Tormach PCNC 440 - How to Measure Lost Motion

Chapter 9
162
UM10372_PCNC440_Manual_0221A
MAiNTENANCE
These adjustments are generally used to address component wear-in over me, but may also be
needed to correct misalignment resulng from misuse, a hard crash of the system, or when some
components are removed or replaced due to damage. The frequency of these procedures depends
on both how the mill is used and how oen.
The adjustments in this secon should not be considered lightly as a wrong adjustment can adversely
aect mill precision. Before making any of the adjustments in this secon, consider the following:
• Why am I making this adjustment?
• How will I measure the eect of this adjustment?
• What unintended consequences may result from this adjustment?
If you do decide to make an adjustment, do not assume where the error is from. The error could
be aributed to a specic problem, or from the combined eect of several problems. Mistakenly
making the wrong adjustment can make maers worse. As a praccal example, if the Z-axis gib
is too loose, it will cause the spindle head to lt slightly downward toward the column. It would
be fairly easy to incorrectly assume that the issue is with the column and base connecon (oen
referred to as tram), and make an adjustment by inserng shims between the column and base.
Instead of correcng the real issue, this adjustment causes the column to slant back to correct for
the head leaning down. Now, the mill is running in a slight parallelogram in addion to a loose head.
9.3.2 Definitions
The following denions are important to the advanced maintenance discussion.
Lost Moon
Lost moon is the dierence between commanded moon and observed moon. This is somemes
referred to as apparent backlash. There are a number of components of lost moon, including
convenonal backlash, bearing compressibility, sliding fricon, and thermal expansion.
Backlash
Backlash is the major component of lost moon in a machine tool axis. It results from the clearance
between moving mechanisms. This is somemes referred to as play. There are two sources of
convenonal backlash that can be adjusted on the PCNC:
• Space between gib and way needed to support an oil lm. This is tuned by ghtening the gib.
• The space between the ball bearings and races in the angular contact bearing pair that
supports the ball screw. This is tuned by increasing the angular contact bearing pair preload.
9.3.2.1 How to Measure Lost Motion
Correctly measuring lost moon is crical to successfully undertaking any of advanced maintenance
procedures detailed in the following secons. Mill setup and tuning is done under no-load
condions. The precision measurements recorded in the Cercate of Inspecon are taken under
no-load condions. The accuracy of a machined feature is not an indicator of machine precision.