Contact versus laser digitising - no contest

Contact probe systems combined with powerful complementary software can offer considerable advantages when compared head to head with laser scanning and digitising technology, explains Andrew Galt.

For the majority of users with a scanning requirement, contact probe systems combined with powerful complementary software can offer considerable advantages when compared head to head with laser scanning and digitising technology, as this article will explain.

In the past the terms, "scanning" and "digitising" were used to distinguish what type of probe was used to obtain 3D data from a physical model or pattern.

"Digitising" was the term used for taking readings of 3D co-ordinate points (from a surface) at predetermined discrete positions using a touch-trigger probe.

"Scanning" was used to describe taking readings of 3D co-ordinate points (from a surface) using an analogue constant contact probe producing a stream of data points based upon the deflection of the stylus tip.

The older, slower method using touch-trigger probes has virtually given way to more sophisticated and efficient techniques using analogue constant contact probes.

This has led to the terms scanning and digitising being used interchangeably, ie not to particularly define what type of probe is used.

The SP range of analogue scanning probes has been designed by Renishaw to suit a variety of applications.

When used with the latest systems these probes can output up to 1000 3D points per second.

The SP2 range is specifically for use on CNC machine tools, usually on larger applications.

The SP600 range has been specifically developed for fine detail and delicate work and is fitted to the Cyclone scanning machine.

On all SP range probes the stylus deflection and hence contact pressure can be chosen to suit the stylus and the particular model being scanned.

In the case of the SP600 this can vary from as low as 10g up to a maximum of 80g.

This very low contact pressure means that extremely small styli can be used without fear of damaging the model or pattern being scanned.

Ruby ball styli as small as 0.5mm in diameter are used routinely on fine detail work such as coining dies and jewellery applications.

In certain cases ball styli as small as 0.3mm and pointed styli with a 0.1mm tip radius are used.

This virtually eliminates any problems associated with sharp internal corners, often used as a criticism of contact probe systems.

The step-over between scan strips is defined by the user and can be set to any value from several millimetres down to 0.005mm.

This combination of an extremely low contact force together with small styli and a fine step-over allows data to be captured from complex surfaces that is unparalleled in terms of detail, quality and accuracy when compared to other systems.

Consequently with the correct combination of stylus diameter and contact pressure, it is possible to successfully scan soft materials such as plaster, clay and plasticine.

A further major advantage of contact systems is that contact-scanned data do not require editing to remove rogue points, which is often seen as a weakness with certain laser systems.

The foregoing outlines some of the fundamental reasons why contact scanning is as good as laser scanning or digitising, now let us look at some other aspects which show why it is actually better.

Most laser scanning and digitising systems usually have a fixed grid format which needs to be defined by the user prior to digitising taking place.

This takes the form of specifying a distance between passes (step-over), and the distance between data points in the direction of scanning, (pitch).

Deep near-vertical faces are quite often encountered in jobs to be scanned.

While representing no difficulty for an analogue contact system, this situation can cause problems for lasers.

For optimum operation the laser beam needs to be nominally at 90 degrees to the unknown surface; suitable for patterns with low relief, but for steep vertical faces the laser head ideally needs to be reoriented and redatumed in order to collect useful data.

With a Renishaw contact-scanning system the user does not have to work out what pitch distance to use when scanning.

The shape of the pattern can control data density automatically; this is achieved by using what is known as a 'chordal tolerance'.

Basically this means that on a truly flat surface, data points will be generated at whatever pitch distance is set.

However, if the surface starts to deviate by more than the chordal tolerance (typically set at 0.010mm), then extra data points will be automatically inserted.

This has the effect of concentrating the data points where they are most useful, i.e sharp changes in geometry, and reducing them where they are not needed, ie flat featureless areas.

2D profiles can be very useful for limiting a scanning or machining area; multiple 2D profiles are often used as a basis to generate CAD models.

Contact systems can capture 2D profiles from unknown shapes in a matter of minutes, something that is just not possible with a laser system.

Laser scanned data often requires extensive post-capture processing in order to improve the characteristic "orange peel" surface.

Although this may improve surface quality it is usually at the expense of accuracy and loss of sharpness on external corners.

The evidence shows that contact scanning systems are generally more accurate than laser-based alternatives, and are readily capable of replicating models to within 0.050mm of the original pattern or model.

Job set-up is a particular area of strength for contact systems.

For example within Renishaw's Tracecut Scanning/CAM software a comprehensive set of datum functions make job set up a straightforward task.

The x-y plane can be simply defined by taking three points, which are contained within that plane; similarly the x and y axes can be skewed to align with the component.

A variety of pattern/component features such as boss, bore, edge etc may then be used as part data to complete setting up the job.

Another feature found in most contact scanning systems, but uncommon in laser systems is 'learn-mode'.

With this technique the scanning system makes use of the data from the previous scanned strip in order to control its own speed.

This form of adaptive control means that the feedrate will be at maximum on smooth areas and will automatically slow where there are sharp changes in geometry.

Renishaw contact systems have both 2D profiling and 3D scanning capability.

2D profiles can be used as limiting boundaries or exclusion zones for scanning or machining, or as templates for setting up 3D scanning areas.

Scanning in 3D can be undertaken in a variety of ways in order to suit the particular shape of the pattern or component.

Scanning paths can be set within an individual grid to be parallel to the x or y axis, at any angle, or radial.

Any number of individual grids of different scanning directions, stepovers etc.

can be defined on any one job.

For example a relatively coarse grid may be used to cover the majority of a pattern, then smaller grids with smaller stepovers can be used to pick out any areas of fine detail.

Undercuts and re-entrant shapes do not present problems for Renishaw contact systems.

The software has scanning algorithms that allow the data for these shapes to be captured.

Subsequent processing automatically removes the re-entrancy and inserts new vertical data from 'the last good' point.

Alternatively, with suitable cutters these re-entrant shapes may be machined using a 'copy-machining' function.

The surface co-ordinate data used in Tracecut are not taken from the ball centre but rather the stylus tip; the data are also stored in this way.

This happens to be the same information that would be required for cutting the part if the same diameter cutter as stylus was used.

Hence cutter compensation calculations for the true surface are only carried out if required and not as a matter of course.

For certain specialist applications a laser systems can be a more viable option than a contact system.

Examples of this would be where very low relief patterns are used such as in the production of coins, medals and jewellery.

However, the advantage would be speed rather than accuracy.

To cater for all requirements the Renishaw Cyclone Series II offers the best of both worlds, an analogue contact probe and a laser probe that can be quickly interchanged to suit the job at hand.

Leading edge technology does not necessarily mean laser based systems; a combination of sophisticated analogue contact probes and powerful software features ensures that contact scanning will continue to deliver the best quality and most accurate solution for the foreseeable future.

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