Arcus-3D-C1 - Granite heated bed

This is the heated bed I used on the C1 and how I made it.

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It seems impossible to get the instructions editor to order my instructions correctly. So I've resorted to numbering them myself. Perhaps when I have an infinite amount of time to figure out how it's screwing up and how to compensate, I'll come back to this. For now, follow them in order of MY numbering.

This is the buildout of the heated bed used on the Arcus 3D C1.

For this project, I was aiming for a 400w bed at 120v.  That is a little aggressive for the size, but I don't like having to deal with part cooling dropping my bed temperature either.

A little Ohm's law says 400W at 120v(RMS) works out to 36 Ohms required resistance.  But... the resistance per foot of nichrome wire actually increases in resistance a bit as you heat it.  So I aimed for 33 Ohms instead.

I have 24 gauge nichrome wire.  So per the above chart I would need 20.6ft of it to hit the target 33 Ohms.

My granite tile was 12x12in.  Laying that out on the tile, leaving 3/4in from all edges resulted in a roughly 21ft path if my cuts are placed 1/2in apart.  I like nice round numbers.

Made the cuts, ran the wire, terminated the ends and encased it all in furnace cement.  Instructions below.

After construction my measured resistance is 33.6Ohms.  Not bad at all for a hand wound ceramic 400w resistor.  :)

  • 1 × 12x12in polished granite floor tile.
  • 22 × 24 gauge nichrome 80 wire
  • 1 × 3950-100K thermistor
  • 2 × 18 gauge silicone insulation wire
  • 1 × Rutland furnace cement

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    1) Pick the right tile!

    This is by far the largest predictor of success for this project.  Look for a tile that you can see a reflection in it that is not distorted. Line the reflection up with the overhead lights (fluorescent tubes) in at least two directions.  Look for straight lines.  Straight lines means you have an actually flat tile.

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    2) Mark the tile.

    Starting 3/4in from the edge, mark off every half inch.

    Rotate 90 degrees and mark off both ends 3/4in from each edge.

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    14) Cure the cement

    Now we get to bake it.  After this step, the cement will be rock hard, so this is also your last chance to clean up the edges or sand off anything you don't like...

    Put it in the over at 250F for about 30 minutes, then raise the temperature to 400F for about 20 minutes.  This is why you need silicone insulation on your wires.

    After baking.

    See those bubbles?  I still didn't get enough of the water out of the cement.  Luckily it looks to be confined to my final skim coat.  If it happens in a groove, it can create a hot-spot on your nichrome and cause it to fail prematurely.

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