Gratuitous Engine Jeweling

While pondering whether I should use the carcass of an old Dell PC to house the stepper drivers and control logic for the LinuxCNC M2 project, I bandsawed a scrap of aluminum sheet to about the right size. It had some truly nasty gouges and bonded-on crud, so I chucked up a wire brush cup in the drill press and had at it:

Machine jeweled baseplate

Machine jeweled baseplate

It’s obvious I haven’t done jeweling in a long time, isn’t it? Even a crude engine jeweling job spiffs things right up, though, even if a cough showcase job like this deserves straighter lines and more precise spacing. The aluminum sheet is far too large for the Sherline, which put CNC right out of consideration, and I’m not up for sufficient crank spinning on the big manual mill.

I match-marked mounting holes directly from the harvested motherboard and drilled them, whereupon I discovered that the aluminum is a dead-soft gummy alloy that doesn’t machine cleanly: it won’t become the final baseplate.

Memo to Self: Use the shop vacuum with the nozzle spinward of the brush, fool.

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  1. #1 by Brent on 2013-03-02 - 09:17

    Aren’t the heat sinks on those drivers on their sides? Maybe place them in a pentagon with a big fan blowing fresh air into the center of the pentagon?

    What do you have in mind for the power supply?

    • #2 by Ed on 2013-03-02 - 10:53

      The frame on those HB-415M drivers is just a L-shaped aluminum sheet, so contact cooling to the support plate through the narrow base should trump air cooling across the un-finned wide side.

      The larger 2M542 drivers have air-cooling fins on the wide side and, at high current, would probably benefit from some directed airflow. We’ll see whether driving an amp or so will require much in the way of additional cooling: the power density is much lower than a Pololu driver!

      There’s a closed-frame 24 VDC supply that fits across the front, making the airflow less wonderful than it could be. I know you’re a big fan of higher voltages, but the stock M2 uses 19 VDC (presumably from a laptop-style brick) and this is in the same range. I’m not convinced higher voltage will bring increased goodness, so I plan some measurements to find out… [grin]

  2. #3 by Brent on 2013-03-02 - 13:20

    Does “M2″ refer to the MakerGear printer? I somehow thought this was for a machining tool.

    I will look forward to voltage vs speed/acceleration capabilities measurements. I’d love to see some numbers to back up or shatter my gut-feelings :)

    On a separate though, I met with an Allegro FAE and we spoke of the A4988 stepper drivers (as used in the Pololu and others) getting damaged. He said that the chip is very reliable wen used properly (of course). He pointed out that the failure mechanism that they have seen the most is hot-unplugging of a stepper motor. When the A4988 kills the current in a winding, it always keeps the di/dt under control, and therefore the voltage reasonable. When an energized winding is hot unplugged the di/dt and voltage go through the roof and that is what damages the A4988.

    An intermittent open caused by a faulty cable between the stepper and the A4988 could cause this condition repeatedly, hammering the poor A4988 into sand, There are diodes in the A4988 but they are designed to handle the reasonable di/dt of normal operation, not the hot-unplug catastrophic events. The 4 outputs could be clamped by 8 big/fast TVS, but the cost of those to protect 4 lines soon gets crazy.

    I also mentioned the current chip’s stray field sensitivity . . . he says they now have a new one that measures the differential field, so it is much less sensitive to stray magnetic fields. Just for fun.

    • #4 by Ed on 2013-03-02 - 15:55

      Does “M2″ refer to the MakerGear printer?

      Yup, the very one.

      The plan is to commission it with the original RAMBO board, get it working as well as can be expected, then do a LinuxCNC conversion (and maybe transplant the RAMBO into the Thing-O-Matic). I want to do some instrumentation / measurement / experimentation / modeling / improvement with a more extensive & easier to modify control system. Think of it as a reprise of my Thing-O-Matic experience, except starting with competently designed machinery.

      I’ve been making noises about LinuxCNC vs. 3D printing for far too long; I want to see what it can do!

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