TM 9-4910-579-14&P
THE COUNTERBORING
The special counterboring tool bit, due to the small amount of cutting done by it, will require much less lapping than
the regular boring tool bit. A special counterboring lapping jig is-available and is recommended, but if very little
counterboring is done, the tool may be lapped freehand on the lapping disc. When lapping freehand, it is important
that you maintain the relief angles that were originally ground on the tool, but this is not too difficult if care is
exercised in placing the tool on the disc and holding the tool steady during the lapping operation.
When lapping the tool in the special counterboring lapping jig, the jig and tool should be positioned and checked on
the lapping disc before the motor is turned on. It is extremely important that you lap the tool so you will retain the
relief behind the cutting edge. The counterboring lapping jig can be used for lapping the face of the tool that is
down in the counterbore and, also, the front or lead face of the tool. You might find, however, that you prefer
putting the tool holder in the 4F position of the large swivel head lapping Jig and lapping the lead edge with a slight
negative angle. The negative angle on the lead edge will keep the tool from digging in while you are hand feeding
for a counterbore and enable the operator to feed the bar more smoothly by hand. See Page - 24- for
counterboring depth boring gage.
In ordering a counterboring tool for this machine specify:
062-1074-05
Counterboring Tool Bit
In ordering special counterboring lapping jig specify:
062-0581-53
Lapping Jig For Counterboring Tool Assembly
BORING CLOSED CYLINDERS
There are many applications where the operator finds it necessary to bore to the bottom of the closed cylinder. In
this type of boring, it is necessary that an offset boring tool be used so that it will extend below the bottom of the
boring head. This tool must be used in special offset tool holder.
In an emergency, standard tool bits can be lapped in the offset tool holder in such a way that they can be used for
offset boring, but it is advisable to order special offset boring tools that have been ground for that particular purpose.
When putting an offset tool or standard boring tool in the offset tool holder, you will find it necessary to file the shank
of the tool so it will conform with the angles on the tool holder.
It is usually better to remove this metal before dressing the tool so the upper end of the tool will not interfere with the
setting of the tool holder in the boring head.
LAPPING THE OFFSET BORING TOOL: #1, #2 and #3 faces on the tool bit are lapped with the large swivel
lapping jig that is used for lapping standard boring tools. Set the number one position the same for both roughing
and finishing.
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