Subgate machining
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    Default Subgate machining

    Hi Guys,

    I've got a project where I will need to reproduce some old injection mould inserts. All of them have subgates( to remove the part from sprue during ejection).

    I've never sparked subgates before, so my question is how would you accurately locate electrode in plane where the inserts sits on an angle. EDM I've got is Joemars jm322 which is manual sinker with programmable Z. I really want to avoid locating it by eye.

    I'll include some photos on Monday showing the old inserts.

    Any help will be appreciated.

    Kamil

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    It's doable given certain conditions. Do you have access to cad? Make your electrode with a small flat on the end rather than a sharp point. You need to know what the end diameter is, so you need to cnc mill/turn it, or manually turn it and check the land diameter with a shadowgraph. Now you know the geometry you can use it as a probe to find the partline face and insert side face by sounding at a specific z. Hopefully the edge where the partline and insert edge is sharp? Then you can sound on that with the bottom of the electrode to set the z first. Do all of this with the insert mounted in the machine on the gate angle. Once you have those numbers draw it all up in cad to find haw far to offset x and z to the final gate geometry. If this does not make sense I can try to get the drawing from the last one we did this way.

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    Hi Kamil:
    Basically there are two ways to set up for subgates. one of which won't work for you because you don't have a full CNC sinker.

    The way that will work for you requires you to tilt the workpiece up into the proper plane so the ram will follow the axis of the subgate.
    Small inserts can go into a vise, bigger ones onto a sine plate.

    Picking up the location is pretty simple.
    Tilt the block and clock it in parallel to the X axis.
    If you are using something like a Rotobore (spins the electrode and is HIGHLY recommended) you can center a pin of a known diameter, touch the edge of the cavity with the side of the pin, add one radius and then add the amount you need to to get the axis of the gate located the proper amount from the edge of the cavity.
    You can calculate this with trigonometry, or you can do as ADFToolmaker recommends (and I do too) you can lay it out in a simple 2D CAD program and just interrogate it.
    If you do it in CAD, it will simplify your life to lay it out with the block tilted and the electrode axis vertical, exactly as it will be when the parts are in the machine.
    Trust me on this; it will save you grief, especially when the angle is close to 45 degrees and it becomes confusing whether you laid it out correctly and is not obvious to visual inspection.
    If you are not using a Rotobore, make or select a pin that has the same body diameter as the pin from which your electrodes are made, stick it in the vee block first to find your subgate axis location, then substitute your electrode to pick up the height.

    A note on subgate electrodes and subgates in general.
    I am a very strong believer in ball ended subgate electrodes sized correctly and burned just deep enough that the tip of the ball penetrates enough to make a round gate orifice of the correct diameter.
    If the ball tipped trode penetrates just the right amount, the shearing edge at the top of the gate will be a straight line (the ball has penetrated to it's equator at the top of the gate) and the bottom will be a bit like a ski jump when seen in cross section.

    There are big advantages to doing it this way:
    1) It's easy to balance the gates on a multi cavity mold...just burn until a target sized gauge pin will pass through the gate... you can get them to match within tenths this way easily, because the orifice is round, not elliptical so you can check it with a gauge pin.

    2) It's easy to pick up the zero height of the trode...just drop it in Z until it touches the angled parting face and call that point Z-zero.
    Subsequent trodes are picked up the same way.

    3) it burns WAY better than a pointed trode because the tip is robust and doesn't evaporate as soon as you put power to it.
    Also you can get some flushing down it and it won't break off and arc in the burn.

    4) The "ski jump" redirects the plastic flow right at the gate orifice and the cosmetic appearance is typically much better because the plastic cannot jet as easily.
    For those of you unfamiliar with jetting, it happens when a small gate opens out directly into a big cavity...the plastic squirts into the cavity without touching the cavity sidewalls until there's enough plastic to provide a backstop to force the plastic out into the sidewalls and slow down the flow.
    This leaves a snaky looking cosmetic blemish and a structural weakness in the part.

    5) the gate is MUCH easier to shear as the pins push it, (because the cross section is round, and the bottom has an acute angle relative to the part sidewall and will break as the top shears) so the mold doesn't run as dirty and the gates last longer before the knife edge is abraded away by the plastic.

    6) it's easier to make the trodes (assuming you have a CNC lathe) because you don't have to turn or grind them to a super fragile dead sharp point.

    So that's it in a nutshell; pretty easy once you wrap your head around setting it up.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining
    Last edited by implmex; 09-21-2019 at 11:56 AM.

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    Implmex, thanks for the in depth explanation of gate geometry. All toolmakers I’ve learned from have never understood the ball ended subgates, so they always encouraged me to just make sharp point ones instead.

    As for the original poster. You can easily eye ball a sub gate in to 0.25mm by eye or alternately grind an intersection flat on the back of the Die that is parallel to the gate angle and turn a parrell diameter on the electrode to edge touch the ground flat and use trigonometry to find the position. You don’t need CAD as basic trig will work easily but It is no doubt easier if you have access to it.

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    To support Marcus' excellent explanation, I'll supply the graphic.

    sub-gates.jpg

    As long as you don't advance the electrode past the radius center-line, the gate opening will be perfectly round regardless of the electrode's included angle, the angle-of-attack or cavity draft-angle.

    Bud
    Last edited by Bud Guitrau; 09-21-2019 at 11:34 PM. Reason: spelling

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    Quote Originally Posted by Bud Guitrau View Post
    To support Marcus' excellent explanation, I'll supply the graphic.

    sub-gates.jpg

    As long as you don't advance the electrode past the radius center-line, the gate opening will be perfectly round regardless of the electrode's included angle, the angle-of-attack or cavity draft-angle.

    Bud
    One fact Implex didn't mention when using a ball type gate detail the gate dia will rapidly change with a location or depth error where the tapered gate will be easier to hold size. A few thousands error will affect the dia. dramatically. Try to gate into a wall or rib detail to minimize jetting or into a gate post if there isn't enough vertical drop to gate into the part but it may need trimming. Good luck.

    Sent from my SM-G960U using Tapatalk

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    Thanks for all your input guys.

    I appreciate both methods ADF and Marcus laid out. Considering my equipment(I've got cnc mill, surface grinder, manual lathe, profile projector and manual sinker( no rotation)) I'll go with ADFs method of putting flat at the tip and pickup sharp corner of the insert as this will locate me in all 3 axis.

    Marcus, thanks for explaining the difference and advantage of round gates over eliptic. I have to admit I've noticed some of our moulds were sparked using ballnosed tip, but I never understood why.
    Inserts I'm doing will have to work together with old inserts which are made to have eliptic gate, so I don't want to risk mixing two types.

    I'll definitely run some trials in the future to see if I can produce ball nosed gates accurately.

    Thanks for all the answers, I've got one less thing to worry about

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    Can't really add anything above cause that about says it all. I ALWAYS drew sub gates in cad for location.

    When I started my shop, I bought a piece of copper tungsten and made up a bunch of 1/4 square x 4 inches long gate electrodes along with a pointed hardened steel one for pickup. If you may be interested, I have a sheet of 1/4 copper tungsten available for exactly this purpose. The shop is now closed and I am liquidating, so if you want, drop me an email. The already ground up electrodes and pickup thingies are already gone, and I may also have some electrode holders available


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