Feed rate Definition

arosura · July 10, 2015

Hi, I'm using CURA, and having fun with it.

Looking for G-code, I have a question.

1. What is the UNIT of the code 'F', in cura? Is it mm/sec?

2. What's the DEFINITION of Feed Rate?

What I know about it is 'The movement speed of print head', but I think there is more meaning.

gr5 · July 10, 2015

Yes it's more complicated. First of all it's in mm/minute so just divide by 60 to get mm/sec.

example: 100mm/sec is feedrate of 6000.

Feedrate is the requested speed for ALL 4 AXES. It's an old term for old CNC and other machines e.g. milling machines and lathes. It is used because certain materials can be cut at certain feed rates.

Anyway - back to 3d printing. It is the requested speed for all 4 axes (X,Y,Z,E) so if you then do a movement in only one axis that axis will go at that speed but is constrained by the max speed for that axis. Also movement is constrained by max jerk setting and by acceleration for that axis. If you make a movement using multiple axes (most often you move X,Y and E at the same time) Marlin will move linearly. It won't move all axes at max speed because then you wouldn't get a straight smooth movement. You want the E axis to speed up at the same time as you speed up the other 2 axes and so on. So the actual movement speed may be slower but Feedrate is the desired speed.

On the control panel for UMO and UM2 you can go into TUNE menu and adjust the requested speed by a certain percentage. Marlin will multiply this by the gcode feedrate to get a REQUESTED speed. The actual speed may be lower, again due to acceleration or max speed on any of the axes that are moving.

It's rare for the Z axis or the E axis max speed to restrict the actual speed. Usually the X and Y axis are the axes going as fast as possible (at the requested feedrage). So most people think of this as the X and Y speed but in reality the "F" parameter controls all 4 axes.