Extruder calibration - need a multiplier of 0.83

I'm printing PLA on an RRP Huxley with Marlin firmware and using a 0.5mm nozzle.
I've calibrated my extruder drive so that when I extrude 100mm of filament, 100mm is used.
I've measure the filament diameter several times and checked it is circular not oval, it is about 1.78 (meant to be 1.75mm).

Plugged all this into Slic3r (0.9.7) but when I print a single walled cube with 0.3mm layer thickness by 0.5mm width I need to reduce the multiplier down to 0.83 to get 0.5mm wall thickness.

I recall reading elsewhere that others have to do the same (of course i can't find those links now).

Is this normal or should I be checking / calibrating something else ?

Thanks, Andrew

Edited 1 time(s). Last edit at 12/17/2012 08:26AM by AndrewC.

... the extruded filament should be slightly thicker than 0.5mm when using a 0.5mm orifice, so when squeezing it down to 0.3mm you have to compensate this 'swelling' too ...

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So it is normal then, and to print dimensionally accurate shapes i need to compensate like this ?

Thanks

Andrew

I have the same issue - I don't see what the point is in calibrating the extruder if that makes it extrude too much.
I get a lot of blobs and have to "tune" my extruder down until it stops blobbing.
I keep thinking though that I should be changing some different aspect of the system and slicer, and leaving the extruder calibrated accurate.
Does anybody know?