FANUC G54.4 workpiece setting error: why COMMON and individual data cannot simply be added
Original 30i-A G54.4 rules relate COMMON and individual setting errors to the table pose during measurement. An original planar counterexample shows the direction error caused by mixing references and distinguishes this operation from adding a common workpiece zero offset.
Contents
Two sets of workholding errors may each be only tens of micrometers, yet adding them directly can give the wrong compensation direction. On five-axis machines with rotary tables, error values also carry the table pose at measurement. Ignoring that condition is equivalent to adding components expressed in different directions as though they shared a column.
COMMON has a shared reference; it is not an ordinary common zero offset
The FANUC 30i/31i/32i-A user's manual B-63944EN/03, §15.5, pages 400–401 (public mirror of the original manual) specifies that G54.4 COMMON entry 00 combines with individual entries 01–07; COMMON does not provide rotational errors. On table-rotation and composite-type five-axis machines, when the two data sets have different registered table measurement positions, the control first converts them to the reference where the table-axis machine coordinate is zero, then combines translational errors. The machine must have the function and the corresponding kinematic configuration.
A planar counterexample that can be calculated by hand
The following illustrates only the mathematical reason for aligning references first. It is not an example of a physical C-axis sign convention or machine data entry. Let measurement frames A and B share an origin, with B's basis rotated 90° relative to A in the right-hand sense, and all rotational errors zero. COMMON translation expressed in A is (0.08,0.04) mm; individual-error components expressed in B are (0.03,−0.02) mm.
B's X direction points along +Y in A, and B's Y direction along −X. The individual error converted to A is therefore (0.02,0.03) mm, giving the correct total (0.10,0.07) mm. Adding the original table columns directly instead gives (0.11,0.02) mm: a difference vector of (0.01,−0.05) mm, about 0.05099 mm long.
| Calculation | Result in reference A |
|---|---|
| Convert the individual error, then combine | (0.10,0.07) mm |
| Incorrectly treat B components as A components | (0.11,0.02) mm |
This is not rounding error. Keeping more decimal places does not correct the directional reference. The real machine's positive C-axis direction, compound-table order and origin relationships must follow its own definitions; the 90° matrix above must not simply be copied.
Why the measurement angle must be more than a note
A reusable measurement record should include displacement components, their reference frame and the table position at measurement. Without pose information, the next person cannot tell whether conversion has already occurred and may rotate the data again. Express two measurements in the same reference before comparing them; matching X and Y column names on a display is not a substitute.
This model deliberately excludes angular errors and different centers of rotation. Those require a full spatial rigid-body transformation; the example must not be extended into componentwise addition of three angles. Further resources are in the OPTION section.