4.3 Acceleration

The print files for any print include directions that determine the speed at which each line segment is printed. As such, different portions of the print may print at different speeds, causing the printer to speed up or slow down as it transitions between speeds. By default, when a change of speed is required, the printer effects the change by carefully accelerating to higher speeds or decelerating to lower speeds. It is possible to turn off this feature by setting the acceleration parameter to “off” (Section 3.7.4.5). There is no practical reason to do this as printing with acceleration enabled — the default — results in smoother, quieter prints that obtain higher speeds. Turning off acceleration when executing commands at speeds in excess of 30–40 mm/s may cause damage to your printer.

4.3.1 Maximum Accelerations

When acceleration is enabled, your printer needs to pick rates at which to accelerate or decelerate when transitioning between the commanded speeds in a print file. The maximum acceleration parameters limit how quickly the transition can occur on each axis: X, Y, Z, A, and B. Note that the A and B axes are the right and left extruders — if you have only one extruder, then there is no B axis.

The maximum for an axis limits both the acceleration and deceleration with one value that is used for both. The default maximum acceleration values are listed in Table 4.1.

If your prints show noticeable overshoot at, say, 90 corners in cubes, then it may well be that your printer is attempting to decelerate faster than its mechanics actually allow. The printer has no idea how fast it can really accelerate or decelerate — it has no built-in feedback system to tell it when it overshoots. As such, specific printers may need to have their maximum acceleration values changed. For example, the defaults use the same value for both the X and Y axes. But, in practice, many printers move far more mass around on their Y axis than their X axis. With the added mass, the Y axis may not be able to accelerate or decelerate as fast as the X axis can. Therefore, the maximum acceleration for the Y axis probably should not be the same as that for the X.2

4.3.2 Maximum Speed Changes

The maximum speed changes regulate the maximum instantaneous changes in speed the printer may effect in transitioning between speeds without carefully accelerating or decelerating through intermediate speeds. This upper limit is imposed by the maximum X, Y, Z, A, and B speed changes as expressed in units of mm/s. Note that, while high values can increase printer vibration and even cause it to jerk about, excessively low values significantly increase print times. Refer to Table 4.1 for a list of the default maximum speed changes.

2If you see overshoot at corners, it may also be a sign that you need to tune the JKN Advance parameters K or K2. If the overshoot occurs even at moderate speeds, then see Chapter 5.