Question: My Elox Astra model A25S power supply is giving us a rougher than normal surface finish. We have determined that the ON-Time switches are at fault. We can get what we want by lowering the ON-Time, but something is definitely wrong. How can we debug this?
Answer: There could be a very simple explanation. The Elox model A25S power supply includes an additional multiplier switch in the center of the thumbwheel assembly. The intention of this switch was to allow ON/OFF settings of greater than 999 µs when using metallic electrodes.
In the U.S., many companies use only graphite and it is not uncommon to ignore the multiplier switch. Some EDM operators may not even know what the multiplier switch does.
It is quite possible that the operator inadvertently changed the multiplier to X2. This would cause the ON and OFF time to essentially double. A programmed value of 100 would behave as 200. This would also explain why you are able to compensate by changing the ON/OFF settings.
If the multiplier switch is correctly set at X1, we need to look a little deeper into the cause. The thumbwheel switch connects to the Elox MVB board through a multi-conductor ribbon cable.
Attach the scope probe tip to TP305 and clip the scope ground clip to the “GND” test point on the MVB board. With the scope connected, turn the power on and look at the waveform. There should be a TTL (5V) logic-high. TP305 will show an inverse signal. For example, if the thumbwheel was set to 100-ON and 30-OFF, you would see the 100-ON as logic low and the 35-OFF as logic high.
Try the programmed value that is not behaving correctly. If the ON/OFF time is not correctly found at TP305, then a malfunction of the MVB assembly has been confirmed. The MVB board, thumbwheel switch assembly and ribbon cable should be serviced as a set.
If the scope reveals that the ON/OFF time is being correctly developed, then the problem likely is caused by peak current relays on the output module that are stuck on. Degraded surface finish can be caused by several factors. ON-Time and peak current are the two most prevalent. If a peak current relay is stuck on, it can cause a poor surface finish. Lowering the ON-Time will compensate and improve the finish. It is easy to misdiagnose an ON-Time problem when the fault lies with the peak current.