Probing Thoughts About, Well, Probing

Sophisticated CNC can do much more than just find the edge for you. It also improves part production and repeatability on prismatic and complex 5-axis jobs alike . . . and it keeps score!

Typically, a programmer will use standard cycles for probing edge, corner, angle and center line positioning on a part. As long as the part is properly fixtured, this method is basically goof-proof, assuming the probe you use is capable of three-axis measurement logging. If, however, you are faced with a misaligned blank, an out-of-spec blank or the need for logging the data for SPC and other offline use by your quality department, the challenges can be many, unless the CNC is capable of handling these conditions.

For example, when you first setup a part and all the above conditions have been probed, you are essentially performing the same steps, each time. Measure, measure and setup offsets, check distances in manual mode, all from the softkeys with standard cycles. Today, however, the more advanced CNC can do all that plus in-process checking with a record of all conditions being logged on a simple screenshot and offloaded to another department for analysis or tracking.

Also, let’s say the wrong part is loaded in a fully automatic, lights-out mode. A fully-featured CNC, through the macro program, can detect that condition and send an automatic alarm with a simple tag such as “wrong part” or “see foreman” or “wrong set-up” instantly displayed on the monitor or a remote terminal.

Another condition might be the tool measurement, where length and diameter are probed at start-up, then remeasured after multiple holes or slots are machined, with instant comparative data logged on the before/after conditions. In this manner, evaluating tool life, tool suppliers, differing tool materials for various jobs and a host of other data management functions can be performed, all using the standard probe cycles on the CNC. This applies to all prismatic parts made on any three-axis machine with standard probe attachments. Whether probes or ring gages, the better CNC can capture all the initial values and save them to R variables. This allows you to create your own file quite easily.

For the more complex parts on a four-axis or five-axis machine, where a multiple measuring ball system might be in use, the advanced CNC today can measure all probed points, import them into a matrix and, when the cycle is completed, all actual data points can move into a running table for “behind the scenes” analysis. Further, with a properly calibrated probe and ball system, multiple rotary axis vectors can be measured and compensated for temperature fluctuations, mechanical deformities and other off-normals, thereby greatly improving process quality.

This is all done in a standard (yes, I said standard) cycle on the CNC. A ball is mounted on the work table for the measurement. The ball is measured with the probes in three angular positions for each rotary axis and the axis vectors in that plane. When all rotary axes have been measured, the complete kinematic calculation is performed.

Best of all, these techniques can be used without writing any custom code on the machine.

Next month, I’ll address some of the macro programming issues that spin off these and other considerations for the most advanced CNC technologies in the job shop and other market segments.