Renishaw units enhance length measuring machine

Select Gauges (2000) has recently teamed up with Renishaw in the installation and further enhancement of its GSIP MUL length measuring machine

As one of the world's leading manufacture of high quality Tungsten Carbide Gauge Blocks, Length Bars, Long Slips, Angle Gauges, Precision Cubes, Toolmakers Flats and several other specialist products, Select Gauges (2000) Ltd, is dedicated to the pursuit of excellence in the field of metrology.

The company has recently teamed up with Renishaw in the installation and further enhancement of its GSIP MUL length measuring machine, and now provides NAMAS calibration at its Torpoint test site, for all makes of 3-D step gauge for use with co-ordinate measuring machines.

The cast iron-based MUL was originally converted in 1994 by TESA-RSD, part of Brown and Sharpe PMI Ltd, soon after purchase, from a micro-optic scale device to a state-of-the-art twin path counting laser interferometer with a 4-metre capacity, as John Kelly, Managing Director, Select Gauges 2000 explained.

'Being a manufacturer of 3-D Step Gauges, Tesa needed the capability to calibrate them, although the MUL will also accept other manufacturer's gauges.

Having purchased the machine they immediately replaced the old manual scale with a Renishaw ML10 laser system.

The MUL is highly accurate - in terms of pitch and straightness.

It is also highly stable and provides for almost perfect smoothness of travel.' Further modifications allow step gauges to be calibrated using a unique precision bridge system with the linear movement controlled via a 3m ball screw actuator.

A pneumatic cylinder on the bridge raises and lowers a Renishaw TP200 high accuracy contact probe as it moves between the gauge steps.

The main X axis drive is via a Panasonic geared motor unit having an output shaft speed of 12rpm.

This equates to a linear speed of 60mm/minute, with the capability of traversing at 0.5mm/minute - the optimum probing speed for the Renishaw TP200 probe.

The motor in turn is connected to an OSP-E50, ball screw driven actuator with a stroke of 3200mm.

Three limit sensors are incorporated in the system, two providing end of stroke detection and the remaining sensor indicates a 'home' or datum position.

Positional feedback is arrived at using a standard Renishaw ML10 laser system using a novel optical set up, which sees two optical reflectors positioned on both 'struts' of the bridge.

This gives improved accuracy over the standard optical set-up.

The ML10 laser head is the core unit of the measurement system, containing a Helium Neon laser tube producing stabilised laser light at 633nm.

The single frequency laser contains sophisticated electronics for stabilisation and to interpolate and count the interference fringes.

This provides true nanometer resolution measurements at feedrates in excess of 1m/s.

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more The Z axis is provided by a vertically mounted AZ32 bore cylinder with a stroke of 10mm.

Limit sensors indicate up and down positions and a rotary encoder, mounted off the output shaft of the OSP actuator, provides continuous feedback and facilitates infinitely variable adjustment of the pitch parameter of the measurement function.

'In a typical measurement sequence,' added Kelly, 'an operator will place a step gauge in a fixed datum position against a backstop; this corresponds to a sensor located on the side of the PSP-E actuator.

The actuator backs off approximately 5mm before the Renishaw TP200 probe descends by way of the vertically mounted pneumatic cylinder.

The main drive then indexes at a fixed rate and passes the probe over the surface of the component.

A signal from the probe then indicates that measurement has been completed, at which point the pneumatic cylinder raises the probe and the actuator will index to the next measurement position.

The cycle continues until the pre-selected number of measurement pitches and passes have been completed.' The Renishaw probe has been interfaced with the ML10 laser and is also connected to software which automatically applies environmental compensation, via the Renishaw EC10 compensation unit, to the measurements recorded by the twin path laser system.

A PLC allows any number of runs to be undertaken (typically about 50 are used) to measure either front or rear faces or a combination of both.

Accuracy is 1.2 microns per metre with a system repeatability of 0.2 microns, and any combination of step increments can be measured up to a total of 3 metres in length.' The 'bespoke' MUL, recently purchased from Tesa-RSD, is sited in an environmentally controlled laboratory within its own double glazed enclosure that ensures a temperature change rate of only 0.01§C per hour.

The enclosure is separated from any item that might affect accuracy, such as the actuator motor which is housed outside the machine.

The Select Gauges 2000, Torpoint laboratory holds the NAMAS Accreditation Number 0105.

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