Smithy Midas 1220 LTD - Manual (Page 75)

Figure 16.5 When cutting multiple threads, increase the lead to
make room for succeeding threads.
Continue this process until the tool is within 0.010" of the finished depth. Brush the
threads regularly to remove chips. After the second cut, check the thread fit using a ring
gauge, a standard nut or mating part, or a screw thread micrometer. It is best to leave
the piece in the chuck and not remove it for testing.
After returning the workpiece to the setup, continue taking 0.001-0.002" cuts. Then check
the fit between each cut. When you thread the nut, it should go on easily but without end
play. When you have the desired fit, chamfer the end of the thread to protect it from
damage. To chamfer is to take a 45° cut off the end of the bolt.
Using the Threading Dial
The threading dial performs the function of
indicating the proper time to engage the
half-nut so that the cutting tool will enter the
same groove of the thread of each successive
cutting pass. This allows the half-nut to be
disengaged resulting in an easier method of
threading. At the end for each consecutive pass
the half-nut can be disengaged and the carriage
can be moved back to the start of the thread
without stopping the spindle or reversing the motor. Disengage the half-nut, back the
cutter away from the workpiece and move the cutter back to the beginning of the thread
with the hand wheel. Turn the cutter the desired amount and you are ready for the next
threading pass. The threading dial is marked with lines numbered 1,2,3,4,5 and 6 and a
single reference line on the housing of the dial. The indicator table shows the selection
for the different thread pitches. Find the desired thread pitch under the "TPI" column and
engage the half-nut at the proper numbers shown on the "SCALE" column of the table.
"1-6" means the half-nut can be engaged on any of the numbered lines 1 through 6. "1,4"
means that the half nut can be engage on 1 or 4 only. "1" indicates that the half-nut can
be engaged on 1 only. For any thread pitch not listed on the chart, cut a test thread and
engage the half nut on 1 or 4 only. If the test cut is not successful, the thread dial annot
be used and the instructions from the previous section should be followed. Cutting of
metric threads cannot be done with the thread dial.
Single Thread Double Thread Triple Thread
Figure 16.6 Threading dial
16: Cutting Threads on your MI-1220 LTD
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