Five axis tool compensation

Modern Machine Shop, Oct, 1990 by Golden E. Herrin

Five Axis Tool Compensation, Part II

As discussed last month in Part I, there is a wide variety of machine geometry configurations which restrict control builders from providing a universal tool compensation solution in CNC's. In addition, there is a lack of agreement among users as to exactly what is expected of 5-axis tool compensation. Generally, CDC (cutter diameter compensation) is expected to adjust the programmed cutter path to compensate for a cutter which is a different diameter than the one programmed (see Figure 1). In this illustration, the location of the workpiece is defined by "G" codes in the part program as being to the right or to the left of the cutter. The automatic CDC feature in the control generates an XYZ cutter path correction which is normal to the edge of the cutter for the amount of cutter diameter deviation dialed in by the operator.

CDC is sometimes referred to by control builders and users alike as CRC (cutter radius compensation). These two terms, CDC and CRC, are normally interchangeable but in some situations where a user may need to correct not only the diameter (or radius) of the cutter but also the radius of the ball on the nose of the cutter, the terms may have different meanings. This feature is more correctly referred to as "Tool Nose Radius Compensation."

Solving the correction vector for tool nose radius compensation is not only a challenge to solve in 3 axis mode but becomes an infinitely more difficult task to solve in the 5 axis mode. In order to set up an offset in the proper direction, the point of contact between the cutter and the workpiece must be known. This point of contact (as illustrated in Figure 2) could be at any point on the cutter diameter or ball. Since the part surface definition necessary to define the point of contact does not normally reside in the CNC, additional information must be supplied or generated internally.

The nose radius compensation presents real challenges, some of which have no valid solution. For example, it is generally accepted that tool compensation should be operator selectable for both undersize and oversize cutters. In a situation where the tool nose is used to form the desired radius in the workpiece (Figure 3), there is no solution for an oversize cutter since path correction can do nothing to make the radius generated be an oversize cutter any smaller. If the correction is for an undersize cutter, the control would be required to generate additional passes to approximate the desired radius on the workpiece. This is more processing capability than available in most CNC's.

No single control builder has yet solved all of the 5-axis tool requirements for all machine configurations although some are willing to create special designs when requested. In addition to special designs, there are other available solutions and some emerging technologies which hold much promise for the future of 5 axis CDC.

One of these solutions applicable to 5 axis TLC is BCL input which is available for most machine configurations. Five axis TLC turns out to be a fringe benefit of BCL in that it requires the postprocessor to be embedded in the control and in an APT programming system, tool length is handled by the postprocessor. The CNC utilizing the embedded post processor recalculates a new tool pivot point to take into account an operator modified tool length. Unfortunately BCL can not handle changes to the cutter diameter since these calculations are made in section III of APT at the host computer level and are not available to the operator.

Another method available in some CNC's to solve 5 axis CDC is POR type CDC. With this version of CDC, POR-axis vectors are generated off line by the APT programming system which correspond to axis vectors for SYZ respectively. These POR vectors are passed on to the CNC in the part program and are used by the automatic CDC feature to calculate offset vectors normal to the work surface. These vectors are always set up in the control but only performs a correction if the operator inputs a cutter diameter deviation.

Each new generation of CNCs brings significantly more computing power to the user. This additional computing power has made possible DOS partitions in certain CNCs which provide a building platform for many enhanced features one of which could be a scaled down version of a 5-axis APT processor complete with an embedded postprocessor. With this capability, not only is shop floor programming possible in a 5-axis mode but changes to the tool length and diameter for 5-axis operation would be equally possible. Just change the length or diameter and reprocess the part program.

Even though 5-axis tool compensation may not be an available feature on your 5-axis machine today, there is an ever increasing amount of computer-aided tools and software waiting to be brought together to solve the 5-axis tool compensation problem. [Figures 1 to 3 Omitted]

GOLDEN E. HERRIN, Control Product Specialist Cincinnati Milacron, Cincinnati, OH 45209