Torn thread - cases where this is actually intentional? Survey/Quiz
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    Default Torn thread - cases where this is actually intentional? Survey/Quiz

    I just came across a manufactured item which would fail QA if the thread form were not torn enough. (in the roots rather than the crests).

    Bit of a turn-around from the usual situation. It's apparently quite expensive to keep the degree of tearing consistently between acceptable bounds, and it certainly rules out the thread being rolled. Must be quite tricky defining the limit samples...

    Anyone come across this or another instance? I'll leave posting the example I encountered for a week or so, to give free reign to the usual bun-fight.

    With any luck someone will have worked in a plant producing this item and might be able to enlighten us on how, for instance, you avoid killing the tool edge on long production runs. (There are quite a few of these items on every product shipped, hundreds in fact)

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    Makes for a hard disassembly. Possibly no salvaging the part either without renewing threads.

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    i'm all ears on this one...

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    I have seen cases where an 11.5 tpi and 12 tpi (bolt/nut) have been used to form a self locking unit. The threads start to bind up after about 4 turns. Was told by an old fella that this intended many moons ago for that purpose, well before thread locker stuff.

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    Could be screws for holding replacement teeth in jawbone? They may need to be textured to help the bone grip the screw.

    - Mike -

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    Some t-nuts have a spoiled bottom thread.

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    Troup you said

    "And it certainly rules out the thread being rolled"
    ---------------------------------
    If It is a bolt thread that you want, then I would have thought that rolling IS the way to go for leaving a torn looking root to the thread.

    The roll tool-bit faces would be manufactured to have a ragged edge on the tooth crest, and a smooth root. When rolled the bolt /screw thread would end up with a smooth crest and a ragged root.

    When I worked in IBM hardware , I came across screws the other way round, ie, the ragged bit was on the thread crests of the screws, and these screws were self tapping into steel or alloy undersized holes.
    -------------------------------------------

    Alan
    I would not fly on an aeroplane with those kind of 'Locknuts'

    When you think about it, initially, there is only ONE thread holding together the items that are bolted together.

    You May remember the Super yacht called DRUM. Belonged to Singer Simon Le Bon. It's Keel fell off it seems because when the keel was bolted on, washers and FULL nuts were screwed on on the inside of the hull and then Half nuts were screwed on as locking nuts. In putting the nuts on in that order, when the locking action was performed with two large spanners, the half nuts ended up taking all the load and the full nuts were left doing nothing.
    There were not enough threads in the half nuts for the job and in heavy weather, a sudden thread failure resulted. This sudden failure then put an IMPACT load on the full nuts which also failed because of that impact, so the keel fell off and the yacht turned over, as it would!!

    11-1/2 and 12 TPI threads locked is an even worse scenario.

    Rgds David C

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    Quote Originally Posted by davycrocket View Post
    Troup you said

    "And it certainly rules out the thread being rolled"
    ---------------------------------
    If it is a bolt thread that you want, then I would have thought that rolling IS the way to go for leaving a torn looking root to the thread.

    The roll tool-bit faces would be manufactured to have a ragged edge on the tooth crest, and a smooth root. When rolled the bolt /screw thread would end up with a smooth crest and a ragged root.
    Davyc

    I 'mis-spoke' -- to copy a dodgy coinage --

    it's really the flanks, rather than the root proper, which need to be torn.

    Sorry about the poor choice of words, people...

    The application is tuning pins for pianos. These (unlike tuning pegs on violins, say) are generally parallel, at least where they insert into the structure of the piano. This makes it difficult to adjust the fit when the hole wears.

    If the 'threads' are rolled, the flanks are too smooth and there is insufficient holding torque developed, when the tuning pins are screwed into the wooden 'pin block', to ensure the string stays in tune over time.

    Something about the 'ratchet' microprofile of a slightly torn finish is evidently ideal to create a self-locking finish into the sort of timber used. A rolled finish, even with a textured roll, cannot provide a steep enough trailing edge to the torn pits.

    The pin block is laminated from specially chosen timber with the grain at a variety of angles, so there is end grain pressing against the 'thread' (which looks more like the helix resulting from the coarse feed used for roughing) and providing the maximum possible independence between the wood's moisture content (which follows prolonged swings in humidity) and holding torque.

    I learned about this because my German grand piano turned 100 years old recently, and while it's in excellent repair, a recent shift into a much drier house than it has been used to has caused recurrent problems with individual strings falling out of tune.

    After the third visit the technician said nothing could be done and it was time for a new piano. At $20k, I begged to differ, and a careful Google search showed, as I suspected, that pins are routinely available in a whole series of oversizes for exactly this contingency. If they weren't, I was going to set up and make a batch, which would mean trying to achieve the right sort of "bad" finish, something I've never had to try to do.

    The quandary for piano makers is that if they drill the holes for the tuning pins to facilitate tuning the piano when new, the holes will wear oversize within a few decades. (Tight holes are a nightmare for tuners, because they're always overshooting the correct angle, whether raising or lowering the pitch)

    It's still generally arranged this way, but it seems to me pretty unsatisfactory when there's a strong cast iron frame right there, from which a decently engineered arrangement could be contrived.

    Several patents have been filed, but I don't think they've been widely accepted. Piano makers (like pianists) tend to be very conservative.

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    Default Interesting (I think) side issue

    Quote Originally Posted by davycrocket View Post
    ....
    You May remember the Super yacht called DRUM. ..... It's Keel fell off it seems because when the keel was bolted on, washers and FULL nuts were screwed on on the inside of the hull and then Half nuts were screwed on as locking nuts. In putting the nuts on in that order, when the locking action was performed with two large spanners, the half nuts ended up taking all the load and the full nuts were left doing nothing.
    There were not enough threads in the half nuts for the job and in heavy weather, a sudden thread failure resulted. This sudden failure then put an IMPACT load on the full nuts which also failed because of that impact, so the keel fell off and the yacht turned over, as it would!!
    Hmm.... I followed this failure with some interest at the time, and I have a somewhat different explanation on file.

    The cross section attached below is taken from the official report, and one reason I was interested (apart from the marine engineering angle which was of professional interest) was that I had sailed on the Southern Ocean leg of the delivery to the UK of Drum's sistership (Lion NZ) which had, at least by design, the identical keel attachment detail.

    We did have occasion to heave to a couple of times under stormsails, (just a trysail, on at least one occasion) about 2500 miles from anywhere, whereas Drum was close inshore when her keel fell off, so I was interested to know whether our keel had been compromised.

    Even more interested was Peter Blake, skipper of Lion.
    His understanding was that <<Drum's keel had snapped off at the hull when welds in the upper keel frame structure failed (see sketch below)..... most maxi keels are fabricated in aluminium. The required amount of lead ballast is securely moulded into the lower part of the aluminium boxwork. The finished product is welded to an aluminium gridwork built into the floor of the yacht and through-bolted into the internal framing..... With Drum, however, it seemed that the work had not been done to specifications. The keel had flexed from side to side and finally snapped off, leaving the keel bolts still in the hull. (My emphasis, seems to be confirmed by second photo)
    The Woolfson Unit at Southampton University, in an independent investigation into the mishap, found that the all-important welds were porous and had insufficient penetration, or none at all.>>

    Lion's keel was removed in any case in the UK prior to the race, as the configuration had been redesigned and a new keel built, lighter and further aft (but not the attachment detail, which was considered satisfactory)

    I'd be interested to know who supplied the explanation you heard, if you can recall? I guess it's conceivable both failure modes were present, but in that case I'd be surprised to see the 'aluminium gridwork' external member sitting flat against the hull all the way along, as it seems to be in the second photo. I'd have thought it would have started to peel of at one end, perhaps triggering the weld failure referred to above.

    Coincidentally, I was looking at Lion NZ just earlier today, and went on board her two days ago. She's just back from commemorating her Sydney Hobart win 25 years ago, her maiden voyage (in the course of which over 100 boats retired or broke, including her main competition).

    This time, forty boats preceded her across the line at Hobart - partly a comment on the weather, which was too light for her liking, but also a telling indication of what has happened to the speeds achievable.

    Boats are routinely losing keels today, but that's another story (canting keels, using engineering which -inexplicacably to me - completely fails to make use of one of the unique capabilities of hydraulic cylinders and their control circuits to limit the bending moments on the keel, and the loads on the ram and attachments)
    Attached Thumbnails Attached Thumbnails keel-attachment-detail-1-drum.jpg   drum-sans-keel.jpg  

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    Quote Originally Posted by HelEx View Post
    Could be screws for holding replacement teeth in jawbone? They may need to be textured to help the bone grip the screw.

    - Mike -
    Well, that's mighty plausible, as well as being very close in principle to the situation I encountered.

    What's more, wood and bone are both organic materials with their own inherent and very tough structures, involving longitudinal tubes.

    I reckon you get the prize .... although I'm not sure what that is ! Muchos kudos, at the very least.

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    There were not enough threads in the half nuts for the job and in heavy weather, a sudden thread failure resulted. This sudden failure then put an IMPACT load on the full nuts which also failed because of that impact, so the keel fell off and the yacht turned over, as it would!!


    Even without Troup's post this mode of failure sounds implausible. The thread in the locking nut only has to plastically deform a very small amount to have the main nut take up the load.

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    Hi again Troup

    The scenario that I described was one that I read somewhere, in a UK Yacht magazine I think. Dont remember where exactly.

    Your reply puts you right at the official report level, so the account that I mistakenly gave needs to be disregarded by readers of this posting.

    And Hdpg
    I can see that you have a point. However Engineering drawings do show the half nut on first, OR 2 full nuts.
    -----------------------------------------------------

    So Alan
    That just leaves me with a determination not to fly on a plane with 12 TPI locknuts on 11and 1/2 TPI bolts.!!
    -------------------------------------------

    Rgds David C

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    I was always under the impression that a lock nut was to be torqued to a value that is less than that of the nut it is locking. I can't remember where I got this from, but it has been with me for many years. Is this so or what? If so, is there any rule of thumb for the relative torques?

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    I have a old clapped out engine lathe that produces a torn finish all the time. The headstock bearing are bad and the shaft wobbles perceptibly from side to side as it turns. This makes it very difficult to get a good finish on some pieces and turning with a cutter having no nose radius inevitably leads to a torn finish regardless of the fineness of feed.

    Are you running a humidifier in the room with your piano? I know with pipe organs it's a requirement in dry climates. Otherwise the only sound they make is a wheeze.

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    Quote Originally Posted by starbolin View Post
    I have a old clapped out engine lathe that produces a torn finish all the time. The headstock bearing are bad and the shaft wobbles perceptibly from side to side as it turns. This makes it very difficult to get a good finish on some pieces and turning with a cutter having no nose radius inevitably leads to a torn finish regardless of the fineness of feed.
    Perhaps you could do a lucrative sideline in piano tuning pins

    Quote Originally Posted by starbolin View Post
    Are you running a humidifier in the room with your piano? I know with pipe organs it's a requirement in dry climates. Otherwise the only sound they make is a wheeze.
    Yup, and it gets through about a gallon of water on a dry day. Might have to rig up a header tank and a humidistat for times when it's left unattended for a period...

    Thanks v much for the tip, btw


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