Taking Full Advantage of Geometry and Wear Offsets

FANUC and other control manufacturers provide two sets of offsets for their turning center controls. One set, the geometry offsets, is used to help with program zero assignment. The other set, the wear offsets, is used to help with sizing adjustments. It is important to fully understand these two offset types since they provide a benefit that many CNC users overlook.

Again, geometry offsets are used to assign program zero. By one means or another, the setup person determines the distances in X and Z from the tool tip at the zero return position to the program zero point. The actual procedure varies based upon whether or not the work shift function is being used, but generally speaking, geometry offsets contain rather large negative values.

Wear offsets, on the other hand, are used to deal with adjustments that cause cutting tools to machine surfaces within their tolerance bands. If, for instance, a finish turning tool is machining a turned diameter 0.002 inch oversize on the initial workpiece, most setup people will make the (-0.002-inch) adjustment in the X axis register of the tool’s wear offset.

In order to understand the benefit presented here, it is important to know that happens when a set of geometry and wear offsets is invoked. With FANUC controls, the first two digits of the 
T word will select the tool station and invoke the geometry offset. The second two digits 
invoke the wear offset. So the word “T0303” indexes the turret to station three, invokes geometry offset number three and invokes wear offset number three.

For each axis, the machine will simply add together the values in the geometry and wear offsets to come up with a total offset. If a value of -12.0272 is in the X-axis geometry offset register and -0.0020 is in the X-axis wear offset register for the offset being invoked, the total offset will be -12.0292.

As stated, the setup person does his/her best to determine the geometry offset values for each tool during setup. A common misconception is that if the geometry offset is perfectly measured, the cutting tool will cut machined surfaces perfectly to size on the first workpiece. But there is almost always a small deviation caused by tool pressure when the first workpiece is machined. This deviation may be small, and the surface may even be within its tolerance band, but most setup people want to set each tool so that machined surfaces are coming out to their target values (commonly the mean value of their tolerance bands).

Most setup people will use the wear offset in which to make initial adjustments. However, the deviation that caused the need for the adjustment (tool pressure) has more to do with program zero assignment than it does with tool wear. Note that this initial adjustment could be made in the geometry offset in exactly the same way it is made in the wear offset. That is, if the turned diameter is coming out 0.002 inch oversize, the tool’s geometry offset could be reduced by 0.002 inch. This way, the production run will start with the geometry offset set perfectly and with the wear offset set at zero.

As the production run continues and finishing tools show signs of wear, additional adjustments must be made. After 50 workpieces, for example, the diameter being machined by the finish turning tool may be growing close to its high limit. The tool is not dull—it is just showing signs of wear, and the operator will reduce its wear offset accordingly. Several such adjustments may be required during the cutting tool’s life.

At some point, the finish turning tool will become dull and its insert will have to be indexed or replaced. After replacement, the operator must also deal with the wear offset for the tool. The new cutting edge will not be in the same location as the cutting edge for the dull tool that has just been replaced. If the insert can be perfectly replaced (as is often possible when indexing inserts), the new cutting edge will be in the same position as the dull tool’s cutting edge was when it was new.

If the initial adjustment has been done in the geometry offset, the operator can simply set the wear offset back to zero and the cutting tool will machine just as the last tool did when it was new. Even if the insert cannot be perfectly replaced (possibly the insert is being replaced with a new one—not just indexed—and inserts are varying in size), the operator will have a point of reference for resetting the offset. In this case, trial machining will probably be necessary, and at least the operator will know how to set the offset prior to trial machining.

Admittedly, how you make the initial adjustment isn’t as important with short production runs when cutting tools will last for the entire run without becoming completely dull. But with larger lots, using this technique can dramatically simplify the task of resetting offsets after dull tool replacement. And if inserts can be perfectly indexed/replaced, this technique can eliminate the need for trial machining, which in turn will reduce production run time.