Understanding Accuracy

There are a number of products in todays measurement world that label themselves as a
Coordinate Measuring Machine. Many of these products have developed accuracy
statements that are similar to the ISO 10360-2. While similar to ISO they tend to overstate
the accuracy of the product compared to the ISO standard. There are 5 primary ways that
are used to confuse users in accuracy statements
1) ( + ) Plus Minus annotation in front of formula. This annotation notates that the
formula is a bandwidth. This basically doubles the formula or the inaccuracy in the
device so if the statement is 2.0 + L/250, it is actually 4.0 + L/125 or on a 8 inch part
the expected accuracy changes from 3.2 microns to 6.4 microns. On a small
machine such as 7106 this is over could be as much a 8 microns
2) Limiting the formula only to one plane. Some companies state their accuracy only
in the XY plane or have one formula for XY plane and another for the XZ, this typically
double but more likely triples the inaccuracy of the device. The problem with this is
while claiming to be a 3D device they are truly only a 2D device.
3) Modifying or adding to the basic formula. This is typically represented by putting an
additional step in the formula, IE (2.0 + 4L/1000) this formula is basically the same
as (2.0 + L/250) but leaves the customer with the impression that the first formula is
more accurate
4) Different accuracy statements for different measurement functions, Examples of
this is one for vision, one for touch, or one for OD and one for ID, surfaces etc. There
are products that are offered today that have more than 11 accuracy statements for
the device, making it difficult for the customer to ascertain if the product will work
for their needs.
5) Not adhering to any standard. There exist products now that state an accuracy and
define their own acceptance tests. Such as measuring a simple ring gage or gage
block without applying any measurement standards for volumetric accuracy.
While the understanding of accuracy statements truly lies with the customer, we at
Masters understand it is our job to educate the customer on various items involved in
measurement to insure they make the correct decisions regarding their measurement
needs