Using the SP600, SP600M and SP600Q

Qualification

The SP600M family of probes require qualification before they are able to give accurate positional data. Renishaw has extensive experience of scanning, and offers support and advice on scanning algorithms and control software. Please contact your local Renishaw subsidiary for further information.

Modes of operation

The SP600M is an analogue output measurement probe and may be used in a variety of ways. Principally, these will be as either a single point measurement or a profile measurement/digitising probe.

Scanning mode

SP600 can be used as a continuous deflection analogue contact scanning probe for profile measurement or for surface digitising purposes. In this case the CMM controller must respond to the deflections of the probe in real time to maintain surface contact, i.e. 6 axes must be controlled simultaneously.

Single point measurement mode: Extrapolate to zero method

Data is acquired whilst in contact and moving normal to the surface, either on the way in or whilst backing off. This is extrapolated to the zero surface position. It has the advantage that the
measurement takes place at zero force, minimising the deflection on probe, stylus and CMM, and additionally that the probe does not need calibration.

Static point measurement mode: Static averaging method.

SP600 can be used to take single points to give increased accuracy whilst reducing the effects of machine vibration by performing static averaging.

The probe’s stylus should be made to contact the workpiece and deflect the stylus to the recommended amount (50 microns). The CMM should be halted and kept nominally stationary.

Whilst the machine is stationary surface position readings should be taken which are then averaged to give one single surface point. The longer the system is kept stationary, the more readings can be
gathered, to give a more accurate result and to average out the effect of machine vibration.

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.

Reorientation

The SP600M probe may be used in different orientations when mounted on a Renishaw PH10M motorised head. The design has been optimised to allow this but it will have some effect on the
distribution of the measuring ranges available in each axis.

When using a SP600 with PH10MQ a PEM25 extension bar is required to achieve A = 97.5° or A = 105° in all B-axis positions

Return to zero

The probe has a nominal absolute centre position where the functions of stylus confi guration and probe orientation cause it to rest. Because of small amounts of internal friction, when the probe is displaced from this zero point, the stylus will not return to exactly the same point on the scale and the axis defl ection readings will show a different value.

This characteristic of probe performance is called RETURN TO ZERO and is a feature of all analogue probes and is not a source of error as the scale system continues to monitor position. Rather it is merely a factor which must be taken into account when designing control software for using the probe. It can be given a value which represents the diameter of a sphere around the nominal zero position within which the probe will return to reset after any displacement.

SP600M has a return to zero value of less than 5 microns after a deflection of 0.5 mm (typically 1 micron). It is important to take this into account as it affects the minimum amount of deflection necessary before the stylus is considered to be in contact with the surface.  Because the stylus can return to a value other than the nominal zero, the CMM must recognise the fact that the range of rest positions of the stylus must not cause the machine motion as the stylus is not necessarily in contact with a surface even though it is “deflected”.

The CMM software should have a parameter for this minimum probe deflection and only deflections above this amount should be considered as the stylus being in contact with a surface.