induced magnetivity in parts
Okay, wire newby here. Our shop got a brand new Agie Charmilles Cut 20p earlier this year, in response to an infkux of orders for a particular copper part we have always had some of the work done by an EDM shop. We were in production of this part the day after it saw power, with a little training from the installer. So got this job, a simple alloy steel ring that gets split on one side 3/8" or so wide at the opening on the OD, 60 degree included angle on the walls, wider to the inside. Doable on a mill, but they'e backed up so the new EDM does the job. The drop becomes magnetic such that it does not drop out of the gap the wire cuts, breaks the wire every part upon exit to the outside of the ring. The drop just sticks there even though it is entirely cut. Also,whether or not related, the ring springs inward considerably when released after cutting, on a 4" ring, springs in .050 on diameter and .10+ at the split.
The rings and drops are strongly magnetic before the erosion process, not at all magnetic before.
Sounds like the springback that you're seeing is simply stress relief. If you were making these rings on the manuals before, you were probably boring the i.d. to size before splitting them. I'm guessing that they relieved then as well, but not to this extent. You were removing the bulk of the material from the center of the stock before splitting, so it was able to settle in somewhat before you split it. From your description, you're cutting the i.d. starting from the outside of the blank, going through the inside, then back to the outside again at the 3/8" gap dimension. Coming in through the outside like that, the stock wants to close up continually while cutting, then when you break through, it relieves all of the stress at once. I imagine some of your wire breaks (maybe most) are caused by the stock springing closed all at once & pinching the wire...might be why the slug won't drop, as well. A better method to would be to drill a wire start hole through an area of the inside finish diameter, then completely cut out the i.d. before cutting through the wall to the outside. Completely blank the inside first. It's still going to relieve, but it should be similar to what you were getting on the mill.
In regards to the magnetism, I've noticed that before while cutting with mine, but it's not very much or frequent. I'm not familiar with Agies, but are you using the same (or similar) cutting parameters as you were with the copper parts you were cutting? If so, this might be the source of the magnetism. I haven't used mine on copper, but I have seen the settings for copper and they are considerably different than the steel settings. If this is the case, contacting Agie about parameters for steel should help with that problem.
Thanks for the reply, toolmaker 35! You are completely correct as to the tool path. Initially, the part would spring inward upon breakthrough, being rather flimsily and conveniently clamped to the worktable and hanging far into the work envelope. My response to that was to support and clamp the part very close to the split such that it could not move upon breakthrough or any other time during the erosion process. then the drop, although completely severed from the remainder of the ring, and though physically able to just fall out of the gap, would purely by its magnetivity cling to the remainder of the ring. And cling there frustrating the machine's efforts to rethread and resume the job that it had already sucessfully completed. Then, whether or not related to being split by this process, upon the ring being released from the clamps very near the split, the ring vigorously springs inward.
My ultimate, I hope, solution to the resultant reduction of the ring diameter and split width was this: I hung the ring from a stud extending from the side of the tombstone on my horizontal machining center, and with an end mill holder that would also fit into the bore opposite of the stud, (below), put it in handwheel mode and stretched the ring open until where it sprung back to was in tolerance on the diameter and the split width, both thankfully somewhat open tolerances. I had to jack the gap open to about .75 for it to spring back to about .35, starting at about .25. We shall see if it creeps back over time... Also the magnetivity is very evident holding two split rings together near their splits, and does not exist holding them together 180 degrees from their splits.
And as regards...
your question about cut parameters, copper vs. steel, no the same parameters were not used. as a matter of fact, the copper parts did not cut at all well using copper parameters, this being copper based but not pure copper; an alloy called Toughmet. Steel parameters were what we were able to make work and tweak from the baseline technology, and on the steel rings that my question was about, the paramater selection was completely independent.