Importance of tool presetting

To most machine tool users, the meaning of endmill presetting is actually closer to endmill length measuring. The endmill is mounted in the toolholder, with the tool length approximately correct. The gage length of the tool is then measured in a presetting station or by a touch-off procedure in the machine tool. The difference between the actual tool length and the tool length used when the NC program was prepared is entered in the CNC as a tool length offset. The control uses the offset to adjust the NC program to correctly position the tool tip.

The U.S. standard for overall length tolerance for endmills is specified in ANSI B94.19-1985. For all endmills except those for heavy-duty use, such as roughing, the overall length tolerance is ±0.062 “, about 1⁄16 “. Many users control the installed tool length with a substantially worse tolerance than that.

Courtesy of BlueSwarf

Figure 1. Shaft collars mounted on endmills permit repeatable tool-length setting with a tight tolerance.

From the point of view of NC programming, the tool is a cylinder and absolutely rigid. If the cylinder length differs from the length that was used during programming, it is a simple matter to adjust the Z-axis position of the machine and place the tool tip in the geometrically correct position. If the geometry of the intersection of the cylinder and workpiece were all that mattered, this strategy would work. However, the length of the tool affects much more than the geometry.

The cutting performance of the machine-and-tool combination strongly depends on tool length (see my June 2009 and March 2010 columns). Tools set at the wrong length can easily cause chatter. Random procedures in tool length setting lead to random machining performance.

Courtesy of BlueSwarf

Figure 2. A shaft collar on an endmill, aligned with a witness mark on the toolholder, provides a repeatable cutting-edge position and improves balance.

Controlling tool length is important, and, fortunately, there are some simple procedures to set the tool length repeatedly with a tolerance at least an order of magnitude better than the ANSI standard.