irregular extrusion

Hello Lovely folks. This is my first post here.

I am a technician at a school in the UK, where we have a student built Printer (ORD BOT I think).
It has worked really well until recently when every few seconds there is a Kerchunk in the extruder stepper motor, resulting in terrible prints.
If I manually extrude via Repetier the flow is smooth and even.
I have replaced the motor, pinch bearing and nozzle, but to no effect. It seems to be something in the drive or the translation from model to g-code to step pulses.
Any help would be massively appreciated as students are queing up.

Thanks in advance.

Allan

The kerchunk sounds like the extruder is skipping. This generally means that there is too much resistance to the filament being pressed into the hot end. This can happen when the nozzle is too close to the surface, or when the hot end isn't hot enough to melt the filament that fast. Sometimes if the motor is being commanded to go faster than the driver can handle.

The motor will skip steps if there's too much resistance. In my printer, the filament will skip at the roller before the motor will skip steps.

The noise is almost certainly proof that the electronics are commanding the motor to turn properly, but that there is a mechanical issue preventing the filament from moving as expected.

If, on the other hand, the noise is a "pop" and you are getting a gap int he extrusion, then that's likely to be moisture absorbed into the filament, and it needs to be dried.

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MBot3D Printer
MakerBot clone Kit from Amazon
Added heated bed.

Leadscrew self-built printer (in progress)
Duet Wifi, Precision Piezo parts

Thanks for your reply Supraguy.

So on the basis that it was resistance in the filament feed I just did a dry run with no filament. I still get the kerchunk! almost as if it is trying to retract while extruding.

Looking at the last couple of tests I did the first few layers are smoother than the later ones.

With no filament it still does it? that eliminates most common issues...

I would start checking for a broken wire on the extruder stepper motor, probably at the Hot end.

A easy way to test would be to bring the hotend to temperature and without filament tell it to extrude some 100mm of filament (G1 E100 F120)
While its doing that wiggle the steppers wires. See if you can cause the noise.

Fixed!

Turns out the fan that cools the control board had stopped, making the stepper driver overheat.
Confirmed by running a test with the replacement fan unplugged until the Kerchunking started. Plug the fan in and it stopped almost straight away. Brilliant!
Test print, perfect.

Thanks to all who read and contributed.