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Wolfie28/10/2010 08:08:11
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502 forum posts
OK heres another question.
 
In two places on my plans there is a call for threads of 40 TPI and 32 TPI. Now I'm sure TPI= Threads Per Inch.
 
OK whats this all about then?? Why are this not BA like all the others. Is it a standard size, what taps and dies do I ask for. Is this a scam so I have to buy yet more tackle
Andrew Johnston28/10/2010 09:01:36
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7061 forum posts
719 photos
Hi Wolfie,
 
Oeeer, where to start. Whole books can, and have, been written on thread standards. Rather than give a discourse on threads in general here are a few pointers:
 
1) Yep, TPI equals threads per inch, mainly applies to imperial threads, metric threads use pitch
 
2) The 32 and 40TPI threads almost certainly refer to ME (Model Engineer) threads. These are of  Whitworth form but finer TPI. As a corollary of this the thread depth is smaller. Consequently small fittings can be made with internal and external threads without looking 'chunky'. The original Whitworth series is pretty coarse, hence the introduction of BSF threads (British Standard Fine) within industry
 
3) BA (British Association) threads start at 0BA (just under a 1/4", actually 6mm and 1mm pitch) and work their way down in size, and up in number, based on a geometric progression. So despite the name BA threads are actually based on metric measurement.
 
4) When all is said and done, don't be afraid to change the design to use a nearby thread size for which you already have the taps and dies
 
Regards,
 
Andrew
ady28/10/2010 09:35:40
612 forum posts
50 photos
A 1mm thread is 25 tpi
0.75mm thread 37tpi
0.5mm 50 tpi
 
simples!
GoCreate28/10/2010 10:11:10
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387 forum posts
119 photos
Hi
I believe BA threads are actually of metric origin and not imperial, their size being determined using a geometric progression going down from 6mm.
 
I think the French are to blame
 
????
 
Nigel
wheeltapper28/10/2010 11:10:49
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424 forum posts
98 photos
Hi
it really depends on the diameter of the material to be threaded.
for instance an M8 bolt is a standard size but you can put the same thread pitch on a piece of steel 3" in diameter.
 
I don't know what your'e building so I'm not sure what the thread applies to.
 
Roy
Wolfie28/10/2010 12:00:14
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502 forum posts
OK thanks all.
 
Right then, so if 0.75mm = 37 TPI, then I can switch 1/4" x 40 TPI for M6 x 0.70 and be close enough?
 
Its for the steam gland thing that goes into the front cylinder cover and carries the piston rod.

Edited By Wolfie on 28/10/2010 12:02:14

Edited By Wolfie on 28/10/2010 12:02:32

Andrew Johnston28/10/2010 12:37:40
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7061 forum posts
719 photos
Hi Wolfie,
 
Errr, no you can't swap to M6 by 0.7mm, because that's not a standard thread. Well, you could get a special tap and die made, but it'd cost an arm and a leg. Ordinary metric threads come in two forms, coarse and fine. If a drawing calls only for the size, then it's understood that it refers to the coarse series. If the fine series is wanted then the pitch needs to be specified as well.
 
As an example, if a drawing calls for M6 it is understood that this is the coarse series, ie, M6 by 1mm. If the drawing says M6x0.75, then that's the fine series.
 
So you could use M6x0.75 as a substitute.
 
Complicated isn't it!
 
Regards,
 
Andrew
Wolfie28/10/2010 13:32:00
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502 forum posts
Yes sorry I was referring to the standard M6 size, but I'm not at home and can't look at my metric tap and die set haha.
 
So M6 x .075 will suffice to stop me buying 40 TPI x 1/4" ?
KWIL28/10/2010 13:55:38
3681 forum posts
70 photos
Yes if you are making the matching part, but if you need to buy something from the trade, probably not.
Stub Mandrel29/10/2010 21:15:46
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4318 forum posts
291 photos
1 articles
Hi Nigel
 
BA was based on Swiss practice, but the  British Standard defined the sizes in inches!
 
Neil
KWIL29/10/2010 23:25:13
3681 forum posts
70 photos
Same applies to plumbing fittings the threads are imperial, but the continent defines them in mm.
Bogstandard30/10/2010 08:17:38
263 forum posts
You shouldn't really play about with mixing up metric and imperial threads on the same job, as metric is defined by pitch (the amount the screw moves forwards in one full revolution) and the imperial system of threads per inch (TPI).
 
But when attempting to find a somewhere near compatible metric thread for replacing an imperial one, it is fairly easy.
 
Divide 25.4 by the pitch of the metric thread.
 
So for a 6mm course (1mm pitch) = 25.4÷1= 25.4TPI
 
and 
 
6mm fine (0.75mm pitch) = 25.4 ÷0.75=33.8TPI
 
So looking at the results, the 6mm fine pitch would be a fairly good replacement for
1/4" x 32TPI.
 
Normally, the main reason for all the different pitches and TPI's is actually to do with the strengths of the parent metals.
 
You would use a coarser pitch in softer materials as there would then be less risk of stripping the threads.
 
You also have to be aware that most imperial threads have a 55º thread angle whereas metric and the US system use a 60º angle, then you come to the famous BA system which for some reason uses 47.5º.
 
I hope this has helped to clear up a few points, and I am sure there will be some who can make it as complicated as you would ever want.
 
 
Bogs
 
Terryd30/10/2010 09:20:46
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1946 forum posts
179 photos
Hi Wolfie,
 
it should also be pointed out that unlike other thread systems ME (Model Engineer) threads use the same tpi on all diameters of thread.  So a 1/8" 40 tpi thread of a given length will have the same number of threads as a 1/2" 40 tpi thread of the same length.  The same applies to 35tpi of course.  This has the advantage that a necessarily short stub thread such as a gland or a screw cap will have the maximum number of threads and often look more in scale. 
 
Also if linear movement is required using a thread (as in a lathe leadscrew) the same can be gained using a 1/8" thread as with a 1/2" diameter thread e.g. a 40 tpi thread gives a convenient 25 thou linear movement per rev and depending on the robustness demanded by the application you can choose your diameter accordingly.
 
However in most other thread systems there is a relationship between the tpi and it's diameter.  hence the larger the thread, the coarser it is  .Although metric fine is a bit different as one pitch covers a range of diameters (e.g. 3mm to 6mm metric fine uses a pitch of 0.5 mm and comes in 0.5 mm increments in diameter etc) and is closer to the ME series.  So you can get away for most purposes with a set of Metric coarse and fine threads, most other high quality engineering countries do pretty well with these for most purposes.
 
As I personally feel closer to my Continental cousins rather than my New World ones my work is mostly Metric if I can possibly avoid imperial although I can work easily in both (often at the same time!) , I use Metric fine and coarse for most purposes, but I do supplement these with a set of ME for special applications.  I have dumped Whit, BSF, BA, ANC and ANF thread systems and all the other special purpose systems which go on ad nauseum and find that these 4 systems provide all I have needed for years.
 
In answer to your other question it is not a deliberate attempt to simply empty your pockets just that these systems have developed according to needs over many long years and are just now being standardised as the 'preferred' systems.
 
Terry
Gordon W30/10/2010 10:21:27
2011 forum posts
Don't forget that whatever thread system you use ,you might need to buy compatible parts, in model eng. these could be in ME, BA, metric or anything in between. I use mostly metric coarse, because that's all I can easily buy locally.
Andrew Johnston30/10/2010 12:18:10
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7061 forum posts
719 photos
But what about the metric constant pitch series; isn't that just the same concept as the ME threads?
 
Regards,
 
Andrew
Terryd30/10/2010 15:14:38
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1946 forum posts
179 photos
As far as I understood Andrew, constant pitch metric threads are only available in larger sizes  Could you point me to some references so that I could do some more research, my efforts with Google etc have drawn a blank on this subject.
 
Terry
chris stephens30/10/2010 16:33:51
1049 forum posts
1 photos
Hi Guys (Terryd),
Some of us are asked to make replacement parts for old equipment be it car, motorcycle or washing machine. Having sets of "old" threading equipment is quite useful.
If making new stuff the first thing I reach for is a Metric tap and die, for convenience  not for some moral crusade.
At present I am making some bolts for a friend's pre-war Humber, the heads are Whit size but the thread is metric fine, and recently I made some bolts for a Lancia that had metric spanner sizes but Whit threads, how do they fit in the equation I wonder?
chriStephens 
Andrew Johnston30/10/2010 17:16:48
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7061 forum posts
719 photos
Hi Terry,
 
Yes, you are correct, Machinery's Handbook lists constant pitch metric threads from 8mm to 300mm.
 
I have used them once, to tap a M10 x1mm hole in the brake system for my glider for a replacement part. The original M8x1.25 bolt kept breaking.
 
Regards,

Andrew
John Olsen30/10/2010 22:59:10
1294 forum posts
108 photos
1 articles
BS3643 as quoted in Tubal Cains "Model Engineers Handbook" lists three metric constant pitch series in sizes that would be useful to us:
4 mm to 5.5 mm with 0.5 mm pitch
4.5mm to 12 mm with 0.75mm pitch
6mm to 18mm with 1mm pitch.
 
I have no idea about general availability of taps for these ranges, although I have a few of them.
 
regards
 John

Wolfie30/10/2010 23:23:16
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502 forum posts
I have used them once, to tap a M10 x1mm hole in the brake system for my glider for a replacement part.
 
Gliders have brakes?? 
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