Firmware ignoring speeds lower then 300mm/min

andy1989

The title really says it all.

If I send something like this G01 X100 F100, the printer/cnc moves at what looks like 300mm/min.

Iv tested this on both my Prusa style printer and MPCNC.

Printer is running on a RAMPS 1.4, CNC on a Rumba. 

Repetier

There is a lower limit due to numerical reasons with timing computations. Here is the relevant code:

    float accel = RMath::max(maxAccelerationMMPerSquareSecond[X_AXIS], maxTravelAccelerationMMPerSquareSecond[X_AXIS]);

    float minimumSpeed = accel * sqrt(2.0f / (axisStepsPerMM[X_AXIS]*accel));

if(maxJerk < 2 * minimumSpeed) {// Enforce minimum start speed if target is faster and jerk too low

maxJerk = 2 * minimumSpeed;

Com::printFLN(PSTR("XY jerk was too low, setting to "),maxJerk);

}

as you see it depends on acceleration.

andy1989

Thanks for the quick reply.

So you wouldn't recommend editing these minimum speeds?

The reason this has come up is for my CNC I want to ramp down into the material very slowly. The g code looks like this.

G01 X2.0712 Y-1.5840 F600

G01 X2.1076 Z-0.1500 F6

G01 X2.0712 Z-0.2000 F6

G01 X20.0000 Y-1.5875 F600

The resulting Z feed rate is not 6mm/min. 

Any sugestions how i could get around this?

If i plug straight down into the material it appears to behave as expected, but ideally id like to be able to ramp down. 

Repetier

If you decrease acceleration the minimum speed goes also down according to the formula and that should be no problem for cnc mode. But F6 seems pretty slow.

andy1989

Thanks for the advice. 

Yep F6 is very very slow. I just kept reducing it further and further expecting to see some change.

I will also consider changing back to 32x micro stepping which will also reduce the min speed.

What is the value of f in this equation?