Extruder seems to have a mind of its own

Hello,
I am posting here because what I am about to describe happens on my Mendel90 although I suspect it is not necessarily a Mendel90 issue....

If I want to remove PLA filament after a print either because I want to change the colour or because I wish to do some maintenance, I wait for the hot end temperature to drop to around 85C, get ready to help the filament on its way out by hand and then issue the following commands manually via Pronterface:

M302 - to enable the extruder motor to function at temperatures below whatever the limit is set to in MARLIN
G1 E-175 F175 - to reverse the filament out of the JHead.

Now and again and more often than not - rather than reverse at the rate I expect, the extruder motor reverses extremely quickly - probably as fast as it can go which is rather disconcerting. To date, this hasn't caused me any problems but I was wondering whether this had happened to anybody else and why it is happening.
Alan

Edited 1 time(s). Last edit at 03/21/2013 04:05AM by Alzibiff.

Hi Alan

I always wind my filament back out by hand because I like to have a feel for whether it is stuck or not. I usually need to add a little manual traction to the filament to get it back out, maybe just a result of the internal profile of the hot end. A few times I tried sending commands for auto-retraction and the hobbed bolt ended up stripping the filament.
Your high speed reverses are reminiscent of an old problem I used to have when there was a mismatch between the Absolute vs. Relative setting for E movement. One is configured in the firmware and the other in the software. Sending a manual command means you need to check which version to send making sure that they are compatible. See here.
For example, when using Kisslicer I have to insert the M83 command because my firmware uses relative E in Absolute mode.

Richard

Thank you for the response Richard. This relative / absolute mode thing did cross my mind yet the phenomenon I described STILL happens if I send an M83 in advance of the retraction code.
I am going to try sending a G1 F175 before the G1 E-175 to see if that makes any difference - to effectively forewarn the blinking thing that I want a 175 feedrate!
On another but similar note, I ran into the relative / absolute E problem yesterday when trying out the latest version of CURA which looks very promising. I started a thread in the Printing section of this forum asking for help which came very quickly from Sublime - worth checking out if you fancy trying yet another slicer!

Alan

Edited 1 time(s). Last edit at 03/22/2013 11:37AM by Alzibiff.