DETERMINING DAMAGE TO COMPUTER NUMERICALLY CONTROLLED
Charles C. Roberts, Jr., Ph.D., P.E.
controlled (CNC) machinery is at the heart of automated manufacturing
throughout the world. CNC machines,
selling for prices in excess of a half million dollars, are manufactured by
many companies, often overseas. A typical example is shown in Figure 1. The
machines are manufactured and tested at the manufacturer’s plant with records
kept as to their precision and accuracy. The machine is then packaged and
shipped to a dealer or customer.
The customer then has the
machine installed and made ready for production. In some instances, after a
small production run, the new owner’s quality control department discovers
severe tolerance problems with the product, suggesting that the machine is
damaged. There is no evidence of an impact on the machine, no scraped paint or
distorted components. The machine owner then makes a claim seeking coverage
under the transit portion of his policy. So how do you prove that the machine
is damaged, and that it is damaged as a result of transit or some other cause?
Analyzing CNC equipment can be a daunting task because of the complexity of the
equipment. However, a few basic concepts will go a long way to help prove or
disprove the existence of damage.
During production of the CNC
machine, testing was probably performed on the precision and accuracy of the
particular machine. The test diagnostic equipment, often a highly precise indicator
attached to the machine, will generate a test pattern similar to that shown in
Figure 2. The machine is programmed to draw
a circle and the precision indicator traces the path of the machine head (machine
head trace). The inner and outer tolerance lines show the limits for an
acceptable test, which often is less than 0.0001 inches. In Figure 2, the
machine accuracy and precision are acceptable. Precision is the ability of the
machine to repeat a particular point location of the head. The accuracy of the
machine is its ability to correctly reflect the location of the machine head. If
a test is performed at the insured’s location and a test pattern like that shown
in Figure 2 results, then it is unlikely that there was any damage to the
machine during transit or any defect in the machine during manufacture.
Figure 3 shows a test pattern
where the machine head trace has been displaced, such that it is out of
tolerance to one side. This is a typical accuracy error. The machine head is
not moving to the exact location of where it is programmed to go, but it is
performing this test repeatedly, so the precision is acceptable. This type of
error can occur from a sudden impact or drop of the machine during transit. Check
out the transit history. Was the machine properly lifted? Figure 4 shows
hoisting instructions for a particular machine using cables to prevent tipping.
If forklifts were used, this may violate shipping instructions. Some machines
are very top heavy and prone to falling over when using fork trucks. Dropping a
machine onto its base may show little physical damage but result in a test
pattern like Figure 3, suggesting severe internal damage.
Figure 5 shows a variable
head trace pattern, which is a result of precision error. The machine cannot
repeat a particular head location, although the location is in the “ball park”
of where it should be. This type of pattern is usually not related to shipping
damage but more likely some problem with manufacture or remanufacturing of the
Working with the CNC machine
manufacturer can aid in obtaining documentation as to the initial condition of
the machine. On the flip side, the manufacturer may end up being a party to a
legal action and may not readily produce information. Working with the
transportation company has similar up and down sides and may yield information
as to the particular handling dynamics of the machine. Finally, a visit with
the insured’s production facility may yield more clues as to the cause of the
problem, which may have nothing to do with the transportation or manufacture of
the CNC machine.
Published in Subrogator