Threading logic check

[heavyg603]

I thought I Understood this but I'm seeing some vastly different data in different calculators so figured I'd lay it out.

I have some thick walled tubing I need to internally thread. Its ID is 1.320

I want to create a 20tpi thread.

When I look at my handy lathe threading chart (this is the classic one out of the atlas manual I think) I follow over to the 20 tpi line and see a single depth of thread at .0379. So by using the correct compound depth I feed to .043. When I finish I indeed have a good thread that has a depth of thread at .0379.

So thinking this through I now have a Minor diameter of 1.320 and a major at 1.3579 ?

None of the calculators I've checked seem to produce this Major diameter. I must of messed up something basic here, can some one educate me a little ?
Thanks in advance!

[earlgo]

Look in the Machinery' handbook under American Standard Unified Thread Form Data. It is a chart in the Threading section.
Therein you will find that the depth of thread for a 20tpi internal thread is .02706. So if you start with your minor bored diameter and add 2x the thread depth you will get an overall OD of the male thread of 1.374. That is nearly 1-3/8-20 which is going to be a special as 1-3/8-18 is the UNEF series.
Most people making an odd sized internal thread make an externally threaded 'master' and then fit the internal thread to the 'master'.
Let us know how you measured your results.
--earlgo

[heavyg603]

That's why I went the calculator route, none of the charts fit the sizes exactly. Was using this tool http://theoreticalmachinist.com/Threads ... erial.aspx
I have only measured the depth of thread on my DRO so far. Original ID was with a set of internal calipers. I really need to get something to do internal threads......and a copy of the handbook.

My next step, once I figure out my dimensions will be to make the male portion. Need to know what the major needs to be to start that of course.

[GlennW]

You are neglecting the fact that you are working with a sharp V tool.

The proper major diameter is calculated using a 1/8p flat, (which is what the calculators are doing) not a sharp V, so your major diameter as cut will be larger.

Also keep in mind that the chart above reflects COMPOUND infeed which is normally roughly at 29 degrees.

[heavyg603]

You are neglecting the fact that you are working with a sharp V tool.

The proper major diameter is calculated using a 1/8p flat, (which is what the calculators are doing) not a sharp V, so your major diameter as cut will be larger.

Also keep in mind that the chart above reflects COMPOUND infeed which is normally roughly at 29 degrees.

Ah ok that makes sense. So the chart I posted with the 20 tpi Single Depth on a V form of .0379, and a compound feed of .043 may be correct ? Is the data in the handbook based on the sharp v or the 1/8p flat ?

[GlennW]

Data in the Handbook should be based on a 1/8p flat at root for internal thread major diameter.

[heavyg603]

Data in the Handbook should be based on a 1/8p flat at root for internal thread major diameter.

Hmm. This could be a challenge. Wish I had a set of calipers with the right anvils to measure this to the root. All Ive seen so far just support external threads

[Harold_V]

I'm at a loss to understand why ANYONE would attempt a thread----any thread--- by judging pitch diameter based on charts, using tool pickup as the reference point. There's more room for error (size of flat, variations in major or minor diameters, error in compound angle) to make the possibility of hitting the desired size nothing more than random. If it matters, use the charts to create a plug gauge, using wires to determine pitch diameter. That way you aren't working blindly.

I've been single pointing threads since 1957 and have never relied upon thread depth as a means of determining size. I practice what I preach. To be fair, I do use that information in determining the depth of a thread relief, assuming one is permissible.

Harold

[Harold_V]

Data in the Handbook should be based on a 1/8p flat at root for internal thread major diameter.

Hmm. This could be a challenge. Wish I had a set of calipers with the right anvils to measure this to the root. All Ive seen so far just support external threads

Calipers?
You've already lost the battle. Calipers do not provide the degree of precision that would make any readings you might establish of value. Tolerance on threads, especially small threads, is quite tight.

Why are you so obsessed with the root diameter? Its tolerance is likely far greater than the pitch diameter, which should be your concern.

Harold

[GlennW]

I'm just explaining why you're calculated numbers don't match up to other published numbers.

The answer to cutting an internal thread to size was already mentioned here:

Most people making an odd sized internal thread make an externally threaded 'master' and then fit the internal thread to the 'master'.

[heavyg603]

Data in the Handbook should be based on a 1/8p flat at root for internal thread major diameter.

Hmm. This could be a challenge. Wish I had a set of calipers with the right anvils to measure this to the root. All Ive seen so far just support external threads

Calipers?
You've already lost the battle. Calipers do not provide the degree of precision that would make any readings you might establish of value. Tolerance on threads, especially small threads, is quite tight.

Why are you so obsessed with the root diameter? Its tolerance is likely far greater than the pitch diameter, which should be your concern.

Harold

Oooook, haven't been doing this for 50 years so trying to figure it out? Not obsessed with the root diameter but all I could grasp to figure out a major diameter for my male part. I have micrometers as well but not internals.

Or maybe I should ask it this way, given I have a internal threaded part, and I want to make a male part to fit, how would you go about it? Or if I go back to the beginning. I have an ID of 1.320 I need to thread and make a male part for.

[GlennW]

Use thread wires for pitch diameter measurement and turn an external threaded plug to use as a gauge, and then cut the internal thread until the plug screws in nicely, or as desired.