Basic lathe threading question

H

hosinsky

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I am confused by the tables and formulas I find in literature and I have two questions:

Metric thread has a flat top. The diameter, say for a M6x1 outside thread where the nominal diameter is 6 mm, is it 6 mm "flat top" to "flat top" or is it 6 mm including the missing points of the threads - should I turn it down to something less than 6 mm before threading it or to 6.0 mm?

What is the thread depth that I should cut, as a function of the thread pitch, if I start with the point of the tool touching the bar I just turned to size?

Thanks
Goran, Canary Islands
 
Goran,

Most threads have a flat crest and this crest is the major diameter of the thread. In other words, your M6 thread should measure 6mm over these flats. The proper way to measure a thread is to measure the pitch diameter of the thread. This is done using a thread micrometer, or thread wires and a standard outside micrometer. The measurement over the wires can be determined from the chart included with the wires, or from one found in other machining references. In a pinch you can use a nut as a sort of thread gauge if thread mics or wires are not available.

Tom
 
The depth to cut the thread (external) is 0.61343 X pitch, taken from the nominal O.D. However the O.D. has a relatively large tolerance so what I do is touch my threading tool to the O.D. and if the O.D. is say 0.005" on diameter undersize I compensate for that by adjusting the crossfeed dial accordingly by making it read -0.005". In your case the depth of thread is 0.61343 mm (0.61343 X 1mm) taken from the 6mm O.D. Rounding of the crests and also the root is inevitable. Depth of thread 0.61343 X pitch applies to both Metric and Unified threads (external). The internal depth of thread is 0.54127 X Pitch taken from [O.D.-2(0.54127P)] The pitch diameter is [O.D. - 2(0.64952 X P)]. I hope this helps clarify and not confuse. Good luck.
OOPS-- depth of thread is 0.54127P for metric
 
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Goran,
I just realized what you meant by the missing points of the crests. Your measurements are from flat top to flat top. The missing points are part of the original design triangle from which the thread is designed. What you are concerned with are the actual (flat top) crests and roots.

UN Thread.jpg
 
The major diameter (in the example above being 6mm) is the largest through-hole diameter that the bold will have to fit through, so in your case, the flat top would have to be 6mm at max (nominal) to be able to fit into a 6mm threaded hole.
 
Just one point to make. In the Unified National thread form and the metric thread form, the top of the thread is always flat. Further the major diameter of the thread is always slightly smaller than the nominal diameter. As an example, an M6 x 1 thread has a major diameter that measures 5.974 to 5.794 mm, and the pitch diameter measures 5.324 to 5.212.

Hope that's some help.

Sandro Di Filippo
 
Thanks for comments and information, especially the thread pitch chart! I now feel that I have cleared my understanding of the threads. The most important parameter seems to be the Pitch Diameter, which needs expensive (for me) equipment to measure.

From the tables at http://www.engineersedge.com/screw_threads_chart.htm and the drawing and formulas at https://en.wikipedia.org/wiki/ISO_metric_screw_thread, basic profile, I have get the following rules which should be sufficient precise for me:

* Workpiece Diameter = Nominal Diameter - 0.1 * Pitch
* With a freshly grounded homemade pointed tool touching the diameter I should enter 0.38 * Pitch (for a diameter of 0.76 * Pitch)

This is not valid for a correctly ground tool with a rounded or flat tip. For testing I will use nuts, although I have found, that all nuts are not born equal.

For us doing Metric threads on a metric lathe without thread dial note that if the (Leadscrew Pitch) / (Pitch) comes out as a whole number you can open and close the half nut anywhere you want, no need for a thread dial. For my lathe, with a 3 mm pitch leadscrew, that means I can cut 0.5, 0.6, 0.75, 1, 1.5 and 3 mm pitch without having to run the tool back without opening the half nut.

This tip I picked up on some list, I do not know who was the originator.

Goran
 
Goran,
I'm sorry but I made a mistake on the depth of thread, it is 0.54127P (metric) not 0.61343P (Unified).

For an M6x1 metric external thread.
I am not sure where you got [* With a freshly grounded homemade pointed tool touching the diameter I should enter 0.38 * Pitch (for a diameter of 0.76 * Pitch)] I suspect you are mixing up 3/8H with 3/8P from Wikipedia but that is only to the pitch diameter not to the root diameter (if that is what you did). Your depth of thread after touching the outside diameter is 0.54127P or 0.54127mm (0.54127P = 0.54127 X 1mm). This is just basic data, so go slightly deeper.

Also, an inexpensive and accurate way to measure your thread is with 3 wires. It is risky to use a nut because if the nut is oversize so will your external thread be oversize, however, after running a tap through the nut it is a good way to check that everything is OK regarding profile, pitch etc.

Here is how to measure your thread using 3 wires, (sometimes you can use 3 drills or ? that are the same size on the shanks).
Pee Dee wire link http://littlemachineshop.com/products/product_view.php?ProductID=1962

Best Wire Size = 0.57735P = 0.57735 X 1 = ) 0.57735mm = 0.0227"
Actual wire size = 0.024" (0.6096mm)
Measurement Over Wires = E - (0.86603P) + 3W [5.35 - 0.86603 + 3 X 0.6096 = 6.3128]
E = Max. Pitch Diameter [Outside Diameter - 0.64952P = 6 - 0.64952 = 5.35]
P = Pitch [1]
W= Actual Wire Size [0.6096mm]

I am mostly familiar with UN Threads and if you like you can read the section "A Practical Method to Cut a UN Thread" here http://un-unr-unj-threads.tripod.com/#R3

When I cut threads I get the following data, then everything goes pretty quick-
D.O.T. (Depth of Thread)
B.W.S. (Best Wire Size)
M.O.W. (Measurement Over Wires)

I hope this is not too confusing and thanks to everyone else for contributing and correcting (me).

One more thing, I always stay away from Maximum Material Condition especially when I am using basic dimensions.

 
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