Choosing the Correct CMM Probe
There are three major brands of probes being manufactured today for coordinate measuring machines: Renishaw, Hexagon and Zeiss. Most people think there are only two types of CMM probing: Touch Trigger and Analogue Scanning. In fact, probing is a much more complex matter and can be the difference between being able to measure a part correctly and inadvertently scrapping good parts.
Let us first take a look at touch trigger probes 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.
Types of Touch Trigger Probes:
- A “Complete Unit” type screws into the robotic head, or, has an auto joint that fits into the robotic head
- A Probe with a Stylus Changing Module is detachable and can be exchanged for different lengths of styli. Module change racks can be utilized with this type of technology and are a huge bonus for users wishing to preserve optimum accuracy. By selecting the correct force module, precision can be optimized for different lengths of styli.
TTP Design and Measuring Technique
The basic touch trigger probe has been around since the 1970s. Its 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 measuring machines.
TTP Styli Length
The manufacturer always calibrates and certifies the touch trigger probe with a given length of stylus, 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 GAUGE PROBES
Strain Gauge probe technology is now appearing on the market and has become widely accepted as a more accurate solution than 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 the strain gauge probing system are twofold: 1) much greater form accuracy is possible, and 2) 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 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.
Scanning 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 while reporting accurate deviation of the feature’s form.
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. They do, however, provide optimum flexibility for users who require that capability.
It's best to consider 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. Furthermore, TTP systems cost less money than scanning probes and, as a rule, are durable.
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.
Increasingly, CMMs are being equipped with both touch trigger probes and scanning probes, providing the machine with optimum flexibility.
At CMMXYZ we have a wealth of knowledge when it comes to choosing the correct probe for you CMM.