Metal-Cutting Tools

Metal-cutting or machining processes are the preferred means of fabricating metal workpieces. Constantly increasing demands regarding precision, service life and machining speed have created a need for increased accuracy in the measurement of tool geometry. This is especially the case where specific dimensions of tools directly influence the form of the parts to be manufactured (for example, the diameter of drill bits and reamers, the thread shape of screw taps or the overall contour of profile cutters). In this connection, it is important to make a distinction between tool presetting and tool measurement. These two tasks involve different accuracy requirements which can be met only by system solutions that differ accordingly.

During presetting, the position of certain tool features is measured relative to the tool holding fixture. This information is used to align the cutting tool in the machine tool or machining center. Since deviations with a magnitude of several micrometers usually cause no problems, devices for presetting tools generally have a relatively low measuring accuracy. They are custom-designed for users of metal cutting tools in production.

Both the tool manufacturer and the user’s incoming inspection department must ensure that the tools meet their geometric requirements. High-precision measuring machines are also required to check this with sufficient accuracy. Tool dimension accuracies of only several micrometers, and sometime even in the submicrometer range, must also be detected in order to generate the correction values required for the manufacture of tools. The accuracy of tool presetting equipment is not sufficient for such precise workpiece measurement. For this reason, coordinate measuring machines with air bearing technology are used. The function-determining cutting edges of the metal-cutting tools can readily be measured using image processing sensors in conjunction with a rotational axis. They are screwed into the axial cutting plane and the high-precision measurement is performed in this position. Other sensors are required to measure free-form surfaces, clearance angles, or the geometry of drill bit tips. One possibility is the use of contact styli. However, since their probing elements are relatively large, it is difficult to measure small tools using styli of this type. The Werth Fiber Probe is very suitable for high-precision measuring tasks, and entire free-form surfaces can be measured using the 3D-Patch software.

The generation of program runs for tool measurement frequently represents a problem. Many tool measurements require extremely fast results. This requirement can be met with parameter programs, for example, general program runs created for entire classes of repetitive feature step drills, twist drills, screw taps and hob cutters). For example, a measuring program for a generic hob cutter compliant with the German standard DIN 3968 is stored in the parameter program for hob cutters. The operator can measure a variety of hob cutters ranging from a simple rack hob to a worm wheel hob by selecting the article number or the individual tool parameters (Fig. 61).

Fig. 61: Tools can easily be measured with parameter programs; a) Rack milling cutter; b) Hob cutter; c) Worm wheel hob; d) Reamer; e) Twist drill; f) Step drill g) Screw tap; h) Deep hole drill.

In order to meet any special requirements regarding measuring speed (large number of geometric elements on the hob cutter; production stoppage due to measurement, etc.), linear-drive coordinate measuring machines are preferred. The swiveling sensors make it possible to view the threads of the hob cutters. Many parameters of the hob tooth’s cutting edge (i.e., the base pitch and the profile form) can be measured optically with high precision. The face parameters and slot pitch are measured with mechanical styli (Fig. 62). This set-up makes it possible to measure and inspect the profile of several hundred teeth in just a few minutes.

Fig. 62: Swiveling probe for hob cutters (Werth Inspector® V).

Another problem occurs when measuring grinding wheels and dressing rolls. Their effective geometry is not defined by a single cutting edge but rather by the superposition of numerous cutting elements (grains) when the grinding wheel is turned. The effective contour is also measured using optical coordinate measuring machines with rotational axes. This is done by measuring the contours of grinding wheels and dressing rolls in various rotational positions and then, based on the measured data, mathematically superimposing a circumscribed envelope.

One type of workpiece typical for optical coordinate measuring machines is an indexable insert. The cutting edges of this tool element were formerly measured using measuring microscopes and measuring projectors. Today this measuring task is performed perfectly by image processing coordinate measuring machines. High-speed scanning is combined with maximum precision. ToleranceFit® software represents an especially suitable evaluation technique. The cutting radii are decisive for the service life of the tools and the quality of the final products. A Foucault Laser Sensor and the 3D-Patch software represent an ideal combination for measuring radii with lengths of only a few micrometers. The 3D-Patch also offers the advantage of being able to measure a large number of points simultaneously.

Optical coordinate measuring machines used for tool measurement are also used to perform direct correction of the manufacturing process. For example, it is possible to measure the contours of step drills with optical coordinate measuring machines and then compare the results with the CAD data. In a manner similar to the one described above for wire EDM, correction data can be generated for the tool grinding process and then fed directly to the controller of the grinding machine. This makes it possible to fulfill even the highest precision requirements for tool manufacture.