HexBed

By evan on Jun 17, 2014
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Bed sitting in place before installing mounting hardware

As some people may know, Chris MacDonald and I have been building a Rostock style 3D printer.

When it came time to pick up a bed, the place I wanted to order from was out of stock. Delta style printers don’t really print on the entire 200mmx200mm surface, so I figured I’d look at making my own heated bed. Having built a Prusa Mendel before, I knew what I wanted in a heated bed; Perfectly flat, solid, adjustable, and able to retain heat.

With those considerations in mind, a ceramic tile fit the bill. The tile is 10mm thick, and has a flat but slight eggshell surface. I cut the tile into a triangle shape, with 310mm from corner ot corner. That was slightly too big and didnt represent the build area so I trimmed it down to a 200mm hexagon.

I picked up some 30gauge steel wire, measured the resistance and determined 2.6m  of wire in two parallel runs would draw 8A @ 12V. (1.5Ohms, and 95W) The wire was taped in a roughly circular pattern around the bed to hold it in place. Then I used JB weld (which is an insulator) to hold the wire down permanently. I let that dry (~5hours) and then removed the tape. I soldered 14Gauge wires (from a PC power cord)  to the leads on the end; and applied JB weld to the rest making sure to cover all exposed wire including the solder joints.

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Bottom of tile, after first round of JBWeld, before tape was removed.

Once that cured overnight, I started the bed up directly attached to the power supply. Success, it heated up. That evening I hooked it up to the RAMPS1.4 board and it tripped the fuse. Then the fuse lit on fire. Strange, but after troubleshooting it the bed was only 1.1Ohms, which meant it was drawing 10.9A at 12V. The fuse was rated to 11A, so it should have tripped, but didn’t and actually started burning. That’s an issue. This was a cheap $12 board from china, I’m assuming there was an issue with the fuse.

Regardless, all was not lost. That fuse only affects the 11A line on the RAMPS board. It can’t control the bed directly anymore, but realistically I’ve always had issues with bed voltage drop through RAMPS. I modified the firmware in pins.h to use a different pin to control the board. Using that pin I trip a 5V relay, which trips the PS-On of a second power supply. The bed has it’s own dedicated power supply, and heating works great.

For mounting, I drilled three holes into the base of the printer, and put a 2″ long 10-24 bolt through from the bottom. I used a nylock nut to hold it in place, and put a wing nut and washer on upside down. Then I drilled three holes in the heat bed, put the heat bed down over the three bolts, put a rubber backed washer and another wing nut. I can adjust the height at each of three points with the bottom wing nut, and lock it down with the top one. Since the top washer has a rubber backing, it will allow for some compression and protect the tile surface. I was able to perfectly level the bed in under five minutes, and the bed is rock solidly attached to the frame.

In retrospect, those holes should have been drilled before applying the epoxy as one hole had to be 10mm further in than the other two to avoid going through the wire.

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First two prints in ABS, glasses and batarang.

The total cost was $3.50 for the tile, $4 for 75m of 30gauge wire, and $1 for a PC power cable. Necessary tools were a soldering iron, tile cutter ($20), 3/16″ tile drill bit ($8.00) and wire stripper.

It’s a relatively cheap alternative to a PCB heated bed if you already have the tools, and still a great value bed if you have to buy them. It’s easily scalable, and judging by how well my first two prints went, with only a quick spray of hairspray, it’s something I’m definitely going to continue to use and refine in the future.

On a side note, huge shout out to E3D their bowden kit is a work of art and well worth every penny.

 

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