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Why Laser Calibration

Laser displacement measurements have become accepted as the standard of comparison for accuracy and repeatability throughout the machine-tool industry. It is stated by the National Bureau of Standards, NBS that "It is our opinion that such devices (meaning the wave length of the h are a priori traceable and that no calibration by NBS is required! However most laser measurement systems are calibrated on a yearly basis and traceable to NIST and can be assumed to have accuracy around the 1 part per million range! (See laser calibration document elsewhere at this web site.

Typically, we buy a new CNC machine tool only after careful investigation of its capabilities and accuracies. We require the manufacturer to prove that the machine can live up to its documentation. Then we set it on the shop floor, run it around the clock year in and year out, subject it to operator and environmental abuse, do repair maintenance, do little or no alignment checks and somehow kid ourselves into believing that it is still living up to its original intent. Still behind all the blinking lights and CRTs lies a basic machine tool, which is prone to wear and misalignments negating the accuracies instilled by the OEM. Most machine-tool builders routinely use the laser to check and fine-tune their machines before shipping and still bring their equipment to the installation site to redo the alignments and accuracies. (Depending on customer requirements). One can only imagine as to the accuracy of older equipment.

The finished work coming off a machine is often the only method used as an index of its performance, but it is impractical or impossible to inspect all the aspects of any one part let alone entire lots. In this age of high quality, zero defect, on time deliveries, the inherent accuracy of the machine tool, to a large part, has to be taken for granted by the production and inspection departments, and guaranteed by the maintenance department or some outside source. The laser is the tool that will due this in an efficient manor with traceable accuracy.

The obvious tangible cost of poor machine performance is the scrap and rework, but the more subtle intangible costs have to be considered:

If for one reason or another an operator is suspect of a machine’s capability to hold the part tolerance, he will spend considerably more time checking prior to finish cuts.

Employees’ morale will suffer trying to make quality parts on a machine that no longer has the capability. Unfortunately, operators are more often blamed than the machines.

Some programmers pride themselves in their ability to get around machine deficiencies by extra steps, which cost time in programming and extended run times.

Downtime of a strategic machine for troubleshooting program errors with the machine’s positioning system being the real culprit.

Poor part geometries causing extra fitting time at assembly.

Extended setup time trying to find sweet spots to machine more precision parts.

NOW THE GOOD NEWS:

Most modern machine controls have what is known as "periodic" or "ball screw" compensation. With the aid of the laser’s ability to measure repeatable machine positioning errors, and this compensation table, we can achieve accuracies far beyond the mechanical capabilities of the machine. Upgrading the quality and accuracy of the machine positioning system, without a major upgrade of the machine!

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