Sizing With Tool Nose Radius Compensation

Most turning center users use wear offsets for the purpose of holding size over the course of a production run. And with most of the tools used on turning centers (turning tools, boring bars, grooving tools, among others), all production run sizing should be done with wear offsets. And since tool nose radius compensation does not help with sizing, most computer aided manufacturing (CAM) system programmers will generate motions based upon the tool nose radius size to be used and will not even use the CNC-control-based feature tool nose radius compensation.

There is at least one time when tool nose radius compensation can help with sizing right at the machine during a production run. Whenever the insert of a tool must machine with over 90 degrees of its cutting edge is probably a good time to think about using tool nose radius compensation for sizing purposes. Consider, for instance, button tools that can machine with up to 180 degrees of their inserts. Look, for example, at the diagram at right.

In essence, this button tool can machine on both sides of the round insert during one cutting motion around the sphere. As the tool begins to wear, the diameter of the sphere being machined by the button tool will change. In the example, normal wear offsets will be of little help when it comes to the sizing required due to tool wear, since adjusting them would have the tendency to make this sphere egg-shaped.

Instead of adjusting wear offsets to hold size in this kind of application, the tool nose radius compensation offset (commonly specified in the offset table under the R register) will be changed. Say, for example, that after 50 workpieces the sphere shape being machined in figure one grows by 0.0004 inch in diameter due to tool wear. In this case, the R register offset value (commonly the tool’s radius) can be reduced by 0.0002 inch. The next time the program is run, the control will keep the tool 0.0002 inch closer to the surface being machined, reducing the diameter of the ball by 0.0004 inch. Sizing with tool nose radius compensation on turning centers in this fashion is similar to sizing the XY motions on machining centers when cutter radius compensation is used.