[SEL] Cutting Metric Threads on Adapted Inch System Lathes

[Richard Allen]

Cutting Metric Threads on Adapted Inch System Lathes
Cutting Odd-Numbered Fine Pitch Threads 

All lathes that are set up in the inch system of measurement that are
adapted for cutting metric threads have an inherent and rather
astounding flaw. With these lathes the standard procedures and
practices that work fine for cutting inch measurement threads will not
work at all when cutting metric threads. When cutting a metric thread
the carriage feed screw timing indicator will no longer indicate when
to engage the carriage feed half nut so that the cutter will follow the
thread once it is established. Thread cutting requires both exact
rotational and positional timing between the head spindle and the
carriage feed screw and when the metric thread adapter gears are
installed the thread timing pickup dial (thread chasing dial) lacks the
necessary gear ratio to show the positional timing aspect between these
two rotating parts. This means that when the carriage feed half nut is
released to stop the carriage motion after the first pass of the
cutting tool is made the positional timing between the head spindle and
carriage feed screw is lost. When the thread timing pickup dial (thread
chasing dial) that is turned by the carriage feed screw shows that it
is in time and ready to pick up the thread again for the subsequent
passes that are needed to complete cutting the thread, the positional
timing is in reality off and the second pass will not coincide with the
thread groove established by the first pass of the thread cutting tool.
This is caused by the gear ratio change gears that are needed to
convert from one inch of carriage travel to 0.3937 inches of carriage
travel. The gear pair used to generate this conversion ratio usually
has 127 and 50 teeth (or a multiple of this ratio), which gives the
exact and precise metric-to-inch ratio of 2.54 to 1. With these gears
the absolute minimum number of turns the head spindle has to make
before the carriage feed screw again comes into positional timing with
it is 127 turns. This is because 127 is a prime number, which means
that it is only divisible by itself and one. This consequently leads to
the great number of turns required before the positional timing comes
into alignment again. With thread pitches that are either a multiple or
fraction of one centimeter, the number of turns gets even greater
before it is in time once more. For a few of the finer pitched threads
the number of turns the head spindle has to make gets up around 22,225
turns before coming up to being timed once more and even at a head
spindle speed of 1000 rpm this takes over twenty-two minutes! Normally,
this timing problem is avoided by simply stopping the lathe, reversing
the head spindle rotation without disengaging any of the carriage feed
drive gears, and then running the lathe backwards until the lathe
carriage backs up to the starting point of the thread. This is where
some major problems come into play. Large capacitor-start induction
motors can only be started about once every five minutes on a
continuous basis because of heat buildup in the starting capacitor bank
and starting windings, and they will very quickly burn out if used this
way. Stopping the motor in order to reverse its direction and then
waiting for it to cool down before starting it again consequently leads
to a great deal of wasted time unless there is a direction reverser
gear and clutch on the motor so it does not have to be stopped, and
most lathes are not set up this way. The alternative to the reverse
gear and clutch is a variable speed reversible DC drive, but they are
not common, either. The carriage drive screw rotation reversing gears
cannot be used to back the carriage up, either, because if they are
disengaged then the timing is lost. Cutting threads on a lathe is time
consuming even with all of the added features that help to make it
easy, so with the additional time required to stop and back up the
lathe for cutting metric threads it becomes intolerable for repetitive
or production work, especially when cutting a long section of fine
threads. For example, it may take two minutes for the carriage to
traverse a long run of very fine threads, so it requires an additional
two minutes to run the lathe backwards to get the tool back to the
starting point. With a similar inch-measurement thread pitch it still
takes around two minutes to run the carriage forward, but it only takes
a few seconds to return the carriage by hand and then wait a short time
longer for the timing marks to line up again before making the next
cut. This disparity caused by running the lathe backwards makes cutting
metric threads take nearly twice as long as inch threads under the very
best of conditions, besides also wearing out the lathe by running it
backwards unnecessarily. There are two ways to remedy this, the first
is to get a lathe that is set up for metric measurement, which then
causes the same rotational timing and time consumption problems when
cutting inch threads. The second, and most practical solution, is to
reset the tool point to the correct timing position after disengaging
the carriage drive and moving the carriage to the starting position by
hand. The tool point timing position is very easily reset by using a
dial indicator in conjunction with the compound feed, which is set at
0° so its travel is parallel with the carriage travel and then is used
to move the cutting tool point back and forth the required small amount
so that it is lined up with the thread groove being machined. This can
be accomplished very easily by making temporary timing marks on the
chuck and an adjacent stationary structure with a felt tip pen. It may
be necessary to “C” clamp a piece of cardboard (or like material such
as a piece of sheet metal, etc.) up close to the chuck in an out of the
way position to draw the stationary timing mark on with which to line
up the mark on the chuck. The steady rest or tailstock can be used to
mount the dial indicator on and the reading taken from the tool holder
or tool bit itself, whichever is most accessible. Before actually
drawing the alignment marks, the carriage has to be backed up slightly
outside of the thread starting position. The carriage feed half nut is
then engaged and the carriage driven up to the start of the thread by
turning the chuck by hand. This is to take all the slack out of the
gears and carriage feed, so be sure to initially back the carriage up
far enough to accomplish this. Draw the timing mark lines after the
carriage is driven up to the starting point. The tool should not
actually be into the cut, just close enough so that about half a turn
of the chuck will bring it into contact with the work piece. Next, zero
the dial indicator off of the tool bit or holder. Make the first cut
and then disengage both the carriage feed and lathe drive motor clutch.
Crank the carriage back into deep contact with the dial indicator,
being careful not to bottom it out or move its position. At this point
engage only the carriage feed half nut. You will probably need to
rotate the chuck by hand to turn the feed screw to get the nut to
close, so don’t force it. Line up the felt tip pen timing marks by
rotating the chuck in the direction it turns when cutting to get all
the slack out of the gears and drives, and then move the compound feed
so the tool bit or holder reads zero on the dial indicator. The tool
point is now at the thread starting point and you did not need to run
the lathe backwards to get there, or run it forwards for twenty minutes
to get the tool and work piece in time again for the next cut. For the
rough work of the first few cuts the tool can be lined up with the
thread by eye, but for the final cuts the dial indicator needs to be
used. With some practice a technique can be developed so the loss of
the 30° feed-in angle that the compound feed is normally provides can
be compensated for by estimating where the bottom of the final cut will
be and advancing the cut toward this point each time the depth is
increased. That way only half of the tool cutting edge will be in
contact with the work piece, preventing tool chatter from a “V” shaped
chip, making for a smoother cut.

Another application for this technique is for cutting those
odd-numbered fine-pitch inch threads that also take a large number of
head spindle revolutions before the positional timing once again comes
into place for engaging the carriage feed half nut. Just follow the
same procedure as above for setting the cutting tool point to its
positional zero and a lot of time can be saved instead of standing idly
by while the lathe does nothing but slowly turn the carriage feed screw
timing indicator into engagement position.

These procedures are initially time consuming to set up and learn, but
once the method is understood it will save a great deal of time for
repetitive thread cutting.