Laser Calibration At The Speed Of Light

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Renishaw’s high-speed data capture technique cuts calibration time 85-95 percent, minimising downtime on key assets. By Rockson Hung, Product Marketing, Renishaw Far East

Whatever your machine or motion system – machining centres, lathes, borers, laser cutters, or CMMs, etc., you need to know it is up to the job before you begin cutting, processing material or measuring components. It is important to gain a detailed picture of how each characteristic of a machine’s positioning performance is varying over time. This helps to assure the quality of the manufactured parts meeting the demands of your customers, particularly those from the aerospace or automotive industries, and enables you to predict when maintenance work will be required for specific machines, and establish contingency plans in advance.In the early days, calibration tests might need to be performed by a highly skilled expert as part of the machine commissioning, and the tests were time-consuming and problematic to carry out. Thus, a simple, easy-to- use and high precision calibration equipment

In the early days, calibration tests might need to be performed by a highly skilled expert as part of the machine commissioning, and the tests were time-consuming and problematic to carry out. Thus, a simple, easy-to- use and high precision calibration equipment is essential – the laser interferometry is a well-established method for measuring distances with great accuracy.

‘Interferometry’ is a measurement method using the phenomenon of interference of waves (usually light, radio or sound waves). The measurements may include those of certain characteristics of the waves themselves and the materials that the waves interact with. In addition, interferometry is used to describe the techniques that use light waves for the study of changes in displacement. This displacement measuring interferometry is extensively used for calibration and mechanical stage motion control in precision machining.

Experience With Precision

The word ‘laser’ implies ‘light speed’ for the casual listener, but witnessing a bi-directional laser calibration run on just one linear axis of a CNC mill conjures up a completely different image – that of something less than the speed of light. A short 1m axis can typically take 30 to 60 minutes as the machine axis moves in 10mm increments and stops to pause and settle before the laser interferometer takes a reading. That pause can be as long as 23 seconds during some tests. The downtime it causes for calibration of a large, high-value asset can make a machine shop owner bite through nails. Depending on machine crashes and the quality system the shop adheres to, a machine may be out of action for several days each year to perform a laser calibration, so machine shop owners are always focused on this Issue.

However, a technique that takes advantage of a feature in a laser interferometer, take Renishaw XL-80 as an example, reduces settling time to just 250 milliseconds after each incremental axis move, cutting calibration times by 85-95 percent. In addition to reducing downtime on critical production assets, this methodology better reflects axis positioning accuracy under actual cutting conditions, where moves are fast and there is virtually no settling time before tool engagement with the work.

Case Study: Quality Tech Services

Quality Tech Services in Byron, Georgia, US, has used the XL-80 laser interferometer technique and instructed new laser buyers on it as well. “As a new calibration service, the first concern we hear when we walk in the door is downtime on a machine,” said Mike Schraufnagel, the company owner.

“Customers will do anything to cut downtime on a critical asset, and our customers who own lasers love this capability. For us, the laser interferometer has been a boon to our efficiency. What normally would take 30 minutes now takes just three. In one afternoon we were able to complete six laser setups and three ballbar set ups on a machine in two hours. This would normally have taken at least a day.”

M-Code And An Open Contact Relay Is All That Is Required

To use the high-speed Renishaw XL-80 calibration technique, a machine’s CNC could use an available M-code which, if not available, is usually a purchasable option to close a ‘normally open’ set of relay contacts for between 10 and 20 milliseconds. Closure of the relay contacts, via an auxiliary I/O cable, will provide a trigger for the laser to record the difference between the laser reading and the target position. M-codes are typically used in CNC for probing cycles, to control coolant and capabilities other than axis positioning moves, so this option is available as standard on many CNC. In others, M-Code may be an option that simply needs to be purchased or turned on by the OEM.

This feature could significantly reduce overall machine downtime by triggering the laser interferometer via a part program. A ‘bounce rejection’ function is added to the laser system to remove the unwanted bounces of the relay contacts, thus allowing the laser to achieve more accurate reading. High-speed data capture by XL-80 can be used to calibrate positioning stages and other equipment where no CNC is involved, it may take some customising of the machine control with an auxiliary board to trigger the laser, but these boards are readily available and well known by people in the controls business.

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