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
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
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
If for one reason or another an
operator is suspect of a machines 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 machines positioning system being the real culprit.
Poor part geometries causing extra fitting time at
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 lasers 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!