Metric Threads

[The Artful Bodger]

One small thing many don't realize is that if you cut threads that are a "multiple" of the lead-screw pitch, you don't need a dial indicator - the half nuts can be engaged anywhere they will lock onto the lead screw. i.e. 8 tpi lead-screw - I can cut 8, 16, 24, 32, 40, 48, 56.... tpi. I haven't checked, but intuitively the same should hold for metric.

A very common metric lead screw is 3mm pitch such as on my generic Chinese 12x36 lathe. If I am reading the tables correctly thread pitches of 0.5, 0.6, 0.75, 1.0, 1.5 and 3.0 can be cut without reference to the treading indicator. Other thread pitches are awkward requiring selection of the correct pinion for the threading indicator. Supplied pinions are 14, 15 and 16 tooth. There are 8 points marked on the indicator.

I have tried using the indicator but never perservered.

[Bob Fisher]

Clasp nuts? Only in the UK. The REAL name is half nuts. Bob.

[rohart]

Paul, are you saying that some lathes have metric thread dials with interchangeable drive gears as standard ?

And Bryce, it would be useful if you provided details of your lathe in case a member has the same machine.

Unless Paul is on the right track, I can't figure out what the reference to 16 is.

[The Artful Bodger]

Richard, my 12x36 metrtic lathe came with three pinions for the threading indicator, 14, 15 and 16 teeth.

Presumably that what the 16 refers to.

[lakeside53]

Emco metric lathes also had 3 or 4 gears on the indicator.

[Don Young]

A thread dial for a metric thread with a metric leadscrew needs to rotate completely around in a distance that is a common multiple of both the leadscrew and the work pitches. For a 1.25MM thread on a 3MM leadscrew the least distance is 15MM, 5 times the leadscrew and 12 times the work. So a gear with any multiple of 5 teeth (15/3) would work. A 15T gear is commonly used. Such calculations show that 3 or 4 gears will cover most common metric pitches.

Actually gears can be calculated for metric pitches with English leadscrews or vice/versa but the gears are mostly so impossibly large that other methods are more practical. Even with correct gearing it would take a very long time for the dial to go around.

[Carld]

I always find it amusing the ridiculous extremes that people use to avoid leaving the half nuts closed when cutting metric threads with an imperial lead screw or vice versa.

[The Artful Bodger]

I always find it amusing the ridiculous extremes that people use to avoid leaving the half nuts closed when cutting metric threads with an imperial lead screw or vice versa.

True, however this is a home shop site and no doubt many of us have single phase motors on our lathes and are loath to stop and start them more than necessary.

[Carld]

Yes, that's true and not many have foot brakes to stop the spindle. You can however thread from the bottom of a thread out with a little creativity and spin it back in by hand if you don't have reverse on the motor. I had a reversible motor on the old Logan I had and it did come in handy at times. I didn't machine with the spindle in reverse though.