A good grade of lubricant should be used on the tool when threads are being cut. Mineral lard oil is a
very good lubricant for threading.
RESETTING THE TOOL
If it is necessary for any reason to remove the tool before the thread is finished, reset the tool to the
gage regardless of the part of the thread already cut. Having the compound rest at an angle of 29
degrees makes it easy to reset the tool if it needs regrinding. The tool is clamped in the tool post after it
is reground. Then it can be set with the center gage as before.
In resetting the tool, proceed as follows: first, reset the tool to the gage; then back the tool away from the
work piece. Start the machine and engage the thread-chasing lever as before. Let the work piece make
two or three turns and shut off the power with the threading lever still engaged.
Adjust the tool into the thread previously cut by moving the compound and cross-feed until the tool is
lined up properly in the thread groove. Back the tool out from the work piece slightly and turn on the
power. When the tool is aligned properly, proceed as before.
There are three classes of fit designated by American Standards now being used. Classes 1A, 2A, and
3A apply to external threads only, and class 1 B, 2B, and 3B apply to internal threads only. The three
classes of fit are:
Class 1. Loose Fit
Class 2. Medium Fit
Class 3. Close Fit
LOOSE FIT
This class possesses the largest allowance and is used where rapid assembly of parts is required and
looseness is not objectionable.
MEDIUM FIT
This is used on the bulk of standard screws, bolts, and nuts. A very small amount of looseness or shake
may be present or if the parts are carefully made no movement can be noted; yet the nut can he
screwed on by hand.
CLOSE FIT
This is used on fasteners where accuracy of fit is highly important, and where no looseness is permitted.