TM 9-4910-638-14&P
MOST IMPORTANT PART OF THE TOOL
The portion 2 is by far the most important part of the tool and its width and condition will determine absolutely the
quality of cylinder wall finish left by the tool in boring. The following rules apply to this part of the tool and, if
observed, will provide the best results possible.
1 - Portion 2 is widened by lapping operation number 2 and narrowed by either 1 or 3 (F or R) preferably 3, because
it is faster.
2 - Widening portion 2 improves the finish but at the same time, increases the tendency of the tool to chatter. The
width of portion 2 MUST, THEREFORE, BE WATCHED CAREFULLY.
3 - The correct width of portion 2 depends on the diameter of cylinder to be bored. For holes up to 3-1/4", the width
of F tool should be about .040". For holes larger than 3-1/4" (and longer than 9") it will be necessary to reduce
this width somewhat, the amount depending on the tendency to chatter at the bottom of the hole. For the largest
holes, the width should be about .020".
Careful examination of the tool before lapping will show a gray line along portion 2. This is the wear that takes place
on this keen edge after it has bored a hole. In lapping, you must remove this gray line in order to restore the KEEN
CUTTING EDGE which is of vital importance in getting a good finish. The keenness of the edge of portion 2 is
especially important.
MISCELLANEOUS SUGGESTIONS ON LAPPING
In all lapping operations, lower the jig very carefully to the lapping disc. IF THE TOOL IS DROPPED ON THE DISC,
BOTH TOOL AND DISC ARE SUBJECT TO DAMAGE.
To get the best results in lapping, oscillate the tool with a short, quick movement. Never lap any more than is
necessary. To facilitate lapping operations, keep the steel filed away from the tungsten carbide , similar to a new
tool.
The lapping at positions 2 and 3 should be done toward the center of the disc. The lapping at positions 1 and 4
should be toward the edge.
CARE OF THE DISC
Keep the felt wick in the lapping disc guard saturated with a good grade of light motor oil (or special lapping disc oil).
If the disc is used dry, the tungsten carbide pulls the diamond powder out of the disc and will shorten its life. The
free use of oil on the disc will not only improve the results, but greatly prolongs disc life. Occasionally the lapping
disc will become gummed up and appear dull. Cleaning with solvent will improve its cutting quality. Do not press felt
wick down tight against the lapping disc as too much pressure will serve as a brake and reduce the usable power of
the motor.
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