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Friday, 21 August 2015

Mendel 3D Printer Problems

My Mendel 3D printer has been working fine for a while, apart from an over temperature moment when a thermistor went open circuit. Recently, though, I've wondered if the platform stepper belt was a bit loose. As things were still being printed I didn't worry too much. The printer is in a box that has filament reel holders and so didn't flex too much when I moved it about. Now I have a new workshop and a dedicated place for the printer to live, I moved it to it's new home. I haven't had time to start it up recently, but yesterday I decided to run it up and install the software on a new control PC. After powering on I popped to the other workshop room for something and when I came back noticed that there was a 'hot PLA' smell. Not good. Here we go again I thought, thermistor problem. Power off, multimeter out to check the thermistor resistance. The problem was obvious when I started to probe the thermistor screw contacts: the wires to the thermistor had fallen out. Not good. The second wire down in this picture was hanging out:

This meant that the thermistor was open circuit and the control electronics seem to always be performing the PID (even when hot end is off) and with the open circuit it drives to max temperature. Which is quite hot.

I reconnected the thermistor wires and started again. No smell this time.
All started off quite well and I tried printing a badge holder. No luck. Twice I got a mis-alignment of later layers, like this:

Platform stepper belt I thought. Turns out I was right, and I have now found out where the M3 nut I found came from. The stepper belt connection blocks are under the bed and I hadn't noticed that one had fallen off. As the connector block has a tooth that engages with the belt teeth, it looks like it had only jumped a tooth or two. 
After removing the bed and re-tightening the connector blocks it looks like the printing is much better, no misaligned layers yet, and a much tidier print. It's working more like I remember from just after I built it.

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