The Ball Lance Micrometer Evaluation

This device, our Part Number BW-100, provides a means for the full calibration of zero to one inch (25.4 mm)micrometers. It also provides a means for calibrating the outside measuring function of dial calipers. In addition to its use for calibration, the Ball Lance is a perfect training tool to teach the proper use of the zero to one inch micrometer. The Ball Lance also provides the final test to prove the qualification of an operator in the use of the zero to one inch micrometer. Before doing any of the size and squareness evaluations of the micrometer, it is imperative that any wear patterns on the gaging surfaces of the micrometer be detected and corrected if required.

There are two ways to measure this wear on the measuring surfaces of a micrometer or a dial caliper. One method that has been widely published, but almost never used, is to use a set of optical parallels and a monochromatic light source. The equipment used to perform this test is quite expensive and very prone to wear. This technique is tedious and it requires considerable operator skill. The combination of the two flatnesses and a squareness the results of these measurements are always in question.

The second way to check for wear is simple, easy to understand and the equipment is very inexpensive. You simply measure the diameter, of a small, very precision ball, in five places between the measuring surfaces of the micrometer. The BALL LANCE provides a very precise, one quarter inch (0.250", 6.35 mm) diameter, long wearing tungsten carbide ball, mounted on the very end of the lance, where it is very accessible for measurement.

This small diameter ball is carefully measured in the very center of the measuring surfaces. It is then measured very near the edges of the measuring surfaces in the front, in the rear, on the left side and on the right side. All five of these measurements should be exactly the same. Any deviation in measurement, indicates worn gaging surfaces and they should be relapped before any further time is spent calibrating the micrometer.

Full instruction for relapping the faces of the micrometer are supplied with our micrometer lapping kit, part number ML-0-1. A perfect zero reading of the one inch micrometer is first confirmed by bringing the two measuring surfaces into contact, by rotating the spindle clockwise. In order to be sure that there is no grease or dirt trapped between the measuring surfaces of the micrometer, the measuring surfaces are closed on a piece of copy paper. The copy paper is pulled a short distance between the micrometer anvils, and then the micrometer is opened and the paper is removed. If this zero reading is off even slightly, the barrel of the micrometer should be rotated according to the instructions of the micrometer manufacturer to correct the error.

The next step is to open the micrometer all the way up by turning the spindle counter clockwise. Now place the micrometer over the one inch diameter ball on the Ball Lance. Rotate the spindle clockwise until the two measuring surfaces come into gentle contact with the surface of the one inch diameter ball. The micrometer should read exactly one inch (25.4 mm). If this reading is off more than the accuracy tolerance of the micrometer, it indicates a lead error of the micrometer screw and there is nothing that can be done to correct it.

Next, move the micrometer over the 0.812 inch (20.6248 mm) diameter ball and again bring the measuring surfaces into gentle contact with the ball. The micrometer should read exactly 0.812 inch. This measurement is much more than just a size check. This dimension is one half of a revolution of the micrometer spindle. This one single check will detect any squareness error of the micrometer. Squareness is one of the major errors in rotating spindle micrometers. This error can be corrected by lapping the flat end of the rotating spindle of the micrometer in a special squaring fixture. This fixture is Part Number MLF (Then add the diameter of micrometer spindle. This information is required because there are several different micrometer spindle diameters in use.) (Note: Part MLF is out of stock and temporarily unavailable, but could be produced again if a customer desperately needed it).

This lapping operation requires a cast iron lapping plate and some three micron lapping abrasive. If you don't have this equipment, it can be purchased as Part Number LAP-12-16. Now place the micrometer over the 0.606" (15.3924 mm) diameter ball and bring the micrometer measuring surfaces into gentle contact with this ball. The micrometer should read exactly 0.606 inch. This measurement, is another squareness check, as it is at 90 degrees to the original zero. Now move the micrometer over the 0.418 (10.6172 mm) diameter ball and bring the micrometer measuring surfaces into gentle contact with this ball. The micrometer should read exactly 0.418 inch. This measurement is the last squareness check as it is 90 degrees in the opposite direction.

The final dimensional check is to bring the measuring surfaces into gentle contact with the one quarter inch (0.250 inch, 6.35 mm) diameter ball on the very end of the Ball Lance. The Federal specification for zero to one inch micrometers only allows a deviation of one ten thousandths of an inch, (0.0001", 0 .00254 mm) throughout the entire range of the micrometer, so the ultimate quality of the master balls for the Ball Wand is required. A long form certification of the Ball Lance master ball diameters can be supplied with traceability to N.I.S.T. , on request, at the time of purchase. Periodic re-calibration of the Ball Wand can be performed again with NIST traceability.

Common Micrometer Errors

The three common errors, that instruments which use lead screws for measurement, are prone to, are drunken threads, out of square measuring surfaces and lead errors.

Drunken Threads

A drunken thread usually has an error, that comes and goes with each revolution of the screw. This error can be very substantial, but because it is almost perfectly repeatable with each revolution it is hard to detect. If a multitude of dimensional standards, that occur at 90 degree rotations of the lead screw, are measured, any drunkenness will show up vividly.

Squareness Errors

Squareness errors of the measuring surfaces will show up using the same series of tests, but the steps must be in specific orientations in relationships to the zero setting

Lead Errors

Lead errors are exemplified over long excursions of the lead screw, so evaluating the distance from the original zero setting to the one inch diameter ball will show up this error best.