UNEF Thead form

I need a reality check. I constructed this 7/8-20 UNEF Class 2A thread from the data I found in the Machinery's Handbook. I can't
believe that the minor diameter lops so much off of the lower part of the form. Is this right?
http://www.grumpyoldgeek.com/images/UNEF_thread.jpg
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wrote:

You should have a flat equal to 1/8 the pitch at the major diameter of the thread. The thread crests should be truncated to about 1/4 pitch at the minor diameter. There are good illustrations in MH, several pages before the tables in my 22nd edition.
Also note, that's a 2B thread -- 2A is the external spec.
--
Ned Simmons

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Ned Simmons wrote:

OK, I need to put the 1/8 flat on the major diameter and then the minor diameter should fall into place.

Got it. Thanks for your help.
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wrote:

Also make sure that the pitch diameter falls in spec.
--
Ned Simmons

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Ned Simmons wrote:

I'll look at that.
Another question, if I'm making a set of drawings for a machine shop to fabricate, do I have to draw the whole form with all the dimensions or can I just spec 7/8-20 UNEF 2B on the drawing?
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wrote:

That's the complete and proper callout for a standard UN thread, there's no need to graphically dimension the thread. You can specify thread depth either in the callout or by dimensioning the appropriate drawing view. Barring any ambiguity, I prefer putting it all in the callout.
Here's what my templates for thru and blind holes look like. (DP) stands in for the ANSI depth symbol, the one that looks like a down arrow with a cap. 7/8-20 UNEF-2B THRU
7/8-20 UNEF-2B (DP) 1.25 MIN TAP DRILL (DP) 2.00 MAX
--
Ned Simmons

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Jim Stewart writes:

Cf the wire sizes and micrometer ranges in my thread data listing.
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On Thu, 23 Oct 2008 10:00:47 -0700, Jim Stewart
<snip>

<snip> ------------- General convention is to simply spec as 7/8-20 UNEF 2B, possibly with spec id [eg . ANSI B1.1] This avoids double dimensioning. More recent convention is to schematically represent the threads on the print using hidden/phantom lines. Be sure to discuss with your supplier to make sure everything is clear such as length of full threads and revise the print as required per the discussion. Also be sure to verify they have recently calibrated go/no-gages and are not checking with an off the shelf nut or screw.
See http://www.draftingzone.com/standards/standard5 / http://www.tpub.com/content/draftsman/14040/css/14040_51.htm http://www.gewinde-normen.de/en/specifications.html http://www.gewinde-normen.de/en/unified-extra-fine-thread.html
Unka' George [George McDuffee] ------------------------------------------- He that will not apply new remedies, must expect new evils: for Time is the greatest innovator: and if Time, of course, alter things to the worse, and wisdom and counsel shall not alter them to the better, what shall be the end?
Francis Bacon (1561-1626), English philosopher, essayist, statesman. Essays, "Of Innovations" (1597-1625).
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Ned Simmons writes:

Which just begs the question: what are the major and minor diameters?
The ideal thread form has no direct application in practical thread cutting. It's kind of like saying holes should be round. It doesn't tell you how out-of-round your real holes can be, or how much over- or under- sized they can be. It is the latter allowances that determine how to drill or bore a real hole.
To cut real standard threads, you must stay within allowances that are rather randomly related to the ideal forms in both sign and magnitude, and which do not necessarily even include the ideal dimensions.
While the tables in Machinery's Handbook are correct, they only tell you the range of allowed results, and not the practical tool shapes and cuts that yield those results.
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On Thu, 23 Oct 2008 00:08:32 -0500, Richard J Kinch

Not quite. It seems you're confusing allowances and tolerances. Allowances are applied to the basic (ideal theoretical) thread form to insure that male and female threads will mate under real world conditions. In other words, the allowances introduce clearance between an assembled screw and nut. The various thread classes are the result of applying specific allowances to the basic thread dimensions.
The tolerances are the acceptable deviations from the dimensions that define a specific thread and class.
--
Ned Simmons

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Ned Simmons writes:

No. Design sizes (what one actually attempts to fabricate) are derived from the "basic" size (the geometric ideals) with allowance applied. The design limits are then subordinately derived by application of tolerances to the design sizes. Thus my point that the basic sizes are of no direct guidance as to practical machining steps; one must apply arbitrary dimensions (allowances) that have no underlying "ideal".
This is why the ISO standard calls allowances by the term "fundamental deviations" in their more rational terminology. Numbers like EI, ES, ei, and es are "fundamental" in that they are arbitrary starting points that have no relation to the ideal thread forms, yet they determine the limits of practical dimensions.
The common UN threads are a surprisingly complex subject. Consider that MH takes five pages just to define the terms. Unless you respect them all, you're just kidding yourself that your results are anything but accidentally within standard.
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On Thu, 23 Oct 2008 16:01:10 -0500, Richard J Kinch

That's what I said.

I don't doubt that's what you meant, but not what you said, specifically, "To cut real standard threads, you must stay within allowances..." The allowances are single values, not ranges that "you must stay within." That's a better description of tolerances.

Which was my point. It may seem like quibbling, but your use would likely confuse the concept of allowances vs. tolerances for someone who hasn't read those pages carefully.
--
Ned Simmons

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Ned Simmons writes:

Yes, an allowance proper is a single value, not a range. But it is a single value within which you must stay, an upper or lower limit or worst or best fit, the tolerance being the specification for how much within. The allowance informally is what I've called that shaded area in MH 16 p 1762 fig 6, because that is where the thread surface is "allowed to be". But I suppose the term "limits" would be more appropriate, although this is not a term in the standards.

Confusing things is a specialty of mine. I've read those pages carefully and am still somewhat confused. But at least now my parts don't come back with "won't fit" complaints.
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On Fri, 24 Oct 2008 01:28:41 -0500, Richard J Kinch

"Limits" makes sense to me.

I certainly don't claim to understand the whole subject either. For instance, the thread tables aren't sufficient to completely specify a good thread. It seems at least one more toleranced dimension is required. I suspect the key is the illustration in my MH that shows the proportions of internal and external thread at maximum material condition, but I've never had a good enough reason to take the time to work through it.
--
Ned Simmons

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Ned Simmons writes:

Quite so. That's why I wrote the software, partly to make sure I understood the specification properly, and partly to yield practical machining specs, as opposed to abstract geometry that are all the standards give.
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Jim Stewart writes:

No. Here is the thread data from my software:
0.875"-20        T H R E A D D A T A
0.8750    D    Basic diameter (in) 0.0500    P    Pitch (in) 0.029        Ideal measuring wire size (in) 0.029    W    First practical measuring wire size (in) 0.0437        Subtract from first wire measurement for actual pitch diameter (in) 0.032    W    Second practical measuring wire size (in) 0.0527        Subtract from second wire measurement for actual pitch diameter (in)         NOTE: interpolated fundamental deviation         INTERP: 1.2700 on [1.0000,1.5000] onto [0.0260,0.0320] --> 0.0292 0.0012    EI    Fundamental deviation (allowance), internal thread (in) -0.0012    es    Fundamental deviation (allowance), external thread (in)         INTERP: 1.2700 on [1.0000,1.5000] onto [0.1600,0.1900] --> 0.1762         NOTE: ext thread pitch dia tolerance above max pitch in table         INTERP: 1.2700 on [0.0000,1.0000] onto [0.0000,0.1180] --> 0.1499 0.0069    TD2    Internal thread pitch diameter tolerance (in) 0.0059    Td2    External thread pitch diameter tolerance (in)         INTERP: 1.2700 on [1.0000,1.5000] onto [0.1800,0.2360] --> 0.2102         INTERP: 1.2700 on [1.0000,1.5000] onto [0.2360,0.3000] --> 0.2706 0.0083    Td    External thread major diameter tolerance (in) 0.0107    TD1    Internal thread minor diameter tolerance (in)
        EXTERNAL: 0.8738    dmax    Major dia, max (in) 0.8656    dmin    Major dia, min (in) 0.8197    d1max    Minor dia, max (in) 0.8047    d1min    Minor dia, min (in) 0.8414    d2max    Pitch dia, max (in) 0.8355    d2min    Pitch dia, min (in)
        INTERNAL: 0.8903    Dmax    Major dia, max (in) 0.8762    Dmin    Major dia, min (in) 0.8327    D1max    Minor dia, max (in) 0.8220    D1min    Minor dia, min (in) 0.8506    D2max    Pitch dia, max (in) 0.8437    D2min    Pitch dia, min (in)
        EXTERNAL THREAD PRACTICAL DATA:
0.8656-0.8738    MAJOR DIAMETER RANGE 0.8792-0.8851    PITCH DIAMETER MEASUREMENT RANGE ON 0.0290 WIRES 0.033-0.039    SHARP-TOOL MINIMUM INFEED (decrease by tool tip blunting)
        INTERNAL THREAD PRACTICAL DATA:
0.823-0.833    BORE 0.033         SHARP-TOOL MINIMUM OUTFEED (decrease by tool tip blunting)
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Richard J Kinch wrote:

Ned got me pointed in the right direction and the problem is solved. Is your software available for download?

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Jim Stewart writes:

It's a miserable 300+ lines of GAWK code. You want that?
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    *I* would like it. I live in unix, and gawk is always within reach -- without even having to install cygwin on a Windows system.
    As long as the overall size of the e-mail is not over 30K it should be fine. If all of the 300 lines are 80 characters long, we only reach 24K, so we are fine.
    Thanks,         DoN.
--
Email: < snipped-for-privacy@d-and-d.com> | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
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DoN. Nichols writes:

See:
http://www.truetex.com/thread.awk
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