TB 9-5210-221-40
(3) If this reading is not the same magnitude and direction as the first reading,
perform b below.
(4) If the two readings are the same magnitude and direction, then the surface plate
is out of level. Bring the surface plate into a level condition by removing half the error as
observed on the TI vial with the surface plate leveling screws. Rotate the TI 180 degrees
and again remove one-half the error with the surface plate leveling screws. Repeat this
technique till no further improvement can be seen.
(5) When the TI can be rotated 180 degrees and there is no apparent change in the
position of the bubble and it is in the center area of the vial, the TI is correctly adjusted.
b. Adjustments
(1) Rotate the TI so that it indicates the greatest error in magnitude and remove the
screw dust cover from over the main vial adjustment screw.
(2) Adjust the vial with this screw to remove one-half the difference between the
two readings. Again rotate the TI 180 degrees and note the reading; then, rotate TI again
and remove one-half the difference with the vial adjustment screw. Repeat this technique
until no further improvement can be seen, then proceed to paragraph 8 a (4) above.
9. Cross Test Vial
a. Performance Check
NOTE
This test is not required if TI has no cross test vial or
equivalent designed to indicate horizontal levelness but at 90
degrees to main axis length.
(1) Place two equal gage block stacks of any convenient height, up to inch on
surface plate. Set a steel parallel on top of the stacks so that about inch of the parallel
extends beyond each stack.
NOTE
The distance from the left edge of one stack to the left edge of
the other must be within 1/4 inch of 10 inches.
(2) Place the center three-inch section of TI utile surface across the steel parallel at
a right angle to its length so TI is supported in a balanced condition.
(3) Allow TI cross test vial bubble to reach a stable indication. If TI bubble is not
totally within the centering lines, increase the height of the appropriate gage block stack
until TI bubble is centered and vial indicates zero slope.
NOTE
Remove TI temporarily while changing height of gage block
stack to avoid unbalanced situations.
