Heated bed screw and earth

Discussion in 'Getting Started' started by Paul Arden, Sep 9, 2019.

  1. Paul Arden

    Paul Arden Well-Known Member

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    My kit did not seem to come with the short screw to affix the earth cable to the heated bed. What are the specifications of this screw? Did everyone else get that screw, I'll dig around but I am fairly certain I don't have it.

    Also, I vaguely recall seeing in a guide somewhere an image where the area around the earth had the coating scratched off to expose the metal for the earth, however all of the guides now just show affixing the screw, was I imagining that? I assume it is not necessary now.
     
  2. Spoon Unit

    Spoon Unit Well-Known Member

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    I didn't find one. I used a 3mm spare socket head I had and fleshed it out with enough washers to prevent it being exposed bed-side.
     
  3. Paul Arden

    Paul Arden Well-Known Member

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    Was afraid of that, finding one that short is going to be fun. Such an unusual screw should definitely be included I think.
     
  4. Spoon Unit

    Spoon Unit Well-Known Member

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  5. Spoon Unit

    Spoon Unit Well-Known Member

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    Someone did report theirs was included, making me wonder if I just used the wrong screw somewhere.
     
  6. Amr

    Amr Well-Known Member

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    I have a habit of measuring the screws against a printed to scale image provided by prusa when ever I am assembling any thing and i can for sure say this screw was in my kit
     
  7. twam

    twam Active Member

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    Also missed the screw. Used a short M3 I had lying around at put so many spacers and a washer until it didn't stick out on the top. I also missed exactly on M6 screw + T-nut for the earth cable installation on the frame. No workaround there so far ;)
     
  8. Paul Arden

    Paul Arden Well-Known Member

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    For that one, I am wondering how this is supposed to provide continuity to the core of the anodised extrusion without using something like a penetrating screw. I posted a question in the documentation about that one. To my mind that basically leaves the frame unearthed.

    I guess it's the same question for the bed and whether the coating needs to be removed around the washer for the earth cable. As mentioned I could swear I saw a picture where that was the case but now I can't find it anywhere.
     
  9. Paul Arden

    Paul Arden Well-Known Member

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    Finally tracked down where I saw the image of the bed with the bit around the connector sanded away, it was on RichRap blog post. Here's the picture he posted.

    [​IMG]

    Interestingly he also seems to have tied the earth from the PSU to the bottom plate which seems to make more sense to me than to the anodised extrusion (unless it to has a portion of anodising removed). Here is where he ran the ground to.

    [​IMG]

    I guess he was working with the earlier printed instructions but I'm definitely concerned with the current documented grounding approach unless I am missing something.

    In the meantime I solved my screw problem with some precision engineering (i.e., a hacksaw and file) :)
     

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  10. Spoon Unit

    Spoon Unit Well-Known Member

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    The thread in the bed looks well bitten in. Isn't this going to provide continuity?
     
  11. Paul Arden

    Paul Arden Well-Known Member

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    For the bed, yes, I’d guess exposing the surface is just a belt and braces approach, however for the extrusion there will be no continuity unless the anodising is broken. I guess a star/lock washer might be enough to dig into the surface, however the guide is just using regular washers.
     
  12. Spoon Unit

    Spoon Unit Well-Known Member

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    Have you actually tested continuity on the extrusion? For me, a signal path is present. Where are you testing from and to?
     
  13. Paul Arden

    Paul Arden Well-Known Member

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    After affixing the ground cable to the extrusion I tested continuity from the cable to various parts of the frame (base, top etc), no luck. I ended up putting a larger (M8) star washer on and screwing it down hard so it slightly bites into the extrusion (attached). I now get full continuity to everywhere on the frame to the PSU chassis.

    I guess if screwed it in hard enough the ordinary washer might do the job (doesn't take much to break the surface), in my first test I probably didn't crank it down enough. Either way I'm positive I have continuity now :)
     

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  14. wHack

    wHack Well-Known Member

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    I didn't get the screw either. I did find that an M3 4mm with one washer was just the right size though.
     
  15. Killercds

    Killercds Member

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    Seems my kit was missing this heated bed ground M3, so I used a spare one laying around. I also seem to be missing an M6 and T-nut for grounding to the frame, so I'm grabbing one from the rear-panel at the moment
     
  16. Greg Holloway

    Greg Holloway Administrator
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    take it from the z-axis motor cover. they don't really need two.
     

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