Introduction: A thread mark on a lathe or a replacement part only makes sense once you know which of the four thread systems it belongs to.
A maintenance technician pulls a damaged part from an imported machine and reads a short stamp on the flange: a diameter followed by a number, with no other explanation. The number could be a metric pitch, a thread count per inch, a gear module, or a diametral pitch, and each reading leads to a different feed setting, a different gear train, and a different thread. Reading thread marks well is less about memorizing numbers and more about sorting them into the right system first. The four systems build on each other in a clear order, and once that order is in place, the numbers start to make sense on their own terms.
Metric and inch marks sit at the bottom of the ladder because both describe the same physical thing: the distance between one thread peak and the next, measured along the axis. A metric mark pairs a nominal diameter with a pitch in millimeters, so an M12 × 1.75 thread is 12 mm across and repeats every 1.75 mm. ISO 724 is the reference engineers use for metric thread diameter and pitch combinations and their tolerance terminology, which is why the pitch value in a metric mark can be looked up rather than guessed. An inch mark pairs a nominal diameter with a thread count, usually written as threads per inch. A 1/2-13 marking means a half-inch diameter with 13 threads in every inch of length. The two systems are mirror images of each other: pitch in millimeters and threads per inch are just reciprocal measurements separated by a unit conversion, since threads per inch equals 25.4 divided by the metric pitch. That reciprocity trips people up. A larger metric pitch number means a coarser thread, while a larger thread-count number means a finer one. For a lathe, these marks translate straight into feed. The lead screw must advance one thread spacing per spindle revolution, so a 2.5 mm metric pitch calls for 2.5 mm of carriage travel per turn, and a 13-pitch inch thread calls for one thirteenth of an inch. That directness is exactly why metric and inch threads occupy the first rung of the ladder: the number on the part is the number the machine has to reproduce.
Module and diametral pitch come from a different world. Both are gear dimensions, not thread spacings, and they describe the size of a toothed wheel rather than the gap between two thread peaks. Module is the pitch diameter in millimeters divided by the number of teeth, so a module 2 gear has 2 mm of pitch diameter for every tooth. When that module is applied to a worm, the resulting axial pitch equals π multiplied by the module — roughly 6.28 mm for a module 2 worm. Diametral pitch runs the same idea in imperial units and in the opposite direction. It counts teeth per inch of pitch diameter, so a diametral pitch of 8 means eight teeth for every inch of pitch circle. The resulting axial pitch is π divided by the diametral pitch, which works out to about 10 mm for that same DP 8 worm. Diametral pitch and module are inversely related to one another, and a larger diametral pitch number produces a finer thread, the reverse of how module behaves. The practical consequence is what matters on a lathe. A metric or inch thread needs a gear train that turns the spindle count into a lead screw count through simple whole-number ratios. A module or diametral pitch thread needs a π factor buried somewhere in that train, because the spacing is not written on the part. Lathes handle this with an approximate π ratio built into the change-gear or gearbox arrangement, and that extra step is why these two groups appear as separate columns on a thread chart rather than being folded into the metric and inch lists. Both numbers describe a gear diameter; the spacing has to be worked out from them. The same principle shows up in power screws. The Acme thread standardized in ASME B1.5 is an imperial trapezoidal profile used for lead screws and similar motion-transfer parts, and its mark follows the imperial pattern of diameter plus threads per inch. The profile differs from a fastening thread, but the numbering system does not. A technician who reads the shape of the mark before reading the number will land in the right system almost every time.
A manual lathe can only cut the pitches its spindle-to-lead-screw train can produce. Metric and inch marks map onto that train directly because the number on the part is already a spacing. Module and diametral pitch marks need the extra π step, which is why a single manual lathe that covers all four groups has to provide four separate ranges rather than one continuous scale. Matching feed to a mark therefore comes down to three moves.
Modern manual lathes list all four groups because the gearing can cover them. The SENMO CQ6280B and CQ6280C, for example, are listed with metric, inch, module, and diametral pitch thread turning plus a choice of metric or inch feed systems, so the same machine handles an imported replacement part stamped in threads per inch and a domestic workpiece marked in millimeters. Both models share the ISO702/II NO. 8 Camlock spindle nose, and the CQ6280C adds a 105 mm spindle bore, but the thread coverage is the same across the pair. When comparing specifications, look for the thread groups to be named one by one; a heavy duty engine lathe supplier that lists only "metric and inch" leaves the module and diametral pitch question open.
The four thread marks form a ladder rather than a list. Metric and inch marks sit at the bottom because the number on the part is the spacing the lathe has to reproduce, one in millimeters and one in threads per inch. Module and diametral pitch sit higher up because they describe a gear diameter first and a spacing second, which is why they need a π step in the feed train and a separate column on the thread chart. The next time a part arrives with an unfamiliar stamp, the useful first question is which group the mark belongs to. The number itself can wait until that is settled.
A: A module thread is marked with a gear module, which is the pitch diameter in millimeters divided by the number of teeth. The axial pitch the lathe actually cuts is π times that module, so a module 2 mark produces roughly 6.28 mm of lead per revolution rather than 2 mm. That is why module work uses a different gear train setting from an ordinary metric pitch.
A: Metric pitch is the distance in millimeters between adjacent thread peaks, so a larger number means a coarser thread. Diametral pitch counts teeth per inch of pitch diameter, and the resulting lead is π divided by that number in inches. A higher diametral pitch therefore gives a finer thread, the opposite of how metric pitch numbers behave.
A: All four groups reduce to a ratio between spindle rotation and lead screw rotation, and a quick-change gearbox or change-gear set can produce many such ratios from one machine. Metric and inch work needs whole-number ratios that reproduce a spacing directly, while module and diametral pitch work adds an approximate π ratio. A lathe built to cover all four simply provides the gearing for each case.
