Perfecting the process

Fortunately the inspection conundrums were short lived.

Earlier this year, the company took delivery of a new turn/grind centre from EMAG. The VSC 400DS had a specific task – to combine turn/grind machining of carbide roll rings and carry out measuring operations to optimise the production process.

By means of an integrated measuring system, the workpieces should be measured between manufacturing steps and the tools checked indirectly. The diameter on the carbide ring is collected with a probe system, compared with the target data and then turned to the correct diameter. If the diameter is finished, the calibre position and depth are measured, then re-measured after pre-turning, turned to size and then the dimensions should be accurate. That was the theory.

Saarstahl AG is am established company that specialises in the production of wire rods, steel rods and semi-finished parts plus contour forgings. Customers are from the automotive, construction, energy, aerospace and general mechanical engineering industries.

“In the beginning we underestimated measurement as we had no previous experience of this approach,” explains Michael Molter, a technician at Saarstahl. “The circumferential grooves on the carbide ring together with the ring grooves on the counter roll are the profile of the roll product, called the calibre. The carbide ring has two calibres and can be mounted on both sides, therefore each front side must be the same distance to the centre line of the calibre otherwise there will be an offset. The calibres of the carbide rings work with wires from 8.5mm to 25mm diameter with a tolerance of ± 0.15mm. 

In-process problems

Previously, every carbide ring was measured offline after machining at a measuring station with a height measuring device. This complex task should have been taken over by the new turn/grind centre through a probe system for machine integrated measurement supplied by EMAG.

To collect the precise position of the calibres, trigger contacts must be carried out on the complex geometries. The first probe provided only moderate repeatability, which was a real problem that created lobing measuring characteristics with different deflection forces.

The staff therefore defined a correction value for each calibre and incorporated it into the following measurement and checked the values offline at the measuring station. Andreas Braun, roll machining manager at Saarstahl explains: “The probe system was accidentally damaged and replaced by a new one from the same manufacturer.

Both probe systems had such wide tolerances that our staff had to re-define the correction values and had to interpolate the data to get the approximate target values. Measurement with the electronic height measuring device on the measuring position failed, too – sometimes the values were within the tolerance, sometimes outside of the tolerance. We had no in-process quality.”

Problem solved

Due to the extent of the problems, a design engineer from EMAG proposed an alternative from measuring specialists Blum-Novotest in Ravensburg – its TC76 probe system. The system was developed to measure workpieces and tools in turning and grinding machines, so no sooner had the system been mounted, all measuring inaccuracies disappeared.

Uwe Fischer, sales engineer at Blum-Novotest elaborates: “Inside the TC76 is the new patented Shark 360 measuring mechanism with a face gear. This guarantees precise trigger contacts and forces in all directions. The probe has precise non-lobing touch characteristics even for measurements with torsion forces on the mechanism.”
Another advantage is the no-wear trigger signal generation of the Blum probe. The TC76 generates an optoelectronic signal by shading a miniature light barrier on the inside of the probe and not according to the tripod principle.

Since being integrated into the turn/grinding centre, the Blum probe system provides repeatable precision and accuracy and Mr Molter can now abandon time-consuming correctional alterations. The values are repeatable and random checks of the carbide rings are performed. So, finally the requested in-process quality was guaranteed. 

Rain or shine

The high measuring speed with the Blum probe (up to 2m/minute) reduced the measuring process time by a reported 40%. The probe checks the tools indirectly via workpiece measurement, so that the manual intermediate measurements are now automated.

The TC76 and the new turn/grinding centre are working effectively together. With the previous machining centre the carbide ring had to be taken from the machine and measured offline, reset and re-machined. With the EMAG machine the complete ring can be turned in one setting. If a cutting plate has wear and the probe displays that the tolerance is exceeded, the turn/grinding machine compensates this wear automatically and turns to the correct size. Measurement is carried out before the final cut and the tool is corrected correspondingly.

The staff at Saarstahl AG can now rely on the probe in hot summer or cold winter weather with no deviations. Staff had no introduction or training period and according to Mr Braun the program was installed and left to work. Neither maintenance nor contact to the Blum service department has been necessary since the installation.

Blum Novotest
www.blum-novotest.com