Calibrated hole making edm sinker
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    Default Calibrated hole making edm sinker

    Hello and good morning to everyone.
    I am new in edm sinking and i have a problem that i
    Wanted to ask if anyone else has experienced also.
    Many times i try to make calibrated holes, for
    Example d3,d5.... That have tolerance 0.01.
    The problem is that many times the holes becomes
    Reverse conical(i dont know if i explain it correctly).
    It comes out like that /\.
    So if i try to make a d5 i try to be 5 on top and 5.01
    At the bottom.on deeper holes though its more than 0.01.
    Can anyone explain why this happens and how can i make a correct hole.
    I tried external flushing/ spindle flushing.
    I tried electrodes that are bought or i made myself in turning machine
    It happens in through holes and blind ones.
    In through holes most of the time its like a barrel.
    I mean that its 5 on top.5.01 at the middle.5 at the bottom.
    Any ideas will be appreciated.

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    Hi kaigamw:
    If your holes are blind holes and are bigger in the middle or on the bottom, you are almost certainly getting overburn from trapped cut debris in your holes, which creates unwanted errors of form.
    There are two common ways to deal with this:
    The first is to vacuum flush, so the cut debris is pulled from the spark gap through the electrode.
    The second is to orbit the electrode so there's more room for the debris to be pushed from the cavity without encroaching on the spark gap and causing unwanted overburn.

    As an additional note, whenever you need excellent geometry in a burned round hole it's worthwhile to also rotate the electrode, either with the C axis of the machine (if it has one) or with an aftermarket gadget like a RotoBore, but when you do so you must be very certain your electrode is very precisely co-axial to the rotating axis or you'll make the problem much worse because the tip of the electrode will describe an arc that's bigger than the trode diameter as soon as the axes are misaligned.
    To make them perfectly co-axial it's common to dress the trodes right on the sinker EDM machine; either by turning them in place if they're graphite, by discharge dressing them if they're copper, or by wire EDM skimming them if you have a super advanced precise hole making machine like some of the Makino and Sarix machines.

    For best quality holes it's worth considering doing all three processes, vacuum flush, orbit and rotate the dressed-in-place trode.
    You also should consider making your pre-drilled hole large enough that your electrode can drop into the predrilled hole without burning anything, and do all of your burning by increasing your orbit and burning only sideways.
    That way the trode gets the most even wear and stays parallel sided.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

    Another possibility that just occurred to me: Are you by chance using "No Wear" settings?
    These operate by controlling the spark duration and intensity such that you are in effect plating some of the cut debris back onto the electrode hopefully to offset the trode wear that occurs during burning.
    You can introduce subtle form errors because this plating effect is time dependent and since the tip of the trode is burning the longest, under some really odd conditions you can make it actually grow in size over time.
    MC

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    Marcus,
    Thank you very much.you ve been really helpful.i ll start experimenting tommorow or monday.i tried rotating but it doesnt get better.
    I havent tried vacuum flush yet.
    Also i have the low wear checked every time.i ll untick it next time. There is just one thing i don t understand and it is the following: "To make them perfectly co-axial it's common to dress the trodes right on the sinker EDM machine; either by turning them in place if they're graphite, by discharge dressing them if they're copper, or by wire EDM skimming them if you have a super advanced precise hole making machine like some of the Makino and Sarix machines.". Can you please elaborate and explain how to make them perfectly co-axial?
    Thanks.

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    Hi again kaigamw:
    There are a few ways to get them coaxial.
    The first is to adjust them with tenths indicators until they run perfectly.
    Some rotary setups have the means to do this; my RotoBore can adjust both concentricity and tilt.
    It's a royal pain in the ass and takes a long time to do but basically you put a tenths clock right up next to the chuck, a second clock at the tip of the trode and fiddle with the adjustments till everything runs as you want.
    So nobody does it that way unless they have to.

    Dressing in situ:
    Graphite electrodes can be turned or cylindrical ground in place either by clamping a carbide turning tool or a grinding head in a vise; spinning the C axis or rotary gadget and using the machine movements, either jogging them or with a program, to dress them to proper diameter.

    Discharge dressing works well for copper because it dresses beautifully but doesn't turn worth crap and you don't want conductive shavings in your dielectric tank anyway.
    Discharge dressing is done against a copper tungsten or tungsten carbide dressing block.
    You reverse your normal polarity so you preferentially wear the electrode over the dressing block, and then you drive the spinning electrode past the surface of the block with a little program.
    The electrode is brought to shape by sparking it against the face of the block.

    Dressing with wire EDM requires a wire EDM attachment that sits in the sinker EDM machine's work tank.
    The electrode is dressed by spinning it and traversing the EDM wire past it using a turn-while-burn strategy where the EDM wire acts as if it were a lathe turning tool.
    Only a very few machines on the market have this sort of attachment, so I mentioned it only for completeness' sake.
    These are for when you want very small holes to sub-micron accuracies, and if you're the lucky owner of one of these machines you certainly don't need someone like me telling you how to use them!
    .

    Almost everyone discharge dresses rotating electrodes against a dressing block, and they accept these electrodes will have to be copper (or sometimes copper tungsten, but copper tungsten trodes are harder to dress).
    It's simple to do and makes the electrode run perfectly true to the C axis.
    You can dress to a given diameter if you want or alternatively just dress until the trode cleans up then jigger the orbit when you burn your job to bring the holes you're ultimately making to proper size.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining


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