Measuring Tool Length For Micron Accuracy

When the smoothness of a mold surface or the dimensions on a part feature are toleranced to the micron level, measuring the length of the cutting tool in the spindle to the same level of precision becomes critical. Makino (Mason, Ohio) has introduced a method for accomplishing this task. The method takes into account not only the position of the tool tip but also the effects of thermal displacement in the spindle. This requires measuring the cutting tool in the spindle at a static (non-rotating) state and then measuring the spindle end while rotating with the cutting tool in place at the speed programmed for machining. Because rotating the spindle inevitably causes some slight thermal growth or deformation, this method can measure that change and, if necessary, factor it into the calculations for tool length compensation values.

Because this method involves contact and non-contact sensing devices, it is called a hybrid automatic tool-length measuring system. According to developers, applying this system allows cutting tools to be changed and still hold variations in dimensional and surface roughness to 1 micron or less from tool to tool. This is possible because the system can adjust for thermal displacement of the spindle rotating at different speeds along with the slight inaccuracies in tool tip position resulting from automatic tool changes. Thus, the results of semifinishing and finishing operations that call for cutting tools of various length are not affected by the tool changes.

This system is available as an option only on the company’s Hyper 2J, V22 and V33 vertical machining centers. After the machine’s automatic tool changer has changed a tool, initiating a macro routine makes the spindle move to the measurement unit mounted on the machine’s table, where a retracting cover exposes a contact sensor. The non-rotating spindle touches the tip of the cutting tool on the pad of the sensor and determines the tool length from the spindle end. Very little force is required to trigger the measurement. Therefore, extremely delicate tools, which may be under a millimeter in diameter, are protected.

The spindle moves away and begins to rotate at the rpm set for that cutting tool. It then moves to a non-contact sensor located adjacent to the contact sensor in the same enclosure. Because the spindle is rotating at speed, the effects of thermal displacement become apparent and can be detected by the non-contact sensor. The macro directs the spindle to make multiple passes at the sensor in 30-second intervals until three consecutive readings do not deviate from each other above a set limit. This ensures that the spindle has reached a stable condition before deriving the length of the spindle end.

By comparing the positions of the tool tip and the spindle end, the system can determine if the tool length value should be adjusted to compensate for apparent spindle growth. For example, if the difference between the non-rotating and rotating tool length is greater than 1 micron or whatever limit the user sets, then an adjusted tool length value will be used in tool comp calculations.

The hybrid system is designed for users who must maintain very accurate blending of surfaces from tool to tool in micromachining applications. In mold work, achieving these smooth surfaces greatly reduces or eliminates polishing.