1. #1
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 19 Hours Ago Has been a member for 4-5 years. Has a total post count of 838. Received thanks 123 times, giving thanks to others 5 times.
    This thread is under the "Other Machines..." heading as it doesn't have its own category - yet?

    I know that some forum members will be aware of the Society of Model and Experimental Engineers (SMEE), a old-established society with mainly UK but also international membership. A small group of members of the SMEE Digital Group have been collaborating on a wire EDM(*) machine for the last couple of years. For those who haven't come across wire EDM, think of a hot-wire cutter as used for foam, but this time using a spark between the wire and the (metal) workpiece. Enough sparks for a long enough time, and the workpiece gets cut away, leaving a cut around 0.1mm wider than the wire used for "cutting" - and this machine uses 0.25mm wire, so that's a pretty fine cut. Move the wire under CNC control, and you can cut intricate and accurate shapes out of difficult-to-cut materials, including hardened metals that are next to impossible to cut otherwise. Wire EDM sits alongside laser and water-jet cutting as a go-to technology for tricky jobs. If the principle sounds easy (which it is), the practice is a bit more tricky. The wire has to keep moving as it gets eaten away as well, while maintaining tension; the spark voltage has to be accurately controlled by adjusting the position of the wire and thus the spark gap - too close and you need to back off, too far away and there's no spark; the whole thing is working in an incredibly noisy electrical environment as all the electronics is running a few centimetres from this spark transmitter.

    If you do a bit of googling on "wire EDM", you will find plenty of big - and I mean pretty big - commercial machines and companies offering wire EDM services. Look hard and you will find a few, a very few, "home-built" wire EDM machines. We now have a working wire EDM machine that is totally self-contained and small enough to go into the back of a car, and as a small demonstration of its capabilities, here are a couple of sample pieces I cut on it earlier this evening:
    Click image for larger version. 

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    The letter E is about 8mm high, the star about 4-5mm across. In fact, I had difficulty in finding the star in the tank (the cutting operation happens submerged in distilled water with a jet directed at the cutting area to flush away cutting debris).

    I know that there are people who have made claims to fabulous machines on this forum but have offered little proof of their existence. Well, you will be able to see this machine in the flesh at the Bristol Model Engineering Exhibition next week, where with a bit of luck and a following wind, I shall have it running on the SMEE stand for all three days of the show. Forum members are cordially invited to come along, and if you can't find me, just ask anyone on the stand. If you are also a wire EDM user, then you are even more welcome to come along and compare our efforts with your commercial machine!


    (*) Electrical Discharge Machining

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  3. #2
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 6 Hours Ago Has been a member for 8-9 years. Has a total post count of 1,703. Received thanks 176 times, giving thanks to others 5 times.
    WEDM is on my desired toy list.

    Are you aware of the plasmaboog yahoo group, or http://www.mechatronicprojects.com/blog/ ?
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  4. #3
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 19 Hours Ago Has been a member for 4-5 years. Has a total post count of 838. Received thanks 123 times, giving thanks to others 5 times.
    Thanks - I had found EDM Yahoo group, but mostly seemed to be concerned with building from a kit of parts available in US. Hadn't seen that blog before - looks as if it started relatively recently.

    I'm currently custodian of our team's machine as I'm taking it to Bristol next week so I shall try to get some pictures, and if the technology isn't too much for me, a clip or two of it cutting.

  5. #4
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 6 Hours Ago Has been a member for 8-9 years. Has a total post count of 1,703. Received thanks 176 times, giving thanks to others 5 times.
    The mechatronics site is fairly new, but I think the author has been working on the machine for a couple years.

    Benny Croonen (IIRC) who started/runs the plasmaboog group has been working on one for a good few years, with the group being a spillover from the EDM group run by Ben Fleming (I've got his original EDM book/plans).

    I've liked reading the periodic posts regarding the problems faced. They're quite a complex machine that needs everything to work nicely together to get a good end result.

    Is there any more info about your machine published anywhere?
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  6. #5
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 19 Hours Ago Has been a member for 4-5 years. Has a total post count of 838. Received thanks 123 times, giving thanks to others 5 times.
    No, nothing published as yet. You are also right about the "complex machine that needs everything to work nicely together to get a good end result" - or even to work at all. The wire feed and tensioner (DC motor with PWM control for wire feed plus stepper for tensioning) is one sub-assembly and includes a solenoid-operated guillotine mechanism for chopping used wire into short lengths as it comes out the end. Then there's the water flow and filtration, the XY movement with ballscrews and profile rails (tank moves on one axis and wire guide assembly on the other) plus stepper motors, and then the electronics. This machine has three PIC embedded microcontrollers in it - one for the keypad and screen, one for wire control, and one for spark generation which also controls the XY motion as these are inter-related. If anything is below par, then nothing works. In testing, a few weeks were wasted chasing electronic faults when it turned out that the deionised water wasn't quite pure enough. It's that bad. Electrical noise has been a perennial problem which is why the main control board is now up to version 9b, there are three separate power supplies, and it's wall-to-wall opto-isolators and so on.

    Cutting area is something like 100mm square (although I need to check this). It uses a variable spark voltage of something like 35-60V although it generally seems happy at about 50V. Sparks are generated from a bank of capacitors switchable from 10uF to 100uF or so (again, need to check but it's something like that). It uses MOSFETs for switching - turn on one to charge capacitor bank, when charged that MOSFET turns off and another turns on to cause the spark. When the capacitor voltage has dropped below some threshold, the spark is turned off and the cycle restarts. The motion control is linked to this; if no spark occurs the wire is advanced and if the voltage drops too low the wire is stopped or even backed up a little. It's a very simple idea but has taken a lot of work to make it function. Wire speed is separately controllable. All the motion control software was custom-written; no use of modified Mach3, LCNC, Kmotion, grbl, or anything else. This does mean that the software has complete control of wire motion so it is possible to backtrack the entire cutting path if needed, for example if a cut ends in the middle of a workpiece rather than cutting its way out again.

    We do not use any fancy commercial EDM components apart from the reel of EDM wire. So, no commercial wire guides, wire contacts, or things like that. Keeps the price down but whether this is going to hang together in the long term remains to be seen. It certainly works at the moment. There is already a long list of "wouldn't it be great if we could..." ideas!

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