Speed fluctuations

Influences

The following parameters/influences must be checked if extreme speed fluctuations occur during processing of programs.

• Data throughput of the NC
• Interpolation point profile
• Dynamic characteristic quantities of the axes
• Slope type
• Active mapping functions (transformations)

Dynamics

Generally, it can be said that high permissible accelerations and short ramp times have a favorable influence on the maximum possible contouring speed. 1.8 provides a list of the dynamic characteristic quantities used by the CNC.

Data throughput

To ensure that a programmed contouring speed can be run, the following influences must also be checked:

• Interpolator buffer size
• Required number of blocks/cycle

The buffer size defines the braking distance and thus the theoretically maximum attainable speed. In the case of the non-linear slope function, the braking distance is fundamentally longer and so a larger buffer and number of records per cycle are needed to achieve the same contouring speed. The number of blocks/cycle needed after stabilization can be estimated on the basis of the feed rate and the cycle time. For example, a travel distance of at least 0.84mm per sampling interval is needed in the case of F=10m/min and a cycle time of 5ms per cycle.

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

Generally, the user can only partly influence the following parameters.

Contour profile and curvature

Due to the accuracy requirements for the workpiece, the parameters are specified by the CAD/CAM system. A workpiece with extreme contour curvature generally leads to relatively short motion blocks and a low speed. If the contour curvature changes considerably, speed fluctuations with a high programmed feed rate cannot be avoided.

It is important that the interpolation point profile is harmonious, especially if NC blocks are short, i.e. the fluctuation width of the relative distances of the individual axes referred to the distance in space of successive blocks must be small (scanning by the CAD/CAM system). Otherwise, it is not possible to operate with a constant contouring speed because the CNC reduces the contouring speed due to resulting curvatures and curvature changes.

A certain amount of smoothing of "noisy" interpolation points can be achieved with the B-spline.

Nevertheless, the resulting contouring sped profiles are not optimal, and the axis speeds fluctuate considerably depending on the motion components of the single axes. It is advisable to use a suitable smoothing algorithm in the CAD/CAM system here.