Friday, August 26, 2011

Glass Heated Build Platform

I finally got the prototype made. It came out really well and even though I have no idea how well it will work yet, visually I think it's great. I have some video of the writing process, but it took almost 30 minutes, so I need to do a bit of editing before I post it. Here are two shots of it right after writing the conductors and before baking:

Just after writing heater element
Both heater and thermistor elements
The first picture shows just the essential part, the heating element. It's about 2.2 Ohms overall so at 12 Volts that should use 5.5 Amps and dissipate 65 Watts. I think that will be enough for the 4"X5" piece of glass. The extra lines you see in the picture are actually just scratches in the X-Y stage.

The next picture shows the completed pattern which includes a very long thin looping conductor which ideally will work as a thermistor using the natural TCR (temperature coefficient of resistance) of the conductor. The higher the resistance, the better range I should get as a function of temperature, hence the need for long and thin. The resistance of the thermistor came out in the neighborhood of 230 Ohms. Once I can fire up and control the heater, I'll have to generate a lookup table to use in my firmware for bed temperature control.

After writing the conductors, the plate was fired for 3 hours at 850°C. Then I was able to solder wires to the conductor pads. I'd like to think of a better interconnect for the next version, but for the prototype, this will be fine.

Now I need to make myself a decent power supply out of one of the old ATX PC supplies I have lying around.

Unfortunately, this past week has not been all good for me on the 3D printing front. I was having a little trouble with backlash, mainly on the X-axis, and in the process of tightening the belt managed to pull off both of the bushing holders on the motor end, and two of the bushing holders from the X-carriage.
Bushing holder separation on X-axis

This is apparently a problem for more than a few people. I believe the belt went slack over time due to the motor mount being designed with a bit too little plastic. Eventually the belt tension pulled the pulley in enough to cause slack. I guess I got a little ham-handed trying to take up the slack and you can see some of the results. The cable tie I had on there was not enough to prevent the detachment of the bushings. Maybe it's me, but I think the desire to reduce the amount of plastic in the printer parts has gone a bit too far. I'm also at a loss to understand why the channel in the X-motor end and X-idler end faces away from the source of tension. I have a hard time believing there isn't a good reason for this orientation, but I'm damned if I can figure it out. I'm really impressed by the work I see on vertically oriented X-axis parts by Emmanuel and the Russians and I would like to do something similar once I get up and running again.

My goal with building the printer was to make parts for other projects, but I'm getting totally sucked in to the idea of improving the design.

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