TM 9-4910-638-14&P
INSTRUCTIONS FOR LAPPING STANDARD TOOL BITS IN OFFSET BORING TOOL HOLDER
Either standard tool bits or special offset tool bits may be used in the offset tool holder. When it is necessary to bore
to the very bottom of a closed cylinder, the special offset tool bit is recommended.
Insert the standard tool bit in the offset tool holder far enough so that it extends above the top of the tool holder. File
off the top of the tool so it is flush with the top of the tool holder. It may not be necessary to file away this metal, but
it is usually better to remove some of it before dressing the tool so the upper end of the tool bit will not interfere with
the setting of the tool holder in the boring head.
Put the offset tool holder in the swivel lapping jig and set the swivel head so the tool holder is approximately parallel
with the top of the lapping disc. Lap this face of the tool until the end or cutting edge of the tool has been relieved
back to the full width of the carboloy insert Swivel the jig assembly around to the #3F position and lap the 3F position
on the tool bit until it forms a sharp line with the first position lapped. It is recommended that the backing metal be
removed with a file that this softer metal will not be dressed into the lapping disc. There is a considerable amount of
lapping necessary in the 3F position and removing the backing metal with a file will materially reduce the length of
time necessary for dressing this position. Swivel the lapping jig head around to the #1 position and lap the tool on
the #1 face just enough to dress off the sharp line. Then swivel to the #2 position and lap the face to the required
width which, of course, in the case of the roughing cutting tool should be held from 5 to 10 thousandths in width.
The #4 position is dressed using the special lapping fixture for offset boring tool.
The preceding instructions are for lapping the roughing cutting tool, but if finishing cutting tool is desired, the same
procedure would be followed with the exception of the #2 position which would be dressed slightly wider.
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