Extruding or not as the case may be…

I found a suitable single pole single throw 12V/40A automotive relay and wired it up as per the previous diagram, however I managed to connect the solenoid backwards at first so nothing happened when I requested power to the heated bed.

This was partly due to translating the pin-out on the relay to the pin-out in the diagram since I had mounted the relay with pin 87 to the right hand side, I had managed to confuse myself regarding the positive and negative sides of the solenoid.

Once I had sorted the wiring including the fly-back diode, the heated bed worked perfectly turning on and off via the relay.
The other thing that you need to do when using an automotive relay is to change the heated bed manager in the firmware from 1 (PID controlled) to 2 (Bang Bang, limited check every HEATED_BED_SET_INTERVAL) where the interval is set to 5000ms.
If you don’t do this then when the temperature for the bed is reached, the relay flaps on and off rapidly and, even with the flyback diode in place, manages to reset the microcontroller.
My 1.75mm PLA filament arrived from sunwoif ebay seller – £11.99 inc p&p for a black 1Kg spool. so it was time to start calibrating the extruder.
The first issue was actually persuading the extruder motor to turn at all. It energised when the other motors did and locked in place so I knew that it was receiving power to both coils, however no matter what I did, I could not persuade it to turn when hitting the extrude button in the Repetier Host GUI or even with the Pronterface GUI.
Eventually it turns out that this is a safety feature and it will only turn if the extruder is at a suitable temperature to melt the plastic so as to avoid damage to the motor – that only took an hour or two of searching and finding many other people with the same problem, largely troubleshot by changing motors, stepper drivers and wiring all for something that was as simple as a line of code that prevents extrusion below a certain temperature.
Having managed to persuade the motor to turn, I now needed to calibrate the motor such that when I ask for 10mm of filament, it actually pushes 10mm.
Before making any adjustments to the firmware, I requested 100mm of filament and the motor promptly pushed out 355mm of filament, so some adjustment is definitely required.
The formula for working out the required number of steps/mm is ((extrude button clicks) * (extrude length per click) * (e_steps_per_mm)) / (length of filament pushed) as shown on Josef Prusa’s Blog entry.
The initial setting in the Repetier firmware for e_steps_per_mm was 453, so my calculation was (1 * 100 * 453) / 355 which gives a new e_steps_per_mm of 123.
Once the firmware was updated and uploaded a request for 100mm of filament resulted in 100mm being pushed, so we were making progress.
The next issue was with the J Head hot end and the thermistor in particular. Initially I had selected type 1 (100k thermistor Epcos B57560G0107F000 – and many other) for both the heated bed and the extruder.
After not being convinced that the extruder was achieving the correct temperatures (PLA refusing to melt) I started to do some research on thermistors and also double check what type I had bought and used on the heated bed.
Turns out that I should be using type 8 (ATC Semitec 104GT-2) for the heated bed as this is what I actually bought from Rapid Online, however I had no idea what the thermistor was in my Chinese J Head.
I compared the resistances of the two thermistors at a number of temperatures setting the temperature with the Chinese thermistor and then comparing the resistance with that obtained from a spare 104GT in the same hole on the J head. Although it looked like it should be a 100K thermistor in that at around the resistance was close to 100K Ohms just like the 104GT, where the 104GT showed 70K Ohms at room temperature (about 22 degrees C) the Chinese one was showing 80K Ohms.
At higher temperatures it was showing nearly double the resistance of the 104GT. Since I don’t have accurate readings for 25 and 85 degrees C it is not easy to work out the required Beta value for this thermistor and from that work out a suitable thermistor table as described on the RepRap Wiki.
In the end I decided to replace the unknown Chinese thermistor with the spare 104GT so at least it would be a known quantity.
The next problem I have is that despite having accurate temperatures at the hot end, my motor is struggling to push the filament and loses steps when the required torque is too high.
I appreciate that greater motor torque is required for an extruder than one of the axes, and even more when using a bowden extruder, however the motor I am using is rated at 4.8Kg-cm/42oz-in holding torque, so more than the recommended 40oz-in mentioned on the RepRap Wiki, although that is with a geared extruder.
I believe part of my problem is that the filament is melting most of the way up the PTFE sleeve in the J Head and then solidifying again at which point it becomes very difficult to push.
If I have the fan turned on, then the hot end struggles to reach temperature, however with it off the PEEK part of the J Head may not be cooled sufficiently for PLA.
I will double check my J Head for the melt chamber size since for 1.75 filament it is supposed to be drilled to 2mm as opposed to 3.5mm for 3mm filament (again from the RepRap Wiki).
It could also be as simple as higher temperatures are required at the hot end – I have been trying 180 degrees C but will try pushing the temperature up to around 200-220 degrees C.

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