Günter Stoffel Medizintechnik GmbH has turned to production metrology from Blum-Novotest for temperature compensation as well as other measuring tasks.

The company turned to Blum because the smallest workpieces produced are less than one millimetre wide and extremely precise. A microscope mounted on the vice – nothing is a more appropriate symbol of the manufacturing method at Stoffel GmbH which specialises

in the development and manufacture of high-quality surgical and endoscopic instruments.

The requirements are extreme, the spoons for the smallest biopsy forceps are 0.8 mm wide, the blade width on the edge of the spoon is just 0.01mm and the two blades must meet precisely when closed. There are many work stages to be carried out by hand, such as deburring, polishing and adjusting for perfect function or riveting of the forceps joint.

Chip removal is carried out at Stoffel in a basement without air conditioning. The machines working there have computational temperature compensation, which calculates the compensation values using data such as travel distance, spindle speed and usage duration. In a normal manufacturing operation, that would function reliably, however, with frequent tool changes, internal compensation reaches its limits.

The problem was discussed during a joint visit by Blum sales representative Erhard Strobel and the responsible ‘W&R’ machine tool dealer. The solution was the Z-Pico touch probe, which facilitates the very precise measurement of the length of the tools and the corresponding compensation of the axes. Inside the probe is a high-precision linear guide which ensures there are absolutely no lateral forces, thereby enabling the measurement of very small, sensitive or long tools.The two measurements for temperature compensation are only configured in the NC program after the rough machining operations, because these do not require a high degree of accuracy.

The finishing tools are measured directly before machining, at which point the temperature behaviour of the tool holder is also considered. As each part is measured using the Z-Pico and the correction factor is continuously adjusted, all temperature fluctuations throughout the day are reliably intercepted. The measuring process requires only a few seconds, because the tools are positioned for measurement shortly before the longest possible tool dimension with a rapid feed rate.

In one of the machining centres, in which predominantly round-handled instruments are manufactured, a Blum LaserControl Micro Compact NT laser measuring system is used instead of the Z-Pico. When machining the round handles, the precision of the Y axis is of particular importance. This meant it was necessary to measure not only in the Z direction but also in the Y direction.
Since Stoffel has been using the Blum laser, an increase in length is no longer an issue, the system corrects both the misalignment of the tool as well as the axis and the table itself. “Some NC operators mill a circular pocket in the surface of the machine table and touch this with a probe, but I think that is too uncertain,” explains Dieter Stoffel. “We have such small chips that one could become trapped in the pocket, thereby distorting the measurement. The laser measuring system is fitted with a special blowing system that cleans the reference tool prior to measurement – so it is always correct.”

The production metrology from Blum has completely proven itself in Wurmlingen. The medical devices produced by Stoffel GmbH have also become more accurate thanks to the Blum measuring systems.

“In the past, we measured manually and then manually entered the compensation values,” reports Dieter Stoffel. “If you made a mistake for any reason, it could lead to catastrophic crashes in the machine. Today, the machine stops automatically when the compensation value is greater than four hundredths.”

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