On Sunday 31 January 2021 08:28:13 andy pugh wrote: > I have started to play around with EDM. The intention is to make an > EDM "grinder" to profile tool steel and possibly carbide cutting tools > using CNC-turned profiles. > > At the moment I am using a cobbled-together XY table with a couple of > STMBL drives. > One of the STMBL analogue inputs is used to measure the current > through the gap. (ie using the voltage across the resistor that a > capacitor is charged through). > Then a PID controller tries to achieve a target current by adjusting > the adaptive feed pin. > > I think that it shows promise, but only partly works. > > I am using a 50R resistor and a 1000uF capacitor. I suspect that this > is too high on both counts. > I am working at 40V and it seems that the tool welds to the work too > readily. > I too have been known to do some EDM, with 50 ohms and a 2 to 10 uf capacitor, paper, oil filled or mylar. Electrolytics have too high an ESR to make a good storage cap though, damping the discharge rate and destroying the sparks explosive effects, hence likely your welding problem.
You also need a liquid medium in the gap, preferably being circulated somehow in order to flush away the debris which will eventually get so contaminated it will short out the gap. I've cut the slots to make a coupling socket to fit the end of a small ball screw, by boring a piece of unhardened A2 to serve as a socket to insert the screw into. I also made a tapered thread at 50 tpi on the outside and used the same tapering technique on both the socket and matching nut's, doing all that machining before cutting the socket walls into petals so the nut, as its drawn up tight on the taper, is squeezing the petals and with a bit of green loctite, has been holding and driving the X screw in my 11x54 Sheldon for about 4 years now. This 3/4" diameter extension shaft locked onto the end of the 8mm ball screw and spinning in Torrington needles and roller washers is spun from the front end by a 3 phase, 2NM stepper servo drive mounted on a slider for belt tension, 20 teeth on the motor, 40 on this shaft projecting out thru the new apron plate bolted to the front face of the carriage, reusing the original hand drive crank for the drive bushing in the face of the carriage. I used a 2NM here because it was short enough to clear the bed. I use the 3NM version on the much bigger Z screw. Machine moves a lot like Casper the ghost now. I have also used EDM to clear broken taps, without damaging the threads the tap cut, quite a few times, but because that's a blind hole, the dielectric fluid doesn't clear the debris well so you do a lot of cleaning and adding new fluid. I've had about as good a result using distilled water (its a good insulator and cuts as clean as using K2, and its a heck of a lot cheaper. To cut the petals, I made an arbor and a 3" brass disk from 32 thou brass. And spun it at about 200 revs in my micromill which has since died. Connected one wire to the workpiece, the other was wrapped around the arbor so the currant pulse did not have to go the the spindle bearings as that will damage those bearings. with 2+ inches of distilled in a plastic tub, and at about a step a second to cut each of the 1" deep slots. I made 3 because the shaft was laying on the flat of one of the nuts installed on the shaft. EDM, unlike turning, leaves no burrs, beautiful cuts. About 2 hours of me standing there with a finger on the x jog key for all 3 slots. There is a preferred polarty, but I'd have to look it up again now. > I suspect that I would get different results if I controlled to the > gap voltage, rather than charging resistor voltage. And probably > better still with some sort of signal processing on that voltage. > > What combination of R, C and V would be typical for a servo-controlled > eroder? I imagine it might be different to a "doorbell" style one. > > Current set up: https://youtu.be/nxpmEFnmK-A Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) If we desire respect for the law, we must first make the law respectable. - Louis D. Brandeis Genes Web page <http://geneslinuxbox.net:6309/gene> _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
