Programming a CMM to probe a slot can be made very simple with automatic routines requiring only a few “taught” points. But what if you don’t have the part and need to program off-line before the part arrives? Or how do you address part variability so that the program works without need for constant supervision to make sure that the slot is properly recognized each time? The ability to view and study a CAD Model while programming and take advantage of the predictable line and circle geometry of the slot provides an answer to these questions.

Recently on a job, a slot served as the tertiary alignment datum on a part we were asked to program for and timing required the program to be ready and running near flawless when parts arrived. But not having the actual part I was faced with the decision of feeding the automatic slot probing strategy some points worked out from geometry, or trying to program something more robust that could handle small shifts of the slot location or orientation and still create a reliable, repeatable datum structure.

It came down to ensuring that all the probings intended to lie on the flat sides would “land” as such and all those intended to lie on the round ends would also find their target areas. But just programming straight off nominal geometry could lead to misrecognition if a prototype with enough deviation arrives.

The approach that worked was first hunting for the slot with early “survey” probings and then using these to refine the subsequent points using formulae. In other words we took probings across either the width or length, calculated a symmetry point and used that to predict accurate positions for probings along the other axis. Depending on which provides a larger target to seek, one may first search for the two straight sides or the tow radii at either end of the slot. A simple spreadsheet was created to plot points anywhere around the entire slot and thus handle any combination of length, width, and orientation.

By Stanley Schnuerer
Magna Powertrain

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