Recently we were asked this question;
"I purchased a new Digital Readout Package from
your salesman and had it professionally installed by your technician. I thought I purchased the most accurate DRO
available but apparently your salesman was wrong. The very first part that I made using the new
DRO was rejected by inspection. Our
inspector said that the part is too large by more than 0.002” and blamed the
new DRO. I ran some tests of my own and
found that he is correct; the table is in fact traveling farther than the
display says it is. I am very frustrated with our purchase decision. What’s going on!?"
The DRO you
purchased is in fact accurate to within 0.0004” over the length of the table
and your part is in fact moving 0.002” too far as described. I understand this statement may sound
contradictory so let me explain. The
very common error you are experiencing is called pitch error which is caused by
your table traveling on an arc instead of a straight line. This arc occurs when the unsupported table
sags as it is traveled from end to end.
Since the part you are machining is clamped a few inches above the
table, it will travel a longer path than the scale resulting in a part that is
too large. The proper corrective action to be taken in this case is to have your table ways professionally scraped. You can also have the Digital Readout laser calibrated if you always machine at the same level above the table.
For a more technical explanation of pitch and yaw error,
continue reading this document…