Precision for medical technology
- Measuring Components

Measuring technology by BLUM at Günter Stoffel Medizintechnik GmbH in Wurmlingen, Germany

When manufacturing very small parts, even a very minor temperature-related fluctuation in the machining centres can mean rejection. This is why Günter Stoffel Medizintechnik GmbH has turned to production metrology from Blum-Novotest for temperature compensation as well as other measuring tasks – after all, the smallest workpieces produced by the company 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. It comprises an interesting mix of state-of-the-art CNC manufacturing and genuine traditional craftsmanship. And 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 0.01 mm 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, this places the highest demands on the abilities of the employees. In order to reduce manual work to a minimum, it has become of immense importance for the company from the city Tuttlingen, Germany, to produce as high-precision parts as possible using the CNC machines. "During assembling, every one hundredth of a millimetre becomes noticeable," explains Managing Director Dieter Stoffel. "Consequently, the demands that we place on the CNC machines is also extremely high."

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' machinery 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. "One problem is our very thin tools," explains the responsible production manager, discussing the advantages of the Z-Pico. "They include a 0.5 mm centre drill, where the tip will break with the slightest wrong load, with the drill becoming two to three micrometres shorter." Naturally, this has a corresponding effect on the precision. The Z-Pico has an extremely low measuring force, making it ideal for use with small tools. 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. "Tools with a diameter of 0.05 mm or more can be registered," stresses Erhard Strobel. "In addition, the switch signal is generated optoelectronically by shading a miniature light barrier inside the device. This principle is wear-free and delivers consistently high measuring precision even after millions of cycles of operation."

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 correction factor generated from the measurement is then written in all tool tables, so that the next roughing operation already benefits from the measurement. 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 systems 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. This is very simple to do with the U-shaped laser measuring system, requiring no other equipment, all that is required is to move the tool in the Y direction in the laser beam to determine a Y correction factor, approached from above, the correction for the Z axis is determined as with the Z-Pico probes.

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 which cleans the reference tool prior to measurement – so it is always correct." The production metrology from BLUM has completely proven itself in Wurmlingen. Whilst previously it was necessary to allow the machine to warm up for 20 minutes, and despite this, a few bad parts were still produced before the desired precision was achieved, the machine now runs empty two or three times in the morning and then, as a rule, good parts are reliably manufactured all day long. The medical devices produced by Stoffel GmbH have also become more accurate thanks to the BLUM measuring systems. "Furthermore, this has meant that the impression which surgeons gain of the instruments quality has once again improved somewhat.

This cooperation with BLUM is also praised within Stoffel, for example, the engineer who installed the systems was not satisfied until the system ran perfectly, in addition, the measuring technology manufacturer is always available with advice and support. With this in mind, it is hardly surprising that Stoffel is completely satisfied with the systems from BLUM, particularly because the major risk from the manual input of compensation values no longer exists due to the BLUM measuring systems communicating with the machine controller. "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."