Arcus-3D-C1 - Granite heated bed

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

MasterOfNull

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Description

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.

Details

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. :)

Components

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

Build Instructions

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1

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.
2

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.
3

3) Cut the parallel grooves.

Clamp a 1x4 over the tile on a suitable workbench and get prepared to be coated in toxic dust. Respirator and eye protection required.

Using an angle grinder with a stone cutting blade, move the 1x4 so when sliding the blade guard along it, you line up with your first mark. Grind with light pressure pulling the grinder towards you against the direction of rotation of the blade. Once you have a straight line cut, the grinder will follow the groove. Go slow and grind each groove down to 1/8th in depth.