# Thread: Over what angle does a screw thread go from zero height to the specified height?

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## Over what angle does a screw thread go from zero height to the specified height?

I am a design engineer for the Navy, please don't hold that against me, and I am currently working on a project for the Marines to defeat IEDs in Afghanistan. I need to know at what point an external thread engages with an internal thread in order for holes on one threaded part to line up with holes on the other threaded part. I figure that happens when the height of the internal and external threads are half the specified thread height. Does that make sense? Trouble is, I don't know where that occurs. The thread height is zero at some point and over some angle finally reaches the full height. I have never seen anything about this but I figure somebody knows something because a tap or die has that designed into it. Can anybody shed some light on this? Thanks.

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Are you talking about "timing" of the two threads?

Ken

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It seems you should be able to figure that out based on the pitch and lead of the screw thread. The pitch is the distance between a point on one thread and the same point on an adjacent thread. On imperial threads, you divide 1 by the threads per inch to get pitch, so 20 TPI would be 1/20 or .050". On metric threads, the second number in the thread designation is the pitch, for instance M6 x 1.0, the major diameter is 6mm and the pitch is 1mm.

The lead is the distance a thread travels in one rotation of the screw. On single start threads, pitch and lead will be the same. On double start threads, the lead will be twice the pitch, triple start will be three times pitch, etc. It should be just math after that. I hope this helps.

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In short, no it does not make sense at all. What kind of engineer are you that you find a "height" in a thread?
Threads have major and minor diameters, pitches, angles, forms , and starts. Possibly more such measurements but they don't have a "height".

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## Thread Height

The thread height is zero at the root of the thread and full height at the major diameter. Inch and metric threads have an angle of 30 degrees per side, measured perpendicular to the axis of the threads. The angle that the threads wind around the circumference is dependent on the thread pitch. It is not clear whether you are wanting the hole to go parallel or perpendicular to the thread axis. If you are wanting to time the threads so interchangeable parts will all line up to the same place when screwed together and seated, you will need to specify where the thread starts on each of the 2 mating parts. On most threading or tapping operations the threads will start at a random place on the circumference. By thread milling or CNC turning, the threads can be started the same place every time. Alternately, if the hole(s) are drilled in a later operation, you could use a jig or fixture to locate the hole(s) in relation to the threaded parts.
Last edited by Toolguy; 01-19-2011 at 11:58 AM.

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Originally Posted by tdmidget
In short, no it does not make sense at all. What kind of engineer are you that you find a "height" in a thread?
Threads have major and minor diameters, pitches, angles, forms , and starts. Possibly more such measurements but they don't have a "height".
Electrical?

David

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## angle

The angle that you are asking about is 30degrees. From root to tip. The rest depends on TPI and diameter. At least that is what I think you are asking,

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tdmidget,
nice going
there are more then one kind of engineer, not all of them are mechanical. But You must missed that one in grammar School.
h12721

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For some reason, I started worrying about safety of our marines.

Jokes aside, Rookie, maybe you want to post a sketch illustrating your inquiry?

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Toolguy's reply is, IMO, correct. And what he asked about perpendicular or parallel is exactly the "confusing" part of the question since visually it could be either and a bit different approach though between the rest of the replies answered pretty much too.

Am I correct in thinking you are interested in if these threaded parts are made separately, as opposed to drilling as a unit, when those holes will align?
Or, perhaps more to the point, if they are first drilled as a unit and then disassembled for later reassembly at what point will those holes line up?
To me, given what you have said (the why in your question), it could act as a simple "timing" device with a few other bits and pieces.

Or it could be a very complicated way to safety wire some parts.

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