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  1. #21
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    I got a PM on my post above stating that I had left out a method. Here is the essential part of my answer.

    I probably left more than one out. There have been many schemes and devices. My point was that a measurement of the OD is not a really good way to determine the actual diameter of a thread. I have looked into this in some detail. The OD of a "standard" external thread is almost never the nominal size: it is always smaller than that. If you measure a million 1/4-20 screws, even the best quality ones, you will not see a single one that measures 0.2500" +/- a tenth or two. All of them will be 0.249" or less, most will be a lot less.

    There is a wide tolerance range for that actual OD. So any OD derived by an actual measurement is going to be just a guess. And that is what is so frustrating about these "what size is this thread" posts. Everybody comes up with guesses. Probably some very good guesses, but still guesses.

    My point is simply that if you want an accurate measurement of an unknown thread and it must be right, as stated by the OP, then you need to measure the pitch diameter, not the OD. Measuring the pitch diameter implies that you are taking measurements on the flanks of the thread and those flanks are the most important element in getting a proper fit. The flat at the crest and the fill at the root can vary by a large amount. And while they are important, they are only of secondary importance when making a mating thread.

    That was my point. I am sure that the device that you were talking about is one way to do this pitch diameter measurement. As I am sure there are others.
    And here is the YouTube video that PM referred to:

    YouTube

    At first glance, it looks like a useful device. I can not find pricing on it. I have many digital calipers and it would be very useful to have one set up for measuring the pitch diameter of threads.

  2. #22
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    Hi to all,

    thanks for all the advice on this.

    I agree on the OD-based measurements, partially. Indeed, most screws are always on the smaller side of the nominal OD, but this doesn't mean that, if you make a screw with a spot-on OD, it won't fit a standard nut. It should fit a standard nut.

    So, in principle, especially for our particular case (BSPP 1 3/8), where the OD and the tooth height are clearly specified, one can use them and be ok. However, in order to do so, tool MUST be perfectly according to specs. And here, imho, lies the most important aspect. If your tool is not pefect, a bit more pointy or not, then you've screwed up any attempt to be based on the OD.

    That's why, imho, going with the pitch diameter is a must when you have to be sure about your fit. (plus, you can overshoot the OD and don't pay that much attention to the tool grind. If you get the pitch diameter right, then the fit will be at least sufficient).

    Based on that, I did try my first at the three-wire method.

    I would like the forums assistance, just a sanity check on my calculations, in case I got something totally wrong.

    Checked page 2001 on the 29th Ed of Machinery's Handbook:



    For my case (pitch = 25.4/11, working in metric), I got a 'best wire size' of 1.3 mm. I didn't have anything at 1.3, but had some 1.4 drill bits, used those.

    So, I used the formula for BSW: M = E – 0.9605P + 3.1657W.
    Pitch = 25.4/11
    W = 1.4
    E = 42.844 mm, derived from BSP Threads Introduction -2 - Egmec Industry Co., Ltd.

    So, I concluded that I needed to measure 45.06 mm on my wires to be on the spot.
    It turned out not that hard to do so...I did thread one piece but I am still reluctant to go through with the rest, I will try to see if I can visit the ship and test the fit. Better be safe than sorry....

    (amazingly, moving in 'tooth height' brought me to the proper three-wire measurement! It seems that the time spent on the D-bit grinder on the tool paid out!)

    Please have a look on the above, and confirm if possible

    Thanks again
    BR,
    Thanos

  3. #23
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    Quote Originally Posted by thanvg View Post
    Hi to all,

    thanks for all the advice on this.

    Please have a look on the above, and confirm if possible

    Thanks again
    BR,
    Thanos
    These might make things easier.

    http://f-m-s.dk/2.01.pdf

    http://f-m-s.dk/2.03.pdf

    http://f-m-s.dk/4.03.pdf

    http://f-m-s.dk/1.09.pdf

    They all come from this link:

    Screw thread types

    I've never understood why those that make threads often don't buy the relevant standards and remove "guesswork". The time saved will often more than pay the cost.

    In your case ISO 228 and the thread is now G and not BSPP.

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    Quote Originally Posted by Gordon B. Clarke View Post
    These might make things easier.

    http://f-m-s.dk/2.01.pdf

    http://f-m-s.dk/2.03.pdf

    http://f-m-s.dk/4.03.pdf

    http://f-m-s.dk/1.09.pdf

    They all come from this link:

    Screw thread types

    I've never understood why those that make threads often don't buy the relevant standards and remove "guesswork". The time saved will often more than pay the cost.

    In your case ISO 228 and the thread is now G and not BSPP.
    Thanks for your reply. These documents are inline with what I've done till now for measuring the thread.
    (though, their catalogs failed to carry the thread under discussion, 1 3/8....)

    Regarding buying 'standards', is it go/no-go gauges you are referring to? I am afraid the income from this work will never come near to the cost of these gauges, and it is highly unlikely I'll ever need them again. If I even find a 1 3/8 gauge....Whereas the three-wire or some other measurement method will be there for the next thread as well.

    Saying this, these flexible-measuring-systems inserts for calipers look really useful! I'll look into how I can get one!

    (right, it's a ISO 228 – G 1 3/8 A thread)

    BR,
    Thanos

    edit: I checked the pricelist...well, this will have to wait until I can make real money from machining I am afraid....

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    Quote Originally Posted by thanvg View Post
    Regarding buying 'standards', is it go/no-go gauges you are referring to?
    BR,
    Thanos
    No. When I write "standards" I mean publications. I don't think I'd ever refer to gauges as "standard". I guess I have what most would regard as a small library and almost all thread standards.

    When I want a quick answer to a specific thread I use

    ThreadPal - dimensions and tolerances for inch and metric screw threads

    Almost always tells me much more than I need to or want to know LOL

    That isn't a complaint - it's a pleasant fact

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    I was confused because you talked about cost. It is annoying that ISO institute sells thread standards in 2019, but what can you do....I guess most standards are open. If not, available online for sure!

    (this doesn't mean that all versions available are complete, though... See what happened with me and the ISO 228 – G 1 3/8 A thread....most charts DON'T have it!)

    BR,
    Thanos

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    Quote Originally Posted by thanvg View Post
    (this doesn't mean that all versions available are complete, though... See what happened with me and the ISO 228 – G 1 3/8 A thread....most charts DON'T have it!)

    BR,
    Thanos
    The main reason charts don't have it is because of the detail and number of pages in the standards. Add to that it's copyrighted.

    There are 2 ISO 228s

    ISO 228-1 Pipe threads where pressure-tight joints are not made on the threads -
    Part 1: Dimensions, tolerances and designation

    ISO 228-2 Pipe threads where pressure-tight joints are not made on the threads -
    Part 2: Verification by means of limit gauges.


    As far as G 1 3/8" goes then, as has been mentioned twice, it is standard pipe thread but not a commonly used standard.

    In ISO 261 standard metric threads are divided into three groups. 1st, 2nd and 3rd choice.

    If anyone goes for 2nd or 3rd choice then they won't be as available as 1st choice. I'd classify your G 1 3/8" as a 2nd class pipe thread.

    If you buy for example an M8 screw then that alone gives all tolerances. Only if you want a non standard pitch or tolerance should that be added.

    http://f-m-s.dk/1.01.pdf

    http://f-m-s.dk/1.02.pdf

    http://f-m-s.dk/1.03.pdf

    Instead of posting as much you should read as much as possible of the link I sent you

    NOT saying you shouldn't post

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    Many thanks Gordon for all the advice!

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    Hi to all,

    an update on this: my first test piece, measuring according to the charts with the three-wire method was sent to the ship for test. Initial reports gave me a scare, since my friend that tried it reported it getting stuck after only half a turn. We discussed about cutting the thread a little deeper and retesting, but first, we revisited the ship with some tools and cleaned the female thread thoroughly.

    Tested it back again, cap screws all the way in and the fit is just perfect.

    Many thanks for all the support, mainly identifying the thread and pushing for a three-wire method measurement before I send it for testing.

    BR,
    Thanos

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    Quote Originally Posted by thanvg View Post
    Initial reports gave me a scare, since my friend that tried it reported it getting stuck after only half a turn. We discussed about cutting the thread a little deeper and retesting, but first, we revisited the ship with some tools and cleaned the female thread thoroughly. Tested it back again, cap screws all the way in and the fit is just perfect.
    That customer may also be in the market for a bespoke thread cleanup tap.

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    Quote Originally Posted by Joe Gwinn View Post


    That customer may also be in the market for a bespoke thread cleanup tap.
    Well, that was my initial thought exactly. I thought their female thread was damaged and I had no intention to modify my thread to be off-spec in order to fit theirs. Even if they insisted, I'd have offered the cleanup tap without charge...

    But, since it cleaned with just a wire brush....we're all happy!

    BR,
    Thanos

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    Hi to all,

    I thought I should post the final results.



    They needed a 22 mm hex as well on top for an allen key. Not the best finish in the world, but it'll do.

    Threads fit very nicely.

    Thanks to all for all the assistance and the advice. I got out a little better from this one.

    BR,
    Thanos


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