TM 9-4910-579-14&P
VARIATIONS IN BORE SIZE
A variation in size from one cylinder to the next can quite often be the fault of the operator, although he is not
conscious of causing this effect. Due to the fact that the tool holder of the machine is spring loaded and the
micrometer is held against the stop plug in the boring head with spring pressure, it is very important that the
operator remove his hand from the micrometer while setting the tool. Force applied to the micrometer in any way
while the tool is being set and locked in place will affect bore size.
Another cause of variations in bore size may be due to using a roughing tool that has worn down until there is very
little carbide left; then using a new finishing tool to make the finish cut. The great variation in position of the boring
point from a worn out roughing tool to a new finishing tool will change the centerline of the cutting face sufficiently to
give a variation in bore size. Because of this, it is advisable to use both roughing and finishing tools with a
comparable carbide in each tool.
Variation in bore size can, also, be caused by a loose thread in the micrometer. The end play on the micrometer
stem between the thread and the thimble can be adjusted to remove this condition. Please refer to Adjustment of
the Micrometer on Page - 24 -.
Variation in bore size can, also, be caused by the micrometer spindle not being held firmly against the backstop with
the ball in the micrometer's stem groove. The tension on this ball can be readjusted and you should check to see
that the stopscrew is locked securely in place. Please refer to Micrometer Spindle Lock Adjustment on Page - 17 -.
Variation in bore size will, also, be quite evident if the operator has scratched the face of the micrometer by rotating
it while the tool was in contact with this face. If such a condition does exist, and does cause trouble, it is
recommended that the micrometer be returned to the factory for relapping of the micrometer face.
As mentioned in a previous paragraph, it is advisable to insert the micrometer with the thumb lockscrew up each
time that the tool is set. Although the face of the micrometer is checked very carefully and extreme accuracy is
maintained between the micrometer stem and the micrometer face, you will find that more accurate settings will be
made if the micrometer is inserted with the thumbscrew up each time the tool is set. The tool being set will be in
contact with the same portion of the micrometer face.
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