FAQ velocity fluctuations

Influencing factors

In general, machining speeds and, as a result, production times depend on the following factors:

• data throughput dependent on the cycle times of the processes in the NC controller
• motion path of motion blocks
• maximum velocities and accelerations of axes
• type of profile generator (slope)
• Path profile: curvatures and uniform distribution of programmed positions.
• Cartesian transformations in the tool coordinate system and kinematics

Dynamics

In general, it can be said that high permissible accelerations and short ramp times have a favourable effect on the maximum possible path velocity. A list of the dynamic parameters used by the CNC is provided in the Parameters section.

Data throughput

The following factors must also be checked to ensure that a programmed path velocity can be achieved:

• interpolator buffer size
• required number of blocks per cycle

The buffer size determines the braking distance and therefore the theoretically maximum achievable velocity. The braking distance is generally longer with the non-linear slope function; therefore, a larger buffer and more blocks per cycle are required here to achieve the same path velocity.

The number of blocks per cycle required after stabilisation can be estimated from the feed rate and the cycle time. For example, at F=10 m/min and a cycle time of 5 ms per cycle, motion paths of at least 0.84 mm per sampling interval are required.

These parameters are specified by the machine manufacturer and cannot be changed by the user.

Contour path and curvature

The parameters are specified by the CAD/CAM system based on the precision requirements for the workpiece. A workpiece with a pronounced contour curvature generally results in relatively short motion blocks and low velocity. If the contour curvature varies significantly, fluctuations in velocity are unavoidable when the feed rate is set high for the path.

It is important that the path of the interpolation points is smooth, especially with short NC blocks. This means that the fluctuation range of the relative motion paths of the individual axes referred to the motion path in space of successive blocks should be as small as possible (scanned by the CAD/CAM system). Otherwise, it is not possible to move at constant path velocity since the resulting curvatures and changes in curvature cause the CNC to reduce path velocity.

A B spline allows a certain degree of smoothing of “noisy” interpolation points.

Nevertheless, the resulting path velocity profiles are not optimal and the axis velocities fluctuate significantly depending on the motion components of the individual axes. In this case, it is recommended to use a suitable smoothing algorithm in the CAD/CAM system.