Table of Contents
Advertisement
SP600 installation and user's guide
www.renishaw.com
Single point measurement mode: THRESHOLD METHODS
There are two types of threshold method as described below. Type1 takes data whilst driving the probe onto the part to a pre-set deflection
threshold, whilst type 2 takes data whilst backing off to the pre-set deflection threshold.
Type 1
A target deflection threshold should be set. The probe is driven onto the part until this target deflection threshold is seen, at which time the
controller simultaneously stores all CMM axes together with the probe deflections - this is the data point.
Type 2
A target deflection should be set. Additionally, an upper target deflection should be set which will enable a back off move to the target
deflection to be executed at a constant velocity. The probe is driven onto the part until the upper target deflection is seen; at which time the
motion should halt and a back off move should commence. When the target deflection is seen, the controller simultaneously stores all CMM
axes together with the probe deflections - this is the data point. This method is more accurate than type 1 above.
Probe deflections
±1 mm (0.04 in) is guaranteed in all axes and in all orientations with a 50 mm (1.97 in) 4.6 g (0.16 oz) stylus.
The probe scales give an absolute positional reading. This means that the probe may be switched off and on and used again without the
need for recalibration, the scale readings will be in the same positions each time.
Maximising performance
The optimum deflection (for accuracy) used during calibration is ideally kept to a minimum (i.e. 200 microns).
Operation limits
The CMM must set operating limits for the probe, these are defined by:
Minimum probe deflection
Below which the CMM regards the stylus as not in contact with a surface.
Maximum probe deflection
Above which the stylus is beyond the probe's measurement range and is likely to overtravel.
Soft limit
A value below the maximum probe deflection. When this deflection is reached, the CMM recognises that the stylus is close to the limits of
the probe's range and that it must take action to re-centre the probe. Renishaw recommends that this limit is set 0.25 mm (0.01 in) below
the maximum probe deflection, although this may not always be possible and may be altered depending upon the CMM control
characteristics, scanning speed etc.
Issued 01 2018
52
Advertisement
Table of Contents
