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Probing

CMM Probing: Separating Reality from Fiction

To most people, there are only two types of CMM probing: Touch Trigger and Analogue Scanning. In fact, it is a much more complex matter and can be the difference between being able to measure a part correctly and inadvertently scrapping good parts. There are three major brands of probing being manufactured today for coordinate measuring machines: Renishaw, Tesa and Zeiss. Let us take a look at touch trigger probes first and find which is best suited to your application.

Touch Trigger Probes (TTP)

Touch trigger probes, or discrete point probes, are the most popular probing system found on a modern coordinate measuring machine. They are ideal for measuring prismatic parts with tolerance dimensions.

There are two major types of Touch Trigger Probes:

  1. A “Complete Unit” type which screws into the robotic head, or, has an auto joint that fits into the robotic head.
  2. A Probe with a Stylus Changing Module: this module is detachable and can be exchanged for different lengths of styli. Module change racks can be  utilised with this type of technology and are a huge bonus for users wishing to preserve optimum accuracy. By selecting the correct force module, one is able to optimize precision for different styli lengths.

TTP Design and Measuring Technique

The basic touch trigger probe has been around since the 1970s. The general design is based on spring loaded kinematic contacts. When the probe is deflected by the part being measured, electrical circuits running through the contacts are broken. This kind of probing is extremely rugged and can be used on manual or DCC coordinate measure machines.

TTP Styli Length

The manufacturer always calibrates and certifies this probe with a given length of styli, in most cases 10 mm. Inaccuracies occur when technicians use styli lengths much greater than this, typically 30 to 50 mm in length, and can't understand why their coordinate measuring machine is giving inaccurate readings.

Strain Gauges

Strain Gauge probe technology is now appearing on the market and has become widely accepted as a more accurate solution over the traditional kinematic probes. On contact with the work piece, the stylus is deflected as the force is applied to the sensor. The benefits of this system are twofold: first, much greater form accuracy is possible, and secondly, inaccuracies due to styli length are drastically reduced as precision is only minutely affected by varying styli lengths. The bottom line is enhanced accuracy on your coordinate measuring machine.

Scanning Probes

Scanning probes provide continuous deflection output that can be synchronized with the position of the DCC CMM. Data is gathered during movement when the stylus is brought into contact with the component to be measured and then moved along the surface of the feature. By using this technique, deflection of the probe is kept within tight tolerance boundaries thereby ensuring maximum accuracy.

Application and Benefits

There are, in fact, several different types of scanning probes, analogue and optical systems prevalent in the market today. We must therefore have an understanding of the benefits a scanning probe provides the CMM technician.

These probes are best used when measuring parts that consist of complex surfaces and because scanning often occurs at speeds of over 300 mm per second, gathering data quickly is no problem. Form deviation is another target for the scanning probe. Features such as cylinders, planes and conical shapes can be measured with extreme precision whilst reporting accurate deviation of the feature’s form.

Versatility

Most scanning probes will also work as a touch trigger probe, but this will usually be much slower than a probe dedicated for that task. This does, however, provide optimum flexibility for users who require that capability.

Summary

In closing then, one must look at the application before choosing the correct probe. If location and position are what you're looking for on three-dimensional machined parts, then a touch trigger probe is the way to go. Touch trigger probes are also a lot easier to program as compared to scanning probes which require more complicated measurement routines. Furthermore, TTP systems cost a lot less money and enable the machine to measure parts faster.

If you are measuring complicated forms, such as dies for plastic injection moulded parts and sheet metal components, then a scanning probe is desirable. Scanning probes shine when it comes to measuring machined parts requiring ultrahigh accuracy in form deviation.The choice is yours, but remember, probing is the one CMM component which, more than any other, will directly affect the accuracy of your system. CMMs don't perform well with substandard or incorrect probing.

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