The inside threads are cut in very much the same manner as the outside
ones. The cutting tool is held in the boring bar and, like all boring
tools, is ground with less clearance than tools used for outside work.
To regulate the depth of each cut, the screw in the adjustable stop is
placed between the stop and the cross-rest. Then by turning the screw
in after a cut has been taken the cross-rest can be drawn back to
permit a deeper cut with the tool.
=Cause of Threads Breaking.=—When cutting threads in cast iron, they
will break if the roughing cuts are too heavy and are liable to if they
are cut to a sharp point. Another cause for the breaking of cast iron
threads is the use of a dull tool, or one with too little clearance.
=Finishing Threads.=—As a general rule cast iron is machined without
using a lubricant, but in finishing threads a little lard oil will aid
in producing a smooth finish.