NEW: Touch Probe TC76-DIGILOG
DIGITAL + ANALOG = DIGILOG
The digilog Revolution
Blum-Novotest introduces the first 25mm analogue touch probe – the TC76-DIGILOG
Touch probes are normally exactly what the name says: probes. If the stylus touches the workpiece surface, a switch signal is generated. Previously, analogue touch probes, i.e. those which 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. In the shape of the 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 25 mm diameter device. In short: a "digilog" touch probe.
Flexibility is important
Flexibility in manufacture is becoming ever more important, not least because the increasing customisation of products is shrinking batch sizes. However, batch sizes are also falling due to “KAIZEN” techniques, because production is no longer taking place on a “stockpile” basis but always when supplies of a component are running low. The more frequently a machining centre is retooled, the more significant the set-up times become. Automatic retooling and calibration of devices, workpieces and tools can save a huge amount of time here.
A measurement system in the machine tool itself enables you to achieve this and more: automatic workpiece zero point detection, inspecting the raw part before machining – in this way undersized blanks can be detected and separated out before machining starts – or even detecting inaccuracies in the clamping of complex or very large components. With the latter, for example in the case of gear wheels for wind turbines, often a 100% quality check is not carried out following manufacture on a coordinate measuring machine, because machines which can measure such huge items are simply not available.
For decades now, Blum-Novotest has been offering touch probes and laser measuring systems for use in machine tools. The touch probes are either mounted in the machining area and the signal relayed by cable to the control, or the touch probe is housed in the tool magazine and exchanged like any milling tool into the spindle and then starts up the workpiece or tool to be measured. In this case, infrared or radio technology is used for data transmission.
Unique measuring mechanism
Blum touch probes operate using a unique measuring mechanism: whereas competitors’ measuring mechanisms are often produced using the three point contact measuring principle, Blum probes work with a measuring plate resting in the shape of a ring on a bearing ring in the touch probe housing. In the three point contact measuring principle, due to the deflection of the stylus a signal is generated if the electric circuit is broken when one of the three legs is lifted. The disadvantage: the probes wear out in the long run, as an electric arc is produced between the mechanical contacts during every switch signal. In the Blum probes, on the other hand, a precision pin is fitted in the centre of the measuring plate which is pushed upwards by the deflection of the stylus into the recording area of the light barrier. The advantage of this is that the measuring force is identical in all probing directions, whereas the conditions in the three point contact measuring principle vary depending on whether the measurement is being made exactly in the direction of a leg or "between" the legs. The patented shark360 face gear, which is used in many Blum touch probes, also prevents the stylus from skewing and thus increases the precision of the system further.
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 can detect the exact position of the measured point in the machining area.
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 which 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 30 millimetre-long stylus up to 1,175 micrometers for a 100 millimetre 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 0.5 metres 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 which 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.
Advantages of analogue measurements
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 are reached; up to 2 m/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, approximately 30 seconds were required for 40 measuring points; with the TC76-DIGILOG, measurement time on the same surface was able to be reduced to 2 to 3 seconds – with 100,000 recorded readings and in higher precision.
A typical application is the inspection of valve seats in car cylinder heads. The valve seat tool determines the shape, so the precision and integrity of the tool is critical to surface quality. Unfortunately, these tools are prone to micro-wear, the times of which cannot be pinpointed, and regularly replacing the tool therefore fails to provide a remedy. Previously, the annular beads on the valve seats produced by micro-wear were only discovered some time after machining on the measuring machine. By the time this had been identified by quality control a number of cylinder heads had already been machined, which then also had to be scrapped. Today, the profile of each valve seat is shut down immediately following the ream process using the TC76-DIGILOG. The bead is often only a tenth of a millimetre wide and a few micrometers high; this makes detection by the digital probing of individual points virtually impossible, as the number of measuring points would have to be extremely high to find these tiny beads reliably. This in turn leads to the measurement lasting too long. By measuring in analogue mode, the DIGILOG probe "feels" beads absolutely reliably, with the result that the machining centre can be stopped immediately and the valve seat tool replaced – only one cylinder head is lost.
Monitoring for machining errors
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 which measure each gear wheel directly after machining and thus safeguard the high level of quality required.
"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, to say nothing of quality checks between the machining stages on the measuring machine. 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, which uses the same axis for measuring, can’t 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 25 mm touch probes with a M16 fastening screw thread – and, of course, digital measurement continues to be possible 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 in its other digital touch probe models; a cable-free model that can simply be exchanged from the tool magazine is due to come onto the market soon.
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