Relevant block length

As a rule, the minimum curve length is limited because of the mathematical resolution of the controller (given by the number of significant digits of a REAL value). At the moment, this length is 31.7 micrometres. Path segments less than this minimum curve length generate an error message and result in abortion of the contouring function at the current block transition.

In addition, the contour can include very short compensation blocks which are inserted by a programming system (CAD/CAM) or by tool radius compensation. After compensation the block retains a continuous path.

To avoid abortion of contouring by these short blocks, a minimum block length can be defined. From this point onwards, polynomial contouring is then relevant for the post-block. Shorter blocks are skipped during active contouring, i.e. contouring is considered in the following block.

Here, a limit for the motion path of the main axes in space as well as a limit can be specified for the motion path of the tracking axes. The block is skipped completely only when both the motion path of the main axes and the motion path of the individual tracking axes are below the specified limit. Polynomial contouring combines the pre-block and the post-block in direction and curvature-continuous function. The initial blocks need not be adjacent (contour need not be continuous).

If a block is skipped, the maximum corner deviation of the main axes and tracking axes can only be approximated. This means, it is assumed that the deviation of the contouring can be ignored in the skipped blocks.

Programing Example

prg_example

Relevant block length

#CONTOUR MODE [DEV, PATH_DEV 5, RELEVANT_PATH 2]

N03 G01 X0 Y0 Z0 C0 F4

N907090 G04 X0.1

N04 X5 G261

N05 Y1

N09 X10 Y3 G260

N907091 Y0

Example of skipping a short block N05 when contouring
Example of skipping a short block N05 when contouring

Special case 1: Sequence of multiple short blocks behind the block transition

If multiple sequential blocks (N20, N30, N40) are shorter than the minimum motion path specified, the blocks are skipped provided the distance to the target point from the last relevant end point (N10) is shorter than the specified minimum motion path. If the target point of the skipped block is outside this envelope curve, the block (N40) is used to calculate the contouring curve even if it is shorter than the specified minimum length. This method permits a slight deviation from the original contour even if multiple sequential blocks are skipped.

Some single blocks (N20, N30 and N40) are too short but the target point is outside the minimum block length.
Some single blocks (N20, N30 and N40) are too short but the target point is outside the minimum block length.

Special case 2: Sequence of multiple short blocks behind the block transition, last block is extremely short

As an exception, the block N40 itself may be shorter than the minimum system-specific length (about 15 micrometres) required for contouring. In this case, the last end point and the new target point are connected by a linear block. This new linear block N20´ is then used to calculate the contouring curve.

Single blocks (N20, N30 and N40) are too short but the sum of all blocks exceeds the minimum system-specific block length.
Single blocks (N20, N30 and N40) are too short but the sum of all blocks exceeds the minimum system-specific block length.

Special case 3: Short blocks before the block transition

If the blocks at the beginning of contouring (before block transition) are already shorter than the minimum system-specific length, the blocks are skipped. The blocks are skipped until the distance between the last valid point and the current target point exceeds the minimum block length. If this is the case, the last end point and the current target point is connected by a linear block N10’. This linear block then is used as the start block for contouring.

Multiple blocks (N10, N20 and N30) are too short but the sum of all blocks exceeds the minimum system-specific block length.
Multiple blocks (N10, N20 and N30) are too short but the sum of all blocks exceeds the minimum system-specific block length.

Special case 4: Deselecting contouring or changing parameterisation

If contouring is deselected when blocks are skipped or the basic conditions for contouring are changed, the current contouring may only be continued up until they are deselected or parameters are changed. After this function, contouring may be continued using the new parameters.

Some single blocks (N20, N30 and N40) are too short but contouring is deselected as of block N20.
Some single blocks (N20, N30 and N40) are too short but contouring is deselected as of block N20.