# Thread: Metric Threads

1. vpt
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Originally Posted by lakeside53
...but retract the tool first, or you'll trash the thread on the back-wind.

Or make one of those fancy self clearing tools.

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it sais to engage the half nuts at 16... WTF

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Originally Posted by Bryce.R
it sais to engage the half nuts at 16... WTF

You need to put different little pinions on the bottom of the threading indicator according to thread pitch (metric). 16 is likely the size of one of them.

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A bit of thread dial theory.

For English threads with an English lead screw, the standard thread dial usually makes one revolution for each four inches of movement along the lead screw or four inches of threading distance. The standard markings of 1 - 2 - 3 - 4 are therefore simply increments in whole inches. Any English thread that has a whole number of threads per inch (no fractions like 3.5 TPI) will repeat after a whole number of inches and therefore you can engage such a thread on any whole number division. When you go to cutting threads with halves (like 3.5 or 13.5 TPI) then the thread will not repeat after one inch: it will be half a thread off. For such threads you can only engage the half nuts after a full two inches of travel or two numbered divisions on the dial. If you have a thread that has quarters (like 2.75 TPI) then you must use a full four inches between engagement points or you must then engage only on the same number that you started on.

For metric threads with a metric lead screw the situation is a bit more complicated due to the fact that they use the lead in mm instead of a threads per ... system. Many metric threads have very odd fractions when measured in threads per mm or threads per cm or threads per meter. You still must travel enough distance on the lead screw to allow those fractions to total up to a whole thread. The situation is further complicated by the larger array of lead screw pitches used on metric lathes. So if you are cutting a metric thread with a 0.7 mm lead on a lathe with a 3 mm lead screw you need to travel a distance that allows that 0.7 thread distance to evenly divide into the 3 mm increments of the lead screw. If my calculations are correct, that only happens after thirty mm or ten revolutions of the lead screw. Different metric pitches will require different synchronizing distances. This explains the need for several gears on a metric threading dial. Metric isn't better for threading: English is.

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Thanks very much Extremely helpful
Originally Posted by Paul Alciatore
A bit of thread dial theory.

For English threads with an English lead screw, the standard thread dial usually makes one revolution for each four inches of movement along the lead screw or four inches of threading distance. The standard markings of 1 - 2 - 3 - 4 are therefore simply increments in whole inches. Any English thread that has a whole number of threads per inch (no fractions like 3.5 TPI) will repeat after a whole number of inches and therefore you can engage such a thread on any whole number division. When you go to cutting threads with halves (like 3.5 or 13.5 TPI) then the thread will not repeat after one inch: it will be half a thread off. For such threads you can only engage the half nuts after a full two inches of travel or two numbered divisions on the dial. If you have a thread that has quarters (like 2.75 TPI) then you must use a full four inches between engagement points or you must then engage only on the same number that you started on.

For metric threads with a metric lead screw the situation is a bit more complicated due to the fact that they use the lead in mm instead of a threads per ... system. Many metric threads have very odd fractions when measured in threads per mm or threads per cm or threads per meter. You still must travel enough distance on the lead screw to allow those fractions to total up to a whole thread. The situation is further complicated by the larger array of lead screw pitches used on metric lathes. So if you are cutting a metric thread with a 0.7 mm lead on a lathe with a 3 mm lead screw you need to travel a distance that allows that 0.7 thread distance to evenly divide into the 3 mm increments of the lead screw. If my calculations are correct, that only happens after thirty mm or ten revolutions of the lead screw. Different metric pitches will require different synchronizing distances. This explains the need for several gears on a metric threading dial. Metric isn't better for threading: English is.

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Thats all true Paul A. however the awkward metric pitches are usually only used in smaller sizes i.e. 6mm and down ('bout 1/4"). From then on the standard pitches are all divisable by .25mm (but I dont know if that makes it easier or not! ).

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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.

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Best of all, a single-tooth clutch running at spindle speed selecting forward and reverse leadscrew rotation, like Hardinge, (mos
Last edited by Hopefuldave; 05-15-2012 at 01:33 PM.

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Originally Posted by lakeside53
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.
That is quite true. Just as you need to use longer synchronizing distances for fractional threads, you only need to use shorter distances for threads that are even in terms of the lead screw. It would seem to be quite obvious that if you are cutting a 8 TPI screw with a 8 TPI lead screw you can engage at any point and the thread will be synchronized. And yes the same thing will apply to metric threads but again the somewhat strange sizes of metric lead screws does make this a bit more complex. I mean, who ever heard of a 3 TPI lead screw, but 3 mm screws are used. 3 mm is close to 8 TPI which is probably the most common lead screw on English style lathes. Some things, like the lead screw's lead just want to be around certain values for a given size lathe. But this does cause some of the awkwardness in metric threading.

Please don't flame me as being opposed to metric. I am ambi-measurement-terous. I use both systems and even combine them on occasion. It is just that neither is perfect and I sometimes enjoy pointing this out on occasion.

As Artful Bodger said, the awkwardness of metric threads tends to be mostly in the small ones. Larger lead threads tend to be more compatible. I have a long term goal/project to develop a universal system for determining when a metric thread can be engaged. I just haven't had the time to devote to it yet. I need a better understanding of the relationships for all sizes of lead screws. Which metric lead screw pitches are better? And why?

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Originally Posted by Paul Alciatore
I have a long term goal/project to develop a universal system for determining when a metric thread can be engaged. I just haven't had the time to devote to it yet. I need a better understanding of the relationships for all sizes of lead screws. Which metric lead screw pitches are better? And why?
Hi Paul, isn't that what the Ainjest Metradial, that I mentioned in post #20, does?

Dave

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