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Blum-Novotest introduces the first 25mm analogue touch probe

Blum-Novotest introduces the first 25mm analogue touch probe

Added to MTDCNC by Blum-Novotest on 02 August 2015

Touch probes are exactly what the name says, probes. If the stylus touches the workpiece surface, a switch signal is generated. Previously, analogue touch probes that measure and analyse the deflection of the stylus and simply do not emit a digital On/Off signal, were very large and costly and for the most part could only be found in coordinate measuring machines. With the new TC76-DIGILOG, Blum-Novotest is introducing the world’s first touch probe for robust use in machine tools, combining digital and analogue technology in a 25mm diameter device. In short, a ''digilog" touch probe.

Until now, touch probes offered both by Blum and its competitors have relayed a digital signal to the machine control. As soon as the machine control detects this switch signal, it reads off the position of the axes and from this it can detect the exact position of the measured point in the machine tool.

Following consultation with customers, Blum has now refined the current technology. The idea is simple, if you can measure the start of the shadowing of the light barrier, it must also be possible to analyse the percentage increase in the shadowing that occurs when the stylus is deflected ever further. This enables an analogue signal to be generated that greatly extends the range of application of measurement in the machining centre and can also save a great deal of time. Digital measurements naturally continue to be possible.

Due to the leverage ratios between the inside of the touch probe and each stylus used, a usable analogue measurement range of 475 micrometers for a 30mm long stylus up to 1,175 micrometers for a 100mm long stylus result.

The signal is routed via a built-in or mounted base, into which the touch probe is screwed, and a cable up to 0.5m long to an A/D converter. From this converter, even relatively long distances can be bridged to the Blum D/A converter. Its output signal is a typical 0-10 volt signal that must continue to be processed by many controls. There are two reasons for this somewhat unusual signal path: analogue transmissions are relatively susceptible to cross feeds and interference caused by electrical emissions, which is why the first part of the cable is kept as short as possible. The following digital section is less susceptible. However, no standard transmission protocol transmits the 50,000 readings per second generated by the TC76-DIGILOG, so on the transmission section to the switch cabinet a protocol unique to Blum is used.

Analogue measurement always has advantages when surfaces or lines are being assessed, for example when a workpiece surface has to be checked for machining errors. When a switching digital probe is used, in such a case a very large number of points has to be measured to achieve an adequate resolution. Whereas an analogue probe can be moved in "scanning" mode over the surface and records more points in a fraction of the time. Astonishing speeds of up to 2m/min are possible with outstanding accuracy. Under test, probes were even moved at 5 m/min while measurement accuracy remained very good. However, for the most part such extreme speeds are not necessary. Previously, in one application 30 seconds were required for 40 measuring points. In comparison the TC76-DIGILOG measurement time on the same surface was reduced to 2 to 3 seconds with 100,000 recorded readings with higher precision.

One manufacturer of gear wheels was tasked with measuring very large gear wheels used in the gear mechanisms of wind turbines. The dimensions of these gear wheels are beyond the measuring abilities of coordinate measuring machines, with the result that previously the gear teeth were only randomly checked for quality. Now, the customer uses Blum DIGILOG probes that measure each gear wheel directly after machining and thus safeguard the high level of quality.

''Today, machining processes in series production are optimised within a range of milliseconds", explains Blum Development Director Wolfgang Reiser. "As measuring processes lasting several minutes or even longer don’t fit into the manufacturing sequence. Production-integrated measurement in the machining centre doesn’t replace quality control on the measuring machine, it supplements it. Measurement in the machine is used for the direct monitoring of the production process, less for overall quality control. A touch probe that uses the same axis for measuring cannot compensate for axis errors of the machining centre.''

Moreover, Blum touch probes have a broader range of uses – automatic calibration of workpieces or zero point detection are examples of this. Temperature drift or imprecisely mounted workpieces can be rapidly detected. When mechanically connected, the TC76-DIGILOG touch probe is compatible with typical 25mm touch probes with a M16 fastening screw thread, enabling digital measurement to continue without restriction.

It’s not just Blum’s specialists who regard the ability to use a touch probe on the machining centre in switch mode, i.e. measuring digitally as well as in analogue mode, as a revolutionary step. Coining the word ''digilog'' for this combination of technologies therefore seemed natural. The additional costs compared with a purely digital touch probe are also limited and Blum-Novotest is even contemplating retrofitting analogue capability into its other digital touch probe models, as a cable-free model that can simply be exchanged from the tool magazine.


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