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| ==Introduction== | | P0d9VT <a href="http://qantyzmpepsz.com/">qantyzmpepsz</a>, [url=http://nrlmjaaiefps.com/]nrlmjaaiefps[/url], [link=http://dwgimatejglc.com/]dwgimatejglc[/link], http://nwukhsimpvjj.com/ |
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| This article is a straight cut and paste of a very useful description posted by Dave Baker on identifying threads - and in this particular case a 14mm thread. The original question:
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| <pre>
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| "Need to find out what this thread is - it's on an air temperature sensor for a car. Its dimensions (male) are:- | |
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| Thread outside diameter Threads per thingie
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| metric 13.8mm 1.5
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| imperial 0.54in 16 tpi"
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| </pre> | |
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| ==Procedure==
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| After 30 years of engineering and accumulating hundreds of tools, thread
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| gauges are something I've never felt a pressing need for. Provided you have
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| an accurate steel rule or preferably a digital vernier and know a few basic
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| facts it's pretty easy to identify most threads. Even if diameter and pitch
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| are similar you always have thread angle to help distinguish types. Occam's
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| Razor says that most threads you'll come across in anything modern are
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| metric so it's best to check for those first.
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| ==Metric Threads== | |
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| O/d will always be a tad under nominal so the bolt doesn't bind in the nut
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| or female thread. For pitch I measure crest to crest over 10 threads and
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| divide by 10. Hopefully it'll be a nice round number of mm or at worst a
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| half mm. You can use a digital vernier by eye across thread crests to at
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| least 0.25mm (10 thou) or better so you can work to an accuracy of 1 thou
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| per thread which is ample. Easiest way is to preset the vernier to the
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| expected measurement i.e. 15mm if you think it's a 1.5mm pitch rather than
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| trying to adjust it manually to line up with the crests. It's actually even
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| quicker with a steel rule marked in 0.5mm increments. If that lines up cock
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| on over 10 pitches and the o/d is just under an integral number of mm then
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| usually you're done.
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| A very useful set of things to have handy is one each of nice new non-rusty
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| bolts in 6mm, 8mm, 10mm and 12mm sizes. The thread pitches on these will be
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| 1mm, 1.25mm, 1.5mm and 1.75mm respectively. That covers most of the normal
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| pitches you'll ever encounter in metric threads apart from the very small
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| sizes of 5mm and under and the big stuff. If you can mesh the appropriate
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| bolt with the teeth of the one you're trying to identify and you can't see
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| any gaps against a good light then job's a good un. The o/d of the bolt is
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| immaterial - for a given pitch the thread form will be exactly the same on
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| any bolt.
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| While I'm on the subject of metric thread pitches I guess I could cover how
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| these really work. The standard pitch for any size metric bolt is always a
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| coarse thread. In fact the coarse pitches were taken pretty much from
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| similar sized UNC bolts which were designed for threads in coarse grained
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| weak brittle materials like cast iron and cast aluminium. Finer pitches like
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| UNF are used in finer grained stronger, or more ductile, materials like
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| steel, forged aluminium, brass, bronze etc and there are metric fine pitches
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| for similar use. However you never see anything coarser than the standard
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| coarse metric pitch even though there may be many finer variants for each
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| size. You'd only need an even coarser pitch for even weaker materials like
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| wood and then either you'd actually use a wood screw or drill right through
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| and use a bolt and nut.
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| Finer pitches create a stronger bolt because the core diameter of the bolt
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| is larger i.e. the pitch depth of the thread is smaller in exact proportion
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| to its length. Bolts that take very high loads such as conrod big end bolts,
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| flywheel bolts etc will always be fine pitch with as big a core as possible.
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| Common sizes for these are 8mm, 9mm and 10mm all of which use a 1mm pitch.
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| The 60 degree tooth angle on metric (and also UNC and UNF) bolts was very
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| cunningly designed for easy use. The top and bottom of the threads are
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| rounded to avoid stress raisers and in such a way that if you deduct the
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| thread pitch from the nominal o/d you always get the tapping drill size
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| which therefore, near as dammit, will also be the core diameter of the bolt.
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| For example the tapping drill for an 8mm x 1.25mm standard coarse bolt is
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| 8-1.25 = 6.75mm so it's an easy calculation to do in your head and no need
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| to carry a Zeus book around in your pocket all the time. This applies
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| equally to UNC and UNF except that you have to do more calculations because
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| the pitch is shown in tpi not length.
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| For fine pitches there can be many variants but the smallest fine pitch
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| you're ever likely to see is 1mm. There are in fact fine pitch variants even
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| for bolts of 5mm and smaller but you'll never come across them except in
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| very specialised equipment.
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| Your 14mm bolt is quite a good example. The standard coarse pitch is 2mm but
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| by far the most common thread in that size is actually 1.25mm which is the
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| standard spark plug size fitted to nearly every petrol car engine ever made.
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| Only the 18mm x 1.5mm Ford Pinto engine plug and the more recent 12mm, 10mm
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| and even 8mm plugs fitted to motorbike engines and the like will be
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| different. There are also 1.5mm and to a lesser extent 1mm pitches in common
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| use on the 14mm diameter.
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| Now here's a thing and to be honest I've only just thought of it. I said
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| above that coarse pitches are best used in coarse grained and weak materials
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| but all spark plugs are fine pitch and yet they screw straight into the cast
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| iron or cast aluminium of the cylinder head. However when they seize they
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| strip the threads right out of the head which probably wouldn't happen if
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| they were standard coarse pitch better suited to the material they screw
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| into. Of course it's far too late to change that now because it's an
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| industry standard but probably not a very clever one. I digress.
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| ==Non metric threads== | |
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| Once you've ruled out metric then the job gets easier. Imperial threads
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| don't generally have variants. UNC is the standard imperial coarse and UNF
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| the standard imperial fine. On old machinery you might come across Whitworth
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| which is an early coarse or its fine equivalent BSF. Both of these have 55
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| degree thread angles not 60 degree ones. BA can also be found on electrical
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| gear but it's easy to spot because of the very pointy 47.5 degree thread
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| angle. It won't come close to matching a 60 degree metric bolt even though
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| the diameter and pitch can be quite similar. You can actually screw many BA
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| bolts into metric female threads but not vice versa.
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| For hydraulic fittings it's usually NPT or BSP. These are quite distinctive
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| and often tapered.
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| ==Additional Comments==
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| Since Dave's original article above was published, some addition contributions have been added:
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| For more information on American thread forms see [http://www.sizes.com/tools/thread_american.htm here]
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| ==See Also==
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| * [[Special:Allpages|Wiki Contents]]
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| * [[Special:Categories|Wiki Subject Categories]]
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| [[category:fixings]]
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