarm and stopping mechanism quit working about a week ago on one of the machines. When you jog the ram down and continuity is made between the ram and the base, either by way of electrode touching your material or just shorting the ram and base, the alarm should sound and the ram should retract. The machine is doing neither. Once the ram starts going down in jog mode it doesn't stop on its own.

I've checked the alarm switch, traced the circuitry on the control board back to the power supply cabinet and everything seems ok. There is continuity for both ram and base wires to the power supply cabinet as well. I'd continue to list things I've checked but I could be here all day doing so.

If anyone has any ideas please let me know. At this point I'm willing to try just about anything to keep the remaining portion of my rearend intact. My boss has chewed most of it off already however...

Thanks.

With an ohmmeter put one lead on the base and then put the ram in contact. Start tracing the wire back from the place it attaches on the head. You should get a short (0 ohms) as you go back. Trace it all the way back to the PCB. You can do this with the power off. Most likely the signal is isolated via an opto isolator on the PCB. It will (probably) be a small 6 pin DIP package. If it is Pin 1 and 2 are the inputs. With power on you should see 0 volts (not ohms) at pin of the pins and about 1.2 volts at the other.

Do you get any voltage between the base and the ram with the two not touching? Most circuits will have some level of DC to trigger an input. It will be something like +5 or +12. If it is really simple and uses relays it will have the coil voltage on the open circuit.

If you have a schematic of the circuit you can send it to me and I will tell you what to test and what is wrong. If you don't have a schematic then better get ready for more flak from the boss.

I've traced wires physically and on the schematic to a LM311, which seems to be operating normally. I can manually trigger an alarm by shorting 2 and 3 on the LM311 (the inputs). The normal voltage to the inputs with power on (jog mode) is 6 volts to the (-) side and 12 volts to the (+). I'm not sure if that's what's supposed to be there or not.

I do have a schematic for the machine but it's not the exact model. They're ok for the most part however. How would you like me to send them to you? I can edit and make notes on them if you want to take a look at what I've done so far.


Thanks a bunch. I appreciate you helping with this.

An LM 311 is a simple op amp. It sounds like its setup as a 50% comparator. So if you can trigger the alarm at the op amp inputs it has to be something between the sense wires and the input to the op amp. If you can get the schematic into a scanned image B & W with enough resolution that I will be able to read it and see pin numbers then I will be glad to tell you what to look for

I had to break the schematic down into 4 files. Let me know if these will work for you.

you need to turn the machine on and check for some small dc voltage between the head and table.....could be anywhere from 5 to 15 vdc

this is the "edge find" voltage, some people call it "touch off" etc, but the machine senses the loss of this voltage and halts the z movement.....

generally it is always on when not in machining, for safety reasons....

I've progressed a bit on this issue since my last reply although the machine is still having the problem. There is no voltage between head and table. I spent a good portion of Sunday tracing this actually. We're missing some of the blueprints for various boards in the control cabinet as well as the power supply cabinet so it's been rough going.

If you have any suggestions on things to check that'd cause this (lack of DC voltage between head and table) please let me know.

Hi,

Maybe I can help you out. I used to work for Eltee Pulsitron from 1981 to 1986. I was the head of the test floor and Sr. Electronic Tech there for 5 years. At the time that LT closed there doors, I went out on my own and started repairing these machines here & there. I haven't done any in recent years but I still have some parts and components. I remember alot of the machine & circuits. I don't have any schematics but I can make one from a board. If you can make a copy of the schematics, maybe we can do some business. Please Reply to S7KB2JXM@Juno.com