Periodic Calibration

Most companies that produce products have some sort of inspection equipment, and diligently

engage in periodic calibrations of the equipment. They keep sophisticated records documenting

proof of the calibrations. This is a good thing, but does not prevent the production of poor quality

work on inaccurate machine tools. By including machine tools a part of this gage calibration

program, or a preventative maintenance program, better products can be produced, with a lower

cost of quality. A secondary benefit of calibration is an increase in employee awareness of how

machine geometry and accuracy affects product quality.

Machine tool calibration includes a combination of geometry and accuracy checks. Geometry

is the relationship between the different moving members of the machine. Correct geometry is

also a prerequisite to accuracy checks. Accuracy is how closely an axis can position to its target.

Repeatability or precision is the ability to return to the same location, even if that location differs

from the target. The combination of geometry and linear accuracy contribute to spacial or

volumetric accuracy. Three ways of checking calibration are: specific measures using standard

measuring equipment, inspecting a special gage using the machines' resident spindle probe, and

cutting tests such as circle-diamond-square. These tests will be briefly described, however, a

qualified machine tool technician should be consulted for details on how to perform the tests. If

excessive error is found, calibration includes finding and correcting the problem.

Machine Geometry

Machine geometry takes many forms, and it would be difficult and time consuming to measure

all of them each time. There are a few measures that reflect the cumulative effect of many errors,

and these are the ones to be checked. These include: linear axis squares (XY, YZ, ZX), spindle

tram (X and Y), home positions, and rotary axis 16 point checks. To check squares, a granite

square is placed on the machine table, with the edges aligned to the two axis to be checked. A

"tenth" indicator (test indicator measuring graduations of one-ten thousandth of an inch) is

rigidly attached to the spindle. The indicator is run along the square by jogging each axis

independently. The deviation from true squareness is measured, and recorded as deviation over

length, as in .0003 over 12 inches. Tram is a measure ofthe spindle centerline angle relative to

the Z axis. One method of checking uses a test bar (a long accurate cylinder) mounted in the

spindle and a "tenth" indicator fixed to the machine table. The indicator is set to contact on the X

side of the test bar to check X tram, and on the Y side of the test bar to check Y tram. The Z axis

is moved to get an indicator reading along the length of the bar. TIR (total indicator reading) is

recorded as deviation over length. Home positions are the origin of the axis, or the "zero"

position. These need to be verified, especially if the machine has rotary axis. The technique

varies greatly depending on machine configuration. The 16 point check measures the deviation of

the rotary axis face and rotary axis centerline from the plane perpendicular to its axis. It is done

with an indicator mounted to the spindle, and a gage block free to move around the face of the

rotary table. The indicator is set to zero on the gage block resting on one comer of the rotary

face. The rotary is indexed 90 degrees and the indicator is moved over the gage block again.

Readings are taken at all four 90 degree index positions. Then the gage block is moved to another

comer, and that comer is indexed to all four positions. When finished, you have 16 readings

representing the four comers at four different index positions. The total range of the readings

indicates the error.