Difference between revisions of "Identifying a thread"

Revision as of 08:37, 15 April 2010

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

Difference between revisions of "Identifying a thread"

Revision as of 08:37, 15 April 2010

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