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  1. #151
    Quote Originally Posted by Neale View Post
    Why leave the 24V supply floating wrt earth? Take the 0V connection to the control box star point and all will be well. Can't see any reason not to ground it. That's what would normally be done - my control box has a 24V smps and a 5V+12V smps and all the 0V connections go straight to the star earth.
    Is that really recommended? Doesn't it defeat the purpose of the galvanic isolation?

  2. #152
    Not quite sure what the issue is here. If the output of a psu - any psu - is left floating them you can guarantee what volts you will see between the terminals but volts wrt ground is undetermined. It will be somewhere between earth and full mains depending on leakage, transformer internal coupling, EMI filler leakage and so on. Take one terminal to ground and everything is now properly referenced to that - no more tingles!

    I don't see that the EMI filter effectiveness or anything else will be affected. In fact, electrically, you are in a much better position because at the moment I would be worrying about the effect of stray voltages on sensitive components. If that "tingle" happens to occur around any high-impedance inputs...

    The reason for leaving the outputs floating is that someone might want a -24V supply so in that case they would ground the +'ve output pin. There might be other reasons but that's one good one.

  3. #153
    I was thinking that the galvanic isolation (floating DC) is there to prevent a current path between AC live and DC, and that when using a probe you temporarily defeat it. I think also galvanic isolation prevents ground loops should a secondary piece of equipment grounded equipment like a PC be connected, not a consideration here.

    But as you say you might get a tingle.

  4. #154
    There are a few reasons for the galvanic isolation. That's actually just a posh name for saying that there are no connections from the live/neutral terminals to the output terminals. I mentioned one reason for this earlier - so that the same smps could be used as a +24V source or a -24V source with one output terminal grounded. Another reason (and someone else mentioned this in a post a little while back) for industrial equipment is that if the 24V supply rails are floating with respect to ground, then you can put in fault detection mechanisms that check if either rail does get shorted to ground under fault conditions; the machine can then be stopped before any magic smoke is emitted. Or, maybe, you have a motor speed controller that needs a 24V supply, but is connected to the mains supply to the motor with no isolation. However, if neither smps output terminal is grounded, you really need to make sure that none of the control electronics wiring or terminals are touchable (due to the whole thing floating at some undetermined but possibly dangerous voltage with respect to ground).

    On a practical note, this also means that a simple electrical touch-off system for tool height setting is not going to be easy as this depends on the touchplate being connected to one controller input and being shorted to ground by the tool tip via spindle/machine frame, etc. If the controller is not referenced to ground, then this ain't going to work, or you will need to isolate the spindle from the machine frame and provide an additional connection to it - which will all be floating at "it tingles a bit" voltages.

    All these issues disappear if you just take the SMPS 0V connection to ground. I cannot see any downside to this.It is what just about any other home-built machine does.

    External, e.g. PC, connections? Valid point, and might be an issue in an industrial environment where different bits of kit are connected to different supplies. In a domestic situation where all the equipment is probably connected to the same ring main, this is a non-issue. In addition, an RJ45 ethernet connection will be galvanically isolated and I believe that the USB connection spec calls for galvanic isolation as well.

  5. #155
    Quote Originally Posted by Neale View Post
    There are a few reasons for the galvanic isolation. That's actually just a posh name for saying that there are no connections from the live/neutral terminals to the output terminals. I mentioned one reason for this earlier - so that the same smps could be used as a +24V source or a -24V source with one output terminal grounded. Another reason (and someone else mentioned this in a post a little while back) for industrial equipment is that if the 24V supply rails are floating with respect to ground, then you can put in fault detection mechanisms that check if either rail does get shorted to ground under fault conditions; the machine can then be stopped before any magic smoke is emitted. Or, maybe, you have a motor speed controller that needs a 24V supply, but is connected to the mains supply to the motor with no isolation. However, if neither smps output terminal is grounded, you really need to make sure that none of the control electronics wiring or terminals are touchable (due to the whole thing floating at some undetermined but possibly dangerous voltage with respect to ground).

    On a practical note, this also means that a simple electrical touch-off system for tool height setting is not going to be easy as this depends on the touchplate being connected to one controller input and being shorted to ground by the tool tip via spindle/machine frame, etc. If the controller is not referenced to ground, then this ain't going to work, or you will need to isolate the spindle from the machine frame and provide an additional connection to it - which will all be floating at "it tingles a bit" voltages.

    All these issues disappear if you just take the SMPS 0V connection to ground. I cannot see any downside to this.It is what just about any other home-built machine does.

    External, e.g. PC, connections? Valid point, and might be an issue in an industrial environment where different bits of kit are connected to different supplies. In a domestic situation where all the equipment is probably connected to the same ring main, this is a non-issue. In addition, an RJ45 ethernet connection will be galvanically isolated and I believe that the USB connection spec calls for galvanic isolation as well.
    I get your point on the convenience of probing Neale, however if you don't connect DC ground to AC ground and you are using an SMPS there is no current path to AC ground at all, there is no shock risk. You get the tingle from the Y capacitor in the SMPS EMI filter there are usually two, one coupling AC live to DC+ and the other AC neutral to DC-, the capacitors are not allowed to be big enough to carry enough charge to hurt you and they are required to fail to open circuit. If you take any 5v USB charger and hold the shield whilst touching AC ground you will get the same tingle.

    I'm not against connecting AC ground with DC ground but it does come with the risk that an AC live fault in the machine enclosure will fry all your electronics as there is a current path between AC live and DC ground. My last machine didn't have this connection so when probing a temporary DC ground connection was placed on the spindle then removed afterwards. I'm not sure which is more correct.

  6. #156
    Progress update. Today was spent putting some controls in a box and attaching it to the machine. I used an old TV mount and made some crude alterations to it with the welder.

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    I also soldering up the MPG connections which was a massive pain in the rear.

    First moves:

  7. #157
    Great when it reaches that point - gives you the drive to get it finished!

    Seems a little slow when jogging with the MPG - is that a "settings" thing? Mine winds back and forth a bit faster, but there's also a "step size" switch on the MPG that effectively changes jogging speed.

  8. #158
    Yep the DDCS lets you configure the step size per MPG tick for each of the three speeds on the MPG but I haven't configured it yet, so it is defaulted to 4 microns or something ridiculous. Sounds rough on the lowest setting as it is below the precision of the machine.

    Still got to rig up the home switches and solder all the cable shields on, then I can tune the config. Also need to dig out my old VFD and see it it still works, it may have been exposed to the elements.

    Then cut the plate for the bed and get the machine to drill itself....

    Before going much further I will run some complex toolpaths in the air so see if the DDCS is any good, I have no reason to believe that it isn't but want to know sooner rather than later if I need to switch to a PC, hopefully it will be fine.
    Last edited by devmonkey; 20-06-2020 at 06:17 PM.

  9. #159
    I ended up fitting the tiny hiwin grease nipples to the HG15 carriages, these are a pain. Today I knocked up an adaptor from a piece of aluminium, it works but only about 50%.

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    I've read the hiwin instructions for moving the grease point to the side of the carriage by piercing one of the blind holes in the green plastic molding, has anyone tried this? It would greatly simplify greasing my Z axis.

  10. #160
    VFD is working, quite surprised by this as it has been 'outside' for a couple of years. DDCS is controlling the spindle on/off and speed.

    Today I cut the 20mm plate for the bed. The blank weighed 80kg which is too much to safely manoeuvre around the table saw so I just my very old and rough circular saw, lots of fluid, lots of smoke, but it did a pretty good job.
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    I took the edge off with a chamfer bit in a wood router, the bed sitting in place:
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    I took the a rough height map with a DTI clamped to the Z. As expected from bringing the rails into plane with laser and epoxy I have around 1-1.5mm total error (this was the frame error before pouring epoxy). The cross members of the frame are also not perfectly in plane and would require a little shimming under the bed.

    I'd rather have the bed closer to planar that this, although the eventual spoil board skimming will fix it, so I'm contemplating attaching some shimming material to the cross members and surfacing this then bolting the bed plate down on top.

    Another thought is to float the bed plate on some rubber washers and compress them until the DTI reads zero everywhere.
    Last edited by devmonkey; 23-06-2020 at 03:27 PM.

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