Saturday, March 12, 2011

Haven't built the boot cloner yet. Thoughts on measuring heat transfer.

    I've not built the boot cloner, though I have finally gotten the ADAboot bootloader downloaded, and I've got the bootcloner code and correct fuse settings. It's mostly a matter of compiling the boot cloner itself. Todo list for burning bootloader:
        Getting the bootloader reduced to a PROGMEM table.
        Setting the correct fuse/lock values.
        Setting the correct bootloader address.
        Compiling the boot cloner (needs arduino libraries)
        Loading the bootcloner
        building the breadbord.
    As for the thermodynamic testing, I want three thermistors working at the same time. Here's my plan:

    One at the top of the heat sink, testing to see if plastic will soften - i.e. is the heat sink effective enough to actually print with this thing mounted in a plastic bracket?

    One just below the heat sink, testing the effectiveness of the thermal barrier. It's a solid steel bolt, even if it is stainless, so we'll see. Initial non-measured testing looked good, but I'd like some numerical results.

    And of course one in the heater block itself to control it's temperature. I'm planning to insulate the heater block itself, and also record the PWM averages so I can have a "heater watts" readout as well. If I can get a few fans working, I would like to try it out with a few different fans as well, see how much active cooling is really needed.

    My primary concern is the "steepness" of the thermal gradient across the bolt seperating the heat sink from the heating block. I want the softening zone to be within the tappered part of the bolt.

    I want to heat it up quickly and then let it sit for five or six minutes to reach equilibrium, which means I'll need to rework the heating code. Maybe even re-enable PID, which requires the 328 chip I haven't burned a bootloader onto yet. Also my 168 chip has a dead pin, and it's one of the analog pins. I don't have enough spare pins to hook up a third thermistor.

    My goal is to get some answers so some design questions, before they result in a broken extruder, ruined print, and downtime. My questions are do I even need to turn down the center part of the thermal barrier? If so, did I do a good enough job? Is the "softening zone" in the tapered part of the barrel? To answer these, I think that I need to test non-turned-down barrel against the turned one, see if it made any difference and if so how much.

    For my model, I'll assume that the heatsink is a uniform temp, the heater block is a uniform temp, and between is a uniform temperature gradient. Then I'll figure out where the "softening zone" is, and calculate the height of the tapered part of the barrel to see if the "softening zone" is low down enough to be in the tapered part of the barrel.

    Of course, the gradient isn't uniform, and a large part of the point of turning the barrel down is to make it be non-uniform. But a uniform gradient should have the "softening zone" higher up, as the thinnest part of the barrel is the bottom of the turned part due to the taper. So if I can prove that the tapered barrel would have the softening zone low enough even if it had a uniform gradient, I'm in good shape.

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