Sunday, 20 November 2011

Hand-drill geared extruder

I managed to break my original BfB extruder :-(. The acrylic had flexed too many times, and the top retaining bracket broke in half. The screw thread now carefully unscrews itself rather than driving the filament down :-(

So, time to update to a modern wade-style or similar extruder, using a hobbled bolt. Of course, I could have bought the gear drives from the many RepRap shops or ebay, but that's too easy :-).
Instead, I found an old cheap hand drill, with similar sized gearing. It turned out to be on an 8mm shaft.
I added a 5mm to 8mm sleeve to a stepper motor (made from a sex bolt)  and drilled and tapped a 4mm hole for a grub screw - I could then attach the smaller gear to the stepper shaft. The larger gear I cut a small square indent to match a carriage bolt, and cut some teeth into the threads using a dremmel cutting disc. A plastic cutting board and an Ikea bracket made a mount, and three skate bearings (8mm 908) made up a channel for the filament.

It's not pretty, and it's heavier than ABS gearing, but it grips the filament like anything - I can't stop it using my full strength grip, and really forces the filament through. 

I'm currently building a heater block and nozzle to add on the bottom. Hopefully this 'RepStrapStruder' will last long enough for me to print a proper wade's extruder. 

Thursday, 10 November 2011

New Megas

My new Mega boards arrived - all good.
Simple tests worked fine, but after uploading the RepRap host software at 115200 Baud, had problems connecting and even uploading new sketches. I had to press reset and upload almost straightaway, otherwise the upload failed.

A bit of research suggested an updated Atmega 8u2 firmware (the usb chip). (also the new boards appear as /dev/ttyASM0 instead of /dev/ttyUSB0 - update to the latest RXTX libraries.

Borrowed from a forum post by stimmer on this thread

I saw that a new Uno firmware had been uploaded to the Arduino repository. I've been trying it out, it seems pretty good so far, tools menu and serial monitor are much more reliable and it only failed to program once. (Although the Duemilanove was never perfect either)

I got the firmware from here: on the right firmware, then click raw, then save it)

Reflashing the firmware on the 8u2 is a little tricky. Don't try this unless you are prepared to risk bricking your board completely!

You need the dfu-programmer utility:
sudo apt-get install dfu-programmer

Then follow the instructions here to get the Uno into DFU mode:

Then to flash the firmware, do this:
sudo dfu-programmer at90usb82 erase
sudo dfu-programmer at90usb82 flash --debug 1 Arduino-usbserial-uno.hex
sudo dfu-programmer at90usb82 reset

Finally unplug the USB plug, wait a few seconds, plug back in.

Also, a good page on the arduino site

Note, that to put reset the Mega 8u2 chip, the reset pins are a bit different: I found instructions for resetting the mega chip.

My mega's accepted the sketches without problem, and both replicatorG and RepRap host connect well at 115200 baud.

Might help if you're getting unreliable high-speed connections to the arduino and you've got a recent board (/dev/ttyACM0)

Tuesday, 1 November 2011

Arduino Mega board is toast!

I have managed to blow my Arduino mega board.
It stopped uploading, and there was a 'hot' smell - never a good sign. Unplugging everything, the arduino chip was very hot indeed.

After waiting for it to cool, I found that plugging it into the USB socket - nothing else - started to make the mega chip warm up, and it got too hot to touch. :-(
Another circuit board is sacrificed to the gods of knowledge and experience.

Tracing things back, I wondered how it happened. My setup hadn't changed, I'd just been running heater and extruder tests - the steppers weren't plugged in, so it was unlikely to be them.

Looking at the my heater board again, I realised that the output pin of the arduino was connected straight to ground through the MOSFET gate - no current limiting resistor. A bit of digging uncovered a possible problem - the design I'd used as a base (RepRap Gen 2/3) had the same circuit, but used current limiting Mosfets. I'd upgraded mine to larger ones, but checking the data sheet, the Mosfet gate acts a bit like a capacitor. Charging the gate could be done slow(with a resistor) or fast. A current of 2A was given as an example to charge the gate in 14nS.

Oh. The arduino pins can only handle 100mA or so. Still, it only does that when it's switched on or off, so long as I wasn't switching it on and off thousands of times a second - like using PWM.... Damn.

Looking back, it's surprising that it coped as well as it did - it should have gone up in smoke straightaway! I'm impressed it worked at all  - it coped for several hours of testing. Each time it switched, the arduino pin was directly connected to ground - drawing a big current for a few mS, then when it switched off, the mosfet gate would discharge itself straight through the arduinio pin to ground.

I have now redesigned my heater / Mosfet board to incorporate a 560ohm current limiting resistor (between the arduino pin and the mosfet gate), and also a pull-down 100k resistor between the Mosfet gate and ground. It should limit the current to 90mA, and make sure the gates switch off if the board isn't connected.

I've also ordered a couple of new Ardunios - hopefully I'll be more careful with these...